Refrigerator Troubleshooting Diagram

Archive for the ‘Refrigerator service diagnosis and repairs’ Category

Refrigerator Service Diagnosis Simple Steps

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In order to diagnose any refrigeration fault quickly and accurately, a set procedure must be followed. The procedure described here takes the form of seven simple steps. If fully understood, these will prevent expensive call-backs and dissatisfied customers. No attempt to correct a condition should be made until the fault has been found, and therefore a thorough diagnosis is essential.

It is necessary to have the correct tools and instruments with which to carry out the procedure. These include a resistance thermometer, a leak detector, gauges, valve keys, a multitester or avometer and a compressor test cord, together with a complete set of engineering tools.

The seven steps are as follows:
1. Check the actual temperature of the product and compare with that recommended for the product.
2. Check the suction pressure, control switch settings and product classification to establish the temperature difference (TD) between the evaporator and the product.
3. Check the superheat setting of the expansion valve.
4. Check the condensing medium temperature.
5. Check the operating and idle head pressures of the compressor.
6. Check the refrigerant type and charge.
7. Check the drive pulley size on an open type system. If the compressor is hermetic or semi-hermetic, check the operating range; they may be for high, medium or low back pressure operation.

An incorrect size of pulley may be fitted to a drive motor. The refrigerating effect may be acceptable in cooler ambient temperatures, but when ambient temperatures rise the equipment will not have the capacity because of the compressor speed.

The same principle applies to the compressor operating range; older models were selected for specific operating conditions.

The following practical sequence is suggested for covering the first six steps:

(a) Ensure that the product has been stored for sufficient time to have become chilled or frozen (has not recently been deposited) before checking the product temperature. Take the actual temperature of the product and not the air circulating around it. Take care that the temperature has not been affected by the opening of the fixture door.

(b) Fit gauges and calculate the average suction pressure to establish the TD. Consult the classification to establish the product group, and note the TD for the type of evaporator employed in the system. If the TD is incorrect, an adjustment of the temperature control may be all that is necessary to rectify the fault.

(c) Ensure that the system has been operating for a sufficient period to be at an average suction pressure, and check the refrigerant charge. Check the operating head pressure and the idle head pressure. Check the superheat setting of the expansion valve. When checking the refrigerant charge, observe the condition of the evaporator (should be fully frosted) and the liquid sight glass. Do not adjust the expansion valve unless the refrigerant charge is complete and the system is operating at an average suction pressure.

Written by sam

November 10th, 2009 at 8:50 pm

Refrigerator Noise

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The sources of noise are numerous, and in some cases are entirely dependent upon the location and the type of building structure.

Common sources of noise are as follows:
1. Line rattles, that is pipework or control tubing vibrating against an evaporator or condenser shroud or framework.
2. Items of stored products (metal containers) or storage shelves vibrating.
3. Evaporator or condenser fan mountings loose.
4. Condensing unit base mountings or mounting frame loose; inadequate noise suppression from mounting fabric used (Tico pads, for example).
5. Perished rubber mounting grommets, which are especially prone to deterioration if contaminated with oil and refrigerant.
6. Gas pulsations against a rigid pipework design, creating a noise source during operation or when the compressor starts and stops.
7. Incorrect belt tension or belt and drive pulley alignment with open-type condensing units.

Written by sam

November 10th, 2009 at 8:46 pm

Refrigerator System Faults

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Sealed systems such as domestic refrigerators and freezers need to be treated differently, since there may not be provision for fitting gauges unless line tap valves are used.

If the temperature control is by thermostat only and there is no low pressure safety control, continuous running of the unit will result if the system:
1. Is short of refrigerant or has a restriction in the liquid line.
2. Is operating in a high ambient temperature or with restricted air flow over the condenser.
3. Has a faulty fixture door seal, imposing a high evaporator load.
4. Has excessive frost build-up on the evaporator.
5. Is operating with a defrost heater in circuit (has a defective timer), giving a high evaporator load.

Written by sam

November 10th, 2009 at 8:45 pm

Refrigerator Thermostats

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Two types of thermostat are commonly used to control the temperature of a refrigerated space or product by stopping and starting the compressor.

The most popular is the vapour pressure type. This consists of a small bulb or sensing element containing a very volatile liquid charge. The liquid has the ability to vaporize at low temperatures. When the bulb is subjected to a rise in temperature, the pressure generated by the vaporizing liquid will increase.

A capillary connects the bulb to a bellows or diaphragm. The vapour pressure causes the bellows to expand or the diaphragm to flex and operate the switch mechanism, closing the contacts to start the compressor motor. As the sensing element or bulb cools, the pressure in the bellows or diaphragm will decrease. The bellows will contract or the diaphragm will return to its normal position, thus opening the electrical contacts to stop the compressor.

Switch mechanisms have some form of toggle action or a permanent magnet device to ensure rapid and positive make or break of the switch contacts. This prevents arcing, which occurs when electrical current jumps across the minute gap between the contacts. Figure 32 shows toggle and permanent magnet switch arrangements.

The permanent magnet snap action type has a switch contact arm made of a magnetic material (iron or steel), and the magnet attracts the arm towards it. The pressure from the sensing element acts against the magnetic attraction to close the contacts according to the temperature of the element or bulb. As the arm moves closer to the magnet, the magnetic effect increases to cause the snap action.

When the sensing element cools and the switch bellows contract, some force is necessary to open the contacts. However, the magnetic force decreases as soon as the contacts break to allow rapid opening.

The compound bar type of thermostat, more generally called a bimetal element, comprises two dissimilar metals, usually Invar and brass or Invar and steel. Invar is an alloy which has a very low coefficient of expansion, whilst brass and steel have a relatively high coefficient of expansion. When an increase or decrease of temperature is sensed by the bimetal element the length of the Invar will cause the bimetal to warp. This warping action is utilized to open and close the switch contact (Figure 33).

refrigerator-bimetal-thermostat

Thermostats have range and differential adjustments which can be altered in the same manner as previously described for pressure controls. Electrical terminal arrangements will obviously differ by manufacturer, and reference should be made to the literature supplied with the controls.

The range on thermostats used on domestic appliances is altered by turning the cold control. Some of these are marked with a warmer or cooler setting. Others are numerical: the higher the number, the lower the temperature. Adjustment of the differential should not be attempted.

Some domestic appliance thermostats have a two-way switching feature in their design. When cooling is no longer required and the cut-out point of the switch is reached, the contacts to the compressor are opened and the contacts to a defrost heater are closed. This achieves automatic defrosting after every on cycle, to keep a minimal frost build-up on the evaporator at all times for more efficient operation.

Written by sam

November 10th, 2009 at 8:40 pm

Refrigerator Pressure Controls

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There are many different makes of pressure controls. Designs may vary but the principle of operation is basically the same: single pressure models respond to either low or high pressure: dual pressure versions can be activated by both
high and low pressures.

Pressure controls are installed to perform a variety of functions. Most current production controls are designed with three electrical terminals. These enable the control to be used to open an electrical circuit upon a rise in pressure or upon a fall in pressure, depending on the application. Unfortunately, identification of the electrical terminals is not standard, and it is important to know which of the three is the common terminal.

Single pressure controls have two adjusting screws: the range and the differential. Dual pressure versions have three screws: the range and differential for low pressure operation, and a range screw for high pressure operation. This differential is preset at 50 psi or 3.5 bar.

The controls described are commonly employed and will serve to identify adjusting screws and electrical terminals.

Applications

A low pressure control may be used as:
1. A temperature control.
2.  A safety cut-out to prevent overcooling or overfreezing of a product.
3.  A defrost termination control to open the circuit to defrost heaters should the evaporator become completely free of frost before the defrost period has elapsed. This prevents undue pressure build-up in the evaporator.
4.  A compressor cut-out when operating on a pump-down cycle.

table-4

5.  An evaporator fan delay control to prevent warm air and moisture from being circulated to the refrigerated space after defrost.

A high pressure control is employed as a safety cut-out for the compressor in the event of excessive operating pressures developing on the high side of the system during operation. Table 4 shows the settings required.

Example controls

Figure 29 shows two typical controls. The details are as follows.

Ranco type 016: low pressure operation

To complete an electrical circuit on rise in pressure, terminals 1 and 4 must be used. To complete an electrical circuit on fall in pressure, terminals 1 and 2 must be used. The control range is 12 in Hg vacuum to 100 psi (0.4 to 7 bar).

Ranco type O17: dual pressure operation

The control range is 12 in Hg vacuum to 100 psi (0.4 to 7 bar) on the low pressure side, and 100 to 400 psi (7 to 26.5 bar) on the high pressure side. The high pressure automatic reset differential is 50 psi (3.5 bar).

Control setting procedure

For low pressure control:
1 Adjust the range screw A until the compressor cuts in at the selected pressure.
2 Adjust the differential screw B to stop the compressor at the desired cut-out pressure.

Pressure controls

Pressure controls

For high pressure control, adjust the range screw H to a selected cut-out pressure and the control will reset automatically. Check the control operation several times after final adjustments have been made. Always adjust controls with a pressure gauge, using the graduated scales as a guide only. Never lever the control mechanism to actuate it; this can damage the pivot and cause erratic operation or failure. Use the insulated lever if fitted. This procedure can be followed for setting any make of control, such as those in Figure 30.

Pressure controls

Pressure controls

When locking plates are provided, set the control, replace the locking plate to secure the differential screw, and fit the knob to the range screw.

Pressure adjustments

Low pressure control adjustments can be made by front seating the suction service valve to reduce the pressure on the low side of the system. High pressure control adjustments must be made by increasing the operating head pressure, i.e. by blocking off the condenser, thereby restricting the air flow, or by switching off condenser-fans. The head pressure can be quickly increased on water cooled units by shutting off the water supply.

It is dangerous practice to increase the head pressure by front seating the discharge service valve.

Control examples

The examples in Figure 31 illustrate the effects of a reduced range and differential setting with a low pressure control.

Control samples

Control samples

 

Written by sam

November 10th, 2009 at 8:26 pm

Refrigerator Compressor Motor Burn-Out

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Prolonged operation at high discharge pressures and temperatures, excessive motor starting, fluctuating voltage conditions, shortage of refrigerant charge and shortage of oil in the compressor are all possible causes of a motor burn-out.

A burn-out can be defined as the motor winding insulation having been exposed to a critical temperature for a long period.

Refrigerant thermal decomposition

This occurs with R12, R22 and R502 at temperatures in excess of 150o C (302o F) In the presence of hydrogen- containing molecules, thermal decomposition produces hydrochloric and hydrofluoric acids. Phosgene is produced at very high temperatures, but this is decomposed in the presence of oxygen.

Bearing in mind the above, it is important to remove the entire refrigerant charge and reclaim the refrigerant for processing. A recognized reclaim refrigerant cylinder should be used and care taken not to overfill the cylinder.

Acid testing

When the windings insulation breaks down, very high temperatures occur at the short circuited location. In addition, a certain amount of moisture will be released from the windings assembly to further contaminate the system. Following a motor burn-out, the system must be decontaminated before a new compressor is fitted.

When the defective motor has been removed, a test should be made to determine the acid content of the compressor oil. Two methods may be used: litmus paper and burn-out test indicator. A sample of the oil from the defective motor compressor should be taken and tested. If the test indicates acid, then the refrigerant system must be flushed and tested as follows. Flushing is dealt with overleaf.

Litmus paper

Take a sample of the so|vent after flushing and put it into a suitable container. Place the litmus paper in the liquid. If acid is present it will change colour, ranging from pink to red according to the degree of acidity in the sample. The system must then be flushed again and the test repeated.

Indicator

When testing with an indicator it is necessary to charge the system with the liquid solvent ready for flushing and allow to stand for 30 minutes. Then take samples of the solvent, if possible from both the high and the low side of the system.

Add the prescribed amount of indicator to the samples and agitate the mixture: examine for a colour change. The results and necessary actions are as follows:
1. If red or pink, strong acid content: flush again.
2. If orange or yellow, acid content: flush again.
3. If carbonized particles are present in the samples: flush again.
4. If lemon yellow, no acid content: system may be evacuated.

System flushing

When a system has been contaminated, especially following a hermetic motor compressor ‘burn out’, the past practice was to flush the system through with R 11. This practice is no longer acceptable.

Approved burn-out filter driers are available these days to make flushing unnecessary. Instead the system can be cleansed by installing suitable filter driers and carrying out a triple evacuation. The filter driers will absorb moisture and acid content from the system pipework.

Evacuation method

Evacuation method

Evacuation

During the evacuation of the system, evaporator fans and electric defrost heaters may be switched on to raise the temperature of the evaporator. However, extreme care must be taken to avoid overheating by the defrost heaters.

Bum-out drier

After a system has been repaired and evacuated, burn-out driers installed, leak tested and charged with refrigerant it should be operated for a period of 24 hours. An acid test should then be carried out and, if satisfactory, the burn-out driers can be removed and exchanged for normal filter driers. When an acid test reveals contamination new, burn-out driers must be installed and the process repeated.

Burn-out driers installed

Burn-out driers installed

 

Refrigerator Water Cooled Condensers Scale and Corrosion

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Scaling

Any type of water cooled condenser is prone to scaling of the interior surfaces of the water tubing. The rate at which the scale forms will depend upon the condensing temperature and the quality of the water circulated. Scaling will be relatively low where condensing temperatures are below 38o C

Shell and tube condensers are fitted with removable end plates to enable cleaning to take place by means of a wire brush. This is satisfactory for mild scale build-up, but heavy scale deposits may necessitate removal by chemical means.

Scale can be removed with descaling agents, which are available in both liquid and powder forms. Solutions of chemicals and water can be used as follows:
1 Muriatic acid 18 per cent and water 82 per cent for rapid descaling.
2 Hydrochloric acid 22 per cent and water 78 per cent for slower but equally effective action.

Descaling processes can be carried out by a gravity flow method or by means of a pump. However, it must be realized that all descaling agents are acid based; therefore any pump must be acid resistant. Never use a system recirculating pump for this purpose. Figure 26 shows layouts for the two methods.

Condenser descaling

Condenser descaling

Descaling solutions can damage flooring, paintwork, clothing and plant life. They are obviously a health hazard, and contact with eyes and skin must be avoided. Adequate protective clothing must be worn, and precautions against spillage must always be taken.

When cleaning condenser coils or tubes the working areas must be well ventilated. A vent pipe is a vital part of the cleaning equipment; it will carry off the toxic fumes which are generated by the chemicals during the cleaning process.

Corrosion and contamination

In addition to scaling, water cooling systems are subject to corrosion from fumes given off by nearby industrial plant. Concentrations of sulphur and salts will be present in the atmosphere. Systems installed in a coastal area are susceptible to corrosion by salt borne by the air.

There will always be the problem of algae growth and bacterial slime. These can only be controlled by regular cleaning and the use of various algaecides. Inhibitors such as Hydrofene have been used over a long period without any ill effect being observed.

Refrigerator High Side Purging

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Like low side purging, the practice of purging from the high side is also unacceptable and must be discontinued. When it is certain that non-condensables are present in a system a leak test must be carried out on the entire system and the leak repaired. If a plant has been recently installed or it has been subject to service or repair it is possible that nitrogen could have been inadvertently left in the system. If air has entered the system in sufficient quantity the effect could possibly be of excessive operating head pressure similar to that of nitrogen left in the system. The removal of a non-condensable can be costly if the operating charge of refrigerant is large. The system would need to be completely discharged, evacuated and then re-charged.

A non-condensable gas will be lighter than refrigerant vapour and will rise to the top of a condenser/receiver when the plant is at rest. Removal of the noncondensable must take place at the highest point. Some condensers/receivers are fitted with a gauge connector or valve for this purpose. This connector may be in the form of a ‘purge screw’ on older type equipment. Removal of a noncondensable can easily be achieved via the gauge port on the high side of the compressor head. The system should be ‘pumped down’ and a short period of time allowed for the non-condensable to rise to the highest point. The procedure for removal is detailed in the section covering recovery/removal of refrigerant. Figure 25 shows the location of the refrigerant and noncondensable (air) in the high side of a system and the gauge connection.

High side purging

High side purging

The removal of non-condensables therefore should be carried out by using a vacuum pump. Purging air from a system charged with refrigerant would also release some of the refrigerant to the atmosphere.

It is the duty of any service or installation engineer to protect the environment by discontinuing the release of CFCs and HCFCs to ttle atmosphere whenever servicing or installing refrigeration equipment. In the case of nitrogen being present in a system, this can be purged from both the low and high side.

Written by sam

November 8th, 2009 at 7:33 pm

Refrigerator Excessive Operating Head Pressure

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This condition is probably more common than faults causing low suction pressures, especially during the summer months when ambient temperatures are higher. Causes and corresponding remedial actions are listed as follows.

Air cooled systems

Causes

Restricted air flow over the condenser is caused by:
1. Condenser fins blocked by an accumulation of dirt and debris drawn in by the condenser fan(s).
2. Inoperative condenser fan(s).
3. Air in the system if a leak has developed on the low side and the compressor has operated with suction pressures below atmospheric.
4. High ambient temperature.
5. An overcharge of refrigerant.

Remedies

1 The most effective method of cleaning condensers is by means of a liquid or foam application which penetrates the build-up of the undesirable deposits on the coil and fin surfaces. Brushing the condenser can produce somewhat limited results because of inability to reach the entire surface area.
2. Replacement of fan(s) may be required, or failure may be due to a loose wire or broken electrical lead.
3. This can be verified by stopping the plant and allowing sufficient time for the condenser to cool to ambient temperature. Then refer to a pressure/temperature table and compare the standing or idle pressure with the pressure given by the table. If the idle pressure is higher than that given by the table, air or non-condensables are present in the system. Running condenser fans can speed up the cooling process.
4. High ambient conditions will require a survey of the installation and location of the condenser. Relocation may overcome the problem and provide larger volumes of fresh air. An extractor fan could be installed to remove the discharged air from the condenser, so preventing recirculation. When multiple units are installed, a baffle arrangement to route a fresh air supply over each condenser must be considered.
5. An overcharge of refrigerant cannot develop and must be the fault of the service or installing engineer. Check the standing or idle head pressure in the same manner as for air in the system. The pressure/temperature relationship should conform to the table. The excess refrigerant must be removed from the system.

Water cooled systems

Causes

Restricted water flow through the condenser is caused by:
1. Scaling of the interior surface of the condenser water tubing.
2. Incorrectly adjusted or defective water regulating valve.
3. Inadequate water supply: malfunction of recirculating water pump, resulting in poor supply and high water temperature.
4. Air in the system.
5. An overcharge of refrigerant.

Remedies

1. Descaling of condenser tubing can be carried out by brushing throughthe tubes of a shell and tube condenser. If the scale deposit is heavy, a chemical method is advisable. A shell and coil condenser can only be cleaned chemically. The cost of descaling must be compared with that of a replacement.
2. Check the water inlet and outlet temperatures of the condenser and the water regulating valve operation.
3. Check the water supply pressure and volume.
4. Air or non-condensables present in a water cooled condenser system can be diagnosed very quickly. The water regulating valve, responding to the high operating head pressure, will be supplying a high volume of water to the condenser. When the plant is stopped, water will continue to flow and reduce the temperature of the refrigerant in the condenser. In a matter of minutes the temperature of the refrigerant will be the same as that of the water (the inlet and outlet temperatures will be equal). Compare the idle head pressure with that given in a pressure/temperature table; a higher than normal pressure denotes the presence of non-condensables.
5. Adopt the same procedure as in remedy 4.

If high operating head pressure conditions cannot be rectified by any of the foregoing, it must be considered that the condenser is undersized. It must be replaced, or a subcooler must be installed.

Removing Refrigerator Compressor Rotary Shaft Seal

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Most smaller open drive and some direct drive semi-hermetic compressors employ this type of crankshaft seal. They are often a cause of noise complaints (squeaking) and refrigerant leaks. Generally this is due to lack of lubrication; when the seal surfaces are dry, wear and scoring of the polished facings occur.

In order to remove this type of seal it is necessary first to remove the compressor flywheel. The larger the compressor, the larger and heavier will be the flywheel. Extra care must be exercised when removing the larger types, which may be castings and easily damaged if dropped. Removal should not be attempted without a suitable extractor. Under no circumstances should a hammer be used to break the bond between shaft, keyway and flywheel boss (not for semi-hermetic compressors).

Systems must be pumped down and isolated electrically. The drive belt guards must be removed to gain access to the flywheel on units with water cooled systems or with a remote condenser. On smaller air cooled units the compressor must be removed from its base, and this is dealt with here.

Open drive units (valves in the head type)

1. Pump down the system, front seat both service valves and isolate the unit electrically.

2. Remove the head retaining bolts and gently raise the compressor head.
3. Withdraw the valve plate assembly and remove the suction reeds from the cylinders.
4. Remove the compressor mounting bolts and belt guard.
5. Slide the compressor body towards the drive motor, release the drive belts from the flywheel and remove the compressor body.
6. Release the locking device on the drive shaft. This could be in the form of two locknuts, a locknut and a tab washer, or a locknut and a pin.
7. Using a suitable extractor, locate the arms around the flywheel boss (avoid locating around the flywheel vee section). Never use a hammer to remove a flywheel.
8. Apply gentle pressure on the extractor to break the bond, then remove the flywheel. With large heavy types it is advisable to tie a rope or cord to the flywheel and secure it in case the bond breaks suddenly.
9. Remove the seal plate retainer bolts and seal plate. The seal will be released by spring pressure in most cases; withdraw the seal from the seal housing.
10. Remove the seal ring from the shaft.

It may be necessary to change the compressor oil when a seal replacement is made, or some oil may be lost during replacement.

The procedure for replacement is as follows. Some manufacturers supply a shaft centring tool to ensure that the seal is correctly aligned on the shaft; this also eliminates uneven pressure being applied by the seal plate and spring during assembly.

1. Locate the seal ring on to the shaft and push fully home to the shaft shoulder.
2. Dip the seal nose in refrigeration oil and locate over the shaft, using the centring tool if available.
3. Replace the seal plate, depressing evenly against the spring. Insert the retaining bolts, and tighten the bolts so as to maintain the correct alignment of the shaft seal assembly.
4. Reassemble the compressor, belts etc. in reverse order. Check the belt tensioning and the alignment of the drive option and flywheel.
5. When reassembly is complete, fit gauges if these have been removed to facilitate the replacement.
6. Crack off both service valves from the front seat positions and purge the compressor through the gauge port unions if there is sufficient refrigerant pressure in the system. If the charge was lost, then evacuate, fit a new filter drier, charge the system and check the oil level.
7. Carry out a leak test.

8. Start the plant and check the system operation.
9. Remove gauges.
10. Clean the compressor and clear the site.

Direct drive units

This type of compressor does not have a flywheel or drive belts; the compressor is linked to the drive motor by means of a coupling. To gain access to the seal housing, this will have to be dismantled before the seal plate and seal assembly can be withdrawn. It may also be necessary to move the drive motor in order to remove the seal assembly.

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