Why 55 degrees?
Convenience. Air temperature of 55 degrees and fully saturated with vapor equals about the same water vapor as 75 degrees 50% humidity, the typical comfort requirement.
For a graphical representation of humid air properties, refer to Apollo BBC's Psychrometric Cart.
Air conditioning systems cool air by pulling it across a cooling coil. In this process, water vapor condenses on the coil, similar to a sweating water glass, and is drained away. Because water is being removed from the air, the air is undergoing dehumidification prior to traveling to occupied spaces.
Air conditioning engineers design cooling systems to blow cold air at 55 degrees F, from ceiling diffusers into the space where it blends with warmer room air. By the time it travels to occupants, the temperature has increased significantly and provides for a comfortable space.
What does Psychrometrics have to do with this?
Cooling equipment can only cool and dehumidify a narrow margin of conditions. What happens when you mix outdoor air into the return air of an air conditioning system? Outdoor air, laden with moisture, dramatically changes the cooling and dehumidification capacity required of the coiling coil. Psychrometrics allows engineers and building operators to select adequate systems and predict their performance.
When sensors and dampers operate out of calibration or the use of a space changes dramatically, the operation and adequacy of air conditioning equipment changes. A detailed psychrometric analysis is required to predict the impact of this change on the indoor environments and occupant comfort.
Psychrometrics can also be applied to heating conditions. Predicting the impact of mixing cold dry air with warm indoor air allows engineers and operators to compensate with other equipment such as humidifiers.
HVAC systems are designed to provide for occupant comfort, but these can also have a dramatic impact on the building enclosure. For more information about this impact and how psychrometrics is used in the analysis of walls, refer to Psychrometrics Page 4.
| Psychrometrics Page 1 - Introduction Page 2 - Indoor Comfort Page 3 - HVAC Systems Page 4 - Building Enclosures Page 5 - Definition of Terms |