Document ybQzZzp64x9ge5znEkqyyyd7n

264 CHAPTER 14 Table 4 of Chapter 8 indicates an- changes commonly used but should be * taken only as a guide. For further discussion of the method see section on Air Change Method in Chapter 8. Crack Length to be Used for Computations . For designers who prefer to use the crack method the basis of calculation is as follows: The amount of crack used for computing the infiltration heat loss should not be less than half of the total crack in the outside walls of the room. For a building having no partitions, whatever wind enters through the cracks on the windward side must leave through the cracks on.the leeward side. Therefore, take one-half the total crack for com-' puting each side and end of the building. In a room with one exposed wall, take all the crack; with two exposed walls, take the wall having the most crack; and with three or four exposed walls, take the wall having the most crack; but in no case take less than half the total crack. The total infiltration loss, of a building having partitions will not be equal to the sum of the infiltration losses of the various rooms, since at any given time infiltration will take place only on the windward side or sides and not on the leeward side, Therefore, if a building has more than one room which is divided by interior walls dr partitions, it is sufficiently accurate to use half of the total infiltration losses for determining the total heat requirements. Latent Heat Loss When it is intended to add moisture to air leaking into a room for the maintenance of proper winter comfort conditions, it is necessary to de termine the heat equivalent to evaporate the required amount of water vapor, which may be calculated by the equation: . . <5> where Hi = heat required to increase moisture content of air leaking into building from To to m-,, Btu per hour. Q.= volume of outside air entering building, cubic feet per hour. d = density of air at temperature U, pounds per cubic foot. - - mi -- vapor density of inside air, grains per pound of dry air. m0 = vapor density of outside air, grains per pound of dry air. hii = 'latent heat of vapor at mi, Btu per poun^l. If the latent heat of vapor (hf&) is assumed to be 1060 Btu per pound, Equation 5 reduces to Hi = 0.0114 Q (mi -r .m0) . (5a) Equations 4a, 4b and 5a may also be used for determining the sensible and latent heat gains due to infiltration in cooling load computations. SELECTION OF WIND VELOCITIES - The effect of wind on the heating requirements of any building should -be given consideration under two heads:' . ; 1. Wind movement increases the heat transmission of walls, .glass, and roof, affecting poor walls to a much greater extent'than good walls. Heating Load 265, 2. Wind movement materially increases the infiltration of cold air through the cracks around doors and windows, and even through the building-materials them selves. Theoretically as a basis for design, the most unfavorable combination , of temperature and wind velocity should be chosen. It is entirely possible that a building might require more heat on a windy day with a moderately low outside temperature than on a quiet day with a much lower outside temperature. However, the combination of wind and temperature which is the worst would differ with different buildings, because wind velocity has a greater effect on buildings which have relatively high infiltration; losses. It would be possible to compute the heating load for a building for several different combinations of temperature and wind velocity which records show to have occurred and to select the worst combination; but designers generally do not feel that such a degree of refinement is justified. Therefore, since Table 1 lists the average velocity of winds occurring at temperatures equalled or exceeded 97J per cent of the winter period for each locality, this value should be the basis for estimating infiltration losses. When using the air change method it will not- be necessary to consider the wind velocities. Designers employing the crack method generally use values corresponding to a 15-mile wind. Due to the small effect of the wind velocity on the transmission, coefficient, the values in Chapter 6, based on a 15-mile wind may be used at all times. Exposure Factors Many designers use empirical exposure factors to increase the calcu lated heat loss of rooms or spaces on the side or sides of the building ex posed to the prevailing winds. However, according to a survey made in 1943, many Guide users have found that the use of exposure factors is not necessary as the Guide method of calculating heat losses provides an ample heat loss allowance. Therefore exposure factors may be regarded as factors of safely for the rooms or spaces exposed to the prevailing winds, to allow for additional capacity for these rooms or spaces, or to balance the radiation, particularly in the case of multi-story buildings. Although the. exposure allowance is frequently assumed to be 15 per cent, the actual allowance to be. made, if any, must to a large extent be a matter of ex perience and judgment of the designer, since there are at present no au thentic test data, available from which rules could be developed for the many conditions encountered in practice. As stated previously, the value of U in the tables of Chapter 6 is based on a wind velocity of 15 mph and the surface resistance for this wind velocity (0.17) is sufficiently low so that higher wind velocities will decrease . the surface resistance to a negligible degree and therefore have only a slight effect on the average over-all coefficient. On the other hand, infiltration losses vary almost directly as the wind velocity, as will be apparent from the factors in Table 2 of Chapter 8. The more exact method therefore would be to differentiate among the various exposures more accurately by calculating the infiltration and transmission losses separately for the different sides of the building, using different assumed wind velocities for the infiltration losses on the various sides. AUXILIARY HEAT SOURCES The heat supplied by persons, lights, motors and' machinery should always be ascertained in the case of theaters, assembly halls, and in dustrial plants, but allowances for such heat sources must be made;only