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

where the diffusion through the gelatin itself will become the limiting factor, i.e., a point may be reached in which the surface layer of the gelatin closest to the gas side is relatively dry, whereas the bottom of the gelatin closest to the base of the film would be relatively wet. The problem is then resolved into a transient diffusion problem in which a humidity gradient is set up through the gelatin. This humidity gradient will only become important when the rate of evaporation has been greatly increased. It was found experimentally that this limit becomes apparent when positivetype stock is dried in a tune shorter than 1 sec or when negative stock is dried in a time shorter than 2 sec. In general, therefore, film driers must be designed in such a fashion that the emulsion of the film is exposed to turbulent air for a period of not less than 2 sec. The actual film speed that can be obtained can, of course, be made as large as one wishes by simply increasing the time during which the film is continuously exposed to turbulent air. With reference to the extremely low temperatures realized in the use of the first model, it can be stated that this was not originally anticipated. It was hoped that as a result of the turbulence the heat transfer between the air and the film would be sufficient to maintain the film at room temperature. However, it was found that approximately 40% of the heat was supplied from the air and 60% of the heat had to be supplied by direct conduction or radiation into the film itself. Consequently, the heating of the back side of the film was necessary to maintain the film at room temperature or higher. With reference to the use of raw film in the experiments, I would like to explain that a large number of experiments were first made with processed film which had been run through Paramount's 35-mm machine. It was found, however, that small changes in the developer or hypo concentrations plus small variations in the thickness and quality of the emulsion itself produced variations in water absorption of the film of approximately 60%. As a result it was not possible to use processed film in the experiments, as consistent data could not be obtained. A number of experiments were then performed in which the total water absorption of the processed film was checked carefully and the total spread in the water absorption figures was carefully noted. The experimental determination of the rate of evaporation at various temperatures and Reynolds numbers of the air was then performed on raw stock which again had been carefully measured as to its water absorption qualities. It was found that the amount of water absorbed in the raw stock was to a great extent determined by the temperature of the water in which it was immersed. During the experiments the temperature of the water was maintained at a point where the water absorption of the raw stock was approximately equal to the water absorption of the processed stock. Adequate corrections were then made in the design of the film drier to produce a safety factor which would compensate for the spread in the water absorption qualities of the processed stock, so that even under the worst conditions processed stock could be dried in the required time. Spot checks were made at various points during the experiments with the processed film to compare the result between processed film and raw stock at a number of points in the curves. With reference to the type of film distortion which was measured and the methods used for measurement, I would like to defer the discussion of this to a later paper which is now in preparation in cooperation with Dr. R. C. Gunter of the Optical Research Laboratory, Boston University. Also, additional details concerning the actual design of film driers and general design charts will be presented in that paper. L. Katz: Drying Film by Turbulent Air 279