Document aJvKQB5v1va36xd8E4wRNQLMX

HEATING VENTILATING AIR CONDITIONING CUIDE 1942 Table 4. Air Changes Taking Place under Average Conditions Exclusive of Air Provided for Ventilation Kind o? Room ob Building Number op Aib Changes Tailing Place peb Houb 1 m 2 2 >3 to *A .2 1 to 2 1 to 2 2 2 to 3 1 Mto3 to be used in connection with Tables 1 and 2 that will allow for both wind velocity and temperature difference: Me = VM* - 1.75 a (1) where Me = VM' + 1.75 b (2), Me = equivalent wind velocity to be used in conjunction with Tables 1 and 2. M = wind velocity upon which infiltration would be determined if tem perature difference were disregarded. a =. distance of windows under consideration from mid-height of building if above, mid-height. b == distance if below mid-height. The coefficient 1.75 allows for about one-half the temperature difference head. For buildings of unusual height, Equation 1 would indicate negative infiltration at the highest stories, which condition may, at times, actually exist. Sealing of Vertical Openings In tall, multi-story buildings, every effort should be made to seal off vertical openings such as stair-wells and elevator shafts from the re mainder of the building. Stair-wells should be equipped with self-closing doors, and in exceptionally high buildings, should be closed off into sections of not over 10 floors each. Plaster cracks should be filled. Elevator enclosures should be tight and solid doors should be used. If the sealing of the vertical openings is made effective, no allowance need be made for the chimney effect. Instead, the greater wind move ment at the greater heights makes it advisable to install additional heating surface on the upper floors above the level of neighboring buildings, this additional surface being increased as the height is increased. One arbitrary rule is to increase the heating surface on floors above neighboring buildings by an amount ranging from 5 per cent to 20 per cent. This extra heating surface is required only on the windward side and on windy days, and hence automatic temperature control is especially desirable with such installations. 122 CHAPTER 5. AIR LEAKAGE In stair-wells that are open through many floor levels although closed off from the remainder of each floor by doors and partitions, the strati fication of air makes it advisable to increase the amount of heating surface at the lower levels and to decrease the amount at higher levels even to the point of omitting all heating surface on the top several floor levels. One rule is to calculate the heating surface of the entire stair-well in the usual way and to place 50 per cent of this in the bottom third, the normal amount in the middle third and the balance in the top third. HEAT EQUIVALENT OF AIR INFILTRATION Sensible Heat Loss The heat required to warm cold outside air, which enters a room by infiltration, to the temperature of the room is given by the equation: Hs = 0.24 Q d(t, - to) (3) where Hs = .heat required to raise temperature of air leaking into building from to to tj Btu per hour. 0.24 = specific heat of air. Q = volume of outside air entering building, cubic feet per hour. d = density of air at temperature to, pounds per cubic foot. li = room air temperature, degrees Fahrenheit. to - outside air temperature, degrees Fahrenheit. 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 determine the heat equivalent to evaporate the required amount of water vapor, which may be calculated by the equation: H'=Qd(-M>7W0-)L (4) 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 t, pounds per cubic foot. Mi = vapor density of inside air, grains per pound of dry air. Mo = vapor density of outside air, grains per pound of dry air. L = latent heat of vapor at Mi, Btu per pound. It is sufficiently accurate to use d = 0.075 lb, in which case Equation 3 reduces to 5 and if the latent heat of vapor is assumed for general condi tions as 1060 Btu per pound Equation 4 reduces to 6. Hs = 0.018 Q(t\- to) Hi = 0.0114 Q {Mi - Mo) (5) (6) Changing the temperature and vapor subscripts in Equations 5 and 6 to (to -- li) and (M0 -- Mi) permits the use of these same formulae for determining the sensible and latent heat gains due to infiltration in cooling load computations. 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. Where buildings 123