Exhibitors Herald (1927)

38 BETTER THEATRES SECTION OE April 16, 1927 for one hour and divide this amount by 0.2S4, the result will be the required displacement in cubic feet per hour. Another method which allows a little more leeway, is to multiply the B. T. U. to be removed in one hour, by the cubic feet required to remove 1 B. T. U. (which is 4) and the result wall also give the cubic feet per hour. From this may be deduced the general principle that the greater the supply fan’s relative displacement, the less liability there will be to cold draughts. * * * In a theatre seating 3,0(X) persons, the heat to be removed in summer weather will be about 1,301,700 B. T. U. per hour, as shown in Table 1, while the air displacement will be 87,(X)0 C. F. M., or 29 C. F. M. per person, as shown in Table 2. Practical experience has demonstrated that 25 C. F. M. per person is about the least amount of air that will give satisfactory results, and 30 C. F. M. is normal. If winter ventilation only is to be figured, the required C. F. M. will be reduced by the cooling effects of the exterior surfaces and the infiltration of air. * i}? ^ Calculations for approximate air displacement and refrigeration required for a theatre seating 3,000 persons, including standees, are shown in Tables 3 and 4. It is assumed that the outside temperature is 90° F., with a relative humidity of 55% ; also that the refrigerated air in the theatre will leave the cooler at a temperature of 59° F. and 100% relative humidity (saturated). Then each cubic foot of fresh air must have the number of grains of water removed, as shown in the first part of Table 3, or 2.57 grains. The second part of Table 3 develops the total quantity of air to be handled, viz., 100,000 C. F. M. It will also be noted in Table 3 that the total fresh (unrecirculated) air is 25,000 C. F. M., this being the sum of the amounts of fresh air supplied to the theatre and the wasted air lost from the lobby. Based on the amounts and the heat supplied by other heat-emitting sources. Table 4 shows that the total approximate refrigeration required will be in the neighborhood of 217 tons. In these calculations the letters used as symbols represent the following: A — Area of surface. S — Seating capacity of auditorium, including standees. B — Body heat in B. T. U. per hour. t — Transmission of heat in B. T. U. per square foot per hour per degree difference in temperature. d — Degrees of temperature range. TaS/t S 7^,000 C KM. or S9°K (V 7is,ooo c.KM. or eo‘Kr r/) /OO, OOO C KM. <7/ TT SP’r ^ j S9°rsss' 64.f'K H. C. — Hung plaster ceiling; approx. ^ transmission. W— Watts of electric current. w — B. T. U. per watt, or 3.415. f — Electrical factor — approx. % load. R — Percentage of recirculated air. F — Cubic feet of air raised 1° F. per B. T. U. T — Tons of refrigeration in B. T. U. per hour. P — Pounds of water respired per person per hour. L — Latent heat of water per pound in B. T. U. per hour (approx.). G — Grains of water per pound. g — -Grains of water condensed per cubic foot of air. * m — -Minutes per hour. E — Exterior air temperature in degrees F. D — Dehumidified air temperature in degrees F. ^ ^ ^ The resulting temperature evolved by mixing the fresh and recirculated air is computed in Table 5, and figures out M)/i° F. This is obtained by allowing 75% of the air to pass through the cooler and 25% of the air to pass around the cooler, with the assumed temperature of the recirculated air coming back from the theatre as 80° F. There is also a rule of thumb in more or less common use for determining the amount of refrigeration required, this being 70 tons per 1,000 persons, but this rule is modified by any unusual factors, such as extremely large lobbies, or other extreme features. In the particular case under consideration, 3,000 X 70 would equal 210 tons, against a computed amount of 217 tons. ^ ^ The “downward” system of ventilation is generally accepted today as the best method to accomplish cooling in an even and desirable manner; it also seems to opf erate satisfactorily for heating when properly designed. With “downward” ventilation all the incoming air enters through the upper inlets, and all the outgoing air leaves through the lower outlets. The ventilating system, however, whether used for heating or cooling, must be correctly planned or it will be far from satisfactory. Tempered air, either cooled or heated, is introduced through the main auditorium ceiling, through lattices, grilles, or through openings in the ornamented plaster work, and at widely distributed points ; also, through the ceiling under the balcony for the rear portion of the orchestra floor. (See Fig. 1, for schematic arrangement.) Air is exhausted through mushrooms under orchestra seats into a basement plenum chamber, and through grilles or registers in the vertical portion of the balcony steps into a balcony plenum chamber. Where ceilings are low— as in the rear of the orchestra under the balcony — extreme care must be exercised to prevent objectionable draughts. Individual ducts to each outlet in such locations should be equipped with volume dampers, to control the amount of air going to the outlet and the outlet velocity should be kept very low, certainly not over 200 f . p. m. Reflectors or diffusers, which cause the aid to spread out horizontally at ceiling perforations, are a preventive of draughts. * * * In one theatre a rather novel difficulty was encountered, caused -by the stratification of the air. For this particular building, the supply fan for the auditorium supplied air through a stack of Vento heaters 6 ft. high, into a masonry main supply duct 10 ft. high. From this main supply duct, branch ducts distributed the fresh air to all parts of the house. A thermometer test of the air in the supply duct showed a difference of 10° F. between the temperature at the top of the duct and the temperature at the bottom. As the air was distributed under the orchestra, the warm air floating on the top came out of the first mushrooms in the rear of the auditorium, while the cooler air, near the bottom, traveled along the duct and came out at the front near the orchestra pit. Another result was a difference of 5° F. between the temperature at one side of the auditorium and the temperature at the other. * Jjf * These undesirable features were rectified by the introduction of deflecting branch ducts which took air at different heights from the main supply duct, so as to equalize the average temperature of air delivered to all parts of the auditorium. 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