Handbook of air conditioning and refrigeration / Shan K

3. Mean radiant temperature T rad is another weighted component of the operating temperature T o. It
affects the sensible heat loss from the human body.
4. Relative humidity of the ambient air � a is the dominating factor that determines the difference
p sk, s � p a in the evaporative loss from the skin surface. Air relative humidity becomes important
when the evaporative heat loss due to regulatory sweating is the dominating heat loss from the
human body.
5. Air velocity v a influences the heat transfer coefficient h and the clothing efficiency F cl in the
term in Eq. (4.24) for the sensible heat loss from the human body. It also affects the overall sensible
heat transfer coefficient h s in the evaporative heat loss term and the clothing permeability
i m term in Eq. (4.24).
6. Clothing insulation R cl affects the clothing efficiency F cl, the area ratio f cl, the heat transfer coefficient
h, the clothing permeability index i m, and the overall sensible heat-transfer coefficient h s.
4.7 THERMAL COMFORT
Fanger’s Comfort Equation
Thermal comfort is defined as the state of mind in which one acknowledges satisfaction with regard
to the thermal environment. In terms of sensations, thermal comfort is described as a thermal sensation
of being neither too warm nor too cold, defined by the following seven-point thermal sensation
scale proposed by ASHRAE:
�3 � cold
�2 � cool
�1 � slightly cool
0 � neutral
�1 � slightly warm
�2 � warm
�3 � hot
INDOOR AND OUTDOOR DESIGN CONDITIONS 4.15
A steady-state energy balance is a necessary condition for thermal comfort, but is not sufficient by
itself to establish thermal comfort. Fanger (1970) calculated the heat losses for a comfortable person,
experiencing a neutral sensation, with corresponding skin temperature T sk and regulatory
sweating E rsw. The calculated heat losses L are then compared with the metabolic rate M. If L � M,
the occupant feels comfortable. If L � M, then this person feels cool; and if L � M, then this person
feels warm.
Using the responses of 1396 persons during laboratory experiments at Kansas State University
of the United States and Technical University of Denmark, Fanger developed the following equation
to calculate the predicted mean vote (PMV) in the seven-point thermal sensation scale:
PMV � (0.303e � 0.036M � 0.276)(M � L) (4.25)
In Eq. (4.25), the metabolic rate M and heat losses L are both in Btu/h�ft 2 (W/m 2 ). According to
Fanger’s analysis, the predicted percentage of dissatisfied (PPD) vote for thermal comfort at a
PMV � 0 is 5 percent, and at a PMV ��1 is about 27 percent.
Tables of PMV and comfort charts including various combinations of operating temperature T o,
air velocity v, metabolic rate M, and clothing insulation R cl have been prepared to determine comfortable
conditions conveniently. Fanger’s comfort charts also include relative humidity. Six of his