Cooling Tower Flow Distribution and Variable Flow in Condenser Water Systems

One of my current obsessions is how subtle details regarding how you pipe a cooling tower can make a huge difference in how the flow is distributed.  I’ve been interested in that for a long time actually as the result of an early field experience.   And it looks like the building where that happened  is still there and the towers even seem to be in about the same location all-thought I am sure they must be newer versions relative to the the time I was last on the roof (with, I hope, a better piping arrangement).


The building is in downtown St. Louis and Bill Coad sent me there to figure out why one tower basin was overflowing while the other one was making up.

When I tried my hand at calculating the pressure drop in the two different runs of piping, from the point where they came together at a tee to the point where each run connected to a tower cell, I came up with a difference in pressure drop 0.15 psi.  It was the first time I had tried to do pipe pressure drop calculation for a practical reason and for a while, I thought I had made a mistake.   But had I realized then what I know now, I would have realized that the different water levels were telling me the answer with out having to do the math.  In other words.  the 4 inches of level difference I was seeing in the field is what you get if you multiply 0.15 psi by 2.31 ft.w.c. per psi and 12 inches per foot.

The experience convinced me that symmetrical tower piping, flumes, and basin equalizer lines are critical for multiple cell tower installations because seemingly inconsequential differences in the pressure drop  in the interconnecting piping can make huge differences operationally.  I took a look at a more recent version of a similar situation in a magazine article I wrote for Consulting Specifying Engineer a while back that you can download from FDE’s commissioning resources website if you are interested.

My initial experience and the article I reference are focused on the pressure drops in the piping leaving the towers.  But similar things can happen if the piping is not symmetrically arranged to the hot basins and distribution headers of multiple cell cooling towers (the pipe entering the tower).

Recognizing this has always been important, but I think it may be even more important now due to the energy conservation driven desire to:

  • Design and operate variable flow condenser water systems where we vary the flow to manage head pressure, or
  • Implement an energy savings strategy where we spread the flow that would normally go to one tower cell out over two or more cells to leverage the cube rule for fan energy savings.

If you don’t recognize the importance of uniform flow distribution over the tower cells and the role that piping configuration will play, then your energy savings measure may not deliver the anticipated savings.  In fact, you could also cause damage to the cooling tower fill, significantly shortening its life span and setting up water quality control problems.

The discussion that follows will focus on towers that use a hot basin with orifices to distribute flow over the fill (generally induced draft cross-flow towers), which look like this.

image  Cooling tower farm

Cooling tower 01  DSCN1738

But similar concepts apply to towers that use a manifold with spray nozzles such as induced draft counter-flow towers, which look like this …

Cooling tower piping 02  Cooling tower piping 03

Cooling towers from above  NT Cooling tower yard 05

… and forced draft counter flow towers, which look like this.

Forced Draft Tower 01  Cooling tower 02


That said, there are induced draft cross-flow towers that utilize pressurized feed distribution systems.  In fact the video on our website showing a pressurized distribution system in action is from just such a tower.

Incidentally, you can find more images of different types of cooling towers on the What’s That Thing” page of FDE’s commissioning resources website if you are new to cooling towers.  There is even a couple of pictures of a tower that uses the water jets to induce the air flow through it instead of a fan, which is not a very common configuration.


Our focus in this blog post is going to be on how hot water is typically distributed over the fill in cooling towers and how flow reductions can lead to poor flow distribution if you go to far. The links below will jump you into the post to a particular topic of interest. The “Back to Contents” link at the end of each section will bring you back here.

  • Gravity Feed Hot Basin Type Distribution Systems
  • Pressurized Feed Distribution Systems
  • Nozzle Curves
  • How Gravity Feed Distribution Systems Work
  • How Pressurized Feed Distribution Systems Work
  • Issues Associated with Non-uniform Water Flow Distribution
  • Non-uniform Water Flow Distribution Bottom Lines
  • Weirs and Cups to the Rescue
  • The “Nuggets”

    Gravity Feed Hot Basin Type Distribution Systems

    Cross-flow, induced draft towers typically gravity to distribute water over the tower fill via a hot basin with orifices in it.  Here is a picture of what that type of distribution system looks like out in the field.

    CT-2 West Basin (Notice dry spots)

    Each of the little round black things is an orifice with a deflector plate mounted below the hole.

    A Typical Hot Basin Nozzle

    Here is a close-up of an orifice and nozzle from above (left) and from the side (right) which shows the deflector.  This particular nozzle is from a BAC tower.

    Tower-Nozzel-03_thumb9  Tower-Nozzel-01_thumb9

    Here are pictures of a nozzle from an Evapco tower (to the left) and a Marley tower (to the right, courtesy Ryan Stroupe of the Pacific Energy Center) which are similar, but slightly different from the BAC approach.

    Cooling Tower Spray Nozzel  Hot Basin Orifice

    As you can see, while the details differ from manufacturer to manufacturer, the general idea is the same;  to leave the hot basin, water has to flow by gravity through the orifice and then the stream of water hits the deflector plate and is splashed out over the fill in all directions.

    Back to Contents

    Pressurized Feed Distribution Systems

    This type of distribution system is typically found in cross-flow towers, both forced and induced draft.  This action packed video clip illustrates what that type of design looks like in operation.

    I don’t have many pictures of these types of nozzles, but here is a close-up of some spray nozzles in a distribution header from a small forced draft cross flow tower.

    Spray Nozzels

    The manifolds are sitting vertically leaning against the outside of the tower because they had been removed to repair damage that occurred when the condenser water temperature got out of hand and ruined the fill.   If you look closely, you can see bits of the fill that had been picked up and circulated by the pump caught in the outlet of the nozzles.

    Back to Contents

    Nozzle Curves

    The flow rate through the nozzle is a function of its diameter, the details of its shape (rounded edges on the opening, etc.) and the depth of water over it.  Tower manufacturers can provide you with a set of “nozzle curves” that document the flow that will be achieved with different nozzle sizes and designs at different water depths.  Here is an example of the curves for  Marley NC tower.

    Nozzle Curves

    This is a similar example of the curves that apply to a Baltimore Air Coil Series 1500 and Series 3000 tower.

    Pages from Weir Dams & Flow

    You can get similar curves for the spray nozzles that are used in towers that use a pressurized distribution system where the water is distributed from a set of headers over the fill with nozzles that are basically like shower heads.

    Using the Nozzle Curves to Determine Tower Flow

    If you have the nozzle curves, you can use them to assess the flow rate over the tower.   The trick is to determine the nozzle that is in place and then measure the basin level.   You can then look up the flow rate for the nozzle with that water level over it and multiply it by the number of nozzles in the basin to come up with the gpm going through that basin (assuming the nozzles are not plugged).

    You then repeat that procedure for the other basins on the tower and add up the results to get the total flow for the tower.

    Note that this is also a way to quickly assess if the flow over a tower or number of towers is balanced between the towers and between the basins in the towers.  If  the towers and orifices are all the same, then the flow is likely well balanced if the water levels in all of the basins are the same.

    In contrast, if the level in one hot basin is higher than the others, it is probably getting more flow than the others.  Similarly, a basin with a lower level than the others is probably getting less flow.

    Nozzle Curve Based Flow Assessment Caveats’

    A couple of caveats’;

    One is that you have to have the nozzle curves or do the math on the orifice so you have the relationship between the water level in the basin and the flow rate per nozzle.

    Another thing o take into consideration is that at low flow rates, the water level in the basin will probably vary, being higher near were the water enters the basin and lower at the most distant point relative to where the pipe connects to the basin.  So, you may have to divide the nozzles up and assess them at different water levels or use and average water level.

    And, of course, the nozzles need to be clean.   It is not uncommon for flakes of metal to break loose from the condenser water piping and become lodged in the orifices, especially in older piping that has some corrosion accumulating in it.  A sudden, radical change in water treatment can sometimes trigger this, as can operation at a new, higher flow rate.

    Finally, towers can have weirs or cups in the basin to force water to flow preferentially through some orifices before flowing to others.  More on that in a minute, but first, lets look how the system works in the first place.

    Back to Contents

    How Gravity Feed Distribution Systems Work

    To explain how a hot basin type distribution system works, which is something you have to understand if you want to understand the constraints on varying flow over a tower, I am going to use images from the cooling tower model I use in class exercises.

    If you want to work with this model directly, you can download it from the Cooling Tower Scoping Exercise page on our website.  There are actually a number of retrocommissioning opportunities in the model so you can try your hand at scoping them out if you want to.   You will find the answers and related information available for download on the web site also, as well as a scene guide that will help you navigate through the model.

    Returning to our discussion, in a perfect world, to get uniform distribution over the tower fill from this type of distribution system, you would like to have the water distributed as a sheet of uniform depth across the entire basin area where the nozzles are located.   The problem is that all of the water arrives in a pipe that will connect to the basin at a single point, concentrating a large volume of flow in a small area relative to the area covered by the basin.   To solve that engineering problem, designers of distribution basins of this type use a combination of manifolds and the flow orifices to create the sheet of water and manage it’s depth.

    Setting Up the Discussion

    For the discussion that follows, I will use the image below, which is from the model I mentioned.  Note that the transparent grayish color is how I represented water;  for instance, the area that the “A” points to is completely full of water.   The two arrows on the “D” illustrate the basin water level by pointing to the bottom of the basin and the surface of the water in it.


    Water enters the basin at the piping connection at Point A, which places it inside a triangular shaped manifold (the corrugated metal panel is the hypotenuse of the triangle).

    Since the only way out of the manifold is the slot at the bottom (point “B”), the water is generally forced to spread out across the length of the basin.  Thus, the manifold takes on the roll of creating a sheet of water that covers the width of the basin.

    Once the sheet of water is established, the the size, number, and arrangement of the orifices (the round object at “C” is a typical orifice) take over and generally force the sheet of water to extend across the basin and control its depth.

    Still Not Sure How This Works?   Try This Thought Experiment

    One way to understand how this works is to imagine what would happen if there literally was no bottom to the basin to the left of the slot in the manifold at B.  If that were the case, I think you can imagine that the water would simply cascade out of the manifold making a little water-fall of sorts onto the fill in the area where the “B” is.  The fill further to the left (towards “C” and “D”) would receive little if any flow.

    In contrast, if the basin had a bottom, as it does in the illustration, but it only had orifices at the far left (where the “D” is), then you can probably imagine that the sheet of water created by the slot would have to extend all of the way across the basin to the row of orifices.

    In this case, if the sum of the cross-sectional areas of the orifices tended to be small relative to the cross-sectional area at “B”, then water would tend to “pile up” in the basin;  i.e. the level depth or thickness of the sheet of water would tend to increase.   But, because of how liquids work, as the depth of the water increased, it would provide more pressure to push water through the orifices, which would tend to increase the flow out of the basin.

    And it would also tend to push back a little bit against the flow of water coming in through the slot.  This would be minor;  inches.w.c. of pressure created by the depth of water in the basin vs. ft.w.c. of pressure created by the pump.   At some point, this would come into balance and a steady state condition would be established with the water at a fairly uniform depth across the basin.

    Its a Balancing Act Between Cross Sectional Areas and Small Inch Water Column Pressures

    In the limit, for instance, imagine that the orifices at “D” are pin holes but that 500 or so gpm is coming into the basin.  If that were the case, I suspect you would conclude that the basin would quickly overflow because more water could be delivered by the pump via the manifold than could leave via the orifices.

    Going the other way, you can probably imagine that if the gap at the bottom of the manifold or the orifices (or both) are large relative to the flow rate, then you would tend to get a semi-circular water distribution pattern in the distribution basin, centered on the point where the pipe connects to the manifold.

    Gravity Feed Distribution System Bottom Lines

    The bottom line regarding this type of distribution arrangement is that how well it works depends on on of the flow rate into the manifold, the width of the gap, and the number and arrangement of the orifices in the bottom of the basin.  Even if you get perfect distribution with a given flow rate, gap width, and orifice size and arrangement, if you vary the flow, there will come a point when the desired distribution pattern degrades to the point where some of the fill receives little if any water.

    Back to Contents

    How Pressurized Feed Distribution Systems Work

    If you have taken a shower (and the shower was not a rain barrel shower), then you have a pretty good idea of how a pressurized feed cooling tower distribution system works.   And if you have ever been in a shower with low water pressure, either due to undersized piping or due to a temporary demand for flow in a different part of the system, then you also have a sense of what can go wrong if you reduce the flow rate to the shower.

    For shower heads and spray nozzles to create a useful spray pattern, they need to have some pressure behind them.  If you drop the pressure, the pattern decays and then simply turns to a dribble as the water just sort of falls out of the nozzle or shower head.

    The pictures below, which are snapshots from a short video on our website that shows a pressurized cooling tower distribution system in action, illustrate what happens to the flow pattern achieved as the over-all flow rate is reduced.  This first picture was taken with the tower flow rate estimated to be in the range of 50-60% of its design flow.

    Snapshot - 21

    Notice how the spray nozzles are making little umbrella like spray patterns, generally covering the fill.  Ideally, they should over-lap to completely cover the fill, and I believe that is what we would have seen if we could have increase the flow rate to the design level.

    Here is what the pattern looked like when Gary, the chief engineer, later in the week, switched over to a smaller chiller which probably cut the flow rate to the tower to about 20-30% of its design value (Image from a video courtesy Gary Walters).

    Snapshot - 20

    As you can see, the distribution pattern is not nearly as good and I suspect that parts of the tower fill were starting to run dry, which introduces a number of issues that we will discuss at the end of this post.

    Pressure Feed Distribution System Bottom Lines

    The distribution pattern achieved by this system is also very much a function of the volume of flow delivered to the tower cell relative to its design volume, just as it was for the gravity feed system discussed previously.  With the pressurized system, reducing the volume of flow to a give manifold reduces the pressure in the manifold, which cases the flow pattern to decay and not completely wet the fill.

    Back to Contents

    Issues Associated with Non-uniform Water Flow Distribution

    If the return/hot water flow to a cooling tower is low enough to result in the distribution system failing to fully wet the fill and as a result, some of the fill starts to run dry, a number of problems can emerge.

    Air Flow Short Circuits

    The resistance to air flow of wet fill is higher than that of dry fill.  So, if the fill on a tower starts to run dry, there is a tendency for more of the air to go through that part of the fill vs. the part where the fill is wet.

    Of course it is the air flow over the wet fill that generates the cooling effect, so air that bypasses the wet fill represents fan power that is delivering no meaningful cooling.  In other words, for the current heat rejection requirement, the tower is using more fan energy that it would need to if fill was uniformly wet and all of the air flow was generating cooling.

    imageThere are a couple of video case studies on our commissioning resources web site that illustrate this.   One centers on a cooling tower where a combination of piping configuration and fluid mechanics results in a two cell cooling tower with a gravity feed distribution system spending a significant number of hours in the year with one cell that has little or no flow over it but has the fan running.  In the image to the left, the cell to the top of the tower, closer to the plant has flow, but the cell in the foreground is virtually dry.

    The other centers on a phenomenon that caused one hot basin served from a common header supplying a gravity feed system to run full (left image below) while the other basin ran dry (right image below), even though there were no valves on either side of the tee connecting the incoming header to the hot basins.

    image  image


    If the air velocity through the dry fill becomes high enough, it can cause the  fill to flutter.   If you have venetian blinds, you may have seen a similar phenomenon occur with the blades on a windy day.  In any case, if the fill flutters too much, the movement can lead to cracking and premature failure of the fill.

    Accumulation of Minerals on the Fill

    Even if the cooling tower water quality is properly controlled, if the water flow is so low that on some portions of the fill, the stream of water totally evaporates before it reaches the cold basin, then the minerals in the water are left behind on the fill.  (If you have an aquarium, you are probably familiar with this phenomenon.)

    As the minerals build up, that will tend to make the problem worse.   There are cleaning processes that water treatment companies can perform to remove the minerals, but this is at an added cost above and beyond the normal costs for cooling tower water quality management.

    And, I know of at least one Owner who built a stainless steel pan slightly larger and deeper that a section of the fill in their tower and then bought an extra section of fill so that they could rotate a section of fill out of the tower and soak it in a mild acid solution in their stainless steel pan, to clean it, using the extra section of fill to replace the section removed for cleaning.  So a do-it-yourself approach that probably saves some money but still takes some labor and an initial investment.

    If you let the accumulation of  minerals go unchecked, then eventually, this will happen.

    Fill Failure 03

    This condition cost the Owner of the two cell tower associated with the picture (photo courtesy Sabastian St. John, St. John Consulting), which served a nominal 700 ton plant, about $50,000 to replace the fill.

    That cost is  something that likely would not have been required for another 5-10 years at least if the deposits had not built up.  Some of that cost was because the tower was on the top of a high rise, so getting the ruined fill out and the new fill in was labor intensive.  But even at half that price it’s still a pretty expensive problem.

    Accumulation of Ice on the Fill

    A phenomenon similar to the one that leads to mineral accumulation on the fill of a tower at low distribution flows can also cause ice to accumulate during sub-freezing weather.  The ice accumulation can be even more destructive to the fill than the accumulated minerals due to the weight of the frozen water.  And the build-up can happen much more quickly.

    Back to Contents

    Non-uniform Water Flow Distribution Bottom Lines

    The Driver Behind the Issue

    As I mentioned at the beginning of the post, the desire to save fan and pump energy can cause us to implement strategies that will result in a reduction in water flow over a cooling tower cell.   This is because one of the fan and pump affinity laws states that for a fixed system, the relationship between flow and fan or pump power is cubic.

    Affinity Law - Bhp and Flow

    That means that if I were able to cut the flow rate in half, then I would reduce the power required to one eighth of what it was originally (half of a half of a half).

    Saving Pump Energy By Directly Reducing Condenser Water Flow

    So for example, if I decided that instead of running a steady flow of water through the condenser of a chiller irrespective of the load condition, I would vary the flow to maintain a constant head pressure, then the pump energy I would consume at part load could be drastically reduced, especially if I had a lot of part load hours.

    Saving Fan Energy by Spreading Out the Condenser Water Flow

    Or, I may decide to use two tower cells when one chiller is running instead of one cell.  Assuming a uniform distribution of flow to both cells, this would split the load equally between two cells.  Since the capacity of a cooling tower is nearly linear with air flow, that would mean that with the load for one chiller split between two cells, the air flow rate would be half of what it would be if either cell was used by itself to reject the heat from the chiller.

    In turn, the affinity law cited above (also known as the “cube rule”) would indicate that running the fan for each tower at half speed would reduce that tower fan’s energy to one eighth of what it was at full speed.  Of course, you would be running two fans at that level instead of one at full speed, but in the end, you would have reduced the fan energy to one quarter of what it was with one cell alone serving the chiller (one eighth plus one eighth).

    Capturing the Savings Requires Good Water Flow Distribution Across the Fill

    But if some of the problems associated with low flow rates over cooling towers started to emerge, then I likely will not fully realize the savings anticipated.   That could happen because the air flow short circuits would cause the fan to have to work harder to achieve the same amount of  cooling  as it would achieve if all of the air flow went past wet fill.

    Or, it could happen because the accumulation of minerals or ice on the fill caused its untimely failure, placing a big hit on the operations and maintenance budget.  Or, if the potential to accumulate minerals was recognized and address, it would result in higher on-going maintenance costs because of the added effort and procedures necessary to keep the fill clean.

    Non-Energy Costs Exceed Energy Savings Benefits;  an Example

    On one recent project, I was working with a team in Marriott’s AEP program to assess the cost/benefit of spreading flow out over two tower cells to save fan energy, which was how the system was originally designed. But as a result of non-uniform flow distribution created by the piping geometry, the fill was starting to accumulate minerals because there were times when some of it was running dry.

    This particular plant was in the mild, San Francisco Bay area environment.  That meant that while it occasionally over the course of the year would see its peak load condition, nominally 700 tons, most of the time it was significantly below that.  In fact when the team developed the load profile from measured field data, it revealed that the plant likely spent 80% of its time at 140 tons or less and 90% of the time at 210 tons or less.

    Many Part Load Hours = Low Cooling Tower Fan Energy Consumption

    That meant that even if all of the flow was directed to one cooling tower cell, the fan energy most of the time would have been modest because the VFD equipped fan would not have to run very fast due to the high number of part load hours.

    Certainly, additional savings were achieved by running the flow over both cells and further reducing the fan speeds.  But a lot of times, the low speed limit came into play keeping the fan running at a minimum speed set by the need to maintain lubrication in the gear box, even though that much air flow was not required for cooling the condenser water.

    So at a certain point, the control strategy could no longer optimize the fan speed to the load to capture the theoretical savings that were possible.  In addition, since the fan speed was higher than needed, the fan had to cycle, which added some wear and tear to the system that would otherwise not have been there.

    The Bottom Line

    The bottom line was that when the team took all of these things into consideration, the fan energy savings achieved by running two cells instead of one were more than offset by the added operation and maintenance costs, primarily the added cleaning costs required to minimize the build up of minerals on the tower fill.  And as a result, their recommendation was that automatic control valves be added to allow one tower cell to be associated with one chiller.

    This would increase the flow over the cell and shift the load to one tower fan. But operating in this manner would improving the over-all efficiency of the tower since it’s fill would run fully wet.  And keeping the fill uniformly wet would eliminate the need for spending several thousand dollars on an annual cleaning process to remove the minerals that would accumulate as the result of the non-uniform flow distribution.

    Back to Contents

    Weirs and Cups to the Rescue

    So by this point, hopefully, you understand how cooling tower flow distribution systems work and how at some point, the principles they are based on to provide uniform flow distribution run counter to our desire to save energy.

    But, there are some steps that can be taken to modify the distribution systems so they the towers can accommodate a wider range of flow variations, at least that is the case for towers using gravity type distribution systems.  In general the manufacturers indicate that they can accommodate a 50% reduction in flow rate by using either weirs or cups.


    Several manufactures enhance the range of flow that their towers can accommodate by installing weirs in their basins.   That was the case for this recently installed cross flow, induced draft cooling tower, which is the source of the pictures that follow.


    The air entered the tower from the left and right side in the context of the picture above and exited on top at the center, where the fan was located. There was a hot basin on each side of the fan that distributed water to the fill located below it.

    Water was distributed from a piping connection that was made to a manifold in the center of each basin.   The picture below, which I took while we were opening the basin covers, will give you a sense of that.  You can see the connection to one manifold towards the top of the picture, where we are just getting the first basin cover open.  I am standing on the basin covers for the second basin and you can see the connection to the center manifold in the bottom left corner of the picture.


    The picture below is what I saw when the team I was working with opened the hot basin covers (what I was standing on when I took the preceding photo).  The green pipe in at bottom of the picture below is the pipe in the lower left corner of the picture above.   The basins all had weirs in them in it in addition to the flow distribution nozzles, just like the basin pictured below.


    A weir is just a technical name for a dam and in the picture above, the weir is the metal fence in the basin that is forcing the water to the right side.   That side of the tower is the entering face for the airflow associated with the fill below the basin in the picture and the fill below the basin I was standing over to take the picture.

    That means that the weirs are acting to keep all of the fill on the entering face wet at low flow rates before allowing water to reach  the nozzles serving the fill deeper into the tower.  As the flow comes up, the water level on the entering side of the weir (the right side of the picture above) rises to the top of the weir, wetting the fill immediately below it while the fill further in to the tower is denied water.

    This is an important element in ensuring tower efficiency because if part of the entering face of the fill is running dry, it will be much easier for the air to pass through it, which will reduce the air flow over the wet portion of the fill, which, of course, is where the evaporation that cools the condenser water is taking place.

    If the flow rate continues to come up, the area blocked by the weir fill and then overflows, allowing the fill further into the tower to receive water.  As a result, the entire entering face of the tower will have wet fill as long as the flow rate is high enough for the weir to have an impact.   That in turn means that all of the air flowing through the tower will encounter wet fill.

    At full flow, the water level is a fairly uniform 3-4 inches across the entire basin.  The weir itself is about 2 inches tall.

    This cooling tower had variable flow condenser water and the control process was unstable at the time we were looking at it.  So it was an opportunity to catch weirs in action and we shot a bunch of video.   I am still working to put a narrated version of that together, but for now, I have uploaded the raw footage to our commissioning resources website if you are interested.  The next few pictures are taken from the video and will illustrate in general what happened.

    When I took the picture above, the flow rate had just reached the point where the water level on the entering side of the weir was going to over-flow and start directing water to the fill deeper into the tower.  This is what it looked like when the flow had increased to the point where a significant amount of water was over-flowing and wetting the fill further into the tower.  (Note that these images are from a different basin in the tower relative to the one in the picture above).


    Tower Flow Variation 14

    We had set the cover hold-down bolts upside down into the basin (center of the picture above) so we could use the threads as little level indicators.   In the videos, if you pause them and count threads, you can tell that the levels are changing under different flow conditions.

    In addition, because of the piping configuration, shown below …

    Tower Return Header

    image… due to the dynamics of the flow through the tee, initially, as the flow came up, the level in the basins on the side of the tower served by the branch of the tee came up faster than the level in the basins served by the run of the tee, presumably because at relatively low flow rates, it took slightly more pressure to get the water to flow through the extra feet of pipe and the elbow on the run.

    But as the flow increased and the dynamic losses through the tee started to have an impact, the level in the basins served by the run caught up with the level in the basins served by the branch and then eventually the level in the basins served by the run started to rise faster than the level in the basins served by branch, presumably due to the higher dynamic loss associated with flow through the branch of a tee vs the run of the tee.

    The chart above, based on data from the ASHRAE Handbook illustrates the tee pressure drop phenomenon.


    Another approach to favoring one portion of the cooling tower fill over another is called a “cup”.  Marley uses this approach and while I don’t have any pictures of my own of a tower that has been outfitted with cups (yet), Marley has a great YouTube video that illustrates what they look like and how they work.

    The general principle is the same as for a weir; the cups are arranged so that water has to achieve a significant depth over the nozzles across the entering face of the fill before water is allowed to flow into nozzles serving fill deeper into the tower.

    Back to Contents

    The “Nuggets”

    So, that was a lot of information I guess;  I’m kind of prone to doing that as most of you know.  But there are a couple of nuggets in there.

    One nugget in terms of operations and commissioning is to always/regularly open up the basins on a tower and take a look at how well the flow is being distributed (or not).  What you see may surprise you and be an indicator of an opportunity to improve things or a clue about why you are not achieving the fan energy savings you anticipated via your energy efficiency measure.

    The other nugget is that for cooling towers in particular and open systems in general, it is a game of inches in terms of getting levels between the different basins to balance out.  For instance, a pressure drop difference of  3 or 4 inches (about 0.15 psi) in the return piping leaving two cooling tower basins that are piped in parallel can mean one basin is making up water while the other is overflowing.   If you are curious about that, I wrote an article in CSE magazine a while back that you might find to be useful and you can download it from our commissioning resources website.

    Back to Contents

    So, I guess that ended up being a pretty long discussion for something that started out a while back as answer to a question about air venting.  But hopefully, the information is useful;  if it is, you can thank Kam for asking the question.


    David Sellers
    Senior Engineer – Facility Dynamics Engineering


Posted in Condenser Water Systems, HVAC Fundamentals, Operations and Maintenance, Retrocommissioning Findings | Leave a comment

An Existing Building Commissioning Learning Opportunity

The picture above is a panorama I shot earlier this year in a recently renovated central chilled water plant serving a large high-rise office building in down town San Francisco. The people in the picture are students in the Existing Building Commissioning Workshop Series that I help Ryan Stroupe of the Pacific Energy Center teach. The workshop is a year long, hands on class that is designed to allow the attendees to learn and apply existing building commissioning skills. We have been doing it for 13 years now; time sure flies when you are having fun. (Not to be confused with what frogs are known to say, which is time sure is fun when you are having flies).

The point being that we are about to start the 14th year of the class, so if you are interested in a hands-on, field-based learning opportunity to help you develop existing building commissioning skills, then this may be something you want to consider.

The class is structured around the ten key commissioning skills. Our goal, as the instructors, is that by the end of the class, you have had a chance to try your hand at all of the skills by applying them to a project you work on over the course of the year in a building you have access to.

The work on your project is supplemented by hands-on lab sessions and field activities using the systems in the Pacific Energy Center, SketchUp models, spreadsheets and other software based tools, and even an escape room.

If you are thinking of taking the class, it is important to realize that it is not a casual undertaking. In signing up, you are committing to:

  • Fourteen day long class sessions consisting of twelve regular classes and two site visits to student projects,
  • Completing what amounts to the scoping and investigation phase of a retrocommissioning project in the facility you choose to work on,
  • Monthly homework assignments that are targeted at helping you learn and apply the 10 key skills, and
  • Self-study as needed to hone your expertise, initially on basic concepts common to all projects and then on the skills you need to successfully carry out your own project.

So it is a significant commitment in terms of time and effort.

But most, if not all students indicate the undertaking is well worth it as you can see from the analysis below and the quotes in the updated flyer Ryan recently put together.

Don’t let the Level of class’s fear of Ryan data series scare you off. Much of the success of the class can be attributed to Ryan’s dedication to making it a learning experience of the highest quality. And experience has shown that for that happen, we need a small group of dedicated people with a fairly strong basic skill set. Such a group allows us to focus on developing the more advanced skills with a low student to instructor ratio for the lab sessions.

For Class 8, what the fear was really driven by was the necessary process of winnowing down the group of 60 to 80 people who initially sign up for the class to about 20 to 25 people, a manageable size for the interactive, hand-on focused lab and field activities that commence in earnest around the third session. Ryan is an excellent judge of who is ready for the class and and who is not and gently but firmly manages the task. So the fear was not so much of Ryan as it was of being winnowed out of the class.

The fact is that most of the time, we find ourselves at the appropriate number of students simply by attrition. During the first few sessions, we go through a number of exercises, including a basic skills quiz, an Excel skills quiz, and mystery graph quiz, all of which are intended more to be learning opportunities than test. But the scores also help Ryan, and the students themselves, assess where they stand relative to what it will take to successfully complete the class. If you aren’t quite there yet in terms of your readiness, then it is in everyone’s best interest that you defer for a year and take advantage of some of the other learning opportunities at the Energy Center to become better prepared.

My point is that winnowed once does not mean banned forever. Ryan has put together a very comprehensive set of classes that are offered annually at the PEC and targeted at helping people prepare for the year long class in addition to providing general knowledge on various topics including Excel Skills, HVAC basics, and common HVAC system types.

As a result, anytime Ryan suggests that perhaps a student is not quite ready for the rigors of the class, he also suggests an appropriate course of study that the students can follow so that they are more fully prepared to succeed in the next class series. Several of our most successful students have followed this path; i.e. voluntarily or at Ryan’s suggestion, dropping out for a year, pursing the suggested course of study and re-enrolling the following year to deliver stellar project results.

The class is taught at the Pacific Energy Center, which is in San Francisco. And because of the hands-on, field experience-based approach taken for delivering the class, unlike some of the other classes I am involved with at the PEC, this is one you have to attend in person. So, obviously, living in the Bay Area would be a plus in terms of participating given San Francisco traffic. Sometimes, I think I get back to Portland faster than the folks who attend from Sacramento.

Having said that two of our most enthusiastic participants last year took it upon themselves to travel all the way up from Southern California each month, never missing a session. A number of other students have done the same thing in other years.

Technically, the class is funded by public benefit money from the California Utility System rate payers. As a result, people living or working inside that system are considered first in terms of who can attend. But that does not mean that people from other areas are not considered. In past years, we have had students from Oregon, Illinois, and even New York City come and complete the class.

If you want to get a fuller sense of what the class will be like, consider attending the RCx 101 class on June 6, 2018, either in person or via webinar (select the “Internet” location when you register to take it as a webinar).

For one thing, the RCx 101 class is a prerequisite for taking the workshop series. And even if the year long effort does not seem like something you are ready for or are willing to commit to, the RCx 101 class will get you up to speed on the existing building commissioning process and the basic skills you need if you want to work in that field.

In terms of gaining additional perspective on the class and existing building commissioning, once you have reviewed the Series 14 Class Flyer, you may also find some of the following resources to be of interest.

If you want to take it a step further, then you may even want to consider the following. For an number of years now, we have been working with 3D SketchUp models as a tool for providing a virtual field experience in the classroom and for self study. Pulling that off is taking some time, but in terms of self study, there are two offerings that you might want to explore.

  1. The Chilled Water Plant System Diagram Exercise lets you try your hand at developing a system diagram for the chilled water piping in a central chilled water plant. The system concept and system diagrams are key design, commissioning and ongoing operations Tool.

In fact, my very first assignment when I entered the industry in 1976 was to draw a system diagram for a chilled hot water system serving a pharmacy school in St. Louis, Missouri. It was a great experience and I have been honing that skill ever since.

  1. The Ballroom Air Handling Unit Scoping Exercise uses a SketchUp model of an air handling system mechanical room to allow you to perform a virtual scoping exercise where you try to identify the 21 or so potential EBCx opportunities in the model based on the clues that you see as you explore it.

You can download the model, a Scene’s Guide and an answer list from FDE’s commissioning resources web site and explore to your hearts content. While not as much fun as an actual mechanical room, we hope that working with the model will get make you more productive on your next visit to the real thing

If you are new to SketchUp, The website also has a page where you also will find instructions regarding how to obtain a copy of the free SketchUp software you need to work with the model along downloads of legacy versions of SketchUp and links to tutorials that will expose you to the basics of working with it, which is all you need to do the exercise.

So, if you have found all of this intriguing, follow the link and register for the RCx 101 class. Worst case, you will have spent a day learning about the Existing Building Commissioning process and the skills it takes to work in the field. And you may just find your self “hooked”, opening the door to a very interesting and rewarding career. At least that has been the case for me.

David Sellers, P.E., Senior Engineer

Facility Dynamics Engineering

Visit FDE’s commissioning resources website at

Visit my non-technical blog The Other Side of Life at

Posted in Retrocommissioning Findings | Leave a comment

The Persistence of the Benefits of Commissioning (Plus a Few Pictures of Legacy Instruments)

As you probably have noticed if you follow the blog, I love finding old instruments in my travels. I have even been lucky enough to save a few of them from the dumpster, like this resonant frequency-based tachometer …


… or this Foxboro pneumatic proportional plus integral (PI) controller …



… or this 1970’s vintage central control panel (the state of the art about the time I entered the industry).


Just the other week, I was in a building down in San Francisco that had originally been built in the 1960’s by Bethlehem Steel as their headquarters on the West Coast.

That was of unique interest to me because my grandfather on my mother’s side was a welder for Bethlehem Steel in their Johnstown Pennsylvania plant around that time; who knows, maybe he made some of the welds in the steel for the building when it was being fabricated back then. (The picture is from one of the elevators; they feature different vintage photos related to the building’s history).

The central plant in the facility had been recently upgraded from the original system. But when we got to the basement mechanical space, I was treated to a few more legacy control components, including this 2-pipe indicating temperature transmitter …


… and another central control panel.

The last picture is an interesting juxtaposition of technologies; the two monitors and the black box behind them (a PC) contains many orders of magnitude more information than the legacy control panel behind them. But I still have a soft spot for the legacy panel and was glad to see that it had been retained when the plant was upgraded.

My reason for bringing all of this up is that about a week ago, Steve Briggs, one of the other FDE engineers that I have the privilege of working and teaching with on occasion, sent me a picture from the field of an old seven day timeclock, the type of device we used to schedule equipment back in the “olden days”.

The device was simply an electrically driven clock with a dial that made 1 revolution every 7 days. Small, adjustable “trippers” were mounted on the perimeter of the wheel with little thumb screws and were shaped so that the side visible to you pointed to the time setting you desired and a little lever on the back of them would trip another lever (which is concealed behind the dial in this picture). The concealed lever, in turn, worked a mechanism that would open and close contacts, thus turning things on and off on a schedule.

There were typically two different types of “trippers” (some people called them “dogs” for some reason). On the visible side, they were different colors, usually black and silver so you could tell them apart.

On the back side, the shape of the lever was different with the difference being that one type of tripper would move the concealed lever in a way to turn close the contacts that the clock controlled while the other type of tripper would move the concealed lever in the other direction, opening the contacts back up. The contacts, in turn, could be used to turn equipment on and off on a schedule. You can still find devices similar to this in the hardware store, targeted at controlling the lamp on your end table.

The brass screws you see below the dial are one side of a number of contacts. In other words, if the picture were zoomed out a bit, you would actually see two rows of screws, with each vertical pair corresponding to an independent contact. In this case, I believe the last pair of screws on the far right would the power connection where you landed the 120 vac power to run the clock.

When I saw Steve’s e-mail and the picture, it immediately reminded me of my very first exposure to the concept of persistence of benefits. In other words, it’s one thing to intend to have a building or system do something like operate on a schedule by providing a time clock with a wiring diagram and control sequence that indicates that the clock should start or stop a piece of machinery or cause a certain function to happen at a certain time of day on a certain day of the week.

But it turns out that it is entirely different thing to have that design actually work and remain in operation over time, something I really did not realize until I ran into my first time clock.

Specifically, in the fall of 1979 or so Chuck McClure sent me down to do field work at Kent Library on the South East Missouri State Campus.

Chuck founded McClure Engineering in 1953, the year before I was born. And, based on the recommendation of Dr. Al Black, a mentor and friend from my Park’s College days, Chuck had taken a chance on an Airframe and Power Plant mechanic with some engineering courses to his credit and hired me as an HVAC field technician, which is how I got my start in this industry.

The reason for the field work in Kent Library was that the University was interested in installing some sort of supervisory monitoring and control system to help them understand how their buildings were running from a central location and to allow them to identify operating problems and ultimately, optimize the existing stand alone control systems based on what they were observing. This, of course, was the fore-runner of what we take for granted now in our Direct Digital Control (DDC) systems. But at the time, it was fairly cutting edge.

In those days, large buildings might have central control panels similar to the ones I illustrated above. (And sometimes, the gauges were even right). But very few if any sites with multiple buildings, like a college campus for instance, had all of the buildings networked together and visible from a central location. So, it was exciting to be involved in a project like this, even though at the time, I did not fully comprehend how big a deal it really was. But eventually, Kent Library would become my first design for what we now would call a DDC system (under the watchful eye of Chuck and Al of course).

At the time of the site visit, my goal was to develop field verified diagrams for the existing interlock wiring and pneumatic control systems serving the equipment in the library. Thus, I found myself opening up control panels, junction boxes, motor control centers and wireways tracing out colored wires and copper tubes and trying to figure out what they were connected to and what all of these funny, new to me, electrical relays, switches, and pneumatic gizmos did.

The original library was dedicated in 1939. But all of the equipment I was looking at had been installed in a 1968 project that had been done by Chuck himself. So, I had a pretty good resource at my disposal in terms of trying to understand the design intent of the facility.

One of the things that had attracted me to McClure Engineering when I interviewed there was that they had always had an interest in energy conservation and the responsible use of resources, even before the first energy crisis hit in 1973. In the course of the interview process, Bill Coad pretty much said to me what would eventually become his Energy Conservation is an Ethic  paper and as a result, I left the interview inspired in a way that changed my life.

One of the reasons Chuck and the University had targeted Kent Library for the pilot for a supervisory control system was that it was fairly energy intensive due to the archival storage nature of the application. If you are playing the archival storage game, one of the things you are trying to do is hold very stable temperature and humidity levels and keep the air very, very clean. Avoiding damage by light and vibration are also important. It’s really pretty interesting (in a nerdy sort of way) and the ASHRAE Handbook of Applications contains an entire chapter dedicated to the topic.1

All of those requirements tend to mean that the HVAC systems in archival storage facilities need to run round the clock, especially in the rare book areas, even if nobody is in the facility. But if nobody is in the facility, then one thing you don’t have to do is ventilate; i.e. introduce outdoor air to manage the contaminates introduced into the built environment by human activity.

In climates like Cape Girardeau, Missouri, ventilation loads can be significant because it can be very cold and dry in the winter and very hot and humid in the summer, as illustrated by this bin data plot I created using the Pacific Energy Center psych chart tool.2

As a result, one of the things that Chuck had done in his 1968 control system design was include a time clock that would shut down the minimum outdoor air that was brought into ventilate the building during the unoccupied hours. In other words, even though the systems could not be scheduled, the ventilation could and Chuck designed the clock into the control system to perform that function

Since I was using the original design documents and control submittals for the 1968 project to guide my field effort, one of the things I was looking for was that time clock because we planned to take over that function with the central monitoring and control system. Doing so would allow us to change the schedules by remote commands from the central location rather than by having to put out a work request to have one of the campus technicians visit the building and move the trippers around on the time clock every time the school was not in session or a schedule changed.

Having to do that every once-in-a-while doesn’t sound like a big thing until you consider the number of buildings on a college campus and that each building might have multiple time clocks in it. The overview above will give you a sense of that. Each of the little markers is a building. Kent Library is the yellow marker to the upper right of the clump of red markers at the lower left side of campus.

Eventually, one of the control panels I opened up contained the clock I was looking for. But the problem was that it looked just like the clock in the picture Steve sent to me; i.e. there were no trippers on it. That meant that currently, at the time of my visit, one of Chuck’s energy conservation features was not delivering the intended functionality.

But it was worse than that. There was a small manila envelope sitting in the bottom corner of the control panel. Even thought it was not very large – maybe 1 inch by 2 inches – it was kind of heavy. I broke the seal and opened it up to discovered that it contained the missing time clock trippers. There were 14 of them to be exact; 7 silver ones and 7 black ones.

That was enough of them to program one on and off event for each day of the week, just as Chuck had specified. The problem was, that since they had never been installed on the time clock the ventilation that Chuck had intended to be shut down for about 6 to 8 hours a day on week days, longer on the weekend as I recall, had not happened, not even once, since 1968.

The good news there was that we had just found a significant opportunity to reduce the operating cost of the facility, which would definitely help justify our project. The bad news was that it should have been happening all along.

The incident certainly caught my attention, Chucks too. And as a result of the incident and other insights we were having as a company about the how buildings were being operated, Chuck tapped into my A&P Mechanic background and had me start developing checklists for some of our new projects.

We applied the lists as a tool to help us prevent problems like the one I had uncovered that day in Kent Library. We also made an effort to train the operators about the features of our designs, especially the ones that would help save energy.

And we worked with our clients to help them understand how to monitor the performance of the facility on a day to day basis by using average daily consumption analysis and supplementing their stand-alone control systems with remote monitoring systems like the one I was starting to work on for Kent Library.

As I look back on it now, I realize that a lot of the things we were doing to try to address the lack of persistence of the benefits of Chuck’s design are the same things that are suggested today in the commissioning industry to help ensure the persistence of the benefits of commissioning.

At the time, the commissioning industry was just starting to emerge in Canada and the United states. But since I had not heard about the commissioning industry yet, I thought that all we were trying to do was operate the building properly.

David Sellers, P.E., Senior Engineer

Facility Dynamics Engineering

Visit FDE’s commissioning resources website at

Visit my non-technical blog The Other Side of Life at

  1. It’s Chapter 33 in the most recent, 2015 edition of the Application Handbook. If you happen to be in the Bay Area, there are copies of different editions of the handbooks available in the Pacific Energy Center Resource Center. I think you can even check them out if you want to.
  2. To do the bin part, you need to upgrade the basic chart tool (which is free) to the professional version. But Ryan Stroupe, the Measurement Tools Program Coordinator that I teach with at the PEC, worked out a deal with Hands Down Software, the chart developer, that allows you to upgrade to the professional version at a 30% discount from the normal price.
Posted in Controls, Mentoring and Teaching, Operations and Maintenance, Pneumatic Controls, Retrocommissioning Findings | Leave a comment

Resources for the Resourceful – Utility Analysis Spreadsheet Tool

Authors Note: Since the point in time when I published this post, some of the links have expired. So, I have gone through it and renewed them and also made a few edits.

Linking to the Weather Data Depot in my previous post reminded me of another recently released free resource.  Several months ago, the California Energy Commission released the retrocommissioning toolkit on via the California Commissioning Collaborative’s website. The toolkit includes a number of useful tools, but the one that came to mind in the context of the previous post is the Utility Consumption Analysis Tool (UCAT).  Here is a screen shot of the raw output generated by the tool as used for a recent project.

The tool also plots the data in a basic chart, but I usually copy it into a different spreadsheet so I can have more control over the colors and arrangement, which are locked down in the tool to facilitate its automated operation. Here is what that data looks like in a standard energy analysis spreadsheet that I have, which I use early on in a project to help me understand how energy is used and the potential savings I might anticipate, and thus, the budget.

But bottom line, the arrangement of the spreadsheet allows you to develop the data you need and  plot a normalized graph of average daily energy consumption almost faster than I can type this.

You simply enter billing dates and the consumption for the billing period and the spreadsheet takes care of the normalization process, including filling in for missing months.  Several years of data can be plotted for comparison along with other data that you want to contrast with consumption patterns.

In the example above, I have plotted cooling and heating degree days. The degree data came from the Weather Data Depot site I mentioned and was added to the chart in my spreadsheet tool. But the UCAT tool includes a table that would allow you to add the data directly into the tool, which causes it to plot on the chart the tool generates, along with the energy data.

I could have also plotted percent occupancy, rooms sold, central plant energy, or any other monthly information that I had available for comparison using the empty table.

The bottom line is that the tool makes monitoring and analyzing your facilities energy consumption patterns a matter of a few simple key strokes.  For a discussion of ways to use this type of analysis to target commissioning and efficiency, opportunities and help them persist, see Using Utility Bills and Average Daily Energy Consumption to Target Commissioning Efforts and Track Building Performance, which is a paper I wrote on the topic for the International Conference on Enhanced Building Operations.

David Sellers, P.E., Senior Engineer

Facility Dynamics Engineering

Visit FDE’s commissioning resources website at

Visit my non-technical blog The Other Side of Life at

Posted in Excel Techniques, HVAC Calculations, Operations and Maintenance, Resource List and other Resources, Retrocommissioning Findings | Leave a comment

The AMCA ASET-US Conference and Mentoring

Earlier this year, I was asked to be the plenary speaker at the upcoming AMCA ASET Conference in San Antonio, Texas. As many of you probably know, AMCA is the Air Movement and Control Association International and their mission is to advance the health, growth and integrity of the air movement and control industry. That means they are very involved on a global scale with manufacturers of air system components; things like fans and ducts and louvers and dampers to name a few. So, being provided with the opportunity to attend one of their technical conferences was exciting to me and my only regret in accepting was that I could only stay for one day, having already agreed to teach several classes at my monthly gig at the Pacific Energy Center in San Francisco.

As you can see from the header image, the conference is rapidly approaching. But in talking with Michael Ivanovich1 earlier this week about my presentation, I found out that there still are slots open in the conference. In fact, Michael give me a discount code to share with you that will give you $50 off the registration fee if you are a non-AMCA member, taking it $450 down to $400.


The conference features two different tracks, Air-Systems Design and Air Products & Technologies. You can get a sense of that by visiting the conference web page or downloading the conference brochure. In my perspective, both tracks have interesting topics and great speakers and I will have a tough time choosing where to go for the time I am there. So, if you decide to attend, I don’t think you’ll have a problem keeping yourself busy.

And of course, attending a conference like this puts you in a great position to share what you learn by being a mentor to to the folks back at your home base. Mentoring is extremely important in terms of making the industry and the world a better place and is a major focus of my plenary talk.

For me personally, most if not all of any success I have achieved can be attributed to some very wonderful mentoring experiences like the one I wrote about previously in one of my blog posts. Working on my presentation for this conference has caused me to recall many, many, many of my mentors and experiences and as a result, I plan to start a series of blog posts dedicated to each of them. So stay tuned.

But bottom line on this post, if you work with air handling systems at a technical level as designer, installer, commissioning provider or in operations, this will be a great opportunity to learn a lot from some really knowledgeable people in the industry and earn up to seven Professional Development Hours (PDHs), something most registered professionals need to do to maintain the currency of their license. And like I indicated above, there are still slots open at the conference, rooms available at the hotel, and reasonable airfares available.

Hoping to see you there.

David Sellers, P.E., Senior Engineer

Facility Dynamics Engineering

Visit FDE’s commissioning resources website at

Visit my non-technical blog The Other Side of Life at

  1. Michael is a good friend but also happens to be the Senior Director of Industrial Relations for AMCA and the guy who got me started doing this blog and has mentored me through-out my writing career.
Posted in Air Handling Systems, Mentoring and Teaching | Leave a comment

Howden North America’s Fan Engineering Handbook

Authors Note:  So, some good news on this.  For a while, some of the links below did not work and I could not get information back from Howden regarding how to get a copy of the handbook since my contacts there had moved on.  

But on a lark, I did a search today and discovered that Howden is now making the 9th edition available electronically at no cost.  Just submit this form and you should get an e-mail with a download link.

I don’t know if they will continue to make a printed, leather bound version like the one in the picture available.  But if you are old and sentimental about books (like I am) you can still find used copies of it and earlier editions out there, as I mention in the blog post.  Prices seem to vary all over the place.

Since the electronic version I illustrated in the post is the 9th edition, I suspect the version you will get by following the link will be very similar if not identical.   I submitted the form myself and will update this note if I notice any major differences.


I realized the other day that the Reading and Resource list I posted does not contain information on Howden North America’s  Fan Engineering Handbook, so I’ll use this post to draw your attention to that valuable resource, which is pictured below.

Fan Engineering

Incidentally, folks my age and older may know this book as the Buffalo Fan Engineering Manual.  I believe Howden North America aquired Buffalo Forge at one point, thus the name change.  If you look at the copyright page, it shows Howden Buffalo as the publisher.

Some have observed that the book looks a bit like a bible;  the fact is that it is kind of the “bible” of fan physics and application.  Howden North America’s Fan Engineering handbook is somewhat of a classic in the industry.  The first copy I owned is from the early 1970’s;  I picked it up in a used book store at some point after coveting the copy one of my mentors had, which they generously allowed me to borrow. The copy in the picture is the 9th edition, published in 1999.  However, the book has its roots in work by Dr. Willis Carrier, and was first published in 1914.

I bring all of this up because I recently discovered that Howden has made it much easier to obtain a copy of the book.   Specifically, if you follow the links on their web site, or simply click on this link, you will be take to an page where you can purchase new or used copies.


The only used copy offered on as I write this is going for $85, which is still a good deal considering the content.   But, I suspect you can find copies for less by shopping around a bit on the internet or used book stores.  For instance, I just found a copy of the 8th edition (1983) for $22.99 on the Alibris web site.  Since the principles behind Newtonian physics have not changed much since 1983, I suspect the used 1983 edition represents a good value for someone on a budget.

Having said that, read on as there are some advantages to getting the latest edition.  Interestingly enough, you will notice that the price range for a new copy, currently at least, is from $100 (directly from Howden) to $189 (from a reseller).  My thought is that unless the reseller has a copy signed by Willis Carrier, you’re probably better off ordering from Howden.

What’s not so obvious from the web site is that if you e-mail Howden at before ordering, you can specify your prefernce for an electronic copy of the handbook as a CD instead of the paper copy.  This alone may be worth the price of obtaining the latest copy for a number of reasons I discussed in a previous post, including portability, searchability, physical storage space and weight (especially when traveling), and sustainability.  But, the electronic version of the Fan Engineering Handbook complements all of these features by including interactive content.

Here is a screen shot of the table of contents, which is the starting point for using the electronic version and also illustrates the wealth of information available in the handbook  (both electronic and paper copies).


Clicking on the highlighted topic Fan Testing takes you to the content of the chapter.


Clicking on the hyperlink for the chapter takes you to a bookmarked summary of the chapter content, from which you can jump to the topic of interest.



You’re probably thinking “that’s all nice and everything, but every well structured electronic document does this sort of thing”.  And while that’s true, the electronic version of Fan Engineering has done a very thorough job of it by providing hyperlinks that take you to related relevant content elsewhere in the handbook.  For instance, in the actual handbook, clicking on the equation hyperlinks shown in blue above will take you to those equations, which are in a different chapter.

But better still, the manual includes spreadsheets pertinent to the various topics discussed.  For instance, if while working with the electronic version of the handbook, you click on the little calculator/spreadsheet icon illustrated in the screen shot above, the following Excel spreadsheet opens up.


This is a spreadsheet that assesses flow rate based field data from a pitot tube traverse.  Note that it is set up for both IP (U.S.) and SI (metric) units.  If you wonder exactly how to use it, you just page down a bit.


If you are wondering exactly how the calculations are done, you can simply highlight a cell, and view the formula, just like you can in any other Excel spreadsheet.


If you wonder what some of the “funny” symbols mean, then just use the electronic table of contents to jump to the Appendix of Symbols and Abbreviations


… or use the hyperlinks or .pdf search function to jump to one of the referenced equations (click on the binoculars or press“Ctrl” and “F” together to get a little search window where you can type in what you are looking for).


One word of caution;  as near as I can tell, the spreadsheets are not protected.  In other words, you should save a copy to your project directory using the “Save As” feature of Excel before you modify them or add values.  Otherwise, when you select “Save”, you will over-wright the original spreadsheets in the Fan Engineering directory.  That’s probably not the end of the world, but if you needed a clean copy, I suspect you would have to uninstall and then re-install the CD content.

For me, the electronic version of the CD is well worth the cost (it’s $149 vs. $100 for the paper copy).  And, truth be told, I really appreciate having both the electronic and paper copy.  Obviously, the electronic copy is handy for my field work and teaching.  But for me, there is some sentimental attachment associated with the paper copy; its like having a bit of a tangible connection with one of the founders of our industry.  And some of the affection for the paper copy is my age;  I grew up with paper books and while I have a Kindle Fire, which is wonderful for traveling and in low light conditions and for its functionality beyond just being an e-reader, I still like the feel of a book in my hand as I read it.

As you probably have noted, this is not a free resource, like many of the ones I list in the reading and resources list I posted previously.  But as I have indicated, it’s a classic reference and a valuable resource for anyone involved with HVAC as a profession and well worth the money.  And, there is some good news if you are a student.  Howden offers a $75 student discount on either the CD or the paper copy of the handbook.  To see if you qualify, simply send an e-mail to asking for more information.  I’m sure you’ll get a speedy reply.

So, many thanks to Howden North America for making this resource available and for making it easier for students to obtain a copy of the latest and greatest information on fan engineering from the pens of leaders in the industry.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Posted in Air Handling Systems, Resource List and other Resources | 2 Comments

Pneumatic Controls Resources

Author’s Note: I originally published this post on March 3, 2013.  At that time, all of the resources I mention below were loaded up in “the cloud” on a Google Drive.  Since that time, I have created the commissioning resources website to complement the blog, which makes it much easier to distribute information like this and maintain it.  So, I have moved these resources to that location, specifically, to a page I have created for them under the General Resources page.

I have also added a little silent movie that starts with pictures of various types of pneumatic control hardware that I have taken on various existing building commissioning projects.  That is followed by a set of animations that illustrate how one and two pipe pneumatic controls work.   The video concludes with some slides summarizing the pros and cons of pneumatic control and actuation along with some bottom lines about pneumatic control.

The information in this blog post is still relevant and will provide additional insight into pneumatic control and what is included in the resources, so you if you are new to pneumatic controls, you may find it helpful, along with the post that preceded it titled Pneumatic Controls, First Cost Advantages, and Retrocommissioning Opportunities.


Since the post I recently put up on pneumatic controls (actually even before I did that), people occasionally ask me if there are resources out there that would help them better understand pneumatic controls.  Even though the industry is moving to direct digital control as “business as usual”, if you are working in existing buildings, you will still find yourself confronted with pneumatic control technology.  As a matter of fact, we just ran into a bunch of it during two field classes in the Existing Building Commissioning Workshop Series that I do with Ryan Stroupe  at the Pacific Energy Center (see page two of the linked brochure if you are interested).

Here is a picture of a pneumatic control panel that is currently operating a system in Post Montgomery Center, a large high-rise in downtown San Francisco.

Executive Floor Receiver Controller Panel

Erik Carlson, chief engineer for the facility, currently is working through the building and upgrading panels like this to DDC.  But meanwhile, the operating team needs to keep the existing pneumatics in top form to maintain their LEED Gold status.

Across the Bay on the Berkeley campus, we ran into this pneumatic control panel which had been retained and piped through manual valves to back-up the retrofitted DDC control system serving the facility.

Pneumatic Controls

In this picture, you can see the pneumatic lines from the old control panel coming together through small, manual, isolation valves with the pneumatic lines from the electro-pneumatic circuit boards in the panel to the left (the valves are the small “T” shaped devices right above the panel to the left and in the lines above the panel on the right side, which is the pneumatic control panel in the picture above with the door closed).

Pneumatic to Barrington Valves 01

Here is a close-up of the circuit boards in the panel to the left in the picture above. 

Pneumatic Interface

Basically, the boards respond to signals from the DDC control system to pulse air into or out of the line to the device they control. In the close-up of a typical board below, you can see the main air (nominal 25 psig supply air) coming into the board via the black tube and connecting to a manifold. 

Pneumatic Interface Close Up

Two small solenoid valves (the shiny cylinders mounted on the black cube, which is the manifold) are pulsed by signals from the DDC system. 

Pulsing one valve causes it to open and allow main air to flow into the output line (the yellowish tube with the gauge in it).  Pulsing the other one allows it to bleed air out of the output line.  These short, digital (on-off) signals work together to create the analog (continuously variable) air pressure output from the system that is used to position the final control element, which in this case, is a steam valve.

The bottom line is that to work on existing buildings (both now and for the foreseeable future near as I can tell) you probably need to have a working knowledge of pneumatic controls and some resources. 

Unfortunately, a lot of the really great resources I had available to me when I was learning this stuff  are no longer readily available.  That’s because many of them were mentors who have retired, or in some cases, passed on.   Others are printed materials that seem to have been supplanted by new information on the new technologies. 

So, in an effort to honor the knowledge that was shared with me by the people, and to make the printed information I have available for others, I decided to scan some of my more useful resources and post them on my Google Drive so you can access them if you would like.

This is likely a violation of some sort of copy-right law, and if someone from one of the companies whose literature I have scanned contacts me and tells me to “cease and desist” with my effort, I will have to pull them back off.  But I suspect that will not happen since much of this is from the 1960’s and 1970’s and most people consider the information it contains to be obsolete.

In fact, when I contacted one of the majors a while back to see if they would allow us to post their damper and valve sizing engineering bulletins as .pdfs on the California Commissioning Collaborative web site (I was with PECI back then) they not only said yes, the guy I was talking to asked me to forward copies to him too since they sounded really useful and he could no longer find the originals. 

I still had mine because I kept my application engineering manuals from when I had worked for them.  In the interim, the company had been sold several times and the various documentation and engineering resources were “cleaned up and made current” by the new owners, I guess.

Having said all of that, what follows is a listing of the documents I have put up along with a link to them and a description of what they are about and why I think they are useful.  There is also a link to the folder containing them in the resource links on the home page of the blog.

The Honeywell Gray Manual

This is the one resource on the list that is not on my Google Drive.  That is because you can download it from another location on the web.  I link to it and discuss the “Gray Manual” in a previous post on the topic.  The bottom line is that it contains useful information on a number of aspects of HVAC including chapters on all of the common control technologies, information on HVAC fundamentals, psychrometrics, and common HVAC system types, all presented in a very straightforward, understandable manner.   It’s a great way for somebody new to the industry to gain an introduction to it and a valuable resource for someone who has been in it for a while and needs a “refresher” on a particular topic.

Johnson Controls (JCI) Fundamentals of Pneumatic Control

To be honest, one of the things I like the most about this particular document is more nostalgic than technical, that being the neatly printed signature and date of Phil Sutherlin across the top of the first page.


That’s pretty much how Phil printed all of the time, be it on a drawing that was going out the door or on the sketches and illustrations he made to help me learn things when I asked him questions. 

As a young engineer, I strove to emulate it as well as and the free hand drawing style he used to lay out systems during schematics and design development.  I actually got kind of good at it, and one of the things that bugs me about where technology has taken me is that since I type and click more and print and draw on paper less, I have lost a bit of that.

But I’m sidetracking a bit here.   Nostalgia aside, the value of this particular document is that it contains a lot of very practical information about control theory (meaning that it applies to any control technology) as well as how that theory is implemented in a pneumatic instrument.


JCI Pneumatic Controls O&M Manual

Some of the information in this document is similar  to the Pneumatic Fundamentals documents.  But the focus is more towards helping operators understand how to adjust and maintain the instruments.


There is even a section on replacement parts.  Believe I t or not, we used to actually fix technology when it broke instead of throwing it away in a landfill and buying a new one.  Perhaps a little lesson in sustainable operations there for us today.

MCC Powers Fundamentals of Pneumatic Controls

MCC Powers is now Siemens after going through a number of other owners in the process.  Their Engineering bulletin on pneumatic controls fundamentals is a nice complement to the information in the JCI Document.  While the JCI document includes some diagrams and a lot of pictures of the hardware, the MCC Powers document includes a lot of diagrams with explanations that illustrate how mechanical systems of diaphragms, levers, and springs were used to accomplish control processes like proportional control or floating control or PID.


Now, of course, we do all of this digitally with math.  But for me at least, seeing  and understanding how a mechanical device would accomplish the function was key to understanding how things worked, both then and now. 

I think that is true for a lot of people actually because the “float in a tank” analogy I use when I teach PID  control really seems to connect the dots for a lot of folks.  I know it did for me and it has its roots in some of the discussions in documents like this one.

MCC Powers and JCI Pneumatic Main Sizing Data

This document describes the process used to size the air mains along with related reference tables and sizing charts.   One of the hardest things to find these days is some sort of documentation on the air consumption of various control devices.  That can be useful information in an existing building commissioning process if you are trying to understand if a compressor is running more than it should due to leaks or if the demand from the control instrumentation is the cause.

Eventually, you can find out what you need to know, but you have to dig around a lot in the catalogs from various manufacturers.  And often, the specific device you are looking for is no longer manufactured.  Back when I did a lot of pneumatic control design, I had a table that listed most of the common devices from the MCC Powers product line along with their air consumption rate and I have scanned that table and included it in this set  of documents.


I have found that it is reasonable to assume similar devices from other manufacturers use air at about the same rate in the general case, even though the specific values probably vary from manufacturer to manufacturer and device to device.

In addition, even thought pneumatic controls are receding from the scene, pneumatic actuation is alive and well.  So having the ability to size air mains and compressors is still quite handy out there in the field.

One  note with regard to sizing systems for actuators;  when I first got into this, I was puzzled about why the table above did not include consumption rates for actuators.   It finally dawned on me that it was because unlike a pneumatic controller ,which constantly bleeds air to operate (the link takes you to some slides that illustrate this), pneumatic actuators only use air when they move.  And the volume they require to go full stroke is basically the diameter of the actuator cylinder times its stroke.

That means if you had a system that was only serving devices like actuators, then the size of the compressor and mains is more a function of how many actuators you think might move concurrently and how quickly you need them to move.  That means that you can often get away with a smaller compressor, especially if you complement it with a large storage tank, which can act as a sort of pneumatic flywheel.

MCC Powers RC 195 Receiver Controller System

For me at least, one of the best ways to understand something is to immerse myself in the details of how it works.  This document will allow you to do that for an RC 195, which is the MCC Powers (now Siemens) receiver controller.   This device is still available and in use out there  In fact, the controllers in the Post Montgomery panel at the beginning of this post are RC 195s. 

RC 195 Close Up

The controllers in the UC Berkeley Facility control panel are similar devices but manufactured by Honeywell.

The fundamental difference between a receiver controller and a controller that has the sensor built into it, like the remote bulb controller illustrated below, is that instead of using a sensing element that measured a specific parameter, like temperature, which was an integral part of the controller, receiver controllers were set up to use a 3-15 psig pneumatic input.

image  image

Pneumatic Controller 01   Pneumatic controller 05

In the pictures, the upper row are taken from the JCI Operating Manual and show the parts and a cross section of a controller very similar to the one in the photographs below.  In the photo at the lower left, you can just see the bellows that is part of the temperature sensing assembly (the silver cap shape behind the plate with screws) that shows up connected to the thermal sensor in the illustration above.

Returning to the receiver controller discussion, by using a generic 3-15 psi pneumatic input, receiver controllers could be interfaced with pneumatic transmitters that were designed to measure a number of different HVAC parameters like temperature or humidity or pressure. 

The transmitters generated a signal that varied from 3-15 psi as the parameter they measured varied over a given span like 0-100°F.  They are identical in concept to the 4-20 ma, 1-5 vdc, and 2-10 vdc transmitters we use today with DDC systems.   They just used air pressure to transmit the information instead of electrons. 

If you understand one technology, you can easily understand the other, which is a topic I discuss in an article I wrote for HPAC magazine a while back called Analog Lessons for a Digital World, along with other things I learned working on pneumatic controls but apply every day while working with DDC controls. My bet is the “cross training” works the other way too; i.e. if you understand DDC control fundamentals, you can readily transition to pneumatics once you learn the jargon and hardware.

Here is a picture of a one-pipe temperature transmitter that is piped to one of the controllers in the panel at UC Berkeley, illustrated at the beginning of the post.

One pipe transmitter 03

The generic input capability meant that only one controller design was required and you adapted it to the HVAC process by virtue of the transmitter you selected.

Bottom line is that the RC195 documentation will allow you to get into the details of how one works and how to use it.  In addition, it has been my experience that once you understand a specific device, the knowledge is easily transferred to similar technologies from other manufacturers.

Fisher Process Grade Pneumatic Controllers and MCC Powers Series 200

If you really want to see how a remote bulb type controller works, the product bulletins for these process grade controllers will give you a pretty good picture of that.  Incidentally, the MCC Powers Series 200 controller is the controller that shows up in the pictures in the photo gallery in the post titled Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – #2 – The Controls May be Pneumatic.  

The instructions are hard to find;  I lost my copy when I moved to Portland and the box the the binder with all of my MCC Powers manuals was lost.  The copy I ‘m sharing here was graciously shared with me by Adam Lebavitz of M&M Control Service Inc.  M&M now owns the remaining stock and manufacturing rights to the Series 200, so if you are looking for parts or service on this type of equipment, they are the folks to talk to.

JCI Damper Engineering

One of the advantages of going back into your files and looking for something is that sometimes, you discover something useful that you forgot you had.  This is an example of that and comes from the JCI engineering manual that Phil gave me.

Among other things, this document contains actuating force and stroke data for a number of different actuator types,   including a technical discussion of the topic and a really cool little nomograph that helps you figure out the relationship between lever arm, stroke and angular rotation for a given arrangement.


When I bring this topic up, some folks say its not very useful information given that the industry is moving to centerline drive actuators and eliminating the linkage systems between the actuator and the damper.  Personally, I’m not so sure about that as illustrated by these recent field photographs.

OA Damper - Centerline actuator with Crank 01

Zone Dampers - Opposite End Zone Dampers - Actuator End

The upper photograph is an actuator serving one of a number of outdoor air damper sections in a large (200,000 cfm) economizer equipped AHU.  The lower pictures are from a triple deck multizone unit.  The actuators on the left control all three deck dampers through a menagerie of lever arms, springs, and shafts.  Here is a system diagram of the unit which will help you understand.


Interesting study in kinematics if nothing else.

Another interesting thing is the damper performance curves included towards the end.


I haven’t tried it yet, but I suspect you could manipulate this data into a version of the damper performance data that shows linearity vs. pressure drop that can be found in a number of places like the Honeywell Gray Manual and the MCC Powers Damper Sizing Bulletin I link to in the resources on the blog’s home page.


Granted, the information in this document pertains to older models.  But my bet is that geometric similarity and some engineering judgment would allow you to extrapolate this information to a current application.  That can be particularly useful if you don’t have any other information to work from.

JCI Reference Information

This is one of those collections of useful facts and figures that are handy to have around.  This particular documents includes things like:

  • units conversion constants,
  • basic HVAC and physics equations,
  • a steam table, 
  • tables with the relationship between wet bulb, dry bulb, dew point, and relative humidity,
  • velocity vs. velocity pressure tables,
  • properties of things like glycol and other anti-freeze solutions,
  • standard pneumatic control drawing symbols (well, the JCI standard anyway, which was a pretty good one actually and similar to the other manufacturers),
  • design conditions for a number of places (bear in mind this is probably mid 1960’s data), and
  • summary psychrometric process diagrams for typical HVAC processes.

The latter is one of the reasons I have included it since it is a quick way for someone new to the industry to start to understand what happens in some of our HVAC processes and correlate it with the psych chart.  For example, here is the one for a cooling coil that is doing dehumidification and cooling.


Well, that’s it.

Hope this is useful to some of you.  I know I have found the information to be useful over the course of my career, including the current time frame.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for a recent index to previous posts

Posted in Operations and Maintenance, Pneumatic Controls, Resource List and other Resources, Retrocommissioning Findings | Leave a comment