Document EEyR7Y05zRgywaymvQyKGBm4

American Society of Heating and Ventilating Engineers Guide, 1931 Making the rough assumption that the neutral zone3 is located at mid height of a building, and that the temperature difference is 70 F, the following formulae may be used to determine an equivalent wind velocity to be used in connection with Tables 1 and 2 that will allow for both wind velocity and temperature difference: Me = y/M1 - 1.75 a where Mt = a/M! + 1.75 b 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 neglected. a = distance of windows under consideration from mid-height of building if above mid-height. s b = distance if below mid-height. X / / Fig. 8. Diagram Showing Part of Perimeter of Building (in heavy outline) for Which Infiltration Is to Be Figured at Higher Rate on Account of Exposure to Prevailing Winds The coefficient 1.75 allows for about one-half the temperature difference head. Example. If M = 20, the equivalent wind velocity at a height of 180 ft from the ground for a building 200 ft high would be Me = V202 - 1.75 X 80 = 16 mph For a window on the ground floor: * Me = V 20* 4- 1.75 X 100 = 24 mph For buildings of unusual height, Equation 1 would indicate negative infiltration at the. highest stories which condition may,,at times, actually exist, although probably greater wind velocities should be figured at such extremely high levels. AIR CHANGE METHOD OF ESTIMATING INFILTRATION The amount of air leakage is sometimes roughly estimated by assuming a certain number of air changes per hour for each room, the number of changes assumed being dependent upon the type, use and location of the room, as indicated in Table 3. ,See Neutral Zone in Ventilating by J. E. Emswiler (A.S.H.V.E. Transactions, Vol. 32. 1926). 78 Chapter 4--Heat Losses by Infiltration Am Changes Taking Place under Average Conditions Exclusive Table 3. rtlK of Air Provided for Ventilation Kate or Room oh Building Numbeb or Am Changes Taking Place pee Hour Rooms, 1 side exposed ...... Rooms, 2 sides exposed-----Rooms, 3 sides exposed-----Rooms, 4 sides exposed-----Rooms with no windows or outside doors-- Entrance Halls--................ Reception Halls................... Living Rooms----Dining Rooms...................... Bath Rooms......................... Drug Stores-.................................... Clothing Stores...-.---..-..... --......... Churches, Factories, Lofts, etc...... m1 2 2 Hto H 2 to 3 2 1 to 2 1 to 2 2 2 to 3 1 H to 3 HEAT REQUIRED FOR WARMING UP AIR ENTERING BY INFILTRATION Because of the influx of cold air by infiltration, provision must be made for the heat required to warm this air to room temperature. The heat required is given by the following equation: Hi = 0.24 Q d (t - t0) (3) where Hi = Btu per hour required for heating air leaking into building from outside temperature ta to inside temperature, t. Q = cubic feet of air entering per hour at inside temperature, t. d = density (pounds per cubic feet) of air at inside temperature, t. t = inside temperature at the proper level. to = outside air temperature for which heating system is designed. 0.24 = specific heat of air. It is sufficiently accurate to take d = 0.075 lb in which case the equa tion reduces to Hi = 0.018 Q (t - to) (4) While a heating reserve must- be provided to warm inleaking air on the windward side of a building, this does not necessarily mean that the heating plant must be provided with a reserve capacity, since the inleaking air, warmed at once by adequate heating surface in exposed rooms, will move transversely and upwardly through the building, thus relieving other radiators of a part of their load. The actual loss of heat of a building caused by infiltration is not to be confused with the necessity for pro viding additional heating capacity for a given space. Infiltration is a disturbing factor in the heating of a building, and its maximum effect (maximum in the sense of an average of wind velocity peaks during the heating season, above some reasonably chosen minimum) must be met 79