308 THE SPRAY PROCESS 



fineness, in order to be efficient and to accomplish maximum 

 recovery. 



The percentage of recovery of the solids of milk in the form 

 of milk powder in any given desiccating arrangement then is 

 largely a matter of efficiency of the dust collector, and the effi- 

 ciency of dust collectors must increase as the fineness and flaki- 

 ness of the product increases. The recovery may be materially 

 facilitated, however, by such an arrangement of the desiccating 

 apparatus, as will cause the particles of drying milk to travel 

 in a direction opposite to that of the escaping air, as indicated 

 in the Gray patents. 



Heating the Air. This is done either by the installation 

 and operation of a furnace, similar in principle to not air fur- 

 naces, or by steam coils installed in a closed, insulated vault. 



The hot air furnace makes possible the heating of the air 

 to higher temperatures and it is claimed to be somewhat more 

 economical from the standpoint of fuel consumption. Its dis- 

 advantages are that the temperature is somewhat more difficult 

 to control, it fluctuates rapidly with the condition of the fire. 

 There is also more or less danger of impure air, because in the 

 case of even slight leaks between the fire box and the hot air 

 chamber, soot and ashes tend to be dra\vn into the heated air 

 and are thus blown into the drying chamber where they mix 

 with and deposit in the milk powder. 



Steam coils, enclosed in a vault, have been found less 

 objectionable in this respect. While it is more difficult to attain 

 quite as high a degree of heat by this method, the heated air 

 can be maintained more easily at a uniform temperature and 

 there is no danger of impurities leaking into the air. 



The air is drawn into the furnace or hot air vault from the 

 atmosphere. It may be, but usually is not, filtered by admitting it 

 through an air filter, located at the intake, into the hot air vault. 

 Absorbent cotton or other similar pervious material may be used 

 for this purpose. 



The earlier patents also cover an air drying arrangement in- 

 stalled before the air reaches the hot air vault. This greatly as- 

 sists in controlling and making uniform fhe results of the drying 

 process from one day to another, neutralizing the disturbing ef- 



