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Why Learn Psychrometrics?

By Donald P. Gatley, P.E., Fellow/Life Member ASHRAE

Many young engineers, or engineers in training, don’t possess a good understanding of psychrometrics and how it impacts HVAC, or drying system design and operation, controls, or diagnosis of client comfort and system complaints. Let’s look at why young engineers might lack the motivation to learn psychrometric skills and some ideas that might provide the motivation to acquire these skills. The final part of this article recommends helpful ASHRAE resources to help all engineers expand their knowledge of psychrometrics.

In the 1980s the Psychrometric Chart Shifted to the Back Burner

Until the 1980s the psychrometric chart was the tool of choice for (1) determining state point properties, (2) graphically using space conditions, sensible heat ratio, and outdoor air requirements to determine (and then specify) cooling coil entering and leaving air state points and system air quantity, and (3) providing a visualization of the complete cycle of the individual processes that link to form the air-conditioning cycle.

Computers and graphical psychrometric software programs have taken over Tasks 1 and 2. Using a psych chart for Task 3 is so useful for detecting design problems as well as finding alternative opportunities, but in too many cases is an outstanding, but underused visualization/diagnosis tool.

“During this shift, several reasons emerged for the lack of motivation to acquire psychrometric skills. Here are two:

What is Psychrometrics

Psychrometrics is one of the four sciences underlying ASHRAE’s scope of technical expertise, including refrigeration, heat and mass transfer, and fluid flow. Psychrometrics is a subscience of physics dealing with the properties and processes of moist air. Moist air is defined as a mixture of two gases: dry air and water vapor (the gas phase of H2O).

The water vapor content of air varies from as little as 0.2% by volume to as much as 3.5%, but that small percentage accounts for a significant part of the energy required to operate air-conditioning systems and is associated with many system comfort complaints.

  • Software design tools today can compute the underlying sensible and latent loads and then feed them into psychrometric software that outputs all of the necessary information for air-handling humidification and coil schedules. The time and work required to calculate and plot the room sensible heat ratio, the cooling coil process, and then calculate air quantities and fan heat gain are all done by the computer. This saves the drudgery but also deprives the designer of familiarity and insight into the design.
  • Psychrometrics is taught only one or two days of a thermodynamics course. Thermodynamics is one of the tougher courses for engineers and includes many new concepts and a good deal of integral and differential calculus. Does psychrometrics require all those complex equations and calculus? No! It requires no calculus—and only a little algebra for the rearrangement and substitution of equations.

Importance of Learning Psychrometrics

Reasons it’s important to learn psychrometrics include:

  • Pass the EIT or P.E. exam.
  • Keep the option open to have a career in HVAC, drying or meteorology, or in one of perhaps 100 different industries (such as bakeries, breweries, building science, food storage and packaging, hospitals, natatoriums, and residential air conditioning).
  • Become an expert in your field.
  • Avoid costly design mistakes and angry clients (not to mention bosses).
  • Better understand this moist atmosphere we breathe, live in, fly in, etc.
  • Do the best possible job for your company and our client.
  • Design a more efficient system or an alternative and innovative cycle.
  • Acquire the skills to analyze (diagnose) comfort or performance problems.
  • Help owners correct building and wall moisture problems.
  • Specify better and more complete HVAC control modes.
  • Have a sense of achievement for mastering one of the four underlying fundamental sciences.

Embarrassing and costly design errors that understanding psychrometrics can help avoid include specifying impossible cooling coil leaving air conditions or failing to recognize that system designs for altitudes more than 1,000 ft (304.8 m) different from your normal designs must use psychrometric software and apps and psychrometric charts for the altitude of the project. Both charts, equations, and shortcut convenience equations must be adjusted!

Uses of the Psychrometric Chart

In today’s world it is wise to print the complete-cycle psych chart and to spend a little time studying and thinking about the cycle and any alternatives such as:

  • Can I lower the cooling coil leaving air dry-bulb temperature and thereby lower the fan air quantity to reduce fan horsepower (and perhaps duct size) without significantly impacting the chiller or refrigeration system?
  • Can I replace the present cycle of processes with alternative or supplementary processes, e.g., exhaust air energy recovery, run-around heat exchangers, desiccant drying, return air bypass, and/or direct or indirect evaporative cooling?
  • Are the leaving air conditions of each process achievable? Are the fan-heat gain and supply and return air heat gain correctly portrayed?
  • Can my design maintain comfort conditions at part load? Warning: some buildings will operate at part load for years until it’s fully occupied, and the electronics at each work station are doubled.
  • If electric reheat is used, are controls installed so simultaneous electrical kW demand of the refrigeration plant and electric reheat do not combine to set a monthly or a very costly annual building peak demand?

In addition to displaying the HVAC cycle of processes, the chart can display frequency of annual weather data, comfort zones applicable to the activity levels of your design, and regions that require different control modes as shown below (From Understanding Psychrometrics, 3rd ed.)

four regions of operation schemes--psychrometrics.jpg
Control modes superimposed on a psychrometric chart illustrated by the Hong Kong University of Architecture.

 

ASHRAE Resources for Psychrometrics

This article has avoided definitions of the nine common psychrometric properties and all of the crisscrossing lines on a psych chart. For these I suggest you use some of the many available YouTube online videos. Then, initially learn to focus only on a few of the property lines such as tdb, W, tdp and RH. Here is a list of ASHRAE Journal articles (free for ASHRAE members through the ASHRAE Technology Portal) that can help you in enhancing your psychrometric skills:

  • Gatley, D.P. 2004. “Psychrometric chart celebrates 100th anniversary.” ASHRAE Journal 46(11):16–20.
  • Sherif, S.A. 2002. “Overview of psychrometrics.” ASHRAE Journal 44(7):33–39.
  • Gatley, D. 2008. “Grosvenor humidity chart.” ASHRAE Journal 50(10):60–66.
  • Steve Kavanaugh, B. Hattemer McCrary, K. Woodbury. “Psychrometric spreadsheet.” ASHRAE Journal 48(1):28–32.

In addition, the ASHRAE online store includes books and other resources on psychrometrics.

 

Donald P. Gatley, P.E., is a retired consulting engineer and moisture consultant. He is the author of Understanding Psychrometrics, 3rd Edition, published by ASHRAE.