36 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



suction or low-pressure gage is controlled to a great extent by the 

 expansion valve in the liquid line; consequently opening it will in- 

 crease the low pressure, as the more the valve is opened the more 

 gas will be pumped, providing the machine is run at a constant speed. 

 If the speed of the compressor is increased, the amount of work will 

 be proportionately increased provided the valves are large enough 

 to handle the additional amount of gas, and vice versa. If the ex- 

 pansion valve is opened too wide for the speed at which the machine 

 is running, the suction pipe will frost back to the machine and even 

 the top of the compressor may become covered with frost. For 

 satisfactory operation in a refrigerating plant a slight frost should 

 cover the suction pipe close to the machine. 



If the temperature of the brine in the cooling room is high, a cor- 

 responding high back pressure should be carried, and, on the other 

 hand, a low brine temperature requires a low back pressure. Should 

 a very low brine temperature be required, in addition to a low back 

 pressure, the machine should be run at a faster speed, depending 

 upon the relative capacity of the machine and the work to be done. It 

 is necessary in the maintenance of low temperatures to keep the sys- 

 tem free from air and deleterious gases, and plenty of cooling water 

 should be supplied to the condenser while the plant is in operation. 



For small plants the receiver should be of suitable size to hold all 

 the refrigerating medium in a liquid form. It is usually located 

 just below the condenser, or as near to it as local conditions will 

 allow, so that the liquefied gas may return to the receiver by gravity. 



In order that the amount of refrigerant contained in the system 

 may be seen at a glance, the receiver should be provided with a glass 

 gage similar to that placed on steam boilers to show the amount 

 of water. This is especially true in small plants, where the attend- 

 ant may not have had the necessary experience to enable him to 

 judge of the amount of refrigerant in the system, and by observing 

 the line of the liquid in the glass gage each time before starting up 

 the compressor he will be able to note any loss of the refrigerant 

 which he might not be able to do by other methods. 



Some manufacturers, however, object to placing a gage of this 

 kind on the receiver, as they contend that there is a possibility of 

 loss of refrigerant by the breaking of the glass, and perhaps some 

 danger should anhydrous ammonia be the refrigerant used. There 

 is little danger, however, as the pressure and temperature is nearly 

 constant and automatic ball stop valves may be used that will shut 

 off the flow should the glass break. 



It should be distinctly understood, however, that the foregoing 

 refers to new plants and not to old ones. Under no circumstances 

 should air at high pressure be pumped directly into an old system 

 after repairing, or into one that has been standing idle for some time, 



