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American Society of Heating and Ventilating Engineers Guide, 1937 and walls (Tables 1 and 2, Chapter 6) must be based on the proper wind velocity for a given locality. In the case of tall buildings special attention must be given to infiltration factors. (See Chapter 6). In the past many designers have used empirical exposure factors which were arbitrarily chosen to increase the calculated heat loss on the side or sides of the building exposed to the prevailing winds. It is also possible to differentiate among the various exposures more accurately by calcu lating the infiltration and transmission losses separately for the different sides of the building, using different assumed wind velocities. Recent investigations indicate, however, that the wind direction indicated by Weather Bureau instruments does not always correspond with the direction of actual impact on the building walls, due to deflection by surrounding buildings. The exposure factor, which is still in use by many engineers, is usually taken as 15 per cent, and is added to the calculated heat loss on the side or sides exposed to what is considered the prevailing winter wind. There is a need for actual test data on this point, and pending the time when it can be secured, the question must be left to the judgment of the designing engineer. It should be remembered that the values of U in the tables in Chapter 5 are based on a wind velocity of 15 mph and that the infiltration figures are supposed to be selected from the tables in Chapter 6 to cor respond to the wind velocities given in Table 2 of the present chapter. The Heating, Piping and Air Conditioning Contractors National Associ ation has devised a method3 for calculating the square feet of equivalent direct radiation required in a building. This method makes useof ex posure factors which vary according to the geographical location and the angular situation of the construction in question in reference to pre vailing winds and the velocity of them. ' AUXILIARY HEAT SOURCES The heat supplied by persons, lights, motors and machinery, should always be ascertained in the case of theaters, assembly ' halls, and in dustrial plants, but allowances for such heat sources must be made only after careful consideration of all local-conditions. In many cases, these heat sources should not be allowed to affect the size of the installation at all, although they may have a marked effect on the operation and con trol of the system. In general, it is safe to say that where audiences are involved, the heating installation must have sufficient capacity to bring the building up to the stipulated inside temperature before the audience arrives. In industrial plants, quite a different condition exists, and heat sources, if they are always available during the period of human occu pancy, may be substituted for a portion of the heating installation. In no case should the actual heating installation (exclusive of heat sources) be reduced below that required to maintain at least 40 F in the building. Electric Motors and Machinery Motors and the machinery which they drive, if both are located in the room, convert all of the electrical energy supplied into heat, which is See Standards of Heating. Piping and Air Conditioning Contractors National Association. Chapter 7--Heating Load retained in the room if the product being manufactured is not removed until its temperature is the same as the room temperature. If power is transmitted to the machinery from the outside, then only the heat equivalent of the brake horsepower supplied is used. In the first case the Btu supplied per hour = X 2'546' and in the second case Btu per hour = bhp X 2,546, in which 2,546 is the Btu equivalent of 1 hp-hour. In high-powered mills this is the chief source of heating and it is frequently sufficient to overheat the building even in zero weather, thus requiring cooling by ventilation the year rouTnhde. heat (in Btu per hour) from electric lamps is obtained by multi plying the watts per lamp by the number of lamps and by 3.415. One cubic foot of producer gas gives off about 150 Btu per hour; one cubic foot of illuminating gas gives off about 535 Btu per hour; and one cubic foot of natural gas gives off about 1000 Btu per hour. A Welsbach burner averages 3 cu ft of gas per hour and a fish-tail burner, 5 cu ft per hour. For information concerning the heat supplied by persons, see Chapter 3. In intermittently heated buildings, besides the capacity necessary to care for the normal heat loss which may be calculated according to customary rules, additional capacity should be provided to supply the heat necessary to warm up the cold material of the interior walls, floors, and furnishings. Tests have shown that when a cold building has had its temperature raised to about 60 F from an initial condition of about 0 F, the heat absorbed from the air by the material in the structure, may vary from 50 per cent to 150 per cent of the normal heat loss of the building. It is therefore necessary, in order to heat up a cold building within a reasonable length of time,, to provide such additional capacity. If the interior material is cold when people enter a building, the radiation of heat from the occupants to the cold material will be greater than is ` * r*i_____ o \ WALL SURFACE CONDENSATION4 Condensation on the interior surfaces of buildings is often a serious problem. Water dripping from a ceiling may cause irreparable damage to manufactured articles and machinery. It often results in short-cir cuiting of electric power and lighting systems, necessitating shut-downs and incurring costly repairs. It also causes rotting of wood roof struc tures, corrosion of metal roofs, and spalling and disintegration of gypsum and other types of roof decks not properly protected. Condensation is caused by the contact of the warm humid air in a building with surfaces below the dew-point temperature, and can be remedied in two ways, (1) by increasing the'temperature of such surfaces above the dew-point temperature, or (2) by lowering the humidity. Dehumidification, of course, is not advisable where a high relative humidity is necessary for manufacturing processes. Hence, the only alternative is to increase the surface temperature'by decreasing the inside `Preventing Condensation on Interior Building Surfaces, by Paul D. Close (A.S.H.V.E. Transactions, Vol. 36,1930). 151 K /