The compressor is the most expensive, and functionally the most important component in a refrigeration system. It is important for HVAC technicians to be able to accurately diagnose compressor problems to avoid unnecessary compressor replacements.
The compressor is the heart of the refrigeration system. Through the process of compression, it converts low-temperature, low-pressure refrigerant vapor from the evaporator into a high-temperature, high-pressure vapor.
The high-pressure vapor gives up heat in the condenser where it becomes a high-pressure liquid that is fed to the metering device. In the metering device, the pressure and temperature are reduced.
The low-pressure, low-temperature liquid flows through the evaporator coil where is absorbs heat and boils to a vapor. That vapor is returned to the compressor and the cycle repeats.
The compressor maintains a pressure difference between the high-pressure and low-pressure sides of the system, enabling refrigerant to flow through the system.
Compressors are divided into three groups based on the way they are physically connected to their motors or engines. The three types are open-drive, hermetic, and semi-hermetic.
Open-drive compressors use an external motor or engine to turn the compressor. The motor can be directly coupled or can use belts and pulleys to drive the compressor.
Welded hermetic compressors, often called tin cans, have the compressor and drive motor encased in a welded steel shell. Due to their sealed nature, welded hermetic compressors must be replaced if they fail.
Semi-hermetic compressors have the compressor and drive motor encased in a hermetically sealed bolted shell. If a failure occurs, limited repairs can be made by removing the cylinder heads or end plates.
Various factors, including system design and type of refrigerant call for different types of compressors. Common types of compressors are reciprocating, rotary, scroll, screw, and centrifugal.
Reciprocating compressors use pistons moving up and down in a cylinder to compress refrigerant vapor. Other major components include connecting rods, crankshaft, cylinder heads, and suction and discharge valves.
As the piston moves down during the intake stroke, low cylinder pressure opens the suction valve, allowing the cylinder to fill with refrigerant vapor.
At the bottom of the stroke, the cylinder is full of vapor and the suction and discharge valves are closed.
The upward or compression stroke of the piston compresses the gas to the point that it forces open the discharge valve, releasing the compressed vapor into the discharge line.
The piston contains a compression ring to keep the compressed gas sealed within the cylinder and an oil ring to prevent crankcase oil from entering the cylinder.
Reed and ring-type valves are mounted on valve plates positioned on the top of the cylinder.
Each valve is held closed by spring tension and opened when pressure on the valve exceeds the spring tension.
Open compressors use a leak-proof seal where the crankshaft exits the compressor shell.
A splash lubrication system lubricates a reciprocating compressor by splashing crankcase oil on the cylinder walls and bearing surfaces.
A pressure lubrication system uses an oil pump to force-feed oil to all bearing surfaces.
Rotary compressors are welded hermetic compressors. They use either fixed or rotary vanes to provide a refrigerant seal within the cylinder.
The spring-loaded fixed vane in a stationary vane compressor moves up and down, isolating the suction and discharge sides of the compressor as the out-of-round rotor turns in the cylinder.
An opposing pair of spring-loaded vanes on the out-of round rotor slide in and out to conform to the cylinder and isolate the low and high sides of the rotary vane compressor.
Scroll compressors are welded hermetic compressors that use one scroll orbiting within a fixed upper scroll to squeeze refrigerant into decreasingly smaller pockets for compression.
The compliant scroll compressor allows the orbiting scroll to shift position to accommodate liquid refrigerant if it enters the compressor.
Screw compressors are used in large commercial applications. They use rotating screws to compress the refrigerant. As the rotating screws turn, they mesh with each other and compress the gas between them.
Centrifugal compressors are used in very large commercial applications and compress refrigerant using impellers spinning at very high speed.
The load on an HVAC system can vary. When the load is low, the system and the compressor have excess capacity that is not needed.
Capacity control can change the pumping capacity of the compressor in order to match changes in the system load. There are several ways that capacity control can be achieved.
Cycling the compressor on and off as demand increases and decreases is the simplest form of capacity control. A room thermostat cycling an air conditioner on and off is an example of this method.
In multi-compressor systems, cycling some of the compressors on and off is used to control capacity. Managing the oil level in multiple compressor systems is critical.
Cylinder unloading is commonly used for capacity control on open-drive and semi-hermetic reciprocating compressors.
Suction bypass unloading uses a temperature- or pressure-actuated valve to re-circulate suction gas back to the discharge side of the same cylinder.
Suction cutoff unloading is activated in a manner similar to bypass unloading, except the suction gas is prevented from entering the unloaded cylinder rather than being re-circulated through it.
To reduce capacity, hydraulic unloaders hold open the suction valve to prevent compression from occurring in the cylinder.
The hot gas bypass method of capacity control uses a solenoid valve to route some of the discharge gas back into the suction line.
Scroll compressors use a form of bypass for capacity control by changing the volume inside the compressor in which the refrigerant is compressed.
Screw compressors use the intake slide valve method for capacity control by changing the point where the suction entraps the vapor and starts to compress it.
Centrifugal compressors control capacity by opening and closing the inlet guide vanes to the centrifugal compressor.
Positive displacement compressors pump a constant volume of gas at a constant motor speed. The higher the compressor displacement, the greater the capacity.
Changing the speed of the motor that drives the compressor is one way to change system capacity to better match the load.
Capacity modulation on scroll compressors can be achieved by moving the two plates apart (unloaded) and bringing them together (loaded) at pre-determined intervals.
The electric motors used to drive compressors operate in the same way as other motors used in refrigeration systems.
Open-drive compressors must be located in an area where ventilation can cool the motor. The motors in welded hermetic and semi-hermetic compressors are cooled by suction gas.
Open-drive compressors must be properly aligned and coupled to the drive motor to prevent vibration and bearing wear.
The electricity used to run a compressor must be the correct voltage and frequency, and all wiring must comply with national and local codes.
High temperature and excessive current can damage or destroy a compressor motor.
Electrical overload devices designed to prevent operation under these conditions are classified as pilot duty devices or line duty devices. Both types of devices can be used to sense motor current and temperature.
The pilot duty device responds by opening the contactor control circuit. Line duty devices are in series with the load and directly open the line voltage circuit.
Overload devices may be reset manually or may reset automatically.
An external line break overload, often called a klixon, is usually a line duty device and is located near the compressor terminals.
Pilot duty external overloads are not as widely used as line duty versions.
Internal line break overloads are internally wired in series with the common terminal of single-phase motors and respond to temperature and current.
Motor thermostat overloads are usually pilot duty devices. They can be internally or externally mounted. They open if motor windings overheat.
Thermistors imbedded in the motor windings sense motor winding temperature and provide input to an electronic overload module.
Electronic overload modules require a power supply, along with terminals for connection to the compressor circuit.
Electronic overload modules contain normally closed relay contacts that are wired in series in the control circuit.
Internal and external overload protectors are available for three-phase motors.
Electronic controls are available that constantly monitor compressor motor current and will stop compressor operation if preset current levels are exceeded.
Other compressor protection devices protect the compressor from high or low pressures, loss of airflow, and other abnormal conditions.
Excessively high or low refrigerant pressures can damage the compressor. Pressure switches wired in series with the control circuit, stop compressor operation if harmful pressures occur.
The reduction or elimination of airflow across the evaporator or condenser coil can adversely affect system operating pressures.
Oil pressure switches or pressure-sensing transducers wired in series with the control circuit stop compressor operation if oil pressure in the compressor is inadequate.
A lockout relay prevents out-of-sequence compressor operation if a protective device opens in the control circuit. The relay must be reset manually.
A compressor anti-short cycle timer is used to allow time for system pressures to equalize before attempting to start the compressor again.
An electronic head pressure control maintains head pressure at low outdoor temperatures by slowing down the speed of the condenser fan motor.
Compressor motors often draw high current and develop a large voltage drop during startup. Reduced–voltage starting methods control starting current and limit the voltage drop to an acceptable level.
Common methods of reduced-voltage starting include primary resistor or reactor, autotransformer, wye-delta, part winding, and solid-state reduced-voltage starters. Read paragraph 8.0.0 in its entirety for a better understanding of reduced-voltage starting methods.
Many mechanical refrigeration system problems, if not corrected, will eventually result in the failure of the compressor.
Slugging occurs when a compressor tries to compress liquid refrigerant or oil instead of refrigerant vapor.
When a compressor attempts to compress liquid refrigerant, pressures in excess of 1,000 psi can be reached that will blow out head gaskets and break valves.
Flooding is the continuous return of liquid refrigerant through the suction line to the operating compressor caused by overcharging or other system problems.
Flooding dilutes the oil and causes foaming. This reduces the lubricating ability of the oil, resulting in failed bearings. Read paragraph 9.2.0 for a better understanding of the causes of flooding.
Flooded starts occur when the oil absorbs refrigerant during the compressor off-cycle. When the compressor starts, the diluted oil cannot adequately lubricate the bearings.
Flooded starts can be minimized by warming the compressor oil with a crankcase heater. This boils off any liquid refrigerant in the oil.
Refrigeration systems are designed to contain only oil and refrigerant. Anything else in the system such as dirt, air, or water is considered a contaminant.
Contaminants can enter a system during installation or as the result of a compressor motor electrical burnout.
Air is considered a system contaminant because it is non-condensable and it can contain damaging moisture.
Moisture in a system can combine with other elements to form damaging acids. It can also freeze and plug the orifice in the metering device.
Acid can form in the system as a result of moisture combining with other elements or when a compressor motor burns out.
Acid can be detected in a system with an acid/moisture test kit or by testing the compressor oil for acid.
Dirt usually enters the system during installation due to poor installation practices such as brazing copper tubes without a nitrogen purge.
System dirt can be eliminated by good installation practices, installing a filter-drier, and by evacuating the system to remove air and moisture.
Compressor motor electrical failures can be linked to the failure of other electrical components or to problems with the compressor’s power supply.
The equipment nameplate contains valuable information about the power supply voltage, as well as the voltage and current values of all electrical motors in the equipment.
Motor damage can result if the compressor is operated outside the voltage range specified on the nameplate.
For single- and dual-voltage rated motors, the acceptable voltage range is ±10% of the stated voltage. For example, a 230V motor can safely operate between 207V and 253V (±10% of 230V).
The voltage imbalance between any two legs of a three-phase power supply must not exceed 2%.
Current imbalance between any two legs of a three-phase power supply must not exceed 10%. Voltage imbalance will cause current imbalance, but current imbalance may also occur without a voltage imbalance.
High head pressure and high temperature within the compressor can cause oil and refrigerant to break down or form acids if moisture is present.
Before condemning the compressor, system checkout procedures must be performed to make sure the compressor is bad.
During a preliminary inspection, your senses of sight, sound, touch, and smell can help identify problems.
Analyzing system operating conditions helps determine if the problem lies with the compressor or somewhere else in the system.
Measurements and readings obtained during an operational check should be recorded and compared to any previously obtained readings.
After actual conditions in the system are know, they should be analyzed by comparing them to system pressures and temperatures that are known to be normal.
It is a good idea to record the operating temperatures and pressures of a system when it is operating normally. Those readings can later provide a benchmark for troubleshooting.
Final electrical and mechanical checks should be made on the compressor to verify if the compressor has, in fact, failed.
Electrical checks include the power supply, all electrical connections, compressor-related electrical components, and the internal overload. Check the motor for opens, shorts or grounds.
Mechanical reasons for failure include loss of charge, loss of lubrication, broken valves or rings, and physical damage.
Compressors that won’t start and are thought to be seized should have all compressor-related electrical components checked as well as the supply voltage.
The procedures for replacing a failed compressor are different, depending on the failure mode.
Before attempting to replace a compressor due to a mechanical or electrical failure, turn off power and lock and tag the equipment so no one can start it.
Recover all refrigerant from the system before disconnecting any wiring from the compressor.
Cut the compressor from the system using tubing cutters. Do not use a torch to remove the compressor.
Electrical failures involving motor windings are often called burnouts. Burnouts can be mild or severe. The procedures for changing the compressor are different, based on the severity of the burnout.
The severity of the burnout can be determined by using an acid/moisture test kit or by testing the oil with an acid test kit. The amount of acid determines the severity of the burnout.
The replacement procedure after a mild burnout is the same as if the compressor experienced a mechanical failure, including the installation of an oversized filter-drier and a triple evacuation of the system before charging.
A severe burnout requires several additional steps including purging the system piping with dry nitrogen in the opposite direction to flow.
Contaminated system components such as the metering device, reversing valve, and accumulator must be removed and cleaned or replaced.