by Frank Eisenhower, PE, LEED BD+C | Principal, Mechanical Engineer

Did you ever wonder why the first chapter in the ASHRAE Fundamentals Handbook is titled Psychrometrics? Well, in simple terms, these principles are the basis of all air-conditioning processes.

Psychrometrics is the field of engineering that studies the thermodynamic properties of air and water vapor and how they inter-relate in our atmosphere. The thermodynamic properties include dry bulb temperature and relative humidity (as reported by your local news), wet bulb temperature, humidity ratio, enthalpy, dew point temperature and others.  

If  you remember your high school years in Physics class, a practical device to measure dry bulb temperature and wet bulb temperature is a sling psychrometer. This apparatus contains two thermometers, one with a wetted cotton wick covering the bulb. One looks very silly ‘slinging’ this rotating device in the air, but it provides two valuable thermodynamic properties of air, dry and wet bulb temperature.  

To facilitate engineering calculations, HVAC engineers utilize a graphical representation of these properties on a chart called the Psychrometric Chart (developed by Robert Mollier in the early 1900’s).  Many of the thermodynamic properties of air are represented by a series of lines on a graph, similar to the chart below. The power of the graph lies in the fact that any two air streams mixed together and plotted on the graph, can be represented by a straight line. This is why your HVAC Engineer will sometimes make the statement, “if it is possible on the psychrometric chart, it is possible in your building.  The graph does not lie.” The resultant graph is used to determine the heating and cooling capacities required for your HVAC equipment.

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