Handbook of air conditioning and refrigeration / Shan K

4.14 CHAPTER FOUR
Effective Temperature
The effective temperature ET* is the temperature of an environment that causes the same total heat
loss from the skin surface as in an actual environment of an operative temperature equal to ET* and
at a relative humidity of 50 percent. And ET* can be calculated as
ET* � T o � w ski m LR (0.5p ET, s) (4.23)
where p ET, s � saturated water vapor pressure at ET*, psia (kPa abs.). The right-hand side of
Eq. (4.23) describes the conditions of the indoor air regarding the total heat loss from the human
body. The same value of the combination T o � w ski mLR(0.5p ET, s) results in the same amount of total
heat loss from the skin surface, if other parameters remain the same. Theoretically, total skin
wetness w sk and clothing permeability index i m are constants for a specific ET* line.
Because the effective temperature is based on the operative temperature T o, it is a combined index
of T a, T rad, and p a. In an indoor air temperature below 77°F (25°C), the constant-ET* lines are
nearly parallel to the skin temperature lines for sedentary occupants with a clothing insulation of
0.6 clo; therefore, ET* values are reliable indexes to indicate thermal sensations at normal indoor
air temperature during low activity levels. The term effective temperature was originally proposed
by Houghton and Yaglou in 1923. A new definition of ET* and its mathematical expression were
developed by Gagge et al. in 1971. It is the environmental index commonly used in specifying and
assessing thermal comfort requirements.
4.6 FACTORS AFFECTING THERMAL COMFORT
Daily experience and many laboratory experiments all show that thermal comfort occurs only under
these conditions:
1. There is a steady-state thermal equilibrium between the human body and the environment; i.e.,
heat storage of the body core S cr and the skin surface S sk are both equal to zero.
2. Regulatory sweating is maintained at a low level.
From the heat balance equation at steady-state thermal equilibrium Eq. (4.1) we have
M � W � C � R � E sk � E res
Let E res � 0.1M and the mechanical efficiency � � 0.05M. From Eq. (4.11), C � R can be determined.
Also, from Eqs. (4.15), (4.16), and (4.20), E sk is a known value. If we substitute into
Eq. (4.1), the heat balance equation at steady-state thermal equilibrium can be expressed as
or
M(1 � 0.05 � 0.1) � F cl f clh(T sk � T o) � w sk i m LR h s(p sk, s � p a)
0.85M � F cl f cl h(T sk � T o) � w sk i mLRh s( p sk, s � p a) (4.24)
In Eq. (4.24), the physiological and environmental factors that affect the balance—the metabolic
rate and the heat losses on the two sides of the equation—are as follows:
1. Metabolic rate M determines the magnitude of the heat energy that must be released from the
human body, i.e., the left-hand side of the equation.
2. Indoor air temperature T a is a weighted component of the operating temperature T o. It also
affects the sensible heat loss and the vapor pressure of indoor air p a in the calculation of the
evaporative loss from the skin surface.