9 2 



Picking, Drying, and Packing Hops. [may, 



less Welsh coal (anthracite) is burnt. This fire heats the 

 air, which, together with the products of combustion of the 

 fire, passes upwards through the hops, and in doing so 

 carries off moisture from the hops and so dry them. 



Theory of Hop-Drying, — When water is exposed to dry 

 air, the water gradually changes to a state of water-vapour 

 and passes away into the air. Similarly, when hops are 

 exposed to warm dry air, the moisture which they contain 

 changes to vapour and the hops become dry. 



The rate of drying chiefly depends: — (i) upon the tem- 

 perature of the air, and (ii) upon the rate at which the air 

 passes through the hops. 



(i) The amount of water that a given volume of air can 

 take up in the form of water-vapour is dependent upon the 

 temperature of the air. If the air is cold, a given volume 

 can take up comparatively little water-vapour, whereas, if 

 the air is warm, the same volume can take up much greater 

 quantities. Thus 10,000 cubic feet of dry air at a temperature 

 of 6o° F. can take up about 8J lb. of water as water- vapour 

 before becoming saturated. If the temperature of the air is 

 raised from 6o° F. to ioo° F., then 10,000 cubic feet can 

 take up about 32 lb. of water- vapour before becoming satur- 

 ated, i.e., nearly four times as much. Consequently, hops 

 dry much more quickly in a draught of hot air than in one 

 of cold. 



We have seen that air at 6o° F. cannot contain so much 

 water-vapour as air at ioo° F. ; if, therefore, air saturated 

 with water- vapour at ioo° F. be cooled to 6o° F., some of 

 the water- vapour is condensed, and a "mist" is formed 

 which settles in the form of "dew." Under certain condi- 

 tions, this state of affairs may occur during hop-drying, and, 

 as it results in serious damage to the hops, it must be carefully 

 guarded against. 



(ii) The rate at which drying proceeds is proportional to 

 the rate at which the air passes through the hops, or, as it 

 is called, the strength of the draught. It is due to this factor 

 alone, namely, the strength of the draught, that much larger 

 quantities of hops can be dried upon oasts in which a strong 

 draught is artificially produced by a fan. 



Let us now examine what happens when, at the beginning 



