Page 226 - Industrial Ventilation Design Guidebook

S. i THERMAL COMFORT I 87

FIGURE 5.9 Nonuniformity limits to avoid discomfort.

moving papers, hair, and other light objects, and the pressure of the air
speed itself may affect some people.

5.1.6 Spatial and Temporal Nonuniformity

The thermal parameters for comfort should be relatively uniform both spatially
and temporally. Variations in heat flow from the body make the physiological
temperature regulation more difficult. Nonuniform thermal conditions can
lead to nonuniform skin temperatures. The active elements of the regulatory
system may need to make more adjustments and work harder in order to keep
thermal skin and body temperatures stable. To avoid discomfort from environ-
mental nonuniformities, the temperature difference between feet and head
should be less than about 3 °C (Fig. 5.9) and the mean surface temperature or
radiant difference from one side of the body to the other should not be greater
then about 10 °C.
Similarly, with cycling temperatures, large fast cycles can cause discom-
fort. To avoid this, if the time to complete one cycle is less than 15 minutes
and the peak-to-peak temperature variation is greater than 1.1 °C, the average
rate of temperature change should be less than 2.2 °C/h (Fig. 5.9). Very slow
rates of temperature change (dT/dt < 0.5 °C/h) are much less difficult to
adjust to and the change can go unnoticed until the temperature is beyond the
comfort zone temperature.
Local air motion is another thermal nonuniformity that can cause a local
cooling of the skin and the feeling of a draft. Draft discomfort from local air
motion increases as the air temperature decreases below skin temperature.
Fluctuations in the local air motion increase the perception of drafts and
should be avoided. The unsteadiness of air motion is often described in terms
of its turbulence intensity (Tu):

where v is the average air speed of the draft and SD,, is its standard deviation.
In spaces with forced air systems, the turbulence intensity is typically between
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0.3 and O.6. That level of turbulent intensity generally limits maximum air
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speeds to < 0.2 m/s for occupants in cool environments. However, in warm
environments turbulence intensity is desirable as it increases the cooling effec-
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tiveness of the air motion.