CANNING SARDINES 119 
TABLE 6.—Relative evaporation rate from water surfaces, due to air motion?) 
) 





; : Rate of : Rate of 
Air are OF pyater evapo- Air A ea? water evapo- 
Sa ration SEMUEGS ration 
Pieeeee 2 5 __ = - | 25 square feet... 1.5 || 1,000 feet per minute ve- | Any area_____- 4,8 
250 feet per minute veloc- |____- dps StS 3.5 locity. 
ity. 2,500 feet per minute ve- |___-- do-= +5 a 8.1 
800 feet per minute veloc- | Any area____-- 4.5 locity. 
ity. 4,000 feet per minute ve- |____- COs ce 12.6 
locity. | 



1 From ‘‘ High-Temperature Drying.’’ By Burt S. Harrison. American Society of Heating and Venti- 
lating Engineers’ Guide for 1922, p. 50. New York. 
TABLE 7.—Relative evaporation rate from water surface, due to heat! 



Relative Relative 
Temperature of water and air, °F. fee ot Temperature of water and air, °F. cee 
ration ration 
1 27 
2 44 
4 71 
8 135 
16 165 






1 From p. 51 of paper referred to in Table 6. 
An effect known as “‘casehardening”’ frequently is encountered in 
drying substances that give up their moisture slowly. This is 
brought about by such rapid drying of the surface of the material 
that further drying is hindered. This usually is prevented by using 
high-humidity air. 
Since the nature of the material to be dried has its influence on the 
method and rate of drying, it is important to consider partial dehydra- 
tion of fish with this idea in mind. The fish are usually taken out of 
the brine or steam chest and placed in the drier. These fish are cov- 
ered with more or less free water, which is removed easily. The rest 
of the water, however, is within the cells that make up the flesh of the 
fish. This water must reach the surface and vaporize before it can be 
removed. In fact, water not only has to reach the surface of the fish, 
but, after vaporizing, it must pass through a stationary film of air 
surrounding the fish. The water passes through the fish and water 
vapor through this film of air by a process known as “‘diffusion.”” The 
rate of diffusion is slow, but it increases as the temperature of the fish 
is raised. Decreasing the thickness of the air film also increases 
diffusion. It likewise increases the rate of heat transfer from the air 
to the fish, because heat also must pass through the film. The thick- 
ness of this film of air is decreased as the air velocity over the fish is 
increased. However, if the rate of diffusion is slow, increased air 
velocity will have little effect in increasing moisture loss other than 
that brought about by greater heat transfer to the fish at the higher 
velocity, 
