98 



MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



heaters can be used without impairing the efficiency of the system. 

 This factor is especially important with oil, because oil requires a 

 considerable amount of excess air to insure complete combustion and 

 thus prevent smoking. As a rule, heavy oil can be completely burned 

 with 250 percent of excess air. More than this amount of air may 

 chill the flame and produce deleterious effects. The design of the sec- 

 ondary air opening varies with the burner used and should be provided 

 for in accordance with the manufacturer's recommendations. 



Indirect Heating System 



Indirect heating systems can be divided into two general types. 

 First, and simplest from the standpoint of equipment, are those with 

 heat-exchange surfaces receiving heat directly from flue gases or 

 products of combustion. The second, a somewhat more complicated 

 type, employs a medium such as a liquid or vapor to transfer heat 

 from its source to the heat-exchange surfaces. 



COLO AJR TEMPERING FOR 



TEMPERATURE ADJUSTMENT OR CONTROL 



COAL- BURNING FURNACE 



COAL STOKER 



FURNACE FLUE GAS FOR 

 HEATING DRYING AIR 



IN DEHYDRATER 



^— HEAT EXCHANGE 

 TUBES-3 IN SERIES 



JZF 'U.0E 



LOW 

 VANES 



Figuke 54. — Coal-burning, single-truck cabinet dehydrator. 



The first type, as one might expect, is inefficient and difficult to con- 

 trol. It can, however, utilize a variety of fuels. Temperature regu- 

 lation is difficult even when gas or oil is used. However, complete 

 combustion is not essential for successful performance of the indirect- 

 heated dehydrator. Some smoking can be tolerated, because the prod- 

 ucts of combustion do not come in direct contact with the drying air. 



If coal or wood is used with this system, a fair degree of control can 

 be obtained by either wasting some of the products of combustion to 

 atmosphere before they come in contact with the heat-exchange sur- 

 faces, or by diluting and cooling the flue gases with outside air prior to 

 their flowing over these surfaces. Obviously these methods of control 

 are wasteful. Although this type of indirect heating has its applica- 

 tions, it is of little value in commercial use. A typical example of a 

 small unit employing this system of heating is shown in figure 54. 

 This unit employs an automatic coal-fired stoker. 



The second type of indirect system, on the other hand, offers practi- 

 cally ideal control characteristics. The heat-transfer medium is usu- 

 ally steam, but may be hot water or oil. This method of heating offers 

 a clean, even source of heat with any type of fuel, either liquid, solid, or 

 gaseous. The boiler or accumulator is a source of heat that can be used 

 instantaneously to cope with variations in demand. Regulation of 



