EVAPORATION 253 



pressures, the temperature in a steam space is always rather 

 lower than would be supposed from the pressure indicated 

 by the gauge. 



The transference of heat is influenced by the velocity of 

 both the heating fluid and the fluid being heated over the 

 heating surface. The more rapidly each fluid moves, the 

 more rapid is the transference of heat, because a greater 

 number of particles of both fluids are brought to the heating 

 surface in any given time. This is popularly known as 

 the effect of " circulation," and is illustrated by the advantage 

 of stirring a liquid being heated in bulk. In the film evapo- 

 rators the circulation is through tubes at high speed (up to 

 2 miles a minute) , and the maximum effect in this sense is 

 thus obtained. The increase in heat transference is not 

 directly proportional to the increase in velocity, but in 

 a lower ratio, sometimes approximately the square root of 

 the velocity. In such a case, if either velocity be quad- 

 rupled, the heat transference is doubled. Other advantages 

 of high velocity are that the heating steam more readily 

 sweeps away condensed steam from the heating surface, 

 and the high-speed film similarly " scours " away " incrusta- 

 tions " on the interior of the tubes. 



The transference of heat is also proportional to the con- 

 ductivity 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 illus- 

 trate this point (calories per hour through i sq. metre of 

 metal i metre thick, with a temperature difference of i C.) : 



Copper .. ..330 

 Iron . . . . 56 

 Steel . . 22-40 



Tin .. 54 



Zinc . . . . 105 

 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 



