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50
BRITISH KINEMATOGRAPHY
Vol. 16, No. 2
The important points revealed by this formula are :— (i) The only quantity which can be varied in practice to any great extent is the velocity. Unfortunately due to the 0.75 power, to increase the cooling requires an even greater increase of air velocity, and in addition there is an upper limit to the velocity. In practice this is not the theoretical one of the speed of sound in air, but arises from the fact that at a certain velocity the slide becomes unstable and begins to vibrate. Even so, the rate of cooling could be perhaps doubled above our present limit ; and if the light output of the projector is to be increased further the size of the gate must be increased — to perhaps whole-plate size for half a million lumens, (ii) The value of T is determined by the temperature which the slide will withstand without breaking (estimated very roughly at 150°C.) and also by the air temperature. Compared with using air at room temperature, a slight improvement might be obtained by using pre-cooled air. It is, however, difficult to arrange for the supply of a fair quantity of air at a temperature below 0°C, for the cooling equipment becomes covered in ice and frost. Rather than go to this trouble, it is considered to be far preferable to use uncooled air and increase the velocity a little.
(iii) It is some advantage to reduce the thickness of the air passages. They must not be made so small as to require too great a pressure to force the air through the slide cooler, nor to be ndow seriously affected by the usual
variations in thickness of the gkss of the slide.
Fig. 8. Diagrammatic cross
sectioned view of slide-cooler. Glass windows close to each side of the slide form thin channels through which air is blown at a very high velocity.
Design of a Slide-Cooler
A slide-cooler embodying these principles12 is shown diagrammatically in Fig. 8. The air is supplied at a small pressure through a nozzle which contracts in the plan view shown, and gets wider in side view until it covers the height of the slide. Another nozzle connects to an exhaust pipe, and providing it is of small angle as shown recovers much of the velocity pressure head at the slide.
The particular advantages of using the glass windows are (i) to obtain the high air velocity ; (ii) to make it uniform all over the slide ; (iii) to obtain silence ; and (iv) they probably make the boundary layer thinner. By using surface-coated optical glass for the windows the light loss is only 6 per cent. One window is fitted in a door to permit insertion of the slide, which is clamped along its upper and lower edges. Along each side-edge there is a thin metal strip to give streamlined air-flow at the leading and trailing edges of the slide. Air Supply System
Consideration was first given to obtaining the air from an impellor pump or compressor, but it was found to be impractical to silence such machines sufficiently to permit their use on the projector, which is used unblimped and unboothed on an ordinary sound-recording stage.