Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.4 Thermal Comfort, Metabolic Rate, and Clothing Insulation 329

11.4 Thermal Comfort, Metabolic Rate, and Clothing Insulation

Human thermal comfort is of special importance to bioengineers and mechanical engineers.

For example, mechanical engineers design the heating, ventilating, and air-conditioning (HVAC)

systems for homes, public buildings, hospitals, and manufacturing facilities. When sizing the

HVAC systems, the engineer must design not only for the building’s heat losses or gains but

also for an environment that occupants feel comfortable within. What makes us thermally com-

fortable in an environment? As you know, the temperature of the environment and the

humidity of the air are among the important factors that define thermal comfort. For

example, most of us feel comfortable in a room that has a temperature of 70F and a relative

humidity of 40 to 50%. Of course, if you are exercising on a treadmill and watching TV, then

perhaps you feel more comfortable in a room with a temperature lower than 70F, s a y 5 0to

60 F. Thus, the level of activity is also an important factor. In general, the amount of energy that

a person generates depends on the person’s age, gender, size, and activity level. A person’s body

temperature is controlled by (1) convective and radiative heat transfer to the surroundings,

(2) sweating, (3) respiration by breathing surrounding air and exhaling it at near the body’s tem-

perature, (4) blood circulation near the surface of the skin, and (5) metabolic rate. Metabolic

rate determines the rate of conversion of chemical to thermal energy within a person’s body.

The metabolic rate depends on the person’s activity level. A unit commonly used to express the

metabolic rate for an average person under sedentary conditions, per unit surface area, is called

met; 1 met is equal to 58.2 W/m

2

or, in U.S. Customary units, 1 met18.4 Btu /hft

2

.For

an average person, a heat transfer surface area of 1.82 m

2

or 19.6 ft

2

was assumed when defining

the unit of met. Table 11.8 shows the metabolic rate for various activities. As you expect, cloth-

ing also affects thermal comfort. A unit that is generally used to express the insulating value of

clothing is calledclo. 1 clo is equal to 0.155 m

2

C / W, or, in U.S. Customary units, 1 clo

0.88Fft

2

h / Btu. Table 11.9 on page 331 shows the insulating values of various clothing.

Example 11.13 Use Table 11.8 to calculate the amount of energy dissipated by an average adult person doing

the following things: (a) driving a car for 3 h, (b) sleeping for 8 h, (c) walking at the speed of

3 mph on a level surface for 2 h, (d) dancing for 2 h.

(a) Using average values, the amount of energy dissipated by an average adult person driving a

car for 3 h is

(b) Using average values, the amount of energy dissipated by an average adult person sleeping

for 8 h is

(c) Walking at the speed of 3 mph on a level surface for 2 h:

48 a

Btu

h#ft

2 b119.6 ft

2

21 2 h 2 1882 Btu

13 a

Btu

h#ft

2 b119.6 ft

2

21 8 h 2 2038 Btu

a

18  37

2

ba

Btu

h#ft

2 b119.6 ft

2

21 3 h 2 1617 Btu

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