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

to avoid the drying conditions which produce these defects. While it is desirable to dry films to the point where the moisture remaining in them is the same as the moisture which they would have when in equilibrium with air at 70 F and a relative humidity (R.H.) in the range from 45% to 60%, a survey of actual laboratory practice has shown that the difficulties noted previously are not encountered when the equilibrium moisture content ranges from as low as 30% R.H. to as high as 60% R.H. The usual type of film drier uses air in this relative humidity range at temperatures below 100 F. This automatically prevents excessive over-drying. Underdrying is avoided by providing a liberal factor of safety in the drying time. The very nature of the rapid-drying processes precludes both of these safeguards to maintaining the drying within the proper range. It was felt desirable not only to determine if black-and-white film, normally processed, could be dried rapidly without harmful effects but in what range the equilibrium moisture content of the dried film must fall to insure satisfactory results. Realizing that the very nature of a rapid-drying process would preclude the automatic safeguard of over-drying which is present in conventional machines and that a liberal factor of safety on the drying time would partially defeat the purpose of rapid drying, it was decided that performance data should be obtained on a type of drier considered the most suitable for operation in conjunction with conventional processing machines in commercial laboratories. This work, therefore, includes a brief discussion of various methods of obtaining rapid drying, several of which were investigated by Ives and Kunz,2 as well as data on drying film under a variety of air conditions. The term "rapid drying" is used, of course, in relation to the usual drying times obtained in commercial practice on conventional machines which for positive films range from 8 to 20 min and from 15 to 40 min for negative films.8 However, any method which resulted in shortening this time to any extent might be called "rapid." For the purposes of this discussion, however, we propose to limit it as applying only to those methods which reduce the drying times to 10% or less of that obtained in the best commercial practice. For positive films this means a maximum drying time of about 1 min and a maximum of 1^ min for negative films. Elementary Drying Principles Before discussing the various methods of drying, a few of the fundamental principles of drying ought to be emphasized. The removal of water from a material can be accomplished in two ways. If the water is standing as fine drops on a surface, it can be mechanically removed by such means as scraping, shaking, centrifuges, or air knives. The other method is by evaporation. If the material containing the water is hygroscopic, then only the surface water can be removed mechanically and the remainder must be removed by evaporation. If the material to be dried contains more water than it would if it were in equilibrium with the air around it, water will be slowly evaporated from it until equilibrium conditions are reached. This natural evaporation is a result of the water-vapor pressure in the material being higher than the vapor pressure of the water in the air. The change is slow because only temperature convection currents cause air movement near the material and therefore the water must diffuse through the air. Diagrammatically it looks as shown in Fig. 1, in which the layer of air above the material is shaded to show concentrations of water vapor. The air at the surface is saturated and the surface of the material is at the wetbulb temperature of the air. This saturation drops off with the distance February 1953 Journal of the SMPTE Vol. 60