308 Tut Spray Prociss 
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 
ina 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 hot 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 drawn 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 the results of the drying 
process from one day to another, neutralizing the disturbing ef- 
