Document v6zV5n9kj7nRmQa8yv9Yj9gp8

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 be used. If weatherstripped, the values may be reduced one-half. A single door which is frequently opened, such as might be found in a store, should have a value applied which is three times that for a well fitted door. This extra allowance is for opening and closing losses and is kept from being greater by the fact that doors are not used as much in the coldest and windiest weather. The infiltration rate through swinging and revolving doors is generally a matter of judgment by the engineer making cooling load determinations and in the absence of adequate research data the values given in Table 3 represent current engineering practice. These values are based on the average number of persons in a room at a specified time, which may also be the same occupancy assumed for determining the outside ventilation requirements outlined in Chapters 2 and 7. Table 3. Infiltration Through Outside Doors for Cooling Loads Expressed in Cubic Feet per Minute per Person in Room Application Paib 36 in. Swinging Doobs, Single Entrance Bank..................................................................................................... Hospital Room.................................................................................... Office................................................................................................... Office Building......................................... ........................................... Shoe Store.............................................................................. . 7.5 4.5 7.0 60 25 0 8.0 2.5 7.0 2.5 3.5 5.0 3.5 3.0 2.0 2.5 2.5 3.5 _"For doors located in only one wall or where doors in other walls are of revolving type. ^Vestibules with double pair swinging doors, infiltration may be assumed 75 per cent of swinging door values. Infiltration for 72 In. revolving doors may be assumed 60 per cent of swinging door values. SELECTION OF WIND VELOCITY Although all authorities do not agree upon the value of the wind veloc ity that should be chosen for any given locality, it is common engineering practice to use the average wind velocity during the three coldest months of the year. Average wind velocities for the months of'December, January and February for various cities in the United States and Canada are given in Table 2, Chapter 6. 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 WeatJier Bureau records, which would result in the maximum heat demand. Since the proportion of transmission and infiltration losses varies with the construction and is 120 CHAPTER 5. AIR LEAKAGE 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, with modification explained in Chapter 6, 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 with the plan of the structure. (See discussion of wind movement in Chapter 42 and Table 2 in Chapter 6.) CRACK LENCTH 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 oil 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. 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 4. 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. 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 121