Psych Charts and Economizers

I recently upgraded my laptop and have been in the process of discovering software packages that I had on my old one and need to reinstall on my new one. One package I recently realized was missing was the Greenheck CAPS software I have mentioned in past posts.In the course of reloading CAPS, I went to the Greenheck website to check for updates and discovered that Greenheck now offers a free electronic psychrometric chart.

As you can see, the quality of the chart is quite nice, comparable, for instance, to some of the electronic charts that are out there for purchase like Akton Psychrometrics, which I happen to use.

In terms of functionality, you can do all of the basic things that you typically would want to do with a psych chart in the field. For instance, one of the questions I ask in a class I do about the design, commissioning, and operation of economizers is:

You have an air handling system equipped with an integrated economizer cycle (See Footnote 1) serving a space that you are trying to maintain at 72°F 50% relative humidity. To do that, you have to deliver air from the discharge of the cooling coil at 51.8/51.6°F tdb/twb.

The system has a minimum outdoor air requirement of 18% of the supply flow. It’s a beautiful 58°F spring day outside with relative humidities in the low 50% range. Because of the integrated economizer feature, the control system has the system operating on 100% outdoor air so that the machinery only has to cool 58°F air to the required discharge temperature instead of the mix of return air and outdoor air that you would have if the system was operating in a recirculation/minimum outdoor air mode.

Then, it starts to rain. For best energy efficiency, should your economizer remain on 100% outdoor air or should you revert to minimum outdoor air?

To answer the question, you need to compare the energy content of the air stream that would pass through the cooling coil if you remained on 100% outdoor air with the energy content if the air stream that would pass through the cooling coil if you were to revert to recirculation with minimum outdoor air.

Luckily for us, the energy content if air can be assessed in terms of enthalpy, and enthalpy is one of the parameters plotted on a psych chart. And better yet, you don’t have to be able to measure or know the enthalpy of the air you are dealing with to use the chart. You just need to know two of the other, more readily measurable parameters, like dry bulb and wet bulb temperature or dry bulb temperature and relative humidity.

And, since the state and energy content of the air stream you would get if you mixed minimum outdoor air with return air would lie somewhere between the two conditions, all you really need to do answer the question is compare the enthalpy of the air in the space at your targeted condition to the enthalpy of the outdoor air. In other words, you just have to plot those two points on the psych chart and compare the results.

To do that with Greenheck’s chart (or the Akton chart I have for that matter), you simply open the State Point and Processes window (located in the Analysis drop-down menu) …

… and put in the data for your two points. Then you can either read the result directly by “eyeballing” the chart …

… or from the information that shows up in the State Point and Processes window when you highlight the point in question …

… or you can put your cursor over the point and read the information from the little windows along the left margin of the chart that show the current parameters for the point under the cursor (I had my cursor over the “Space” point for this screen shot).

As you can see from the illustrations, the enthalpy or energy content of the outdoor air when it is raining is about 25.1 Btu/lb where as the enthalpy of the space air is 26.4 Btu/lb. That means that the system will use less energy if you stay on 100% outdoor air despite the fact that the relative humidity of the outdoor air is 100% due to the rain.

This is a bit counter intuitive for some folks, mostly because its easy to be mislead by the term relative humidity. We generally associated high relative humidity levels with hot, muggy, high energy content air. But, you have to keep in mind that the term is RELATIVE humidity; i.e. a measure of the amount of water in the air Relative to what it could hold if it was saturated at the current temperature.

For instance, the relative humidity at the North or South pole is probably close to 100% most of the time simply because the air is so cold that you can’t keep much moisture in suspension. But a 100% relative humidity day in Key West Florida would have a very different feel to it.

So, bottom line, if you are in the bussines, you probably need a psych chart. And now, thanks to Greenheck, you can have the benefit of an electronic psych chart that lets you quickly and accurately plot your data and paste it into reports for free.

For a relatively modest cost, you can also upgrade the Greenheck chart to provide more functionality than the basic functions provided by the free of charge version. In a subsequent post, I’ll look at what some of those features are but in the meantime, visit the Greeheck site and download your copy of their free tool.

And incidentally, if the whole concept of psychrometrics and using a psych chart is a but of a mystery to you, the Honeywell Gray Manual, another free on-line resource, has a great, very understandable discussion of both topics in the Psychrometric Chart Fundamentals chapter.

And, for those who want to know more about economizers, integrated or otherwise, there are a couple of useful free resources out there to help you out.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for an index to previous posts
Footnote 1 – An integrated economizer cycle is a a process where the economizer is allowed to remain in operation, bringing in 100% outdoor air, even if the air is above the required discharge temperature set point and thus, must be cooled to some extent by a mechanical cooling process.

In general terms, the idea is that if you have for instance, a 72°F/50% relative humidity space, it will be cheaper to cool outdoor air with an energy content below that associated with 72°F/50% RH that it will be too cool the space’s 72°F/50% RH return air mixed with the minimum outdoor air requirement even though the outdoor air is above the current cooling coil discharge temperature requirement.

The trick is in figuring out the energy content of the two air streams, which is where the psych chart and ethalpy controlled economizers come in to the picture.

This entry was posted in Controls, Economizers, Psychrometrics. Bookmark the permalink.

4 Responses to Psych Charts and Economizers

  1. Mike Hart says:

    David has made a good point on his questions about the economizer operation, however it is not necessarily correct to assume 100% relative humidity when it is raining. It may be 100% in the cloud, but on the ground, it is normally less than 100% and in practice the rain sometime dehumidifies the amibient air because the water is cooler than the ambient temperature. Think about how many times you have been rained on and you had a chilled sensation. It works like the “air washers” used back in the 50’s and 60’s, cold water spayed into the air will cool and dehumidify.
    Good article.

  2. Hi Mike,
    I hadn’t noticed this comment before, but you make a good point. At the time I first did that example, I was literally thinking about what was happening outside in Oregon, and when the rain moved in, we also got socked in with fog. So, the relative humidity really was 100%. But as you point out, that’s not always the case for a number of reasons. Thanks for clarifying that.

  3. Thanks David. Very informative.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s