As previously stated, there may be more than one fault in a system and some may persist until such time as a total breakdown or component failure occurs, to the detriment of the product being stored.
For example, after a leak has been discovered and repaired, the system is ready to be charged with refrigerant: reference has already been made to a check of operating pressures and a full sight glass as indications that the system is fully charged. However, this does not necessarily mean that the evaporator is correctly flooded and is operating to its full capacity. To make certain of this, the superheat setting of the expansion valve must be checked. An incorrectly adjusted valve can lead to starvation of liquid refrigerant to the evaporator, or to flooding of the evaporator with the possibility of liquid refrigerant entering the compressor to cause damage.
There is also the possibility of a restricted refrigerant flow, which again will result in starvation of liquid to the evaporator.
Other faults, less evident when an engineer is in attendance, can come to light after the repair: refrigerant leakage from a shaft seal may be due to another component failure; an inefficient compressor can lead to loss of duty and extended running time, with unnecessary expense to the customer.
Checking a compressor operation is simple, but will entail adjustment of some controls (see page 51).
The various faults and symptoms are dealt with in this chapter.
Procedure
It is recommended that a quick acting indicating thermometer be used. Two thermometer probes must be firmly attached to pipework free of frost or ice to ensure good thermal conductivity, one at the evaporator inlet and the other as close as possible to the thermal element or bulb of the expansion valve (see Figure 18). The temperature of the refrigerant passing through the pipework at these two points can now be quickly and accurately taken.
The temperature difference between the two points is the amount of super- heating taking place within the evaporator. A very wide differential means that the coil is starved of liquid and subject to excessive superheating, and that the evaporator is not fully active. Very little or no temperature difference means very little superheating; the evaporator is fully active over its entire surface, but some frosting back could occur at certain times during the system operation.
It is important that the expansion valve is adjusted to give the correct amount of superheating during commissioning, when a replacement has been made and, as good insurance, when a system has been charged.