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American Society of Heating and Ventilating Engineers Guide, 1937 angle of about 90 deg. The windows that are to be figured for prevailing and non-prevailing winds will ordinarily each occupy about one-half the perimeter of the structure, the proportion varying to a considerable extent with the plan of the structure. (See discussion of wind movement in Chapter 4.) CRACK LENGTH USED FOR COMPUTATIONS In no case should the. amount of crack used for computation be less than half of the total crack in the outside walls of the room. Thus, 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. 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. Table 3. Air Changes Taking Place under Average Conditions Exclusivb of Air Provided for Ventilation Kind or Rook or Building Number or Air Changes Taking Place per Hour 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 Clotting Stores Churches, Factories, I.ofts, etc. 1 IX 2 2 X to % 2 to 3 2 1 to 2 1 to 2 2 2 to 3 1 to 3 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. This method may be used to advantage as a check on the calculations made in the more exact manner. MULTI STORY BUILDINGS In tall buildings, infiltration may be considerably influenced by tem perature difference or chimney effect which will operate to produce a head that will add to the effect of the wind at lower levels and subtract from it at higher levels. On the other hand,'the wind velocity at lower levels may be somewhat abated by surrounding obstructions. Further more, the chimney effect is reduced in multi-story buildings by the partial isolation of floors preventing free upward movement, so that wind and temperature difference may seldom cooperate to the fullest extent. 138 Chapter 6--Air Leakage Making the rough assumption that the neutral zone is located at midheieht 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 = VM1 - 1.75 a (1) Me = VM1 + 1.75 b (2) where 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. Stairwells 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 high altitudes 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. Heating Surface for Stair-Wells In stair-wells that are open through many floor levels although closed offfrom 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. 139