Factors depend on Human Comfort:-
Air temperature (dry bulb temperature or DBT)
Humidity
(relative humidity RH)
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Air movement
(velocity fpm or m/s)
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Humidity (relative humidity RH):
The atmosphere always contains moisture in the form of water vapor. The maximum amount of water vapor that may be contained in the air depends on the temperature. Higher the temperature of the air, the more water vapor may be contained. At high temperatures and high moisture contents extreme discomfort is experienced as the evaporation of moisture from the body into the atmosphere by the process of perspiration
becomes difficult. Saturated air at 100% prevents any evaporative cooling.
Internal Quality of Air:
The air quality is important. In order to control the air purity the air supply to the space is filtered. The degree of filtration will depend on specified requirements for the environment within the room. In addition to the dust control, the air-quality demands precise temperature and RH levels. Too low RH irritates respiratory organs and dust populations increase rapidly at RH levels above 50% and fungal amplifications might occur above 65% RH.
Air movement (velocity m/s):
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The air movement can produce different thermal effects at different air temperatures, in the following two ways:
It provides cooling through evaporation at low humidity levels and at higher humidity’s above 85%, air movement cannot help add vapor to the already highly saturated air.
It increases convective heat loss, as long as the temperature of the moving air is less than the skin temperature.
Air temperature (DBT):
The dry-bulb temperature is the temperature of the air around us and is the most important of all of the above factors. The human body's primary response is towards the changes in temperatures and it is this temperature that we attempt to keep within comfort conditions while designing structures for habitation.
Comfort zone:
It is an area plotted on the psychometric chart that pertains to those conditions of dry-bulb temperature, wet-bulb temperature, wind speeds etc. in which most people wearing specified cloths and involved in specific activity will feel comfortable.
i.e., neither too cold nor too warm. The comfort range of temperature varies between 21 to 25°C dry bulb
temperatures and 45 - 65% relative humidity. This applies mainly to summer air-conditioning. During cold winters the comfort condition would be in the range of 18 to 20°C dry bulb temperature and relative humidity of a minimum of 30%.
Studies of personal comfort have shown that relative humidity ranges between 30% and 65% can be considered 'comfortable' depending on activity.