Document 0qVv8RGVBKoJ44YmN1ONzmrmO

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 Theoretically as a basis for design, the most unfavorable combination of temperature and wind velocity should be chosen. It is entirely possible that a building might require more heat on a windy day with a moderately low outside temperature than on a quiet day with a much lower outside temperature. However, the combination of wind and temperature which is-the worst would differ with different buildings, because wind velocity has a greater effect on buildings which have relatively high infiltration losses. It would be possible to work out the heating load for a building for several different combinations of temperature and wind velocity which records show to have occurred and to select the worst combination; but designers generally do not feel that such a degree of refinement is justified. Therefore, pending further studies of actual buildings, it is recommended that the average wind movement in any locality during December, January and February be provided for in computing (1) the heat trans mission of a building, and (2) the heat required to take care of the infiltra tion of outside air. The first condition is readily taken care of, as explained in Chapter 5, by using a surface coefficientfQ for the outside wall surface which is based on the proper wind , velocity. In case specific data are lacking for any given locality, it is sufficiently accurate to use an average wind velocity of approximately 15 mph which is the velocity upon which the heat trans mission coefficient tables in Chapter 5 are. based. In a similar manner, the heat allowance for infiltration through cracks 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 show, 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 method2 for calculating the square feet of equivalent direct radiation required in a building. This method makes use of ex- *See Standards of Heating. Piping and Air Conditioning Contractors National Association. 134 CHAPTER 7. HEATINC LOAD 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 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 = horsepower x 2546, and Efficiency of motor in the second case Btu per hour = bhp X 2546, in which 2546 is the Btu equivalent of 1 hp-hr. In some 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 round. The 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 about 535 Btu per hour; and one cubic foot of natural gas 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 135