Refrigerator Troubleshooting Diagram

Archive for the ‘Compressors’ Category

Multicylinder compressors

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In the first century of development, compressors for higher capacity were made larger, having cylinder bores up to 375 mm, and running at speeds up to 400 rev/min. The resulting component parts were heavy and cumbersome. To take advantage of larger-scale production methods and provide interchangeability of parts, modern compressors tend to be multicylinder, with bores not larger than 175 mm and running at higher shaft speeds. Machines of four, six and eight cylinders are common. These are arranged in a multibank configuration with two, three or four connecting rods on the same throw of the crankshaft to give a short, rigid machine (see Figure 4.4).

This construction gives a large number of common parts – pistons, connecting rods, loose liners and valves – through a range of compressors, and such parts can be replaced if worn or damaged without removing the compressor body from its installation.

Compressors for small systems will be simpler, of two, three or four cylinders (see Figure 4.5).

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April 13th, 2011 at 5:20 am

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Winter Shutdown of Water-Cooled Compressors

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Where water-cooled refrigeration and air-conditioning systems are not operated in the winter, it is necessary to service them before discontinuing their operation. They should be pumped down, water drained from the condensers and piping, belts loosened, and motors oiled. The compressor should be thoroughly tested for leaks, and any found should be corrected.

To pump down and discontinue service, attach head and suction gages, and close the liquid shutoff valve at the receiver. Run the compressor until a suction pressure of from 2 to 5 psig is obtained. If, on stopping the compressor, the suction pressure rises, repeat until this pressure is steady. Close the suction and discharge shutoff valves to confine the refrigerant charge in the receiver. Open the main switch and be sure to tag it with a warning not to start the compressor without first opening the discharge and suction stop valves, because damage to the compressor may result. Tighten all packing nuts on valve stems. Cap all valve stems tightly.

The condenser is usually of sufficient capacity to hold the complete charge. However, if it is not, the overcharge must be pumped into an empty drum. Never fill a receiver completely with a liquid refrigerant (indicated by a rise in head-gage pressure above the safety cutout pressure) so that no gas space exists. If the compressor room temperature increases, there is danger of rupturing the receiver. Shut off the water supply to the compressor, and disconnect the outlet water line from the condenser. This will allow the water to drain from the condenser. After the water has all drained out, connect the air line or an automobile tire pump to the blowout plug, and proceed to force the remaining water (if any) out of the condenser. Condensers with removable heads can be effectively drained by shutting off the waste supply and removing the condenser heads, thus allowing all the water to drain.

On compressors of large capacity, it will be necessary to arrange for a connection to accommodate the airline, or pump. After all water has been forced from the condenser, it is recommended that all water lines be disconnected from the condenser and the water valves drained by removing the drain plug from the bottom of the water valves. For frequent and quick draining, it is suggested that a tee and drain valve be installed between the water-regulating valve and condenser inlet. The safest, surest way to prepare condensers for winter shutdown is to flush them out with an antifreeze solution. After the condenser has been treated with the antifreeze solution and drained, the condenser inlet and outlet water connections should be plugged in order to maintain the remaining antifreeze solution in the condenser. Remove the belts from the compressor motor, but not from the flywheel, so that they will not take a permanent set. It is also advisable to oil the motor to protect the bearings from corrosion.

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February 14th, 2011 at 12:06 am

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Compressor Startup

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When starting a water-cooled compressor after winter shutdown, the following procedure is suggested:

1. Install a pressure gauge on the condenser, and check for pressure. If there is no pressure in the condenser, determine how the charge was lost and repair the leak before opening any valves.
2. Permit the refrigerant to flow through the system. To do this, open the liquid-line valves.
3. Check the entire system for refrigerant leaks, using a halide leak detector.
4. If the system is gas tight, connect the water lines, open the hand valve in the water lines, and open the discharge and suction valves.
5. Install the belts and check for correct tension after a short period of operation.
6. Check the motor for oiling. If old oil is heavy or dirty, replace with new oil of the proper viscosity.

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February 14th, 2011 at 12:04 am

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Compressor Replacement

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The reinstallation of a compressor is roughly a reversal of the process of removing it. First, place the mounting pads and washers in position. Place the compressor carefully on the base in the same position it occupied before removal. Bolt the compressor in place. Replace the suction and discharge valves, using new gaskets. After the compressor and valves are bolted in place, the compressor must be evacuated to remove the air.

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February 14th, 2011 at 12:02 am

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Compressor Removal

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To remove a compressor from the unit, proceed as follows:

1. Attach a compound gage to the suction-line valve. Close the suction-line shutoff valve and run the compressor until 20 to 25 inches of vacuum is obtained. Then close the discharge shutoff valve. Before removing any fittings, crank the suctionline valve to bring the gage reading back to zero.

2. Before removing the service valves, loosen the pressure gauge and relieve the pressure in the head of the compressor. Remove the belt and cap screws holding the suction, and discharge valves to the compressor. If the compressor is to be taken away from the premises for repairs, place service valves over both the discharge and suction-line openings to prevent air and moisture from entering and oil from leaking out. Loosen the four nuts that hold the compressor to the base, and bend the tubing away from the compressor just enough to permit the assembly to be lifted out. Care should be taken not to loosen the mounting pads and washers on which the compressor rests.

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February 14th, 2011 at 12:02 am

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Stuck Compressor

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The reason for a compressor being stuck-up is usually the result of moisture in the system, or a lack of lubrication. When this occurs, the compressor should be thoroughly cleaned out. The compressor should be completely disassembled and the parts thoroughly cleaned and refitted. New oil and refrigerant should be put into the cleaned system. A tight compressor will result when a cylinder head, seal cover, or similar part has been removed and not replaced carefully, or when the screws have been tightened unevenly.

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February 14th, 2011 at 12:00 am

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Compressor Knocks

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A knock in the compressor may be caused by a loose connecting rod, eccentric strip and rod, eccentric disk, piston pin, crankshaft, or too much oil in the system. A compressor knock can be determined by placing the point of a screwdriver against the crankcase and your ear against the handle. A knock can then very easily be heard. It will not be possible to determine what causes the knock before the compressor is disassembled. Sometimes it may be possible to determine a looseness of the aforementioned parts without completely disassembling the compressor.

First, remove the cylinder head and valve plate to expose the head of the piston. Now, start the motor and press down on the top of the piston with the finger. Any looseness can be felt at each stroke of the piston. The loose part should be replaced. It is always good to check the compressor oil level first before analyzing and determining the compressor repairs. Oil knocks are usually caused by adding too much oil in servicing.

It should never be necessary to add oil to a system unless there has been a leakage of oil. A low charge is sometimes diagnosed as lack of oil. Always ensure that a low oil level is actually caused by lack of oil rather than a low charge before adding oil to the system.

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February 13th, 2011 at 11:59 pm

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Compressor Shaft-Seal Leaks

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To determine whether a compressor is leaking at the seal, proceed as follows. Close the shutoff valves by turning the stem clockwise as far as possible. To ensure adequate refrigerant pressure in the compressor crankcase and on the seal-bearing face, attach a refrigerant drum containing the correct refrigerant to the suction shutoff valve outlet port. In making this connection, there should be a gage in the line from the drum to the compressor to determine accurately the pressure in the compressor.

Test pressures for this purpose should be approximately 70 to 80 lb. If the compressor is located in a cool location, it may be necessary to raise the pressure in the drum by adding heat. In this process, care must be taken not to exceed 100 psi, since greater pressure may damage the bellows assembly of the seal. With this pressure on the crankcase of the compressor, test for leaks with a halide torch, moving the finder tube close around the seal nut, crankshaft, seal-plate gasket, and where the seal comes in contact with the seal plate. If this does not disclose a leak, turn the flywheel over slowly by hand, holding the finder tube close to the aforementioned parts.

After a leak has been detected, locate the exact place where it is leaking, if possible. If the leak is around the seal-plate gasket, replace the gasket. If the leak should be at the seal seat or nut, replace with a new seal plate, gasket, and seal assembly. To replace the seal, observe the following instructions:

1. Remove the compressor from the condensing unit in the usual manner.
2. Remove the flywheel, using a puller. Leave the flywheel nut on the crankshaft so that the wheel puller will not distort the threads.
3. Remove the seal guard, seal nut, and seal assembly.
4. Remove the seal plate and gasket.
5. When assembling the seal, put a small quantity of clean compressor oil on the seal face (both plate and seal).
6. Reverse these operations to reassemble. Make sure the seal plate is bolted in place and the seal guard is at the top.

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February 13th, 2011 at 11:57 pm

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Compressor Flow-Back Protection

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In most air-conditioning and refrigeration systems, it is possible to control the refrigerant feed so that liquid refrigerant does not return to the compressor during operation. Some systems have poor operation of the expansion valve. Also, some have sudden changes in loading and will periodically return liquid refrigerant to the compressor. This is not usually in sufficient quantities to do real damage. However, there are certain systems that, by their very design, will periodically flood the compressor with excessive amounts of liquid refrigerant. Principally, these prove to be systems in which there is a periodic reversal of the refrigerant cycle to do one of the following:

  • Provide hot gas for defrosting an evaporator.
  • Furnish heat for release purposes in ice-making apparatus.
  • Change from cooling to heating cycle, or vice versa, in heat pump systems.

These systems are almost identical in operation and in the effect on the compressor. Consider an air-source heat pump for a typical analysis of what happens. The flow back occurs when reversing the cycle in either direction. During the cooling cycle, the outdoor coils act as condensers and have warm liquid continually draining from them. The lines to which the thermal expansion-valve bulbs are strapped are now hot-gas lines and are hot. When the cycle reverses to heating, the outdoor coils change from condensers to evaporators. The liquid that has been draining from them during the condensing cycle is now dumped into the suction line. Of even greater significance, the thermal expansion valves also open wide as the result of their bulbs being on warm lines (lines that were previously hot-gas lines). Therefore, these valves will flow through until control is re-established at the bulbs. The two effects constitute a substantial liquid flow back through the suction line.

When there is no way to control the hazard of periodic flow back of substantial proportions through the suction line, it is necessary to take measures to protect the compressor against it; otherwise, compressor life will be materially shortened. The most satisfactory method appears to be a trap arrangement that catches the liquid flow back and may do one of the following:

  • Meter it slowly into the suction line when it is cleaned up with a liquid-suction heat inter changer.
  • Evaporate 100 percent of the liquid in the trap itself and automatically return the oil to the suction line.

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February 13th, 2011 at 11:55 pm

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Centrifugal Compressors

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Centrifugal compressors are essentially high-speed machines and best suited for steam-turbine drives. Because they are designed for the same high speed as the turbine, they may be directly connected. Where high-pressure steam is generated, the turbine can act as a reducing valve, and the low-pressure steam that leaves the turbine can be used for heating and other purposes. In smaller sizes, a great many applications are driven by electric motors and equipped with gear-type speed increasers. Centrifugal compressors are used with low-pressure refrigerants, and both evaporator and condenser usually work below atmospheric pressure.

Compression of the refrigerant is accomplished by means of centrifugal force. Because of this, centrifugal compressors are best suited for large refrigerant volumes and low-pressure differentials. They are well-suited to low-temperature refrigerating cycles, especially those using petroleum or halogenated hydrocarbons as a refrigerant.

Centrifugal-compressor installations are economically advantageous where steam-turbine drive is permitted, since the installation and labor required for such an installation is relatively small compared to that required for a comparable gas-engine–driven compressor plant. This is mainly because of the compactness and lightness of the unit compared to the amount of horsepower involved. In addition, a centrifugal compressor will occupy only a fraction of the space ordinarily required for refrigerating equipment. Refrigerating units of the centrifugal type are available in various sizes varying from 100 to 2500 tons capacity, and are manufactured for use with electric motors, steam turbines, or internal-combustion engines.

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February 13th, 2011 at 11:52 pm

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