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If the psychrometric chart were a person, you might say its importance to the history of the HVAC&R profession is akin to that of James Harrison, Willis Carrier or Lord Kelvin himself.
That is to say, essential. “Psychrometrics is a sub-science of physics dealing with the properties and processes of moist air,” says ASHRAE. “Moist air is defined as a mixture of two gases: dry air and water vapour (the gas phase of H2O). The term comes from the Greek psuchron meaning “cold”, and “metron” meaning “means of measurement”. Although the principles of psychrometry apply to any physical system consisting of gas-vapour mixtures, the most common system of interest is the mixture of water vapour and air, because of its application in HVAC and meteorology. A psychrometric chart is a graphical representation of the psychrometric processes of air. Psychrometric processes include physical and thermodynamic properties such as dry-bulb temperature, wet-bulb temperature, humidity, enthalpy, and air density. Pioneered by Carrier in 1904, the style of psychrometric chart with which we are familiar graphically depicts several parameters. These are dry-bulb temperature, wet-bulb temperature, dew-point temperature, relative humidity, humidity ratio, specific enthalpy, and specific volume. |
The psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters, one of which must be the pressure. Changes in state, such as when two air streams mix, can be modelled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level.
“The water vapour content of air varies from as little as 0.2 per cent by volume to as much as 3.5 per cent,” says Donald P. Gatley, author of foundation text Understanding Psychrometrics. “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.” Gatley says many young engineers these days don’t have a good understanding of psychrometrics and how it impacts HVAC. Advances in software in the 1980s, he says, and the low priority allocated to teaching it in university courses has de-emphasised its importance. However, he says there are many reasons why it remains important to understand psychrometrics. Not least of these are that such expertise helps avoid costly design mistakes, allows for the best possible job, and provides the skill to analyse comfort and performance problems and therefore provide solutions. |
AIRAH sells both A3- and A4-sized psychrometric pads, available through our online store. AIRAH members can also log in to their account to access a high-res downloadable PDF of the AIRAH Psychrometric Chart. |