Tuesday, December 2, 2014

Domestic refrigerators and freezers

 Domestic refrigerators and freezers

Because of the numerous designs of these appliances, a description of the system pipework layout is helpful to the service engineer.

In comparison with commercial or industrial installations, domestic systems are more prone to motor failure and subsequent contamination because few users appreciate the need for adequate air circulation. The method of charging these systems is different because of the minute operating refrigerant charge involved, and the decontamination and evacuation procedure also differs to some extent.

In modern production methods, domestic appliances are assembled and insulated in situ, with an expanded foam insulant. The foam insulant sets and bonds itself to the components it is intended to insulate. The welding technique of fusing aluminium to components and system tubing is often the source of refrigerant leakage. To gain access to the leak area is uneconomical and sometimes not possible without damaging the appliance structure. Overall, therefore, servicing may be somewhat limited.

Many service and repair outlets will not undertake repairs to an evaporator which has developed a leak. A large amount of moisture could have been drawn into the system after the leak had developed by the continuous running of the compressor. This will contaminate the compressor motor, which could fail after a comparatively short operating period following the repair or replacement of the evaporator.

Whenever a system has been subject to moisture contamination it must be

thoroughly evacuated. It is often advisable to do this under workshop conditions rather than on customer premises, and this again raises the question of economical viability.

Appliance systems

There are numerous refrigerator and freezer system arrangements. Figures 52 55 show the basic refrigeration circuits for a conventional

imagerefrigerator, a two-door refrigerator/freezer, an upright freezer and a chest freezer.

Components and operations
Capillary restrictor

Domestic refrigerators and freezers do not employ a mechanical device as a refrigerant flow control. Instead a capillary restrictor meters refrigerant liquid

imageto the evaporator and maintains a pressure differential whilst the compressor is operating.

Basically the capillary restrictor is a small tube. The refrigerant flow rate is determined by the length of the tubing and the internal diameter of the bore. Refrigerant will continue to flow through the capillary when the compressor stops until the pressures in the system (high side and low side) equalize.

The capillary is normally located after the filter drier; it is sometimes formed into a tight coil around the suction line. Figure 56 shows a typical domestic refrigerator arrangement. It will be noted that the capillary actually passes through the inside of the suction line to the evaporator and thereby provides a heat exchange feature which improves compressor performance.

imageAccumulator

Suction line accumulators are employed to prevent frosting back along the suction line after off cycles. This is because the relatively small refrigerant charges in modern refrigerators and freezers are difficult to control accurately, and some overspill from the evaporator occurs when the compressor stops.

Accumulators are shown in Figures 52 55.

Freezer door heater and oil cooler

Figure 57 shows a freezer system diagram with door frame heater and oil pre-cooler.

Hot gas defrosting

Figure 58 shows the normal refrigeration and defrost cycles in an appliance using hot gas defrosting.

Solenoid flow control for refrigerator/freezer

Figure 59 shows the refrigeration system and Figure 60 the electrical circuit for solenoid flow control.

When the refrigerator thermostat is made and the freezer thermostat is open circuit, the relay is energized through contacts 1 and 3. The compressor operates but the solenoid is not energized, and the refrigerant flows through the refrigerator evaporator and the freezer evaporator.

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When the freezer thermostat is made and the refrigerator thermostat is open circuit, the compressor operates and the solenoid is energized through contacts 1 and 2. The refrigerator evaporator is by-passed.

When the refrigerator thermostat makes whilst the compressor is operating, the relay is energized. This opens contacts 1 and 2, and refrigerant will again flow to the refrigerator evaporator as well as the freezer evaporator.

Electrical faults

The diagnosis of electrical faults on components and compressors has already been dealt with in Chapter 7. The following procedure may be carried out to quickly pinpoint the faulty component and to eliminate unnecessary dismantling. The electrical circuit is tested for continuity from the terminal board of the circuit. The typical electrical circuit in Figure 61 has been chosen for the test procedure.

Steps 1 and 2 of the test procedure can be disregarded if the refrigerator or freezer has an interior light. This will come on when the appliance door is opened, unless the light bulb itself is defective.

The procedure is as follows:

1 Test for continuity from points 1 to 2, which will test the whole circuit.

2 Test from point 3 to points 1 and 2 to reveal any earth fault.

3 Test from points 1 to 8, which eliminates half of the circuit.

4 If the fault is in section 1 to 8, test between points 1 and 5 to determine if the fault is due to a loose connection, a defective cable or a blown fuse.

5 Should the fault be located in section 5 to 8, then the thermostat must be considered inoperative because the interior light and door switch will be isolated when the door is closed.

image6 If the fault is in the compressor half of the circuit, it should be located in a similar manner by dividing into sections until it has been finally determined.

When an ohmmeter is used to test for continuity, it should normally be set to read off the ð1 scale. If the total resistance of the circuit is in excess of 100 ohms, the ð10 scale will be required; reset to the ð1 scale when the faulty section is isolated to less than 100 ohms.

Figure 62 shows a typical refrigerator/freezer electrical circuit.

Decontaminating domestic systems

With liquid flushing using R11 no longer permissible the previously described dilution method can be carried out (see Chapter 16).

Refrigerant charging in domestic appliances

The principles of refrigerant charging have been covered in Chapter 4.

Domestic refrigerators and freezers operate with very small refrigerant charges, and the charge must be administered accurately. This can be best

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achieved by using a visual charging cylinder, sometimes called a dial-a-charge (Figure 64).

This is basically a small refrigerant cylinder with a liquid-indicating sight glass or tube. In front of the sight glass and surrounding the cylinder and sight glass is a rotating screen upon which are graduated scales for the common refrigerants, R12, R22 and R502. Above the refrigerant scales is a temperature range.

Fitted to the top of the cylinder assembly are a pressure gauge; a purging valve, which is normally a Schraeder type; and, if a heater is incorporated, a pressure relief valve for the cylinder protection. At the base of the cylinder is a charging valve and hose connection.

Filling the cylinder

1 Obtain a service cylinder of the correct refrigerant. Connect it to the visual charger with a suitable hose.

2 Open the valve on the service cylinder and invert the cylinder.

3 Slacken the hose connection at the charging valve to purge air from the hose. Tighten the hose connection.

4 Open the charging valve to allow refrigerant liquid to flow by gravity into the cylinder. Observe the sight glass: when liquid flow ceases, depress the purge valve and gently vent off a small amount of refrigerant vapour. When a pressure difference between the charging cylinder and the service cylinder is created, the liquid will begin to flow again. Repeat this operation until the required amount of refrigerant is in charging cylinder.

5 Close the valves on both the visual charger and the service cylinder.

6 Disconnect the service cylinder.

Charging the system

1 Assuming a line tap valve has been installed and the system evacuated, connect the hose to the charging valve and the line tap valve.

2 Open the charging valve and slacken off the hose connection to purge air from the hose. Close the valve.

3 Note the ambient temperature. Rotate the screen until the required refrig- erant scale lines up with the ambient temperature on the temperature scale.

4 Note the level of the refrigerant in the sight glass.

5 Open the line tap valve fully and slowly meter the prescribed amount of refrigerant into the system. If the visual charger has a heater this can be energized before opening the line tap valve to create a pressure difference between the charging cylinder and the system.

6 When the charge has been administered as indicated by the sight glass, close the charging valve.

7 Allow a few minutes for the liquid in the hose to vaporize and the system pressure to equalize. Then close the line tap valve.

8 Disconnect the visual charger and leak test the system.

Alternative method of charging

If a refrigerant charge cannot be accurately measured by using a visual charger, it must be drawn into the system by the compressor from the low side. It is imperative that this is carried out slowly to eliminate the risk of overcharging, which could damage the compressor.

By allowing small amounts of refrigerant vapour into the system, and observing the frost line on the evaporator, overcharging can be prevented.

When the frost line reaches the location of the thermostat bulb it is always advisable to stop charging and allow sufficient time for the system to reach average evaporating temperature and start to cycle. Should frosting back occur, purge off the surplus refrigerant in small amounts from the line tap valve connection.

Reference to the compressor nameplate should be made to determine the running current.

Refrigerant charges are normally stamped on the refrigerator model plate. The charge may be given in ounces or grams according to the age of the refrigerator. These quantities can be easily converted: 1 ounce D 28.35 grams. For example, 150 grams is equivalent to 150/28.35 D 5.29 ounces; 6.5 ounces is equivalent to 6.5 ð 28.35 D 184.27 grams.

Remember, an undercharge of refrigerant will result in long running periods or continuous running. An overcharge of refrigerant will result in frosting back, overheating, increased running costs and could possibly damage the compressor.

The domestic absorption system

The modern domestic absorption refrigerator or ‘gas refrigerator’ operates on the principle of using heat to produce cold. It employs a mixture of hydrogen and ammonia with water (aqua-ammonia) as the cooling agent: water has an affinity for ammonia, and the hydrogen speeds up the process of evaporation.

The heat source may be town gas or, in the case of smaller systems designed for use in caravans, camp sites etc., propane, butane or even paraffin. An all-electric version has a small heater element to provide heating.

The valve assemblies which regulate the gas flow incorporate a small pilot jet to ignite the gas, as the valve is energized electrically. The evaporating temperature is controlled by a thermostat which in turn energizes the gas valve or heater element, depending upon the setting of the thermostat (cold control).

Operation

When the thermostat energizes the valve or heater, heat is applied to the ammonia and water mixture in the generator. The liquid then boils off the pass through a small tube (percolator tube), in much the same way as coffee in a percolator, to enter the separator or rectifier. From there it circulates by gravity.

imageThe ammonia vaporizes faster than the water and the aqua-ammonia separate in the rectifier; the ammonia vapour rises to the condenser and the water drains back to the absorber to be recirculated. The lighter ammonia vapour rising from the condenser coils changes back to a liquid to drain into the evaporator, where it mixes with the hydrogen vapour from the absorber.

Heat from the stored product in the food compartment of the refrigerator will vaporize the ammonia liquid in the evaporator. The mixture of ammonia vapour and hydrogen, being heavier than either of the two gases alone, passes to the absorber where it meets the water coming from the rectifier. The ammonia is then absorbed by the water and, when the water has absorbed as much vapour as it can hold, the vapour returns to the evaporator.

clip_image003[1]At maximum working conditions (maximum heat at generator) the pressure on the high side of the system (condenser and absorber) will be approximately 14 bar with a relative temperature of 36 °C. Assuming that the refrigerator is in an average ambient temperature of 21 °C, the heat rejection will be quite reasonable with a 15 ° temperature (36 21 °C). Pressure on the low side of the system will be 12.6 bar hydrogen and 1.2 bar ammonia, so the relative temperature of the evaporator will be -15 °C. At this temperature, heat exchange from the food products to the evaporator will be at an average 3 °C, producing a 12 ° temperature difference [3 to - 15 D 12 °].

ervicing

Since this is a completely sealed system containing two gases, very little can be achieved by the service engineer except the replacement of a thermostat, valve assembly or heater element.

The pilot jet and valve should be kept clean and the refrigerator should be carefully levelled at the time of installation and checked when service is carried out.

Figure 65 shows the absorption system cycle.