Journal of the Society of Motion Picture and Television Engineers (1950-1954)

duction, the water on the surface of the film will be approximately at the wetbulb temperature and its vapor pressure will therefore be the same as for saturated air at this wet-bulb temperature, while the air itself will have a vapor pressure corresponding to the dew point temperature.4 This difference in vapor pressure is sometimes referred to as the "driving force" and is roughly proportional to the difference between wet and drybulb temperatures. When the drying air is heated the vapor-pressure difference is increased causing a higher rate of evaporation. A higher surface temperature with the accompanying increase in rate of evaporation can also be obtained by heating the material. The foregoing remarks have been made with respect to the constant-rate phase of the drying of a material. In the drying of many materials, and film is one of them, the final drying is not at a constant rate but at a falling rate. This is a result of the rate of moisture diffusion through the film to the surface being slower than the potential rate of evaporation. The same factors, air velocity and vapor-pressure difference, affect the drying in this stage but their action is modified by the diffusion and equilibrium moisture characteristics of the film. In the rapid drying of film as much as two-thirds of the total drying time may be in the falling-rate phase. As a result the type and thickness of both support and emulsion have a large effect on the drying time of the film under a given set of drying conditions. As a brief summary, then, it can be said that drying is a heat transfer as well as an evaporation or mass transfer operation. In the drying of motion picture film it is partially done at a constant rate of moisture removal and partially at a falling rate. In both phases the vapor-pressure difference and air velocity will affect the speed of drying, but in the latter phase the characteristics of the film will also affect this speed. Special Requirements for a Commercial Rapid Film Drier In considering possible methods of rapid drying which might be used in the design of a commercial film drier there are a few special limitations and requirements which do not arise in the design of conventional machines which should be considered. Recognizing the fact that short drying times are frequently obtained at elevated temperatures, it was realized that it would be extremely difficult to control a film drier so that the film did not approach the high temperatures to which it was subjected. Because the film support softens at 240 F, it is considered important that the film drier be so designed that the film is never subjected to this temperature while in the dry state. Some films are made quite brittle if subjected to temperatures much lower than this and the temperature limit on driers for these films should be set accordingly. In the case of film breaks or other accidental stoppages the heat source in the machine must not constitute a fire hazard. The machine should have some ready means of adjusting the drying rate, independently of the film speed. A satisfactory rapid commercial film drier must be economical and reliable. The cost of drying film is not a major cost of the processing. However, a rapid drier should not require an extraordinarily high initial investment nor should its cost of operation be several times the operational cost of conventional driers. It should be con trolled easily and require a minimum o attention during operation. With these requirements in mind th merits of the possible methods can be considered. The basic differences in methods o drying have to do with the method o transferring the heat to the materia In drying film the three basic hea transfer methods can be used, namely 88 February 1953 Journal of the SMPTE Vol. 60