MIL-HDBK-1011/2
APPENDIX A (continued)
Section 1:
PEOPLE AND COMFORT
1.1
Comfort Criteria. Thermal comfort is maintained when the body is
in thermal equilibrium with its surroundings. The human body exchanges heat
with the environment through convection, radiation, evaporation, and through
conduction to solid objects. The primary environmental factors affecting
these heat exchanges are: air temperature, surrounding surface temperatures
(mean radiant temperatures or MRT), humidity, solar radiation from the sun and
sky, and air motion. The primary personal factors affecting the heat
exchanges are activity level (equivalent to metabolic rate and measured in
mets) and clothing insulation (measured in clo). Current research does not
indicate significant differences in the perception of comfort due to
differences in age (of adults), nationality, or sex.
1.2
The Effect of Air Movement. Air movement influences bodily heat
balance by affecting the rate of convective heat transfer between skin and
air, and the rate of bodily cooling through evaporation of skin moisture.
1.3
Acceptable Comfort Zone. The acceptable comfort zone shall be as
prescribed by ASHRAE Standard 55. Eighty percent or more of building
occupants will find this zone thermally acceptable in still air and shade
conditions. The standard is based on the concept of operative temperature,
to, in which air temperature and radiant temperature are linked as follows:
t+o, = (h+c, t+a, + h+r, t+r, )/(h+c, + h+r, )
EQUATION:
(3)
h+c, is the heat transfer coefficient of air,
where:
h+r, is the heat transfer coefficient of mean radiant
temperatures,
t+a, is the temperature of the air, and
t+r, is the mean radiant temperature.
Figure 4 in Section 2 gives the acceptable range of operative temperature and
humidity for persons in typical summer (0.35 to 0.6 clo) and winter (0.8 to
1.2 clo) clothing at near sedentary (< 1.2 met) activity levels. See Tables
A-1 and A-2 for ranges of activity levels and typical clothing.
1.3.1
Comfort, Humidity, and Condensation. It is possible to have
moisture condensation in a building below the humidity maximum of 95 percent
relative humidity shown in Figure A-1. ASHRAE Standard 55-1981 has a lower
maximum (0.012 humidity ratio) that is based on avoiding condensation and mold
growth in ducts of centrally air conditioned buildings rather than on human
thermal comfort requirements. In naturally ventilated buildings, surface
temperatures are closer to the ambient air temperature than in the ducts of
mechanically air conditioned buildings. This reduces the potential for
condensation and mold growth, allowing the higher acceptable humidity limit.
Figure A-1 also gives air
velocities required to allow occupants to
feel comfortable at temperature and
humidity conditions above the still air
comfort zone; this figure is called
a "bioclimatic chart" because it plots the
comfort boundaries over an extended
range of environmental conditions.
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