56 



CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE 



Table 8. — Relative humidity {percent) of atmosphere by wet- and dry-bulb 



thermometers 





Relative humidity when depression (° F.) of wet-bulb thermometer « is 





Air temperature 









(°F.) 

























0.5 



1.0 



1.5 



2.0 



2.5 



3.0 



3.5 



4.0 



4.5 



5.0 



20 



92 



85 



77 



70 



62 



55 



48 



40 



33 



26 



25 



94 



87 



81 



74 



68 



62 



55 



49 



43 



37 



29 



94 



88 



83 



77 



72 



66 



60 



55 



50 



44 



30 



94 



89 



83 



78 



73 



67 



62 



56 



51 



46 



31 



94 



89 



84 



78 



73 



68 



63 



58 



52 



47 



32 



95 



90 



84 



79 



74 



69 



64 



59 



54 



49 



33 



95 



90 



85 



80 



75 



70 



65 



60 



56 



51 



34 



95 



90 



86 



81 



76 



71 



66 



62 



57 



52 



35 



95 



91 



86 



81 



77 



72 



67 



63 



58 



54 



36 



95 



91 



86 



82 



77 



73 



68 



64 



60 



55 



40 



96 



92 



87 



83 



79 



75 



71 



68 



64 



60 



45 



96 



93 



89 



86 



82 



78 



74 



71 



67 



64 



50 



96 



93 



90 



87 



83 



80 



77 



74 



71 



67 







i Difference between dry-bulb and wet-bulb readings. "Water should not be freezing on the wet bulb 

 while a reading is made. The humidities shown in this table apply only when the air is moving rapidly 

 past the thermometers, as with the sling or aspirating psychrometer. 



MAXIMUM USE OF EQUIPMENT 



If during the cooling period it is necessary to shut off some of 

 the compressors to avoid localized freezing at some points while 

 fruit temperatures are too high at others, the capacity of the equip- 

 ment is not being used to full advantage and some means for better 

 distribution of the refrigeration should be found. This usually may be 

 done by improving the air circulation or increasing its volume. While 

 ample circulation cannot compensate for inadequate refrigeration, 

 it does permit maximum use of the refrigeration available. 



Pending the time when the air circulation system can be over- 

 hauled to give maximum use of the compressors, the management 

 may take temporary steps to prevent freezing at local points dur- 

 ing the cooling period. These usually involve removal of fruit or 

 covering it where air is introduced and employing portable fans 

 to accelerate the movement of air away from the cold spots towards 

 points where fruit temperatures are high. 



Operating Efficiency 



KEEPING EQUIPMENT BALANCED 



To get the best results from a plant the various steps in the me- 

 chanical removal of heat must be balanced. That is, the heat picked 

 up in the room must be transferred in succession from the fruit to the 

 air, from the air to the cooling coils, from the coils to the compres- 

 sor, and from the compressor to the condenser, where it is discharged 

 to the cooling water. If in one or more of these steps the quantity 

 of heat that can be transferred is unduly restricted, the equipment 

 performing the other steps cannot be worked to greatest capacity. 

 The condenser is doing its part if the head pressure is not exces- 

 sive; and the cooling coils are not unduly limiting the capacity of 

 the plant if the suction pressure is well up. It is less simple to 

 know whether the air circulation system is in balance with the rest 

 of the equipment. 



