VEGETABLE AND FRUIT DEHYDRATION 



47 



why the final stage of drying a fruit or vegetable proceeds so much 

 more slowly than the initial stages. 



Returning to the example of evaporation at 100° F., we find that 

 as evaporation proceeds the liquid cools down below 100° unless addi- 

 tional heat is supplied. Actually, 1,037 B. t. u. 3 would have to be 

 supplied to the liquid at 100° F. for every pound of water evaporated. 



Table 7. — Properties of water vapor and air 



Temperature (° F.) 



Vapor 

 pressure 

 of water 



Latent 

 heat of 

 vaporiza- 

 tion 



Absolute 

 humidity at 

 saturation J 



Specific volume ' 



Dry air 



Saturated air 



. . 



Inches of 

 mercury 

 0. 0375 

 .063 

 .103 

 .165 

 .248 

 .362 

 .52 

 .74 

 1.03 

 1.42 

 1.93 

 2.59 

 3.44 

 4.52 

 5.88 

 7.57 

 9.65 

 12.20 

 15.29 

 19.01 

 23.46 

 28.75 

 29.92 

 35.0 

 42.3 

 50.8 

 60.7 

 72.1 

 85.2 

 100.1 

 117.2 

 136.4 



B. t. u./lb. 



1,094 



1,088 



1,083 



1,077 



1,071 



1,066 



1,060 



1,054 



1,049 



1,043 



1,037 



1,032 



1,026 



1,020 



1,014 



1.008 



1,002 



996 



990 



984 



978 



972 



970 



965 



959 



952 



945 



938 



931 



924 



917 



910 



Lb. vapor jib. 

 dry air 



0. 00079 

 . 00132 

 . 00216 

 . 00347 

 .0052 

 .0077 

 .0111 

 .0158 

 .0223 

 .0312 

 .0432 

 .0595 

 .0815 

 .1114 

 .1532 

 .2122 

 .2987 

 .432 

 .658 



1.098 



2.30 

 13.4 



Cu.ft./lb. 

 11.58 

 11.83 

 12.09 

 12.34 

 12.59 

 12.84 

 13.10 

 13.35 

 13.60 

 13.86 

 14.11 

 14.36 

 14.62 

 14.88 

 15.13 

 15.39 

 15.64 

 15.90 

 16.16 

 16.41 

 16.67 

 16.92 

 16.97 

 17.1 

 17.4 

 17.6 

 17.9 

 18.1 

 18.4 

 18.6 

 18.9 

 19.1 



Cu.ft.jlb. 

 dry air 



11.59 



10 



11.86 



20 



30 



12.13 

 12.41 



40. 



12.70 



50 



13.00 



60 



13.33 



70.. 



13.69 



80 



14.09 



90. 



14.55 



100 



15.08 



110. . 



15.72 



120 



16.52 



130. 



17.53 



140 



18.84 



150 



20.60 



160 



23.09 



170 



26.84 



180 



33.04 



190 



45.00 



200. . . 



77.2 



210 



382 



212, . 





220 







230. 







240 







250 







260 . 







270 







280 . 







290 







300 . 













1 Barometric pressure, 29.92 inches of mercury. 



This quantity of heat that is absorbed when a pound of water vaporizes 

 is known as the latent heat of vaporization. It varies slightly with 

 temperature (see table 7), but under the usual conditions of dehydra- 

 tion it is approximately 980 to 1,050 B. t. u. per pound of water evapo- 

 rated. A considerable proportion of the heat supplied to a dehydrator 

 goes to furnish this latent heat. 



Properties of Air and Water Vapor 



According to the definition given above, air is "saturated" with 

 water vapor when the partial pressure of the vapor in it equals the 

 vapor pressure of water at that temperature. Saturated air is just 

 on the point of becoming foggy ; if there is actual fog, the saturated 

 air is carrying an additional quantity of water in the form of liquid 

 droplets. 



The "dew point" of any given mixture of water vapor and dry air 

 is the temperature at which it would become saturated with water 



3 A British thermal unit is the quantity of heat required to raise 1 pound of water 1° F. 

 in temperature. 



