October 2019 — A properly working refrigeration system has the correct water temperature, the correct amount of water flow (if it’s a water-cooled system), and sufficient airflow across the evaporator or condenser. Common problems involve improper suction pressure, head pressure, superheat, and subcooling. However, other problems can arise. Following are some miscellaneous issues that may occur in a refrigeration system.
Various problems can 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 result from either an overcharge of refrigerant or the presence of noncondensables. A quick way to determine if the problem is a result of noncondensables 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.
High oil level
A high oil level usually occurs when someone has added oil when it is not needed. While the system is running, some oil will leave the compressor with the refrigerant (an oil separator will help return the oil) and travel into the system. If refrigerant velocity is slow, or if improper or no oil traps have been used, the oil will not return to the compressor properly. When this happens, you might be inclined to add oil when you see that the oil level at the sight glass is low. Before adding oil, however, check for signs of leakage. If the oil is low, it may have leaked out. If it did not leak out, then it must still be in the system. It is also important to be aware that if liquid refrigerant has gotten into the sump of the compressor, it will raise the oil level, giving a false reading.
Listed below are the effects of too much oil in the system:
- The evaporator becomes oil-logged. Oil retards the heat-absorbing efficiency of the evaporator by coating its inside piping.
- The compressor tries to compress the liquid oil, leading to valve damage.
- In some cases the oil level is so high that as the crankshaft turns, it hits the oil, leading to increased compressor amperage and possible damage to the crankshaft and wrist pin/ connecting rod.
Low oil level
When oil leaks out along with the refrigerant, or is not cycled back at the required rate, the system shuts down as a result of low oil pressure. Normally, the velocity of the refrigerant pushes the oil back to the compressor. In cases where oil leaves the compressor and gets trapped in the system, the oil could build up to the point of sending back large slugs of oil to the compressor, causing damage.
Oil traps are used to aid the return of the proper amount of oil to the compressor. Oil traps are typically found on vertical risers, starting at the bottom of the riser and added every 20 feet or so. The oil trap is designed to collect oil. The oil slowly builds up in the trap, leaving less and less area in which the refrigerant can flow. As the refrigerant velocity increases (due to travelling through a smaller area), it slowly carries a little oil back to the compressor so as not to slug the compressor with too much oil (allow a large amount of oil all at once). In some cases, you might choose to install an oil separator to alleviate the problem.
If you discover that oil has leaked out, the leak needs to be fixed and the correct amount of oil added to the system. Because most hermetic compressors have no sight glass, the sound of metal grinding against metal is the sure sign that the oil level is low.
Though restrictions can be found any place 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 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 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
Most manufacturers recommend 400 cfm per ton of airflow across the evaporator coil for air conditioning systems and 450 cfm per ton for heat pump systems. If these amounts are not achieved, the system will not work properly and may cause damage to the compressor. An example of this is liquid refrigerant being sent back to the compressor because there was not enough airflow to boil it all off in the evaporator.
Airflow across the condenser coil is also very important because a reduction in airflow can increase head pressure, lower system capacity, and create a high compression ratio, all of which may lead to compressor failure.
Whether or not you are a refrigeration technician, being able to at least ask the right questions can help you troubleshoot a system to maintain proper building operations and good tenant relations.
This article is adapted from BOMI International’s Refrigeration Systems and Accessories course, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.