People
The average amount of heat given off by a person depends on the level of activity, and can range from about 100 W for a resting person to more than 500 W for a physically very active person. Typical rates of heat dissipation by people are given in Table 8 for various activities in various application areas. Note that latent heat constitutes about one-third of the total heat dissipated during resting, but rises to almost two-thirds the level during heavy physical work. Also, about 30 percent of the sensible heat is lost by convection and the remaining 70 percent by radiation. The latent and convective sensible heat losses represent the “instant” cooling load for people since they need to be removed immediately. The radiative sensible heat, on the other hand, is first absorbed by the surrounding surfaces and then released gradually with some delay.
If the moisture leaving an average resting person’s body in one day were collected and condensed it would fill a 1-L container.
It is interesting to note that an average person dissipates latent heat at a minimum rate of 30 W while resting. Noting that the enthalpy of vaporization of water at 33ºC is 2424 kJ/kg, the amount of water an average person loses a day by evaporation at the skin and the lungs is (Fig. 27)
which justifies the sound advice that a person must drink at least 1 L of water every day. Therefore, a family of four will supply 4 L of water a day to the air in the house while just resting. This amount will be much higher during heavy work.
Heat given off by people usually constitutes a significant fraction of the sensible and latent heat gain of a building, and may dominate the cooling load in high occupancy buildings such as theaters and concert halls. The rate of heat gain from people given in Table 8 is quite accurate, but there is consider-able uncertainty in the internal load due to people because of the difficulty in predicting the number of occupants in a building at any given time. The design cooling load of a building should be determined assuming full occupancy. In the absence of better data, the number of occupants can be estimated on the basis of one occupant per 1 m2 in auditoriums, 2.5 m2 in schools, 3–5 m2 in retail stores, and 10–15 m2 in offices.