PSYCHOMETRICS - BASIC DEFINITION AND TUTORIALS

What is Psychometrics?


Psychrometrics is the science of the properties of moist air, i.e., air mixed with water vapor. This subset of thermodynamics is important to the HVAC industry since air is the primary environment for all HVAC work. 


Whereas oxygen, nitrogen, and other components of dry air behave similarly in only a vapor phase in the HVAC temperature range, water will undergo a change of state in the same temperature range based on pressure, or in the same pressure range based on temperature.

In the human comfort temperature range, the comfort of people and the quality of the environment for health, for structures, and for preservation of materials are also related to the moisture in the air. Control of the moist-air condition is a primary objective of the HVAC system. 


Remember the following:

1. Air is considered to be saturated with moisture when the evaporation of water into the air at a given temperature and atmospheric pressure is offset by a concurrent condensation of water vapor to liquid. Cooling of saturated air results in dew, fog, rain, or snow. Warm air can hold more moisture than cold air.

2. Percent relative humidity measures how much water vapor is in the air compared to how much there would be if the air were saturated at the same temperature. 


The adjective relative is appropriate because the absolute amount of water that air can hold is relative to both temperature and barometric pressure. 


Changes in barometric pressure related to altitude or to weather conditions affect the moisture-holding capacity of air.

3. A psychrometric chart which presents properties of mixtures of moist air on a single graph is a most useful tool for quantitatively calculating and analyzing HVAC processes. Familiarity and facility with these charts are a must for the HVAC designer.

4. It is impossible to remove moisture from air in a heat exchange cooling process without bringing the air near to the saturation line. Moisture may be removed by desiccants without approaching saturation.

5. Optimum conditions for human health and comfort range from 70 to 75 F and 40 to 50 percent relative humidity. In terms of perceived comfort, a little higher relative humidity can offset a little lower ambient temperature.

6. Moist air in cold climates is a problem and a liability for building designers. Since the inside environment usually is moister than the outside air, insulation and vapor barriers are required to prevent condensation in the structural cavities. 


Failure to respect this liability may lead to early deterioration of a building. Swimming pools and humidified buildings (hospitals, etc.) are particularly vulnerable.

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