5 minute read
REFRIGERATION
Throughout the lifetime of a refrigeration system there is a need for regular maintenance and repairs. Unfortunately, many systems only receive the attention they need when they break down. There are a number of checks that need to be done regularly to prevent system failure. For this discussion, we will focus on a split system, where there is a condensing unit along with one or more evaporators.
Coil surface
Ensure that the condensing and evaporating coils are clean. A dirty evaporator will tend to reduce the suction pressure, while a dirty condenser will tend to increase the discharge pressure. Both conditions contribute to a loss in efficiency. Capacity and efficiency will also drop if the defrost cycle is not working correctly. If necessary, the defrost should be adjusted so all frost is removed from the coil. Additionally, the defrost cycle should allow sufficient drip and drain time to prevent refreezing of the water. Drain pans and drain lines should be cleaned if necessary. Where electric defrost heaters are used, confirm that all defrost and drain line heaters are operating during the defrost cycle.
Check the compressor oil level and appearance. Generally, the oil level should be at approximately one-half sight glass. If the oil is low, do not add any until it is determined why the level is low. If there are no signs of oil on the external surface of any of the l system components or piping, it is quite likely the oil is held up somewhere else within the system. In this case, the refrigerant charge, superheat and subcooling values should be checked.
In a system with multiple evaporators, it is possible one or more evaporators is starved of refrigerant. In o this case, there may be insufficient liquid available to feed the evaporators. By confirming the presence of liquid by visually inspecting the sight glass just upstream of the expansion valve or by calculating the subcooling, it is relatively easy to confirm whether the expansion valve has 100 per cent liquid available to it.
There are other reasons why the coil may be starved of refrigerant, such as an incorrect setting of the expansion valve, partial or complete loss of the charge in the element or head of the valve, high load, and so on.
If the oil is dirty, it should be changed. However, before changing the oil, evaluate the condition and colour of the oil. If the oil has a greyish colour to it then the compressor has experienced some wear. Although it is possible to change the oil, the wear to the compressor is irreversible and will likely continue to take place until the oil pressure drops to a point where it is not possible to keep the compressor online. When changing the oil, clean
Compressor oil level and condition
LOW-TEMP CONDENSING UNIT WITH CRANKCASE PRESSURE REGULATING VALVE.
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Phil Boudreau Phil is the Ontario sales manager for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at pboudreau@bitzer.ca.
the magnetic plug and the screen if there is one. A dirty screen will restrict oil flow to the inlet of the oil pump. A magnetic plug that has a lot of metallic debris on it is also an indication of internal compressor wear. For a comprehensive analysis of the oil, a sample can be sent to an oil testing facility. In this case, it is recommended to provide the oil testing laboratory with a copy of the data sheet listing the allowable limits. This information is available from your oil supplier.
COMPRESSOR WITH MECHANICAL CAPACITY CONTROL UNLOADING ON MEDIUM-TEMP RACK.
Oil pressure
On compressors that utilize an oil pump, the net oil pressure should be checked. The oil pressure can be affected by many factors such as a large amount of refrigerant in the oil, low oil, dirty screen, worn compressor, incorrect oil viscosity, among others. To check the net oil pressure, locate the two ports that provide access to the pump inlet and outlet pressures. The pump inlet is basically the crankcase pressure. Never use the suction line pressure to calculate the net oil pressure. Typically, there will be a suction screen after the suction service valve. This screen along with any variation between the motor barrel and crankcase pressures may result in an incorrect reading.
Piping
Inspect the entire piping network for any visible damage or traces of oil and correct if necessary. Insulation on suction lines should be checked. If necessary, replace the insulation to reduce the amount of heat ingress from the surroundings. This also applies to the subcooled liquid lines in two-stage compressor systems.
Contactor and pressure controls
Contactors are generally used to energize the compressor. The contactor is a mechanical device which can fail. It may not pull in evenly or has contacts that may become pitted and/or burned over time. A faulty contactor can also result in a single-phasing condition, which can destroy a motor. Contactors should be checked for voltage drop across the contacts. If unsure of its condition, replace the contactor.
Ensure pressure controls are set to a value that will not allow the compressor to operate outside of its intended operating envelope. Generally, the low-pressure control is set to the highest value that is practical and the high-pressure control is set to the lowest practical value to support the application.
It is also a good idea to visually inspect all the electrical wiring to ensure there are no loose connections or signs of overheating.
Motors and fan blades
Fan blades that are showing signs of cracking or warpage, or that are separating from the hub, must be replaced. A damaged fan blade will not be correctly balanced and will place additional stress on the motor bearings. Also ensure that all fan blades are sitting correctly in their venturi so they will provide proper air flow. A noisy motor is usually a sign of failing motor bearings. Even though the motor may be working, it must be replaced.
Operating conditions
It is important to evaluate the operating conditions of the system. The suction and discharge pressures along with the suction and discharge temperatures, should be recorded along with the voltage and amperage. Use the compressor manufacturer’s software to compare the actual conditions to the expected conditions. Any anomalies will need to be investigated. For example, as mentioned already, a higher-than-expected discharge temperature is likely the sign of a worn compressor for a given set of conditions. A higher amperage can be due to low voltage, voltage imbalance, defective contactor, high load on the system, and so on.
