International projectionist (Jan-Dec 1950)

AIR COOLING of Motion Picture Film for Higher Screen Illumination By F. J. KOLB, Jr. Eastman Kodak Company The second installment of a series of three articles which detail the experimental setup utilizing high-velocity air jets directed at the film aperture and which reportedly resulted in an increase of from 30 to 60 per cent in permissible flux and in attainable screen illumination. FOR these experiments, a radiation source for producing more energy than the present commercial projection lamphouses was necessary. The Peerless Hy-Candescent lamphouse was specially modified to operate with watercooled positive jaws and permit the burning of experimental trims at currents and radiation intensities up to 65% in excess of present commercial equipment, providing an instantaneous net flux of 1.65 watts per square mm and a mean net flux of 0.99 watt per square min. Vycor protection plates for the condensers were used, since these experimental trims burning close to the condenser tend to cause excessive pitting and spattering of the silica condenser lens. Furthermore, a % inch thickness of Vycor is effective in preventing solarization and the resulting loss in transmission of the silica condenser. Our test projector is a Simplex E-7 with the gate modified to permit the introduction of air-cooling nozzles. The projector was operated with a 60% rear shutter and over-all shutter transmission to the screen of 50%. Air Compressor Used The air supply was obtained from the pressure tank of an air compressor, with the air throttled down so that it entered the nozzles at a pressure of only a few pounds per square inch. For the pressure and flow rates found necessary in the particular equipment tested, a rotary blower would have been a more satisfactory and less expensive source of lowpressure air. A number of nozzles of different de fJ. Soc. Mot. Pict. Eng., Dec.. 1949. sign were tested for cooling efficiency. These nozzles range from 0.007 to 0.14 square inch in area and were found to operate best with a free-air velocity at the nozzles from 250 to 500 feet per second. It should be pointed out that the choice of a suitable nozzle is a compromise among cooling efficiency, ease of mounting and direction, method of construction, space available, and freedom from interference with the normal projector performance. The nozzles used in this test were chosen for their ease of installation in the Simplex E-7 projector. We feel it is undoubtedly true that more efficient nozzles with a lower noise level could be designed and built, particularly if one were free to alter the rear of the projector in order to make room for an optimum installation of the nozzles. Cooling of the Film In order to measure how much cooling is actually obtained by air jets directed at the film in the aperture, measurements were made leading to two series of data. In the first, an approximate heat-transfer coefficient was determined for film cooled by high-velocity air; in order to make such a determination, it is necessary to know the film temperature — and as a result, measurements had to be made at very low radiation intensities such that film could be left stationary in the gate and its temperature measured with an attached thermocouple. A second set of data was obtained making use of an experimental film containing an indicator that changed visually when its temperature threshold had been exceeded. Measurements of the approximate heat-transfer coefficient at low radiation levels (in engineering units for convenient correlation with published data on air cooling) are shown in Fig. 1. These data must be taken as indicative only, and while we expect the basic conclusions drawn from these data to apply to all methods of air cooling, the quantitative values of the heat-transfer coefficient FIGURE 1 Cooling cine film by high -velocity air. Heat transfer coefficient as a function of air velocity for several types of nozzles. (Gross coefficients not corrected for losses by radiation.) Coefficients were measured at low radiation flux densities. ^ V • '<3 .«* ® sy ®S NAT URAL CONVE< :tion 50 IOO 200 AIR VELOCITY— FtVsEC. Nozzle Curve 1 2 3 Size 0.015" X 1.06' 0.052" X 1.20" 0.126" X 1.10" Description Slit Slit Slit 4 0.077" diameter single nozzle 5 0.124" diameter single nozzle 6 0.018" diameter row of 28 holes 7 0.040" diameter row of 17 holes INTERNATIONAL PROJECTIONIST • February 1950 17