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, 
