18 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 



In this discussion a cubic foot of space has been used as the basis 

 of the calculations. In analyzing the heat quantities in the drying 

 operation it will be easier to use 1 pound of dry air as a basis, with 

 its accompanying moisture, and follow it through its various stages. 

 Its volume will therefore not remain fixed, but will change with 

 every change in temperature, and consequently the degree of satura- 

 tion produced by a definite amount of moisture accompanying it will 

 depend upon the volume which it occupies. 



THEORETICAL ANALYSIS OF HEAT QUANTITIES. 



For this purpose* the simplest way will be to follow a pound of 

 dry air through a drying cycle as a basis for computations. While 

 in reality the vapor does not enter the air like water in a sponge, but 

 occupies the same space whether air is present or not, we may, for 

 convenience, conceive of a pound of air as containing a certain amount 

 of vapor, which, in reality, means that the space occupied by a 

 pound of dry air under given conditions contains a certain amount of 

 vapor. 



VAPOR AND AIR IN MIXTURE. 



As already explained, the total pressure always is the sum of the 

 individual pressures of the air alone plus the vapor alone. Thus 

 we may speak of a pound of air as being wholly or partially satu- 

 rated with vapor, meaning that it is the space occupied by the pound 

 of air which is in this condition of vapor. If a pound of air said in 

 this sense to contain a given weight of vapor is heated a given amount 

 under a pressure of one atmosphere, both air and vapor will expand 

 the same amount, so that at the new temperature both will occupy 

 the same relative amount of space; the pound of air, however, will 

 still contain the same w^eight of vapor. The amount of vapor con- 

 tained in a pound of air alone, when it is saturated, can not be used 

 as the divisor in obtaining the relative humidity when compared 

 to the amount of vapor actually contained in the pound of air alone, 

 because when the air is saturated the pressure of the air alone will 

 have been reduced, corresponding to the increase in the vapor pres- 

 sure (since the sum of the two make up one atmosphere), so that for 

 a pound of air a much greater space is required, and, consequently, an 

 equivalently greater weight of vapor to occupy this larger space. 

 For relative humidity it is necessary to compare the weights of vapor 

 which occupy the same amount of space when partially or wholly 

 saturated, or, better still, to compare the vapor pressures. 



CYCLE IN DRYING OPERATION OF 1 POUND OF AIR. 



In following the pound of dry air through its cycle of opera- 

 tion, let the air enter the heater either from outside or from the 

 spray chamber at temperature t^, and let it contain d^ pounds of 



