When a plant has been charged it must be established that the charge is in fact complete. The disappearance of bubbles in a sight glass does not necessarily mean that the evaporator is correctly flooded. Continued adding of refrigerant when bubbles show in the sight glass can also result in the system being overcharged.
Shortage of refrigerant in evaporator
A situation could arise where the sight glass shows full of liquid yet a shortage of refrigerant is evident in the evaporator. This may be due to a number of factors.
Refrigerant liquid can flash off in long liquid line runs. The ideal location for a sight glass is just before the expansion valve, although it is common practice to install them close to the condensing unit. Ideally two sight glasses should be installed, one near the condensing unit and the other before the expansion valve. This will determine whether a solid column of liquid reaches the expansion valve.
Restricted refrigerant flow to the evaporator can be caused by the following:
1. A partial blockage may occur in the filter drier, thereby creating a pressure drop in the liquid line. If the sight glass is located after the filter drier, bubbles will be evident. When a sight glass is installed before the filter drier, a pressure drop will exist just the same but bubbles will not be visible. In each case a temperature difference will exist either side of the restriction.
2. An expansion valve filter or screen may be blocked by undesirable substances circulating around the system which have passed through the filter drier, such as moisture or particles of the desiccant in the filter drier. Carbon may build up in the fine mesh of the valve screen.
3 The thermostatic expansion valve may be incorrectly adjusted or defective through partial loss of the phial charge, and therefore will not open sufficiently. A total loss of the valve phial charge will result in a complete blockage and a starved evaporator. Some expansion valves have replaceable cartridges and screens. The cartridge is stamped with an orifice size; too small an orifice can result in a starved evaporator.
4 Some plants employ evaporator defrost systems which incorporate a magnetic valve or solenoid valve. This valve, installed in the liquid line, will stop refrigerant flow to the evaporator when a defrost period is initiated by a timing device. This enables the evaporator to be evacuated of refrigerant so that, when the defrost heaters have completely cleared the evaporator of frost, an excessive build-up of pressure will be prevented during the period of continued application of heat.
Each of these conditions will result in lower than normal operating pressures.
When there is a shortage of refrigerant, pressures will be lower than normal. However, the reduction in pressure may be so slight as not to be readily detected, other than by a loss of evaporating capacity and the longer running time of the unit. If the shortage is considerable, both suction and discharge pressures will be very low; if the system temperature control consists of a thermostat only, the unit will run continuously with poor refrigerating effect. When a low pressure switch is in circuit, the compressor will short cycle (cut in and out quickly) on this control.
Likewise, any restriction in the refrigerant supply will produce the same symptoms on the low side of the system. It is possible for the compressor to operate continuously on a deep vacuum if a low pressure switch is not used. A complete blockage, preventing refrigerant from entering the evaporator, will cause a low pressure switch to stop the compressor; it will not restart because there will be insufficient pressure rise from the evaporator to actuate the
High suction and low discharge pressures
This condition is usually due to a fault within the compressor, such as broken valve reeds or incorrect seating of the valve reeds.
If the suction reeds are at fault, some of the discharged vapour will be forced back into the suction side of the compressor as the piston reaches the top of its stroke.
Faulty discharge reeds will contribute to longer running time, poor refrigeration effect and lower than normal operating head pressure. During an off cycle the discharged vapour will leak back into the cylinder(s), causing a pressure rise which will actuate a low pressure switch and quickly restart the compressor if this is the temperature control.
Figure 17 shows the passage of refrigerant through compressor valves with serviceable and defective valve reeds.
Compressor valves can become distorted and fractured if liquid refrigerant is allowed to enter the cylinder; this is known as liquid slugging. For this reason service cylinders should never be inverted during the low side charging of a system.
Valve seats and reeds can become pitted if metal filings or small particles of grit are allowed to enter pipework during installation or repairs. They may circulate with the refrigerant and become trapped between the reeds and valve seats when compression takes place.
Excessive wear on pistons and cylinder walls will also contribute to a loss of compression. This is most likely to be experienced on a plant which has been in service for a long period, but could be related to lack of proper lubrication of the compressor.
A shortage of refrigerant can be the cause of oil starvation in the compressor and low pressure in the evaporator. The reduced amount of liquid refrigerant flowing through the evaporator is insufficient to move the oil and return it to the compressor, and it remains entrained in the evaporator.