Document gaYXK4vwDrQboV4JQ5Xx5Bqr9

246 CHkPTER 14 1948 Guide where B -- air leakage per (hour) (foot of crack) (Chapter 8) for the.wind velocity and type of windows or door crack involved multiplied by 0.018. L = length of window or door crack to be taken into consideration, feet. Example 4 What is the infiltration heat loss per hour through the crack of a 3 x 5 ft average, double-hung, non-weatherstripped, wood window, based on a wind velocity of 15 mph? Assume inside and outside temperatures to be 70 F and zero respectively. Solution. According to Table 2, Chapter 8, the air leakage through a window of this type (based on 14 in. crack and in. clearance) is 39 cu ft per foot of crack per hour. Therefore, B = 39 X 0.018 = 0.70. The length of crack (L) is (2 X 5) + (3 X 3), or 19 ft; t = 70 and ft> = 0. Substituting in Equation 4b, Hs = 0.70 X 19 X (70 -- 0) = 931 Btu per hour. Number of Air Changes to be used for Computations In the opinion of many engineers the crack method is inferior in practical results to the air change method. Estimates of the number of air changes require careful judgment regarding construction and conditions under consideration, that is; whether they are better than, or worse than, the average conditions assumed in Table 4, 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 or 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 determinethe heat equivalent to evaporate the required amount of water vapor, which may be calculated by the equation: = ' (5) where H\ = heat required to increase moisture content of air leaking into building from mo to mi, Btu per hour. Q = volume of outside air entering building, cubic feet per hour. d = density of air at temperature ft, pounds per cubic foot. Heating Goad 247. i = vapor density of inside air, grains per pound of dry air. mo = vapor density of outside air, grains per pound of dry air. Afg = latent heat of vapor at mi, Btu per pound. If the latent heat of vapor (hfg) is assumed to be 1060 Btu per pound, Equation 5 reduces to Hi = 0.0114 Q (mi -- mo) (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: el. Wind movement increases the heat transmission of walls, glass, and roof, affecting poor,walls to a much greater extent than good walls. 2. Wind movement materially increases the infiltration of cold air through the cracks around doors and windows, and even through the building materials themselves. : 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 97per 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 used 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. Exposnre 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 exposed 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 re garded as factors of safety for the rooms or spaces exposed to the pre vailing 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 experience and judgment of the designer, since there are at present no authentic test data available from which rules could be developed for the many conditions encountered in practice.