Refrigerator Troubleshooting Diagram

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Refrigerator Evacuation

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It is imperative, with halogen refrigeration systems, that all traces of air, non-condensibles and moisture be removed. If this is not achieved then the presence of air or non-condensibles will cause abnormally high discharge pressures and increased temperatures, resulting in the conditions relating to high operating pressures previously explained.

Air in the system will also mean that a certain amount of moisture contained in the air will be circulated with the refrigerant under operating conditions. This moisture could freeze at the orifice of an expansion valve or liquid capillary to prevent refrigerant flow to the evaporator, should the filter drier become saturated.

When a system has been pressure leak tested, traces of nitrogen may also be present to further aggravate the high discharge pressure condition.

There are two ways in which a system can be evacuated, the deep vacuum method and the dilution method.

Deep vacuum method

To meet the requirements of a contaminant-free system, a good vacuum pump is necessary. Under normal ambient temperatures a vacuum of 2 torr should be achieved with a single deep-vacuum cycle.

The length of a deep-vacuum cycle can vary considerably: the larger the installation, the longer the cycle. This may be left to the discretion of the commissioning engineer, as stipulated by company policy, or a specific period may be requested by the customer. Obviously a large high vacuum pump will expedite the procedure. It is not unusual for a system to be left on vacuum for 24 or 48 hours, or even for several days, to ensure that it is completely free from contaminants.

The advantages of a deep vacuum are that (a) there will not be any appreciable loss of refrigerant other than the final trace charge administered while leak testing, and (b) it is possible to reclaim a trace charge of refrigerant from a large system (see Chapter 16 relating to contaminants and refrigerant recovery). Also, the immediate environment will not be polluted by refrigerant vapour so that it is difficult to carry out a final leak test when the system is charged. This will be evident when comparison is made with the dilution method.

Dilution method

The dilution method or triple evacuation should be carried out using OFN (oxygen free nitrogen) and not with a trace charge.

1. The initial nitrogen charge should be left in the system for at least 15 to 30 minutes. It can then be re-evacuated to a vacuum of 5 Torr.
2. This vacuum is then broken with another OFN charge allowing time for it to circulate the system.
3. Re-evacuate and charge the system with refrigerant.

This repetition may appear to be unnecessary but after a single or double evacuation small pockets of non-condensables may still be entrained in the system pipework or controls. By repeatedly breaking the vacuum with OFN these pockets will be dispersed or diluted by the OFN.

After each evacuation the pump should be switched off and, after a few minutes settling period, a vacuum reading taken. The system should then be left for another 30 minutes and another reading taken. A rise in pressure means that there is still a certain amount of moisture present.

Under no circumstances should the system compressor be used for the evacuation of the system.

A comparison of vacuum gauge graduations is given in Figure 107. Note that 1 torr = 1 mm Hg = 1000/zm Hg and micrometres are referred to as microns.

vacuum-pump

Figure 108 shows a typical arrangement for connecting a vacuum pump for deep evacuation.

Typical deep vacuum pump arrangement

Typical deep vacuum pump arrangement

During the evacuation of the system the evaporator fan(s) may be operated and defrost systems switched to the heating cycle in order to raise the temperature in the evaporator. Heaters must not remain energized for excessive periods in case of overheating of the evaporator and possible damage. It is also very important to ensure that no parts of the system are isolated from the vacuum pump.

Figure 109 shows a triple evacuation arrangement. When the pump is operating, the isolator valve must be open, the service valves on the compressor in the midway position, the liquid shut-off valve at the receiver open, and the refrigerant cylinder valve closed. Both valves on the gauge manifold must be open. When breaking the vacuum with refrigerant vapour pressure, ensure that the pump isolating valve is closed.

Triple evacuation arrangement

Triple evacuation arrangement

Table 7 shows the pressure/temperature relationship for water. When evacuating a system, remember that there must be an adequate temperature difference .between the ambient temperature and that of the water to provide the heat necessary to vaporize the water.

table-7

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