Chapter 19. -REFRIGERATION AND AIR CONDITIONING PLANTS 



168.4X 

 Figure 19- 11. -Water regulating valve (cross 

 section.) 



PRESSURE 

 INDICATED IN BUCK 



TEMPERATURE INDICATED 

 IN RED 



47.102 

 Figure 19-12.— Compound R-12 pressure gage. 



foot of air. Specific humidity is the weight of 

 water vapor (in grains) per pound of air. It 

 should be noted that the weight refers only to 

 the weight of the moisture which is present in 

 the vapor state; it does not include moisture 

 that may be present in the liquid state. 



Relative humidity is the ratio of the weight 

 of water vapor in a sample of air to the weight 

 of water vapor which that same sample of air 

 would hold if saturated at the existing tem- 

 perature. This ratio is usually stated as a 

 percentage. For example, when air is fully satu- 

 rated, its relative humidity is 100 percent. 

 When air contains no moisture at all, its relative 

 humidity is zero percent. When air is half 

 saturated— that is, holding half as much mois- 

 ture as it is capable of holding at the existing 

 temperature— its relative humidity is 50 per- 

 cent. 



Relative humidity, rather than absolute hu- 

 midity or specific humidity, is the factor that 

 affects comfort. This is true because it is the 

 relative humidity that affects evaporation. Mois- 

 ture tends to travel from regions of greater 

 wetness to regions of lesser wetness. If the air 

 above a liquid is saturated, the liquid and the 

 vapor are in equilibrium contact and no further 

 evaporation can take place. If the air above the 

 liquid is only partly saturated, some evapora- 

 tion can take place. 



A specific example may illustrate the differ- 

 ence between absolute or specific humidity and 

 relative humidity. If the specific humidity of 

 the air is 120 grains per pound and the tem- 

 perature of the air is 76' F, the relative humidity 

 is nearly 90 percent— that is, the air is nearly 

 saturated. With a relative humidity of 90 per- 

 cent, the body may perspire freely but the 

 perspiration does not evaporate rapidly; hence 

 there is a general feeling of discomfort. 



If the temperature of the air is 86° F, how- 

 ever, with the specific humidity remaining 

 constant at 120 grains per pound, the relative 

 humidity is only 64 percent. Although the 

 amount of moisture in the air is the same as 

 before, the relatively humidity is lower because 

 at 86 °F the air is capable of holding more water 

 vapor than it can hold at 76° F. The body can 

 therefore evaporate excess moisture and the 

 general feeling of comfort is much greater even 

 though the temperature is 10 degrees higher. 



TEMPERATURE. -When testing the effec- 

 tiveness of air conditioning equipment and when 

 checking the humidity of spaces, two different 



485 



