Refrigerator Troubleshooting Diagram

Archive for the ‘Refrigerator service diagnosis and repairs’ Category

Removing Refrigerator Compressor Valve Plate Assembly

without comments

On most compressors this will be a relatively simple task, but the procedure will differ with compressor design. The following is the procedure for a compressor with the valves in the head:

1. Start the plant, front seat the suction service valve and reduce the crankcase pressure to 3 psig or 0.2 bar if possible.
2. Stop the plant, isolate electrically and front seat the discharge service valve.
3. Slacken off the compressor head bolts slowly to release the high pressure from the discharge side of the head. Remove the bolts.
4. Gently raise the compressor head, with suction and discharge lines intact, sufficiently high to be able to withdraw the valve plate assembly. Care must be taken to avoid fracture of the pipework unions.
5. If the cylinder head and valve plate gaskets are damaged when the valve plate is removed, they must be replaced.
6. Distorted or broken reeds will obviously have to be replaced. Some manufacturers will recommend replacement of the entire valve plate assembly; replacement kits are available complete with gaskets.

Should the valve seats on the valve plate be eroded or pitted and a replacement is not readily available, they can be made serviceable by reseating or lapping. This is generally regarded as a workshop practice. The method is as follows:

1. Obtain some valve grinding paste or carborundum powder and some polishing compound. Spread a liberal amount of the paste on to a lapping block or a hard flat surface such as a polished steel plate or sheet of glass.
2. Remove the discharge reed retainers and reeds from the valve plate.
3. Place the valve plate on the pasted surface, exert a firm even pressure and move the valve plate in a figure-of-eight motion. Continue until the valve seats are returned to an original finish. When carborundum powder is used it should be mixed with refrigeration oil.
4. When reseating is complete, remove all traces of carborundum and paste from the valve plate with spirit or paraffin.
5. Repeat the operation with the polishing compound and clean as before.
6 Reassemble dry with new reeds if required. If the original reeds are used, they should be inverted so that the seat contact is made to the unused side of the reed.

The procedure for replacing the valve plate assembly is as follows. If gaskets are re-used they should be perfectly clean and dry.

1. Once the valve plate is located, replace the head bolts and screw them down finger tight.
2. Tighten the bolts diagonally across the head, care being taken not to over tighten and strip the threads.
3. Crack off the suction and discharge service valves from the front seat positions.
4. Leak test the compressor and pipework unions.
5. Purge the compressor through the gauge port unions.
6. Set the service valves to the operating positions and start the plant.
7. Operate the plant and observe pressures; these should now be normal.
8. Carry out the compressor pump test.
9. Reset the low pressure control.
10. Remove gauges, replace valve seal caps and wipe the compressor clean of oil.
11. Make a final leak test.
12. Clear the site.

Refrigerator Compressor Efficiency Test

without comments

This is also referred to as a compressor pump test. It is carried out when the functions of the compressor suction and discharge valves are suspect; this will be indicated by high suction and low discharge pressures. The test should be carried out with the plant running at an operating head pressure of at least 100 psig or 6.5 bar if possible in order to prove the efficiency of the valves.

The procedure is as follows:
1. Front seat the suction service valve and note the cut-out pressure of the low pressure control.-The control must be reset to its original cut-out pressure after the test.
2. Alter the range of the low pressure control so that the compressor is drawing a vacuum.
3. Reduce the low side pressure to at least 20 in Hg vacuum or 0.7 bar.
4. Stop the compressor and observe the pressure rise on the compound gauge for 2 minutes.

If the suction reeds are seating properly and cylinder/piston wear is not excessive, the pull-down to a vacuum should be rapid. The front seating of the suction service valve isolates pressure coming from the evaporator, and only the compressor crankcase is being evacuated of the refrigerant vapour. A compressor is deemed reasonably efficient if the 20 in vacuum is achieved. If it is not possible to draw this vacuum, then the suction reeds are defective.

When a 20 in vacuum is achieved, the pressure rise should be minimal; the entire vacuum should not be lost over the 2 minute observation period. When the vacuum is drawn and the plant switched off, if a rapid rise in pressure is observed on the compound gauge then refrigerant vapour is leaking into the cylinder(s) via the discharge valve(s). The valves must be inspected for distortion of reeds or faulty seating.

Moisture in Refrigerator

without comments

When a system pipework is opened to atmosphere during a replacement operation, it is possible that air will enter the system. Air contains moisture, and only a small amount of moisture in a system which has a capillary for the refrigerant control can result in that moisture freezing. This leads to a complete loss of refrigeration.

Filter driers are normally capable of dealing with small quantities of moisture. However, it is recommended that a drier be changed when leaks are detected, especially on the low side.

A complete loss of the refrigerant charge owing to leakage can result in a compressor operating on vacuum, drawing in air. In this case the drier could become saturated and moisture will circulate through it. When this occurs, freezing can take place at the expansion valve; this is tantamount to a complete blockage in the liquid line.

When the symptoms of plant operation indicate a complete blockage, and no temperature difference is obvious at the filter drier, it is natural to suspect that the expansion valve or perhaps a solenoid valve is at fault. The simple expedient of applying a cloth dipped in hot water will determine the presence of moisture. Warming up the expansion valve will melt the ice in the valve and the flow of refrigerant will resume, but only until such time as the temperature at the expansion valve is low enough to form ice and restrict the liquid flow once more. A blowtorch should never be used for this purpose, for obvious reasons.

In most cases, fitting a new filter drier will overcome the problem. However, it is stressed that this drier should be removed after a suitable running period and another new drier fitted. Should the condition persist after a drier has been replaced, the system must be discharged of refrigerant, evacuated and then recharged.

Written by sam

November 8th, 2009 at 6:47 pm

Replacing Refrigerator Filter Drier

without comments

Assuming the restriction in the drier is only partial, and that a low pressure cut-out control is in circuit, the following procedure should be adopted:

1. Note the pressure at which the compressor cuts out. The control must be reset to the original setting after the filter drier is replaced.
2. Alter the range of the low pressure control and pump down the system as previously described. Ensure that the suction service valve is front seated when this is completed.
3. Isolate the plant electrically.
4. Remove the defective filter drier and replace it. Take care that the new one is installed correctly; note the direction of flow.
5. Open the liquid shut-off valve at the receiver.
6. Leak test the joints which have been disturbed.
7. Loosen the connection on the suction line at the inlet to the compressor and allow a small amount of vapour to escape, tighten connection.

8. Set the suction service valve to the operating position and operate the plant.
9. Reset the low pressure control to its original cut-out setting.
10. Remove gauges, fit service valve caps and carry out a final leak test.
11. Clear the site of debris etc.

Should the filter drier be completely blocked, which is improbable, then the replacement procedure will be the same as that given earlier for the replacement of a thermostatic expansion valve with a complete loss of charge.

Refrigerator Pressure Drop

without comments

If an internal equalizing expansion valve is substituted with an external equalizing valve, an additional connection must be provided for the external equalizing capillary.

External equalizing capillary

External equalizing capillary

The capillary connection must be installed in the suction line as illustrated in Figure 24. The connection is a ı88 in flare half union, brazed into the suction line 150 to 200 mm downstream from the expansion valve bulb location.

Written by sam

November 8th, 2009 at 8:43 am

Refrigerator Thermal Charge Complete Loss

without comments

When this condition exists there is effectively a total blockage in the high side of the system (liquid line). The pressure in the low side of the system may be negative, or that determined by the cut-out setting of the low pressure control. The system is therefore ‘pumped down’ from the point of the blockage, in this case the expansion valve, and there is no need for the pumping down procedure to be carried out. There is, however, a liquid line full of liquid; unless an isolating valve has been installed, this liquid must be removed. If there is a liquid charging valve at the receiver with an appropriate connector, it may be possible to transfer the liquid into an empty refrigerant cylinder. If not, it must be reclaimed. The following procedure should be adopted:

1. Isolate the plant electrically and close the liquid shut-off valve at the receiver.
2.  Front seat the suction service valve.

3.  Release the thermal bulb from the clamp at the outlet of the evaporator.
4.  Ensure that the working area is well ventilated. If any food products are in the immediate vicinity they should be removed or covered as a protection against contamination.
5. Check the orifice size of the new valve, and check that it is suitable for use with the system refrigerant.
6. Remove the defective valve and install the replacement. Remove the old bulb clamp and fit the thermal bulb of the replacement valve to the evaporator outlet in the original location, using the new clamp provided with the new valve.
7. Open the liquid shut-off valve at the receiver.
8. Leak test the connections which have been disturbed.
9. Set the suction service valve to the operating position and start the plant.
10. Operate the plant until pressures have stabilized. Check the refrigerant charge via the sight glass and operating pressures.
11. Adjust the valve to give the recommended degree of superheating.
12. Remove gauges, cap service valves and carry out a final leak test.
13 Clear the site of debris, oil etc.

The system may be evacuated through the gauge port on the compressor. Crack off the suction service valve from the front seat position and allow a small amount of vapour to enter the compressor crankcase. Front seat the suction service valve again, slacken off the gauge port union and evacuate the compressor. After evacuating, set the suction service valve to the operating position and continue the above procedure at step 12.

Replacing Refrigerator Thermostatic Expansion Valve

with one comment

There are a number of reasons for replacing an expansion valve:
1. Mechanical failure of the valve: possible seizure of the valve stem carriage; worn valve seat and letting by; defective or wrong sized valve orifice cartridge.
2. Partial loss of the thermal charge: insufficient pressure exerted by the power head in response to temperature changes, resulting in an erratic valve operation.

3. Complete loss of the thermal charge from the power head: the spring pressure will close the valve and refrigerant flow to the evaporator will cease.
4. An excessive pressure drop across the evaporator is discovered: an external equalizing type has to be substituted.

Written by sam

November 8th, 2009 at 7:58 am

Refrigerator Distributor Refrigerant Control

without comments

Some commercial evaporators are designed with series-parallel and parallel tubing circuits, which means that they have more than one refrigerant circuit. Refrigerant from a single outlet expansion valve is directed to the circuits by means of a distributor, which in most cases is part of the evaporator assembly. These exist in many forms; two types are shown in Figure 22.

Distributor refrigerant control

Distributor refrigerant control

A multi-outlet expansion valve can also be used to perform a similar function, feeding two refrigerant circuits. Such a valve is shown in Figure 23.

Double outlet expansion valve

Double outlet expansion valve

 

Refrigerator Suction Line Frosting

without comments

When flooding of the evaporator takes place, frosting along the suction line will be evident. This is caused by the liquid refrigerant boiling off and reducing the temperature of the suction line. Frosting along the suction line can also occur when there is a pressure drop through the evaporator and the suction line temperature is reduced to below 0o C

With low temperature applications the returning suction vapour may be at a temperature well below 0o C resulting in suction line frosting. It is common practice to insulate suction lines of low temperature installations in order to overcome this problem.

Written by sam

November 8th, 2009 at 7:48 am

Refrigerator External Equalizing

without comments

A low evaporating pressure can be due to pressure drop through the evaporator caused by friction, the length of the evaporator tubing, and the number of return bends employed in the evaporator design. If this occurs, the saturation temperature of the refrigerant may be lower at the outlet of the evaporator than at the inlet.

When an evaporator has a pressure drop in excess of 5 psig or 0.3 bar it is essential to employ a thermostatic expansion valve with an external equalizer. This type of valve is designed to compensate for pressure drop. The equalizer is a small tube or capillary which is connected beneath the expansion valve bellows or diaphragm; the other end is installed in the suction line at the outlet of the evaporator. In operation the equalizing tube provides the same pressure as that in the suction line at the thermal bulb location whilst the compressor is running. This equalizing of pressure will permit accurate superheat adjustments.

The expansion valve equalizing connection should be located 150-200 mm beyond the thermal bulb of the valve on the compressor side, as shown in Figure 20.

To further explain the significance of pressure drop and the need for the use of an external equalizing expansion valve, Figure 21 shows the effect of pressure drop with an internal equalizing expansion valve and how external equalizing compensates for the pressure drop. The calculations, pressures and temperatures are for refrigerant R12.

Expansion valve with equalizer

Expansion valve with equalizer

Internal and external equalizing expansion valves

Internal and external equalizing expansion valves

 

You might also likeclose