Document KGqvg9JeO1xqn1Dd9nRzOO11x

Heating Ventilating Air Conditioning Guide 1939 In considering both the transmission and infiltration losses, the more ^ exact procedure would be to select the outside temperature and the wind ' velocity corresponding thereto, based on Weather Bureau records, which would result in the maximum heat demand. Since the proportion of transmission and infiltration losses varies with the construction and is different for every building, the proper combination of temperature and : wind velocity to be selected would be different for every type of building, even in the same locality. Furthermore, such a procedure would necessi tate a laborious cut-and-try process in every case in order to determine the worst combination of conditions for the building under consideration. It would also be necessary to consider heat lag due to heat capacity in the case of heavy masonry walls, and other factors, to arrive at the most accurate solution of the problem. Although heat capacity should be con sidered wherever possible, it is seldom possible to accurately determine the worst combination of outside temperature and wind velocity for a given building and locality. The usual procedure, as explained in Chapter 7, is to select an outside temperature which is not more than 15 F above the lowest recorded, and the average wind velocity during the months of December, January and February. The direction of prevailing winds may usually be included within an 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 1 with the plan of the structure; (See discussion of wind movement in Chapter 36 and Table 2 in Chapter 7). CRACK LENGTH USED FOR COMPUTATIONS In no case should the amount of crack used for computation be less j 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, j 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 t through the cracks on the windward side must leave through the cracks i on the leeward side. Therefore, take one-half the total crack for com puting each side and end of the building. I The amount of air leakage is sometimes roughly estimated by 'assuming j 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 j room, as indicated in Table 4. This method may be used to advantage as i a check on the calculations made in the more exact manner. \; MULTI STORY BUILDINGS In tall buildings, infiltration may be considerably influenced by tem- ;r , perature difference or chimney effect which will operate to produce a 't < head that will add to the effect of the wind at lower levels and subtract f from it at higher levels. On the other hand, the wind velocity at lower f levels may be somewhat abated by surrounding obstructions. Further- j more, the chimney effect is reduced in multi-story buildings by the partial > isolation of floors preventing free upward movement, so that wind and ; 126 IL Am Changes Taking Place under Average Conditions Exclusive Table 4. AIR ^ 0F a,r Provided for Ventilation . . __________ orKind Room or Building 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.. Number op Air Changes Taxing Place per Hour 1 2 2 Hto u 2 to 3 2 1 to 2 1 to 2 2 2 to 3 1 ys to 3 temperature difference may seldom cooperate to the fullest extent. Making the rough assumption that the neutral zone 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: Mc = VM1 - 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 127