The transference of heat is also proportional to the conductivity of the metal forming the heating surface. For gelatine liquors, copper tubes are almost invariably employed, the advantage being great even when price is taken into consideration. The following conductivity coefficients illustrate this point (calories per hour through 1 sq. metre of metal 1 metre thick, with a temperature difference of 1° C.):—
| Copper | 330 |
| Tin | 54 |
| Iron | 56 |
| Zinc | 105 |
| Steel | 22-40 |
| Lead | 28 |
The coefficient of heat transmission decreases the more with increasing thickness of wall, the worse conductor is the metal. For copper tubes, however, this decrease is usually unimportant.
The transference of heat is also much influenced by the viscosity of the liquor being evaporated; the greater the viscosity, the lower the coefficient of heat transmission. Unfortunately for this process of evaporation, gelatine sols are exceedingly viscous, and thus the difficulty in obtaining a concentrated sol is thus greatly enhanced.
The transference of heat is often greatly hindered by incrustations of the tubes, which incrustations generally conduct heat very badly. Thus the relative heat conductivities of copper and chalk are as 1000:5.
The amount of heat transferred is of course determined also by the area of the heating surface. The amount of evaporation needed thus determines the number of tubes (of standard size) in the evaporator, and thus the capacity of the machine. An evaporator should have its heating surface area chosen with a view to the duty required of it.
In practice the working of an evaporator is often not a very difficult matter, and large numbers of machines are operated by unskilled labour. Troubles generally arise from inconstant steam pressure, incrustation, leakages of air, which reduce the vacuum, the temperature head, and hinder heat transmission. For the evaporation of gelatine liquors the Yaryan, the Kestner, and the Blair-Campbell film evaporators are the most widely used. The velocity of the liquor through some of these machines is so great that occasionally no vacuum is used. The temperature obtained is high (200° F.), but the time is very short, if rapid cooling of the evaporated liquor is arranged.
REFERENCES.
"Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand. Scott, Greenwood & Son (1916 Ed.).
"Evaporation," by E. Kappeschaar. Norman Rodger (1914).
"Evaporation in the Chemical Industry," by J.A. Reavell, M.I.Mech.E., J.S.C.I., 1918, April 11th.
"Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59.
"Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29.
"Notes on Condensing Plant," J.M. Newton, B.Sc., J. Junior Inst. Engineers, Aug., 1912.