304 THE SPRAY PROCESS 



milk, without precondensing, was sprayed, and while this pro- 

 cedure is entirely feasible, it was soon found that it was more 

 economical to remove a considerable portion of the water of the 

 fluid milk and to reduce the product to a concentration of about 

 4:1 or 4.5:1 before spraying. This is accomplished by condens- 

 ing the fluid milk by any of the methods for condensing as de- 

 scribed under the "Manufacture of Condensed Milk" in this 

 volume. In general practice the vacuum pan is used for this 

 purpose in most of the milk powder plants. 



Effect of Pre-condensing on Economy of Manufacture. 

 The chief advantage and purpose of pre-condensing', instead 

 of spraying the fluid, or uncondensed milk, lies in the greater 

 economy of operation in the case of pre-condensing. 



The fluid milk contains more water than the condensed 

 milk; more water must be removed during the spraying process, 

 hence less milk can be desiccated in equipment of the same capa- 

 city and in the same space of time than in the case of spraying 

 pre-condensed milk. The pre-condensing therefore means greater 

 capacity of the available equipment- shorter hours and greater 

 economy of operation. 



Again, the fuel requirements are greater in the process of 

 desiccating by the spray method than by evaporation in the 

 vacuum pan or the film method. The comparative efficiency 

 of evaporating water by means of air and in vacuum, is well 

 understood. The heat-transmitting coefficient of air is much 

 lower than that of steam and metal heating surfaces. The 

 heat applied in the form of heated air is less completely utilized 

 than the heat applied in the form of steam in copper jackets and 

 coils, hence in evaporation by heated air there is greater waste 

 of heat and fuel. The various factors which enter into the dry- 

 ing by means of air and the resulting losses of heat transferred 

 are discussed in detail by E. Hausbrand 1 in his revised treatise 

 entitled "Drying by Means of Air and Steam." 



Effect of Pre-condensing on Bulkiness of Spray Milk 

 Powder. Other conditions, such as orifice of spray nozzle, 

 pressure of milk, and temperature to which the milk is preheated 

 being the same, the milk powder made by spraying fluid or un- 

 condensed milk, is somewhat more bulky than that made by 



1 Hausbrand. "Drying by Means of Air and Steam," 1901. 



