Journal of the Society of Motion Picture Engineers (1930-1949)

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REPORT OP PROJECTION COMMITTEE 445 system should exhaust 76.2 Ib. of gas or 1456 cu. ft. per minute. This is in the ratio of one cu. ft. per minute for each 53 watts of total connected load, and this ratio might be used to calculate the ventilation requirements for smaller installations. At this point it should be mentioned that to draw this relatively large quantity of air through a lamphouse without affecting the stability of the arc will require very careful lamphouse design. Ventpipes should be as large as possible near the lamphouse (pipes eleven inches in diameter have been used on 100 ampere arcs with decidedly beneficial results) but the system as a whole should have sufficient "ventilation resistance," so that atmospheric conditions will have but a negligible effect. The ventilation characteristics of some high intensity lamphouses have been improved by moving the ventpipe forward from the center of the top to a position directly over the positive flame. In the case of the rheostat room the problem is slightly different. Here, due to the need for accessibility, the use of tightly fitting flues is not practical, and lower temperatures must prevail, but only the maximum wattage to be dissipated for an extended period of time need be considered. For the large installation before mentioned, assuming a line voltage of 115, this will be about as follows: 2 Spots each 100 amps, and 55 volts 11,000 watts 1 Flood 150 amps, and 35 volts 5,250 watts 2 Effects each 100 amps, and 55 volts 11,000 watts 1 Projector 150 amps, and 35 volts 5,250 watts Total 32,500 watts This is equivalent to 1850 Btu. per minute and, assuming a temperature rise of 30 °F., the system should have a capacity of 3475 cu. ft. per minute, or one cubic foot per minute for each 9.3 watts. This ratio would, of course, apply for any type of converting or controlling equipment merely by applying it to the total watts lost over a period of time. The constants used in the foregoing calculations were taken from Babcock & Wilcox' Steam, and are as follows: Approximate specific heat of flue gas at 300 °F. = 0.25 1 pound air at 100 °F. = 14.1 cu. ft. 1 pound air at 300 °F. = 19.1 cu. ft. 1 kilowatt hour = 3413 Btu.