Psychrometric chart provides a graphic relationship of the state or condition of the air at any particular time. It displays the properties of air: dry bulb temperature, wet bulb temperature, dew point temperature, and relative humidity. Given any two of these properties, the other two can be determined using the chart. The chart’s usefulness lies beyond the mere representation of these elementary properties – it also describes the air’s moisture content, energy content, specific volume, and more.
The psychrometric chart conveys an amazing amount of information about air. It provides an invaluable aid in illustrating and diagnosing environmental problems such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation. A psychrometric chart also helps in calculating and analyzing the work and energy transfer of various air-conditioning processes.
In practical applications, the most common psychrometric analysis made by HVAC contractors involves measuring the dry and wet bulb temperatures of air entering and leaving a cooling coil. If these temperatures are known along with the volumetric air flow rate through the coil, the cooling capacity of a unit can be verified. Using the dry and wet bulb temperature information, two points can be located on a psych chart and the corresponding enthalpy values read for them. The total BTUH cooling capacity can then be determined. Contractors often have to perform this calculation to prove that their equipment is working satisfactorily.