20 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



The latent heat of evaporation ranges from 1,035.6 B. t. u. at 100° 

 F. to 977.8 B. t. u. at 200° per pound of water, but for ordinary 

 purposes of calculation it may be considered as 1,000 B. t. u. per 

 pound of water evaporated. The total heat of evaporation required 

 per pound of water evaporated under the conditions assumed will be 

 100 + 1,000, or 1,100 B. t. u., and for 6.32 pounds per minute it will 

 be 6.32X1,100, or 6,952 B. t. u. per minute, on an average. The 

 actual amount of heat which must be supplied by the fuel and carried 

 by the air will have to be much larger to compensate for the heat losses 

 in the drying system. 



HEAT LOSSES 



The principal ways in which heat is lost in a drying system without 

 being used as heat of evaporation are (1) through incomplete com- 

 bustion of the fuel, (2) in flue gases escaping from the stack, (3) by 

 radiation through the walls of the system, (4) by air leakage through 

 open seams or when doors are opened during drying operations, (5) 

 through the removal of heated material, trays, and trucks from the 

 drying chamber, and (6) through the necessary discharge of a portion 

 of the air. 



THERMAL EFFICIENCY 



The relation between the amount of heat actually used in the 

 evaporation of water in a drier and the total amount of heat generated 

 by the fuel is called the thermal efficiency of the drying system. This 

 ratio, which is expressed in percentage, is determined by dividing the 

 number of heat units required for the total heat of evaporation by the 

 number supplied by the fuel consumed during the same period and 

 multiplying the result by 100. If 1,100 B. t. u. be taken as the average 

 quantity of heat required to evaporate 1 pound of water, the thermal 

 efficiency of the whole system can be expressed as follows: Thermal 



~ . Pounds water evaporated X 1,100 B. t. u.. . i . A . 

 efficiency = Un i ts fuel used x B . t . u . per unit X 100. 



The thermal efficiency of the system is the product of the thermal 

 efficiencies of the heater and drying chamber calculated separately. 

 The thermal efficiency of the heater shows the ratio between the heat 

 generated by the fuel and the heat received from the fuel and carried 

 by the air to the drying chamber. The thermal efficiency of the drying 

 chamber gives the ratio between the heat received from the fuel and 

 carried by the air and the total heat of evaporation. 



Tunnel and cabinet driers operating under conditions of partial 

 recirculation should average better than 40- to 50-percent effi- 

 ciency for the drying chamber. The thermal efficiency of the whole 

 system will be influenced largely by the types of heaters (direct heat, 

 direct radiation, and indirect radiation) which are selected for use in 

 such driers. The thermal efficiencies of different heating systems in 

 tunnel and cabinet driers shown in table 4 may be assumed. These 

 values may be used to determine the approximate amount of heat 

 which must be generated and the portion of generated heat which 

 must be carried by the air in the drier being designed. In each case 

 the lower value given is sufficiently conservative to provide a reason- 

 able margin of safety. 



