Page 419 - Industrial Ventilation Design Guidebook

3 74 CHAPTER 6 TARGET LEVELS

This section deals with the ISO standards, which have been or are be-
ing prepared by ISO/TCI59/SC5/WG1 and CEN/TC122/WG11. An over-
view of the standards issued and the documents under preparation is given
in Table 6.2. CEN Technical Report CR 1752 will also be used in this sec-
1
tion. Several of these standards may be used as the basts for the design
and evaluation of buildings, HVAC (heating, ventilation, and air condi-
tioning) systems, and protective equipment (clothing) and the optimization
of work/rest schedules.
The basic philosophy has been to standardize evaluation methods, with
recommended limit values for the different parameters or indices listed in in-
formative annexes. These or other values may then be adapted in national
rules for the thermal environment.

6.3.2 The Thermal Environment
Existing methods for evaluation of the general thermal state of the body, both
in comfort and under heat or cold stress, are based on an analysis of the heat
balance for the human body:

where
S = heat storage in body
M = metabolic heat production
W = external work
C = heat loss by convection
R = heat loss by radiation
E sk = evaporative heat loss from skin
C res = convective heat loss from respiration
E res = evaporative heat loss from respiration
K — heat loss by conduction
The factors influencing this heat balance are activity level (metabolic rate,
2 2
met or W/m ), thermal resistance of clothing 7 cl, (clo or m °C/W), evapo-
2
rative resistance of clothing R ec j (m Pa/W), air temperature t a (°C), mean
radiant temperature t r (°C), air speed v a (m/s), and partial water vapor
pressure p a (Pa). These parameters must be combined so that the thermal
storage is 0, or else the working time has to be limited to avoid too much
strain on the body. For comfort, the mean skin temperature also has to be
within certain limits and the evaporative heat loss must be low. In existing
standards, guidelines, or handbooks, different methods are used to evalu-
ate the general thermal state of the body in moderate environments, cold
environments, and hot environments; however, all are based on the above
heat balance and the listed factors.
Aside from the general thermal state of the body, a person may find the
thermal environment unacceptable or intolerable if local influences on the
body from asymmetric radiation, air velocities, vertical air temperature dif-
ferences, or contact with hot or cold surfaces (floors, machinery, tools, etc.)
are experienced.