With any refrigeration system, there are a slew of miscellaneous problems that may occur, such as incorrect amperage, noncondensables in the system, oil problems, restrictions, and airflow issues. This article describes how to recognize and respond to some of the more common issues.
Problems in the refrigeration system may affect the amperage drawn by the compressor. For example, when noncondensables are in the system, the compressor works harder as a result, and a high amperage reading will be detected. Loose wire connections and overheating can also cause high amperage draw. On the other hand, an inefficient compressor draws lower amperage. The building engineer should consult an experienced electrical contractor to make electrical repairs for any malfunctions related to amperage draw.
Noncondensables in the System
Many system problems can result from either an overcharge of refrigerant or the presence of noncondensables in the system. One quick way to determine if the problem is a result of noncondensabIes in the system is to shut the unit off and allow sufficient time for the system to reach ambient temperature. Next, check the discharge pressure, which should be within 5 psig of the pressure at that ambient temperature. If the pressure is higher, it indicates the presence of noncondensables. The system charge then needs to be recovered, and the system must be evacuated to below 500 microns. In addition, the liquid line filter drier must be replaced. Remember that noncondensables in a system will increase the compression ratio, leading to a shortened life for the compressor and higher energy costs.
Most manufacturers state that as long as you can see oil in the sight glass, the oil level is acceptable. Some, however, specify that oil levels should register from one-third to two-thirds of the sight glass. The one thing all manufacturers agree on is that the oil level should not be above the sight glass, which may make it appear empty. Always refer to the manufacturer’s specific oil level recommendations, and use the specific type of oil that is required.
Though restrictions can be found anyplace in the system, areas where they are more likely to occur include metering devices or filter driers where openings have been reduced. At times, a suction line or a liquid line can become kinked or crushed. The restriction may be so severe that frost occurs; but at other times, the restriction may be slight and not so easy to notice.
Using gauges and temperature probes will provide the information necessary to determine if there is a restriction and how severe it is. Most of the time, the restriction will be found on the liquid line. Some signs to look for are high superheat, low suction pressure, and low compressor amperage draw.
Over the past few years, variable frequency drives and multiple speed motors have become the norm. With the advent of variable frequency drives, adjustable speed drives, variable air volume (VAV) boxes, and inlet guide vanes, the airflow across a DX coil or chilled water coil is constantly changing. When this happens, the refrigeration pressures change. A few items that affect the air side of a system are:
- Dirty filter.
- Dirty evaporator coil.
- Incorrect fan speed.
- Loose or broken belt, or a sheave that needs adjustment
- Closed supply diffuser.
- Closed return diffuser.
- Incorrectly installed ductwork in new construction.
Leaking Liquid Line Solenoid Valve
If the liquid line solenoid valve leaks badly, the compressor may be unable to reduce the suction pressure sufficiently to terminate the pump down cycle. This will cause the compressor to run continuously on its minimum stage of loading. A leaking valve is evidenced by a temperature drop across the valve body and the appearance of flash gas in the liquid line sight glass.
Broken Compressor Valves
Broken compressor valves are generally indicated by a rapid rise in suction pressure immediately after the machine is stopped. If the suction pressure rises faster than 5 psig per minute, the compressor valves are not holding. However, before any action is taken, make certain that the pressure rise is not caused by a pressure leak on the low side of the system, such as a leaking liquid line solenoid valve.
If the thermostatic expansion valve is adjusted to produce too little superheat, liquid refrigerant may pass from the evaporator into the suction line. This is evidenced by an abnormally cold suction line and possible liquid flood back to the compressor. On the other hand, too much superheat results in the feeding of too little refrigerant to the evaporator. This will cause a shortage of evaporator capacity, which is particularly noticeable at system full load conditions. Adjust the superheat carefully, taking suction pressure and temperature readings as accurately as possible. Normally, 12 to 15 degrees F of superheat are satisfactory for most air-conditioning applications.
Inoperative Expansion Valve
If an expansion valve is protected from dirt and other foreign matter by a liquid line filter-drier, the most likely cause for malfunction is loss of charge from the valve power element. When this occurs, the valve remains closed or nearly closed. An inoperative expansion valve is indicated by low evaporator capacity, low suction pressure, high superheat, and frosting at the evaporator inlet. To test for an inoperative power element:
- Stop the compressor.
- Remove the remote bulb from the suction line.
- Place bulb in a container of ice water.
- Start the compressor.
- Remove the bulb from the ice water and warm in hand. At the same time, check the temperature of the suction line at the evaporator. If a rapid reduction in temperature is noted, the element is working properly.
Do not allow liquid to flood through the coil for any longer than is necessary to complete the test. Flood back to the compressor should always be avoided.
Improperly Sized Expansion Valve
An oversized expansion valve will generally overfeed the evaporator at part load, but at full load, it will perform satisfactorily. This is because at part load the valve attempts to control at its superheat setting, but its oversize valve allows liquid to pass too rapidly. This overfeeds the evaporator. In response, the valve closes until normal superheat is reestablished. At this point, the valve again opens to pass another slug of refrigerant. This hunting condition causes the suction pressure to vary and may allow liquid to pass into the suction line.
An undersized valve, on the other hand, controls well at part load but cannot feed enough refrigerant to meet the full load demand. Under conditions of rising system demand, the evaporator capacity falls behind the load, and the superheat rises.
Troubleshooting your refrigeration system isn’t something that can be learned from a thousand-word article, but hopefully you are a little more prepared to find the inefficiencies before they become major problems.
This article is adapted from BOMI International’s course Refrigeration Systems and Accessories. More information regarding this course is available by calling 1-800-235-2664.