2022-10-04 – Author’s note: In reviewing this post yesterday to answer a question that came up, I discovered that some of the psych chart images had their quality degraded for some reason. So, I have replaced them and I believe now, everything is legible.
In answering the question, I also realized that I needed to mention one additional consideration that you would want to address if you used the process discussed, that being the need for good mixing – which is always important – becomes even more important because of the lower set points used in this process. So I added a paragraph about that when I re-posted.
I realize that for most normal people the word “interesting” could in no way, what-so-ever be associated with the words “psychrometric process”. As I often tell folks,
When I say “interesting” you can (and probably should) add the words “in a nerdy sort of way” to the end of my sentence.
That is the case here, so having given fair warning, I am going to proceed.
As some of you likely know, I occasionally write for the Engineers Notebook column in the ASHRAE Journal, usually about twice a year. Last April, I wrote a column titled The Perfect Economizer, which was actually the trigger for the blog post series I am currently working on (and lagging behind on). In any case, the magazine received a letter to the editor in response to it from Mr. C. Mike Scofield , PE, ASHRAE Fellow, President of Conservation Mechanical Systems, Sebastopol, California.
In it, he presented an interesting system configuration and psychrometric process and wondered if I had seen it applied in Portland, which I had not. My editor asked me if I would mind responding to Mike’s question, and I did (published in the September ASHRAE Journal).
If you don’t receive the Journal, you may want to refer to a copy of the letter and my response that I have posted along with the copy of the article on our Commissioning Resources website since the discussion sets the stage for what follows.
What follows is an edited version of the correspondence between Mike and myself subsequent to my initial published response. That happened because I became curious about the details of the process he had plotted on the psych chart he provided and I wanted to understand it better.
Once I understood it, I realized that it was a very clever process, but also an interesting psychrometrics exercise because it makes you think outside the box a bit compared to the psychrometrics of a conventional system. So, I asked Mike if he would mind co-authoring this blog post with me to go into the details of the process so folks could learn from our discussion and he graciously agreed.
This will get a bit long (as usual). The links below will allow you to focus in on the specific content of interest. Each section as a “Back to Contents” link that will return you to this point.
- A Few Resources
- The System and Psych Chart
- The Reason the System Might Be of Interest
- Taking a Closer Look at the Process
- Process Analysis Assumptions and Details
- Multiple Processes, Not a Single Process
- The Air Inside the Building Came from Outside the Building
- The Process Targets a Space Condition Window, not a Point
- The Evaporator Cooler will Produce Near Saturated Air
- The Chilled and Hot Water Coils are Not Active
- A Brief Review of Mixing on a Psych Chart
- The Mixing Dampers are Controlled by the Dry Bulb Temperature Leaving the Evaporative Cooler, Not the Mixed Air Temperature
- The Mixed Air Set Point is Lower than Typically Used
- Good Mixing is Critical to Success
- The Process (Finally)
A Few Resources
The process Mike asked about in his correspondence involves evaporative cooling and humidification. Evaporative cooling is a constant wet bulb process and you can simply accept that as being true. But if you want to understand it in more detail, along with the related concept of adiabatic saturation, I wrote a blog post that explores evaporative cooling in detail, including adiabatic saturation and wet bulb temperature that you can refer to.
If you want to work along with what follows on a psych chart of your own, you can download a free version of an electronic psych chart that Ryan Stroupe of the Pacific Energy Center has made available from the link in this blog post. In addition to providing links to the chart the post illustrates how to plot basic psychrometric processes and also illustrates the features associated with upgrading the chart to the professional version. The process plot examples can also be used if you are working with a paper chart, you simply need to manually plot the points on paper vs. using the tool in the electronic chart to enter them.
Alternatively, I uploaded a blank .pdf chart to the page associated with the Perfect Economizer article on our Commissioning Resources website. There is nothing wrong with using a paper chart. Mike himself is a self-confessed paper chart and slide rule guy, and I did things that way myself for a long time. In fact, I still carry my slide rule around, partly for nostalgia, partly to show folks who have never seen one, and if push comes to shove, no batteries required!
But the electronic chart does have some benefits in terms of being easily reproducible in things like this blog post and other tools that it includes, like the ability to plot TMY data as bin data on the chart, which gives you a “visual” on the climate you are considering.
If you are just learning about psychrometrics and using the psych chart, you may also find the chapter on Psychrometrics in the Honeywell Gray Manual to be useful. And there are a number of slides in resource provided on the Useful HVAC Equations and Concepts page of the Commissioning Resources website that deal with the psych chart and basic psychrometric parameters.
The System and Psych Chart
Here is the system AHU configuration and psych chart that Mike sent with his letter.
Mike’s written description of the illustration was as follows:
Has your team installed and tested a WB airside economizer using a
high saturation efficiency (97% to
99% RH) rigid media adiabatic evaporative cooler/humidifier (AC/H) to mix building return air with outdoor air to produce a supply air dew point that ranges between 45°F DP to 55°F DP during cold and dry ambient conditions?
The psychrometric chart shows a VAV system at 50% fan turndown with an assumed minimum 25% outdoor air to meet
code ventilation requirements. The
high saturation efficiency, at fan turndown to 50% flow, ensures that the delivery DB temperature off the AC/H is within a fraction of 1°F of both the WB and DP temperatures at the saturation curve. A low-cost commercial-grade DB sensor may be used with acceptable accuracy in determining the delivery DP condition of the supply air.
The Reason the System Might Be of Interest
Note that the final element in the system is the evaporative cooler/humidifier. There are a number of reasons that a system of this type might be of interest currently. But Mike brought it up because ASHRAE research suggests that …
… maintaining the space relative humidity between 40% and 60% decreases the bio-burden of infectious particles in the space and decreases the infectivity of many viruses in the air.
One place you can find this is in the ASHRAE Building Readiness information published by the ASHRAE Epidemic Task Force. It is also discussed in the ASHRAE Position Document on Infectious Aerosols (see page 8). And I suspect folks with a healthcare background were not surprised by this since maintaining humidity levels in that range in a health care environment have been a requirement for quite a while for the reason indicated.
But COVID has brought that to the forefront as something that might be considered more generally by designers. and in that context, I suspect the system configuration Mike suggested may merit consideration as long as due consideration was given to the application issues the committee mentions in the Journal’s May 2021 IEQ Applications column. For instance:
- Is the building envelope suitable for an indoor environment with a higher than typical humidity level? Or will condensation on surfaces or inside building assemblies become and issue?
- What will the water that is consumed cost? This will likely vary significantly with the nature of the climate and the local rate structure.
- Related to item 2, does the utility offer a sewer charge credit for water that is supplied to the facility but not discharged to sewer? The sewer charges can be as much or more than the water charges, so having a credit of this type can make a bit impact for evaporative processes like we are discussing.
- Also related to item 2, what will the parasitic losses associated with the added pressure drop in the system and the operation of the evaporative cooler pump cost?
- In addition to varying with climate and rate structure, the pressure drop loss will vary with the flow rate. For a constant volume system, this could be significant. But,
- For a variable volume system with a lot of part load hours, this may not be as big a factor as it seems due to the square law relationship between flow and pressure drop.
COVID and infections control issues aside, there are other reasons you might consider applying this approach. When I did a quick survey of the company to see if anyone had seen the configuration Mike proposed, it turned out that we had. But the applications were driven by the nature of the load and included automotive paint booths, server rooms, and museums. That’s not to say the concept does not have merit for the reason Mike pointed out. It just means that myself and the folks I work with have not seen it applied for that reason (yet).
Taking a Closer Look at the Process
Finally, the part you have all been waiting for. To get started I want to clarify a few of the assumptions and details behind what Mike presented.
Process Analysis Assumptions and Details
There are a number of things you need to understand for the discussion of the process to make sense. But if anyone is still actually reading this at this point, and if said person can hardly wait to read the process discussion and feels fairly comfortable with psychrometrics, then said person may want to skip this section and jump straight to the discussion of the process itself.
Having said that, the following paragraphs kind of lay a foundation for the discussion of the process.
The Line on Mike’s Chart is the Result of a Bunch of Processes, Not a Single Process
Probably the most important thing to recognize is that the heavier black line Mike drew on the psych chart was not one specific psychrometric process. Rather, it is the locus of points representing the leaving conditions from the evaporative cooler that will be produced by a system configured and controlled as he proposed as the outdoor conditions varied. I did not realize this initially, and it is an important point to recognize.
In the course of what follows, Mike and I identify specific points on this line for specific indoor and outdoor conditions The hope is that this will allow you to “connect the dots” and understand the locus of points that Mike presented, which is what it did for me.
The Air Inside the Building Came from Outside the Building
In some ways, this is obvious. But there is an implication to it that I want to highlight, that being that the lower limit on the moisture level in the building is most likely set by the ambient moisture level outside the building.
In other words, most processes that occur in buildings add moisture to the air. Since the air inside the building comes from outside, then the moisture added in the building will tend to raise the dew point and specific humidity of the air inside the building.
There can be exceptions to this. For instance:
- If the facility was hosting a desiccant manufacturers product showcase and all of the vendors had their wares on open display, then potentially, the moisture level inside could be reduced relative to the outside. Or, in a more realistic example,
- For a facility that processed paper and stored the raw material in a warehouse that was maintained at a low temperature relative to the process area which was maintained at a higher temperature and actively humidified, during cold, dry weather, when the raw material was brought in, it would tend to absorb moisture and lower the indoor humidity level.
But most of the time, building processes will add moisture to the air. We can reflect this on the psych chart using a sensible heat ratio (SHR) line, which is the ratio of sensible (heat or temperature changing energy) added to the air by the process occurring in the building relative to the total amount of energy added (both heat and moisture in the form of water vapor, the latter increasing the specific humidity).
A SHR of 1.0 means there is no moisture being added to the air. Increasing latent loads cause the SHR to drop away from 1.0. The chart below illustrates several different sensible heat ratio lines plotted relative to a 72°F/50% RH space.
So, for example, an air handling system was delivering saturated 45°F air at its design flow rate to serve a design load condition for a space with a SHR of 0.9 and a set point of 72°F, then the resulting space condition would be 72°F, 42% RH. If the SHR was 0.8, then the space condition would be 72°F, 46.8% RH. The chart below illustrates these two processes.
The 45°F saturated air could be the result of any number of processes, including:
- The leaving condition from an evaporative cooler, or
- The leaving condition from an active cooling coil coil that was condensing, or
- An air handler supplying 100% outdoor air on a foggy day.
The Process Targets a Space Condition Window, not a Point
In the charts that follow, the trapezoid highlighted in orange represents the space conditions targeted by the process we will discuss, specifically:
- 70-75°F dry bulb temperature
- 40-60% relative humidity
The chart below contrasts the window targeted by the process we are discussing with the 2010 ASHRAE summer (red) and winter (blue) comfort zones.
As you can see, the range we are discussing is a subset of the winter comfort zone, which is the season during which the process would be used.
While most designs target a specific point for calculation purposes, real processes operate over a range that is set by things like the tolerances on the design point and the accuracy of the control process. In this case, the range allows the proposed process to be used over a fairly large range of climate conditions in the Portland area.
If we narrowed the range down, either in terms of temperature or relative humidity, there would be fewer hours were we could use the process in the Portland climate and vice versa. I believe this will become apparent as we move through the details of our discussion.
The Evaporator Cooler will Produce Near Saturated Air
Evaporative coolers are to some extent, field deployments of adiabatic saturators. For a true adiabatic saturator, at its exit, the leaving air is saturated, which means:
- The relative humidity is 100% and
- The dry bulb temperature, dew point temperature, and wet bulb temperature are identical numerical values.
To achieve this, among other things, a true adiabatic saturator needs to be infinitely long, which (I suspect) is one of the reasons you do not run into many of them out in the field. For one thing, they would kind of get in the way. And for another, Owners and Architects – with some justification I might add – are somewhat opposed to infinitely long mechanical rooms.
One of the things that happens when you make your evaporative cooler less than infinitely long is that the air coming off of it is not 100% saturated. But, units can typically produce air with wet bulb temperatures that approach the dry bulb temperature by 3-4°F under design conditions, with efficiencies in the 80% –95% range depending on the specifics of the design.[i]
If you reduce the flow and thus provide more time for the air in the evaporative cooler to be in contact with the media in the cooler, you can approach adiabatic saturation. Mike’s diagram assumed that would happen because he was modeling the application in a VAV system that was at 50% of its design flow and as a result, the saturation efficiency of the evaporative cooler would approach 100%.
The charts that follow make the same assumption for the purposes of illustration. But a real system would generate leaving conditions that are very near but not on the saturation curve of the psych chart. How close the leaving conditions got to saturation would depend on the efficiency of the evaporative cooler at the flow rate that existed at the time. The approach to saturation will improve as the flow rate drops below the design value.
The Chilled and Hot Water Coils are Not Active
Mike’s analysis focused on outdoor conditions when neither preheat nor mechanical cooling would be required to achieve the targeted leaving air condition. In other words:
- The evaporative cooling process alone could deliver the desired leaving air temperature, which in the example, ranges from about 45°F to about 55°F.
- The outdoor conditions are such that the system was never driven to minimum outdoor air when it was cold outside, which is when preheat would be required if the outdoor air temperature continued to drop with out causing the evaporative cooler leaving air temperature to drop.
How many hours this encompasses will vary significantly with climate. In particular, the metrics Mike cites were based on assumptions about applying the process in the Portland, Oregon climate and the analysis and charts that follow use the same assumption.
A Brief Review of Mixing on a Psych Chart
To understand the discussion that we are leading to, it is important you understand how a mixing process shows up on a psych chart, in particular that:
- The mixed condition for two points on the chart will lie on a line that connects them and,
- The mixed point will be proportionally spaced between the two points in direct relationship to the percentage of the mass flow rate associated with each of the points.
This is illustrated below for a number of different mixing percentages, temperatures and humidity levels. Notice how the mixed temperature and its location relative to the two conditions being mixed is the proportional to the minimum outdoor air percentage and the two temperatures that are being mixed.
The Mixing Dampers are Controlled by the Dry Bulb Temperature Leaving the Evaporative Cooler, Not the Mixed Air Temperature
This is really important because, as mentioned previously, for an evaporative cooling process, the leaving air is nearly saturated and as a result, measuring dry bulb temperature will also provide an indication of the wet bulb temperature and dew point temperature.
If the air is saturated, they will be exactly the same. If the air is near saturated, then they will be very close. For example, if the saturation efficiency of the evaporative cooler was 95%, then the leaving wet bulb temperature would likely be with in a degree or less of the leaving dry bulb temperature.
If you consider this for a minute, you will realized that for a given outdoor dry bulb temperature and a given evaporative cooler leaving air temperature set point, where the evaporative cooler leaving air dry bulb temperature is being used to control the mixing dampers;
- Because the air is nearly saturated, the mixed air dampers are also being controlled for a leaving wet bulb temperature that is nearly identical to the dry bulb temperature , and
- As a result of item 1, the mixed air dampers are also operating to maintain a fixed wet bulb temperature set point, and
- The amount of outdoor air brought in to the system will vary with the outdoor wet bulb temperature; on a dry day, the system will bring in less outdoor air to achieve the required set point vs. what it will need to bring in on a moist day.
This is illustrated in the chart below. Note how the outdoor air percentage required to achieve the 45°F saturated leaving air dry bulb/wet bulb temperature varies with the outdoor conditions.
The next chart illustrates what happens in a more conventional mixed air control process, where the mixing dampers are being controlled for a fixed mixed air dry bulb temperature. Note how the outdoor air percentage does not change, even when the outdoor conditions change.
The Mixed Air Set Point is Lower than Typically Used
As you have probably observed, the 45°F supply temperature we are discussing is a lot cooler than we typically use in our systems, all-though you might see temperatures in this range for some special processes.[ii]
Generally speaking, running colder discharge temperatures than needed to satisfy the space dehumidification load will cost you energy when you are doing mechanical cooling.
- For one thing, it will require lower refrigerant temperatures in the coils, which will tend to lower the efficiency of the compressors providing the refrigeration.
- For another, if the minimum flow rate provided by the terminal equipment provides more sensible cooling than needed once they are at minimum flow, you will use unnecessary reheat compared to what would happen with warmer supply air temperatures.
But, if you are not using mechanical cooling, issue 1 enumerated above goes away. That means that as long as a lower supply air temperature does not drive zones into a reheat mode, then for a variable air volume system, there could be a fan energy benefit associated with the lower supply temperature.
In other words, if a zone required 1,000 cfm of 55°F supply air to maintain a 72°F set point, it could also maintain that set point by using about 630 cfm of 45°F air. So, as long as:
- The diffusers would perform with the cooler air, and
- The colder distribution temperatures did not result in condensation issues on the ductwork and related hardware, and
- None of the other zones on the system were driven into a reheat cycle when they would not have been driven into a reheat cycle with warmer supply air,
… then fan energy will be saved.
For Mike’s idea, the colder supply temperature will translate to lower system flow rates. This will tend to push the saturation efficiency of the evaporative cooler to higher values, which means using dry bulb temperature to control the process will provide satisfactory results with out the added first and ongoing cost of some sort of humidity sensor.
Good Mixing is Critical to Success
Achieving thorough mixing in a mixed air plenum is critical to success and is surprisingly hard to achieve. Velocity and temperature stratification are very common, especially if you don’t pay attention to the details. In fact one of my current focuses on the blog is a series of posts looking at this topic.
Since a process using the approach we are discussing may use a mixed air temperature set point that is lower than typically encountered, as discussed in the preceding paragraph, ensuring that the mixed air plenum is designed to promote good mixing will become even more critical. The most serious potential issue, of course is a localized cold spot where temperatures could drop freezing during extreme weather, even though the average mixed air temperature was well above freezing.
The Process (Finally)
What follows is my transcription of the dialog between Mike and myself as we discussed the process he suggested. At the end of it, he indicated that I had “nailed it”. But if there are errors in the transcription that follows, they are totally on me.
For the discussion that follows, I have assumed a space SHR of 0.90. But other SHRs (until you got pretty extreme in terms of space latent load and outside of what you would see for most commercial office buildings) would have similar results.
In general terms, since the system is controlling for the temperature of near saturated air leaving the evaporative cooler:
- The mixing point will lie on the constant wet bulb temperature line associated with the set point.
- The blend of outdoor air and return are required to meet set point will vary as the outdoor conditions vary causing the mixing point to move up and down the constant wet bulb line.
- Once the outdoor wet bulb exceeds the set point, the system will be driven to 100% outdoor air, which will cause the discharge condition from the evaporative cooler to move up the saturation curve.
The following paragraphs illustrate this in more detail.
An Extreme Winter Portland Day
If we start with a somewhat extreme condition for Portland (based on TMY3 data) then the process looks like this.
Controlling the mixed air dampers to deliver 45°F air off the evaporative cooler puts you at about 45% outdoor air and delivers a space at the bottom end of the targeted temperature window and up a bit from the bottom end of the targeted RH window.
A Typical Portland Fall/Winter/Spring Day
If we look at what would happen if the OA was in a more typical but cold range (the left most red squares on the chart), we end up here.
We require a higher percentage of OA (83%) because it is already moist. But since we are modulating the mixing dampers based on what happens after the evaporative cooler to maintain 45°F at that point (remember, for this discussion, because of the saturation efficiency of the evaporative cooler, 45°F dry bulb is about the same as 45°F wet bulb), we just slide up the 45°F wet bulb line and the space condition we deliver (assuming the load – sensible and latent – did not change) remains the same.
A Warm But Dry Portland Fall/Winter/Spring Day
If we look at what happens on a warmer, but dry OA condition, as long as the OA dew point is below the evaporative cooler LAT set point, we still hold the same space conditions. But this time, we need to use more OA because the OA is warmer and dryer.
Moving from Spring to Summer (Summer to Fall Transition Similar, Just Going the Other Way)
If the OA wet bulb rises above the evaporative cooler LAT set point (which is controlling the mixing dampers), it will drive the mixing dampers to the 100% OA position and hold them there.
The control process can not meet its set point and as a result, the evaporative cooler LAT rides up the saturation curve, following the outdoor air wet bulb temperature. Here is what that looks like for a somewhat common condition with a OA wet bulb above the 45°F evaporative cooler LAT set point.
Now, the space temperature and humidity start to drift up because the evaporative cooler LAT starts to drift up, but (assuming the load did not change and the VAV system flow did not change), you are still inside the envelope you targeted. If you really wanted a lower space temperature, you could allow the VAV system to move a bit more air.
Encountering a Limiting Condition
Once the outdoor wet bulb drifts up to 50°F, we reach the limit of what we can do with the current VAV system flow rate (50% of design) assuming the load condition did not change; i.e. at that point the space ends up at the upper limit of the temperature window, but below the humidity limit.
Allowing the System Flow to Increase
If we continue to let the evaporative cooler LAT drift up as the outdoor air wet bulb drifts up, the VAV system could still keep us in our targeted window if it increased the flow rate. When we reached the 55°F upper limit Mike discussed (a common commercial building HVAC system leaving condition) we would end up here.
But, if the load had not changed, we could actually allow the LAT to drift up to about 59°F before the leaving condition was outside the targeted window (assuming the load has not changed and the VAV system is allowed to move more air to accommodate the lower LAT to space temperature difference).
Some Bottom Lines
How you would decide if you should do this and when to do this would be a function of the ability of the envelope to handle higher humidity levels in cold weather, the ability of the operating team to maintain the equipment, utility rates, hours of operation, and climate in addition to a desire to hold indoor conditions in the 40-60% RH range. A totally brilliant idea in location “A” could be a disaster in location “B”.
For instance, if you had an artesian well on your property and the law was written to say you owned the water rights (i.e. free water), what you would do would be totally different from a location where the water rates where high and you also did not get a credit on your sewer bill for water that was evaporated.
And if you did get a credit on your sewer bill for water that was evaporated, then that would also change the financial perspective.
Mike and I talked about using the TMY3 data to look at the water consumption and pump energy for the process in Portland to assess the full cost implication of using this strategy, but neither of us have had the time to do this at this point, so fodder for a future post.
But hopefully, what we have shared will help you “think outside the box” in terms of how we operate our buildings to deliver a clean, safe, comfortable, productive environment as efficiently and sustainably as possible, given the ever changing challenges we face.
[i] For those who are interested, the relationship for saturation efficiency is as indicated below.
[ii] For example, for the make up air systems serving the clean rooms I worked with when I was a facilities engineer/system owner at Komatsu’s Hillsboro plant, we targeted a 46°F leaving air temperature from our cooling coils in order to hit the space relative humidity requirement.