Psychometric Chart and Comfort Air Engineering
A psychrometric chart is a chart drawn between temperature, relative humidity, enthalpy, wet bulb temperature, etc. As in the air conditioning process, the air is undergoing many changes like in temperature, change in energy, and change in humidity, etc. It is very easy to understand and calculate the changes on the basis of a psychrometric chart. The design of the psychrometric chart is based on two simple facts: that the indoor air is a mixture of air as well as water vapor and it contains energy that keeps on changing due to various interactions.
ASHRAE’s standard 55-2004 describes thermal comfort as the state of mind which expresses satisfaction with the thermal environment and is assessed by a subjective evaluation. However state of mind cannot be measured and therefore based on surveys a comfort zone or rectangle is drawn in the psychometric chart at which most of the people will feel comfortable.
The level of comfort of a person is very subjective and depends on the following factors: age, physical activity, health, and clothing. It is further dependent on other factors like air temperature, air velocity, humidity, and radiant temperature. This means that even if the air temperature is 24 deg C (75 deg F), but the humidity is high at 90% a person will not feel comfortable or vice versa. Even if the humidity is at 90 % but the air temperature is 18 deg C (64 deg F), an individual may feel comfortable. However, if both the temperature and humidity are comfortable but the speed of the air is high, a person may feel draught effects and wind chill factor comes in effect. Similarly all the other parameters may be normal, but the subject is sitting in front of an un-tinted window and may feel radiant heat that may make him uncomfortable. Thus all parameters have to be taken into consideration, and the comfort zone is dependent on the extremities of the points from which you feel comfortable to uncomfortable. Thus within a comfort zone most of the people will feel comfortable if all these points are addressed.