Thermal Energy Created by the Human Body

Heat Transfer Design and Engineering
Thermodynamics

Thermal Energy Created by the Human Body

The body is a heat engine. It converts chemical energy of the food consumed into both heat to sustain metabolism and work. The harder the body exercises or works, the greater the need to reject heat in order for the body to maintain thermal balance. The human body primarily rejects heat to the environment from the body surface by convection, radiation, or evaporation.

The total energy production rate of the body can be written as:

Q + W = M Ask

where:

Q = heat production rate
W = rate of work
M = metabolic rate
Ask = total surface area of skin.

The metabolic rate is customarily expressed in units of mets (or M) where:
1 M = 1 met = 18.4 Btu/h ft2 = 58.2 W/m2

Since the area Ask for adults is of the order of 16 to 22 ft2 (1.5 to 2 m2), heat production rates by adults are about 340 Btu/h (110W) for typical indoor activities. Metabolic rates in units of mets for various activities are shown below.

Activity
Btu/hr · ft2
W/m2
meta
Resting
Sleeping
13
40
0.7
Reclining
15
45
0.8
Seated, quiet
18
60
1.0
Standing, relaxed
22
70
1.2
Walking (on level surface)
2.9 ft/s (0.88 m/s)
37
115
2.0
4.4 ft/s (1.3 m/s)
48
150
2.6
5.9 ft/s (1.8 m/s)
70
220
3.8
Office activities
Reading, seated
18
55
1.0
Writing
18
60
1.0
Typing
20
65
1.1
Filing, seated
22
70
1.2
Filing, standing
26
80
1.4
Walking about
31
100
1.7
Lifting, packing
39
120
2.1
Driving/flying
Car driving
18 – 37
60 – 115
1.0 – 2.0
Aircraft, routine
22
70
1.2
Aircraft, instrument landing
33
105
1.8
Aircraft, combat
44
140
2.4
Heavy vehicle
59
185
3.2
Miscellaneous occupational activities
Cooking
29 – 37
95 – 115
1.6 – 2.0
Housecleaning
37 – 63
115 – 200
2.0 – 3.4
Seated, heavy limb movement
41
130
2.2
Machine work
sawing (table saw)
33
105
1.8
light (electrical industry)
37 – 44
115 – 140
2.0 – 2.4
heavy
74
235
4.0
Handling 110-lb (50-kg) bags
74
235
4.0
Pick-and-shovel work
74 – 88
235 – 280
4.0 – 4.8
Miscellaneous leisure activities
Social dancing
44 – 81
140 – 255
2.4 – 4.4
Calisthenics/exercise
55 – 74
175 – 235
3.0 – 4.0
Tennis, singles
66 – 74
210 – 270
3.6 – 4.0
Basketball
90 – 140
290 – 440
5.0 – 7.6
Wrestling, competitive
130 – 160
410 – 505
7.0 – 8.7

a 1 met = 18.43 Btu/hr · ft2 (58.2 W/m2)

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