744 Journal of Agriculture , Victoria. [10 Dec, 1917. 



In the tunnel evaporator, the majority of operators maintain a tem- 

 perature of 160 to 175 degrees in the lower and hotter end of the tunnel, 



while the upper end will be 15 to 25 degrees cooler. Since the fruit is 

 introduced at the upper end, and gradually moved toward the hotter end, 

 it begins to dry at 135 to 150 degrees, and is finished at the higher 



temperature. This is essentially what the operator of the kiln accom- 

 plishes by the first method described in the last paragraph. 



In the Carson-Snyder evaporator, a very different set of conditions 

 prevail." Since the temperature in the upper drying chambers is usually 

 25 or 30 degrees lower than that in the lower ones, it follows that the 

 fruit placed in the upper chamber is dried at a temperature considerably 

 lower than that used in any other evaporator. In physical characters 

 and appearance it is indistinguishable from other fruit, and, as already 

 stated, we possess as yet no information as to whether chemical differences 

 exist. 



In drying prunes and berries, the temperature at the outset should 

 not be allowed to rise about 125 or 130 degrees until the fruits have lost 

 a considerable portion of their water, as otherwise there will be ex]ian- 

 sion and bursting, with consequent dripping. The temperature which 

 may be employed in the later stages of the process will depend upon tlie 

 circulation of air ; if ample air movement can be obtained a temperature 

 of 175 to 180 may be employed in the last half of the drying period, but 

 if the circulation of air is defective the temperature must be kei^t below 

 this point, or the fruit will be partially cooked, or dried at the surtace, 

 while the interior is still too high in water content. 



Relation of Temperature of the Air to its Moisture-carrying 



Capacity. 



It must not be forgotten that the capacity of the air to carry moisture 

 is a function of its temperature, and inpreases rapidly as the temperature 

 is increased. How significant this fact is may at once be seen from 

 consideration of the fact that 1 cubic foot of air at the freezing point 

 can absorb 1-160 part of its weight of water, and that the water-absorb- 

 ing capacity is doubled with every increase of 27 degrees in temperature. 

 This is shown in the following table : — 



Temperature. ' 1 cubic foot of air can absorb — 



32 degrees . . • . • • ■ ■ • • 1-160 its weight 



59 degrees . . . . . . ■ . . . 1-80 



. 86 degrees . . . . . . . . . . 1-40 „ 



113 degrees .. . . . . . . . . 1-20 



140 degrees .. .. .. .. 1-10 „ 



167 degrees . . . . . . . . . . 1-5 „ 



104 degrees .. .. .. .. .. 2-5 „ 



221 degrees . . . . . . 4-5 „ 



If we disregard the expansion of air with increasing temperature, 

 which we may do since it amounts to only 1-490 of the volume for each 

 degree rise of temperature, it will be seen that air raised from 86 degrees 

 to 167 degrees has had its moisture-earying capacity increased eightfold, 

 whereas if the temperature be raised to 140 degrees the moisture-carry- 

 ing capacity will be increased only fourfold. It is, therefore, easily 

 seen that in drying any substance not easily injured by heating choice 

 may be made between the use of a very large volume of air moderately 

 heated or a much smaller volume of air raised to a higher temperature. 



