EVAPORATION 311 



in temperatxire (water-jacketed crystallizer), one may change (the hot 

 body in a steam boiler and the cold body in a juice heater), or both may 

 remain at uniform temperature (condensing vapour and boihng liquid in 

 evaporators and vacuum pans). 



The passage of heat by conduction through a partition is controlled by 

 the following circumstances : — 



I. The mean temperature difference between the hot and the cold body. If 

 the hot and cold bodies do not change in temperature, as with condensing 

 steam and boiling liquid, the mean temperature difference offers no difficulty 

 in definition ; if, however, one or both temperatures vary, as with a hot 

 liquid and a cold liquid, the mean temperature difference is given by the 

 expression 



/ T 



{T,na..— r„.,,,) ^ [log, ~ 



where T„^^ and r,„,„ are the greatest and the least temperature differences 

 (%e = 2-3025 %io)- 



2. The resistance to the passage of heat from the hot body to the 

 partition, through the partition, and from the partition to the cold body. 



3. The rapidit3^ of movement (or circulation) of the hot and cold 

 bodies. 



4. The area of the partition. 



The influence of these factors is discussed below. 



According to experimental observations, when the temperature differ- 

 ences are small and near to each other on the thermometric scale, and when 

 the physical properties of the bodies on either side of the partition do not 

 vary much vni\\ change of temperature, the rate of heat transfer is ver\' 

 nearly proportional to the temperature difference. Thus with a range of 

 temperature, say from 200° F. to 220° F., with condensing steam on one side 

 and juice of 15° Brix on the other, about twice as much heat will pass 

 with steam at 218° F. and juice at 208° F. as with steam at 220° F. and 

 juice at 215° F. ; but it does not follow that proportionality will obtain as 

 between one system at 230° F. and another at 150° F., or when the tempera- 

 ture differences to be compared differ greatly in magnitude. It is, however, 

 certain that as the position of the temperature difference rises in the absolute 

 scale of temperature the rate of transfer also increases. This is very marked 

 as between the first and last cells of a multiple effect evaporator, and is also 

 in this case probably due to causes such as viscosity of the s^Tup, as well as 

 to position in the scale. 



* The writer examined this point as regards the first and last cells of a 

 vertical submerged tube quadruple effect apparatus, taking as the tempera- 

 ture differences the value C^ — Jx or C^ — C^ and C4 — VS^ ; the rate 

 of heat transference was obtained by observing the time required to fill a 

 tank with the water discharged from the first cell. If the rate of heat 

 transference is proportional to the temperature difference, then T (Cj — C2) 

 = constant, where T is the time taken to fill the tank. Some results follow 

 below, the temperature being in F.° and time in seconds. These experiments 

 were made in a factory and not in an engineering laboratory. 



* For the significance of these expressions see p. 321- 



