This article is based on information from RSES.
Service managers involved in conducting in-house training sessions on the ever-important issue of air flow and HVAC system performance often find that technicians are somewhat intimidated when they need to review a psychrometric chart. (Or, they’re of the opinion that they don’t need to know much about psychrometrics to do their job.) Since there are six separate sets of lines that are used to plot conditions once temperatures in a given situation are established, the chart can be daunting. However, it can be demystified by reviewing the sets of lines individually in skeletal illustrations. In Figure One are the dry bulb temperature lines of a psychrometric chart.
These lines are properly referred to as “Constant Dry Bulb Temperature Lines” because any point on any given line, each line represents the temperature listed on the scale at the bottom of the chart. The lines shown below in Figure Two, are also referred to as constant temperature lines, and they are identified as Wet Bulb Lines.
These lines run down from the temperature scale on the curved section of the chart at an angle of approximately 30-degrees off of horizontal. The next set of lines shown in Figure Three are the “Constant Relative Humidity Lines”
The top curved line on the chart shown as the 100% relative humidity line is also known as the saturation line, meaning that we no longer have water vapor in the air, but liquid condensing from it. In most situations, many people are comfortable when the humidity level is around 50%. It is usually recommended that the humidity level in a building remain between 40% and 60%.
These first three sets of lines are the basic ones that allows a technician to understand the process of taking both a wet bulb and dry bulb measurement in a given area, and then plotting the relative humidity conditions in the space. This is one of the most fundamental checks a technician can perform in the process of evaluating the performance of HVAC equipment and advising the customer (in language they can understand) what service may be required in order to ensure that their system is performing as efficiently as possible.
Going beyond these three sets of lines; the Constant Dry Bulb Temperature Lines that run from the bottom of the chart to the curved top, the Constant Wet Bulb Temperature Lines that run from an angle of approximately 30 degrees from the curved line to the right, and the Constant Relative Humidity Lines that follow the pattern of the curve line, the fourth set of lines we want to consider are the “Constant Dew Point Lines”. (See Figure Four)
The dew point temperature numbers are the same ones used for the wet bulb temperature scale, but the lines coming from the numbers run directly horizontal rather than at a diagonal. These lines also correspond directly to another listing to the right of the chart which expresses moisture level on the very fine scale of grains per pound. This scale is also known as the Specific Humidity Scale. This set of lines on the psychrometric chart, shown in Figure Five, run down from the curved line at an angle of about 60 degrees, and they are the “Constant Specific Volume Lines”.
The point you want to drive home in your training sessions is that these lines represent the idea that air has a certain density that changes as the temperature and water vapor level changes. And this scale is built on the fact that 1 lb. of air at a saturation temperature of 65-degrees F has a specific volume of 13.50/ft3lb (13.50 cubic feet per pound).
This brings us to the sixth set of lines on the chart that run from the curved line to points on a numbered scale above the chart, shown in Figure Six, and they are known as the “Constant Enthalpy Lines”. The term Enthalpy means “total heat content”, and these lines are simply extensions of the dew point temperature lines.
The scale shows the total heat content measured in BTU/lb. which is the common measurement of heat content used in the HVACR industry. The two kinds of heat that make up total heat are Sensible Heat, which is heat that can be measured, and Latent Heat, which is also known as “hidden heat” and is defined as heat that brings about a change in state, but not a change in temperature.
Before we explore an example of the processes of handling sensible and latent heat, we’ll bring together all the lines we’ve discussed so far; the complete psychrometric chart appears as shown in Figure Seven.
Once a technician has a grasp of the basic structure of the psychrometric chart (understanding that there are six separate sets of lines as we discussed) the next concept to discuss is that the chart can be used to evaluate the performance of a comfort cooling system regard to the removal of sensible and latent heat. Sensible heat, since it is simply defined as heat that can be measured, relates to the change in the temperature of an air sample. Latent heat, since it is defined as heat that brings about a change in state but not a change in temperature, is related to change in the level of water vapor in an air sample.
One simple way to explain the concept of the removal of these two kinds of heat and how the psychrometric chart can illustrate the level of a performance of a system relative to comfort via both temperature and relative humidity, is to consider something known as “State Point”, shown in Figure Eight.
The two temperatures plotted in this example are 80-degrees dry bulb (red line) and 67-degrees wet bulb (blue line), and the point at which the two lines converge is just above the 50% relative humidity line. The idea to keep in mind here is that once this baseline is established, the psychrometric chart can be used to illustrate the amount of sensible and latent heat that is being removed in a given situation. If the level of sensible heat removal is too high and the removal of latent heat is too low, that won’t result in maximum comfort. (See Figure Nine)
In this example we’ve achieved a significant drop in dry bulb temperature all the way from 80-degrees down to 60-degrees, and from the state point of 67-degrees wet bulb down to 56.3-degrees, but what we haven’t accomplished is a balanced removal of heat. This is indicated by the fact that our relative humidity is now 80%. In Figure Ten, though, we have a more balanced removal of heat that will result in a more comfortable situation.
Here, we’ve only dropped the dry bulb temperature from 80-degrees down to 75-degrees, and the wet bulb temperature from 67-degrees down to 61-degrees, but in the process, we have accomplished a more balanced removal of heat because our relative humidity is now at 45%.
When you present a simplified explanation of the psychrometric chart and add an explanation of a fundamental psychrometric process that applies to the common customer complaint of “not cooling enough”, it’s an important step toward getting the technician to understand the performance of the HVAC system from a whole building perspective, rather than just as a the ‘box’ that provides the cool air.