30 MASS. EXPERIMENT STATION BULLETIN 360 



admitted to the coils so that the return pipe to the compressor is frosted up to the 

 compressor. It is not, however, good practice to allow frost to form on the cyl- 

 inders of the machine. There are various ways in which the flow of refrigerant 

 to the coils is controlled. One method is by means of a float valve which floods 

 the coils to a fixed level. This arrangement is very desirable and is in general 

 use in ammonia systems. It allows the evaporating coils to operate at maximum 

 capacity. Another method is by means of an automatic expansion valve. This 

 type of valve is adjustable. A third method is by means of a hand-controlled 

 expansion valve. With this arrangement any desired amount of refrigerant can 

 be admitted to the coils, and the proper setting is obtained by watching the 

 formation of frost on the suction pipe at its entrance to the compressor. 



The capacity of the compressor is increased as the suction pressure is increased 

 because of the greater weight of a cylinderful of refrigerant at the higher pressure. 

 The aim, therefore, should be to carry as high suction pressure as possible to 

 assure maximum output of the compressor. Another advantage from high suction 

 pressure is the fact that it means high refrigerant temperature, which gives less 

 frosting of coils than lower temperatures with less dehumidification within the 

 room. The capacity of the evaporating units is, however, reduced as the pressure 

 in the evaporating coils is increased. Ordinarily the refrigerant is carried from 

 20° to 25° colder than the room temperature. 



. High Discharge Pressure 



The discharge pressure is dependent on the following factors: (a) size of con- 

 denser, (b) load carried on machine, (c) temperature and quantity of cooling 

 water, (d) presence of non-condensable gases in system, (e) cleanness of con- 

 denser, and (f) amount of refrigerant in system. The operator should be informed 

 on the discharge pressure which can be expected at different seasons of the year, 

 in order that he may know whether the condensing system is functioning properly. 

 If the pressure seems excessive the operator will first check on the amount of 

 water which is being circulated. If the water is circulating normally and if the 

 increase in pressure has been gradual, it may be that the condenser needs cleaning. 

 The frequency of cleaning depends upon the condition of the cooling water. If 

 the water carries a large amount of sediment, it will naturally foul the condenser 

 more rapidly than if clear water is used. While the condenser can usually be 

 cleaned without disconnecting any of the piping which carries refrigerant, it is 

 desirable that a service man be on hand when an operator first dismantles the 

 condenser. Later the operator should be able to do this job without such help. 



If high pressure is not caused by lack of water, high water temperature, or a 

 fouled condenser, it may be due to an excess of refrigerant or to non-condensable 

 gases in the system. In the first case, it is of course necessary to remove some of 

 the refrigerant; and in the second case the non-condensable gases are drawn off 

 by "purging." Either of these conditions decreases the eiifective cooling surface 

 within the condenser and should be corrected by a service man. Excessive 

 pressure on the discharge side of the system puts unnecessary wear and tear on 

 the equipment, and increases power consumption. 



Leaks of Refrigerant 



Ammonia leaks are easily detected by the distinctive odor of ammonia. Some 

 other refrigerants, however, have very little if any odor and leaks may not be 

 apparent. For these it is necessary to use certain specific tests. 



Such a test may have to be used to locate the point at which ammonia is escap- 

 ing, even though it is obvious that a leak exists. A wet piece of red litmus paper 



