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.
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.
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.
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.
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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.
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.
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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.
This is a similar example of the curves that apply to a Baltimore Air Coil Series 1500 and Series 3000 tower.
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.
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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.
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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.
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).
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.
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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.
There 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.
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.
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.
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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.
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.
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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).
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 …
… 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.
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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.
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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.
Senior Engineer – Facility Dynamics Engineering