50 MANAGEMENT OF DAIRY PLANTS 



outer wall becomes heated and rises, its place being taken by 

 the cold air against the inside wall. As this in turn becomes 

 warm, it forces its way upward; the other air, having gradually 

 cooled, drops to the bottom; thus a continuous circulation is 

 set up inside the air space itself and heat is constantly trans- 

 ported by convection. 



3. By Conduction. — This mode of transmission is accom- 

 plished by the transfer of heat from one molecule to another. 

 It is the mode oftenest to be reckoned with in problems of in- 

 sulation. The heat conductivity of dense substances, such as 

 metals whose molecules are heavy and close together, is high; 

 the conductivity of lighter material, such as wood, is less; while 

 that of the gases is extremely low. An absolute vacuum would 

 be an absolute insulator, but that is practically impossible on 

 a large scale. Material containing the greatest amount of air 

 divided in the smallest air spaces forms the best insulating 

 material. 



Formula for Estimating Heat Conduction. — Of course, 

 there is no building material or form of construction which gives 

 perfect insulation. The total amount of heat which any given 

 insulation will transmit in 24 hours is the measure of the re- 

 frigeration lost through imperfect insulation in that time, and 

 hence of the refrigeration that must be appUed every 24 hours 

 to offset that loss and keep the storage room at a given degree 

 of temperature. For calculating this loss and consequently 

 needed offset, the following formula may be used: 



R =fn (t — I') British thermal units ' 



In the foregoing formula, / represents the total square feet 

 of insulating surface surrounding a given storage room — walls, 

 floor, ceihng, windows, and doors; n is the number of B. T. U. 

 of heat transmitted through such surface every 24 hours for 

 each square foot, for each degree Fahrenheit of difference in 

 temperature between inside and outside of wall; / is the average 

 temperature outside the wall, and /' is the temperature to be 



' A B. T. U. is the amount of heat required to raise the temperature of i pound 

 of water i° F., at its greatest density. 



