nee BULLETIN 1335, U. S. DEPARTMENT OF AGRICULTURE 
FREE CROSS-SECTIONAL AREA OF TUNNEL 
If 3 inches of vertical space be allowed each tray, and 1 inch of 
this represents the thickness of the tray, there will be 2 inches of 
2 inches Xx 36 inches X 2 x 25 ieiaats 
i4A » which is 
25 square feet of free cross-sectional area, through which the air can 
pe provided all spaces on all sides of the trucks are occupied by 
affles. This figure (25 square feet) will be used later in calculating 
the air velocities through the tunnel. 
open vertical space per tray, or 
QUANTITY OF WATER EVAPORATED 
If the tunnel is to have a capacity for drying 7 tons, or 14,000 
pounds, of prunes each 24 hours, assuming that 35 pounds of the 
dried product will be obtained from every 100 pounds of the fresh 
fruit, there will be evaporated 14,000 x 0.65, or 9,100 pounds of water 
each 24 hours—an average of 6.32 pounds a minute. A definite 
amount of heat will be required to bring about the evaporation of this 
quantity of water. 
HEAT REQUIREMENTS FOR EVAPORATION OF WATER 
The requirement for sensible heat will be 1 B. t. u. per pound of 
water evaporated from the material per degree increase in its tem- 
perature. The actual temperatures of the products during drying 
lie above the wet bulb temperature and approach the dry bulb tem- 
perature as the drying progresses and the rate of evaporation decreases. 
For safety in calculation it may be assumed that the product becomes 
heated to the dry bulb temperature at the hot end of the tunnel. 
In the example considered, increasing the temperature of the fruit 
from 60° F. outside temperature to 160° F. in the tunnel makes the 
sensible heat requirement 100 B. t. u. per pound of water evaporated. 
The latent heat of evaporation ranges from 1,035.6 B. t. u. at 100° 
F. to 977.8 B. t. u. at 200° F. 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 
Pe pound of water evaporated under the conditions assumed will 
e 100 +1,600, or 1,100 B. t. u., and for 6.32 pounds per minute it will 
be 6.32 X 1,100, or 6,952 B. t. u. per minute, on the average. The 
actual amount of heat which must be supplied by the fuel and car- 
ried by the air will 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 throug 
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. ; 
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