Chapter_03_Thermal_comfort_and_Heat_stess - Section 3.5...

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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 50-kg bags 423 364 playing tennis 432 372 playing basketball 657 565 heavy exercise 900 774
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Thermodynamic analysis of the human body: The first law of thermodynamics for a stationary system is dU QW dt = ± ± (3-36) 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 + = ± ± (3-37) 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. (3-37). Note also that under conditions in which the person is doing thermodynamic work, M in Eq. (3-37) 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. (3-37) thus becomes cond Q ± conv Q ± res Q ± rad Q ± evap Q ± cond conv res rad evap MQ Q Q Q Q 0 +++ + += ±±± ± ± ± (3-38) All terms in Eq. (3-38) 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 ± (3-39) 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 =+ ± ) (3-40) 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 steady-state heat transfer term.
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Chapter_03_Thermal_comfort_and_Heat_stess - Section 3.5...

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