Section 3.5 Thermal Comfort and Heat Stress
Table 3.6
Metabolic rate as a function of physical activity for a 70 kg adult man
(abstracted from ASHRAE, 1997).
activity
metabolic rate
(W)
metabolic rate
(kcal/hr)
sleeping
72
62
seated, quiet
108
93
standing, relaxed
126
108
walking about the office
180
155
seated, heavy limb movement
234
201
flying a combat aircraft
252
217
walking on level surface at 1.2 m/s
270
232
housecleaning
284
244
driving a heavy vehicle
333
286
calisthenics/exercise
369
317
heavy machine work
423
364
handling 50kg bags
423
364
playing tennis
432
372
playing basketball
657
565
heavy exercise
900
774
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View Full DocumentThermodynamic analysis of the human body:
The first law of thermodynamics for a stationary system is
dU
QW
dt
=
−
±
±
(336)
where U is the internal energy of the system,
is the rate of heat transfer
into
the system, and
is
the rate of work being done
by
the system, following the standard convention in thermodynamics.
Q
±
W
±
Since the body’s temperature does not change, the body’s internal energy decreases at a rate equal to
(dU/dt = 
), and this same rate of change of energy must be transferred from the body in the
form of heat transfer to the environment. Thus, the first law of thermodynamics for this system
simplifies to
M
±
M
±
MQ0
+
=
±
±
(337)
which states that all of the energy associated with metabolism is rejected ultimately into the ambient
environment as heat. Note that since
is positive,
must be negative in order to satisfy Eq. (337).
Note also that under conditions in which the person is doing thermodynamic work, M in Eq. (337) is
replaced by

.
M
±
Q
±
±
M
±
W
±
Five types of heat transfer into the body are considered: conduction, convection, respiration,
radiation, and evaporation, which are denoted as
,
,
,
, and
respectively.
Eq. (337) thus becomes
cond
Q
±
conv
Q
±
res
Q
±
rad
Q
±
evap
Q
±
cond
conv
res
rad
evap
MQ
Q
Q
Q
Q
0
+++
+
+=
±±±
±
±
±
(338)
All terms in Eq. (338) have dimensions of energy per unit time, i.e. power. Each of these heat transfer
terms is considered separately below.
Conduction
: Conduction heat transfer is by direct contact with solid surfaces, such as chairs, the floor,
etc. Since the surface area in such contact is small, and since the materials are generally good
insulators, with clothing between the body and the material providing further insulation, conduction
heat transfer is negligible in the present analysis.
cond
Q0
≈
±
(339)
Convection
: Energy transferred into the body by convection depends on the velocity of air passing
over the body and the temperature difference between skin and air. Wadden and Scheff (1987) suggest
an expression of the following form, for
expressed now in units of kcal/min:
conv
Q
±
( )
(
0.67
conv
s
a
a
s
Q
KA
0.0325 0.1066U
T
T
=+
±
)
−
(340)
where

K
= fraction of skin exposed to atmosphere

U
a
= ambient air velocity (m/s)

T
a
, T
s
= ambient and skin temperatures; for a first approximation, T
s
= 35.0
°
C (308.15 K)
Respiration
: The process of breathing in and out involves mass transfer as well as heat transfer, and
technically should not be considered as part of thermodynamic closed system analysis. Nevertheless,
one can approximate the net rate of heat transfer due to respiration as a steadystate heat transfer term.
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 Fall '07
 CIMBALA
 Heat, Heat Transfer, Stress, metabolic rate, ambient air temperature

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