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4 
DEHYDRATION OF FRUITS AND VEGETABLES 93 
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 gen- 
erated 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: 
Pounds water evaporated x 1,100 Bate u. 
Units fuel used x B. t. u. per unit 
Thermal efficiency = < 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 per cent efficiency 
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 5 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. 
TaBLeE 5.—Assumed thermal efficiencies of different heating systems 
Thermal efficiency 
i Drying 
Heating system Gheuniser bahia 
(tunnel | Heater svete 
or cabi- ; 
net) 
Per cent | Per cent | Per cent 
LDV RSYO RUNES IF Le. Set eh ri ae arg sap pens em Oe gees) ee 40-50 | 90-100 36-50 
Oe Cima ti atlGMe eee ne TNA CR EOT SES OEE ED a ae ee Bsa k 40-50 | 80-90 32-45 
PACE GEILACT AHL O TEE thet wey a oe eee ee ee ES to 40-50 60- 70 24-35 
HEAT TO BE GENERATED BY THE FUEL 
If the heater selected is one of the direct radiation type, burning 
ordinary fuel oil, then, employing the data in Table 5, the thermal 
efficiency .of the whole system may be expected to be at least 32 
per cent. In other words, the total heat of evaporation will equal 
32 per cent of the total heat which must be generated. 
