Applied Psychrometrics – Introducing the Psychrometric Chart

Psychrometrics is the field of engineering concerned with the determination of the physical and thermodynamic properties of gas-vapor mixtures.  For the purposes of the discussions on this blog, which are primarily related to Heating, Ventilating and Air Conditioning Systems (HVAC), we can narrow that definition down and say it is the field of engineering concerned with the determination of the physical and thermodynamic properties of moist air.

I have touched on this concept previously in the string of blog posts that I wrote on the topic of Saturated Multiphase Systems.  In those posts, I looked at a number of different types of thermodynamic diagrams and where they come from.  A psychrometric chart is simply a type of thermodynamic diagram that focuses on moist air. 

If you are going to be working with building systems and HVAC it is important to understand and learn how to apply the psychrometric principles associated with moist air.  A psychrometric chart is a tool that we can use for doing just that. Here is a typical example, which is a screen shot from the Akton Psychrometrics software package, which is an electronic psych chart.


Pretty scary looking, right?  Fortunately, instead of using a psych chart, there are a number of equations that have been developed that allow us to directly compute the properties of interest as long as we apply them within the constraints that apply to them.

  For example, here is one you can use to come up with specific volume.


And here is the one for enthalpy, a property we use a lot in HVAC calculations.


At this point, I bet the psych chart is not looking half bad.  And that is my point.  The psych chart (and other thermodynamic diagrams) allow us to analyze some fairly complex phenomenon  and visualize processes via a graphical technique rather than via calculations.   That can save us a lot of time and can also be a lot less intimidating.

As a result, I will be using a psych chart extensively in the Applied Psychrometrics blog posts.  The purpose of this post is to introduce concept of the psychrometric chart.  In the next few posts, I will take a look at the various axis of the chart. Then Iwill start looking at how you use it to solve field problems and analyze processes.

The bottom line is that if you are going to be working in this industry, you probably need to get comfortable with using a psych chart and that is not as tough as it sounds.  Hopefully, this string of posts will help you with that.  But there are also a number of other resources that you might find helpful.

The Honeywell Gray Manual

The Honeywell Gray Manual was the text book Honeywell would to provide years ago to entry level engineers as they were groomed to work in the HVAC controls industry.  As a result, it includes a very good chapter on how to use the psych chart along with chapters on HVAC basics before moving on to control theory

The Greenheck Electronic Psych Chart

Greenheck provides a free electronic psych chart on their website that will allow you to do the basics like plot points.  A lot can be said for simply working with the chart, perhaps using it to try the examples you read about in the Honeywell Gray Manual.

And you for a modest fee, you can upgrade to their Professional version, which gives you a very comprehensive tool that will serve you well at a bargain price compared to some of the other options like the Akton Chart I have or the ASHRAE electronic chart.

Principles of Refrigeration

The first 5 chapters of Principles of Refrigeration by Roy Dossat discuss most of the physical principles you need to be familiar with in the HVAC business before moving on to focus on refrigeration, which is the primary topic of the book.  Each chapter builds on the previous and the 5th chapter is focused on the psychrometric properties of air.

The book is well written and discusses things in layman’s terms.  The most comprehensive and detailed version of this book is the current edition. But, the original edition is available at no cost from internet archive and is more than adequate in terms of providing a basic level of understanding.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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