Page 228 - Industrial Ventilation Design Guidebook

5.1 THERMAL COMFORT i 89

At higher air speeds h c > h r, convective heat loss becomes greater than radia-
tion and T 0 approaches T a. For such conditions Eq. (5.20) is recommended: 9

where A depends on air speed (v}:

The above indicates that to maintain a constant level of comfort when MRT
decreases, T a must be increased an equal amount. This is the difficulty of pe-
rimeter zones. In many such environments the air and surface temperatures
differ and operative temperature is a convenient way to characterize the envi-
ronment.
How is mean radiant temperature (MRT) determined? One could calcu-
late or measure the surface temperatures of the room and calculate MRT from

where T rn is the absolute temperature (K) of the radiating surface « and Pp_ n
6 18
is the angle factor from the person to surface «, ' and Fp_ n is the fraction of
radiation leaving p that strikes n.
If the surface temperatures are not widely different, Eq. (5.21) can be sim-
plified to

At a location MRT and T 0 are often measured with a sphere or ellipsoid repre-
senting the person, as shown in Fig. 5.10. In the diagram the energy balance
on the globe at steady state is q c — q r, or

FIGURE 5.10 The determination of MRT.