Heating, Ventilating, Air-Conditioning&

Refrigeration Technology

SECTION 9 DOMESTIC APPLIANCES

UNIT 45DOMESTIC REFRIGERATORS

UNIT OBJECTIVES After studying this unit, the reader should be able to

• Describe refrigeration as it applies to domestic refrigerators

• Describe the compressors, condensers, metering devices and

evaporators that are used on domestic refrigerators

• Describe the various methods used to defrost domestic refrigerators

• Explain how condensate is removed from the refrigerator

• Describe the controls and electric components used on refrigerators

• Discuss various service procedures used on domestic refrigerators

REFRIGERATION

• Heat enters the refrigerator by conduction, convection and the introduction of warm food

• Warm food in the box raises the temperature• Air in the box is circulated over the cold coil• The air in the box transfers sensible and latent

heat to the cold refrigeration coil• The evaporator dehumidifies the air, causing

frost to accumulate on the coil surface

Freezer Compartme

nt

Freezer Compartme

nt

Refrigerator Compartme

nt

The load on the system

increases as warm

product is added to the

box

Heat is also introduced to the

box when the door is opened

through the doors and gaskets that

seal the box

Heat infiltration increases the

temperature of the air in the box

NATURAL DRAFT EVAPORATORS

• Normally of the stamped plate variety• Product in the frozen food compartment

may be in direct contact with the evaporator• Evaporators can have manual or automatic

defrost• Most evaporators have an accumulator at

the outlet• Evaporators are not equipped with air filters

Capillary TubeSuction Line

Stamped-plate Evaporator

Freezer compartment

Product in contact with the evaporator coil

EVAPORATOR DEFROST

• Manual Defrost– Unit must be turned off and emptied– Room heat, a pan of water or small

heater is used to melt accumulated frost– Never use sharp objects to remove frost

• Automatic Defrost– Accomplished with internal or external

heat– Hot gas or electric heating elements

Frost on Coil

NEVER USE SHARP OBJECTS TO REMOVE ICE FROM THE COIL

THE COMPRESSOR

• Pumps heat-laden, vapor refrigerant from the evaporator to the condenser

• Usually of the hermetically sealed, reciprocating or rotary, variety, ranging from 1/10 to 1/3 horsepower

• Compressors are located at the bottom of the unit

• Compressors can have oil cooler lines• Refrigerant lines connected to the

compressor are often made of steel or copper

THE CONDENSER

• Air-cooled variety• Can be of the natural or forced draft

variety• Natural draft condensers

– Located at the back of the unit– Must have proper airflow across it

• Forced draft condensers– Usually located under the unit at the back– Must have a set air pattern

BACK OF REFRIGERATOR

Suction line

Natural-draft condenser

Compressor

DEFROST CONDENSATE, AUTOMATIC DEFROST

• Domestic refrigerators are low-temperature appliances

• Frost will accumulate on the evaporator coil

• Evaporator coil must be defrosted• The compressor and condenser help to

evaporate the water that accumulates during defrost

• Defrost water pan should be periodically cleaned

Condenser Coil

Discharge line run to bottom of condensate

pan

Condensate Line

Condensate Pan

COMPRESSOR OIL COOLERS

• Helps keep compressor oil cool• Line leaves the compressor and makes

several passes through the condenser• The line then returns to the compressor• Air passes over the line just as air passes

over the coils of the condenser coil• Oil cooler lines are closed loops

BACK OF REFRIGERATOR

Suction line

Chimney-type condenser

Oil Cooler Line

Compressor

METERING DEVICE

• Domestic refrigerators use capillary tubes• Capillary tube is usually fastened to the suction

line for increased heat exchange rate• Capillary tube can also be run inside the suction

line• Refrigerant flow through the capillary tube is

determined by the length of the tube, the bore size and the pressure difference across the tube

• Designed for indoor operation

Suction Line Capillary TubeCapillary tube connected to the

suction line

Capillary tube run inside the suction line

Suction Line

Suction Gas

Capillary Tube

High Temperature, High Pressure

Refrigerant

Heat is transferred from the refrigerant in the capillary tube to the refrigerant in the

suction line

THE DOMESTIC REFRIGERATED BOX

• Modern refrigerators have magnetic gasket material around the door for a better seal

• Condensate flows from the box, through a trap to the condensate pan

• The trap prevents air from entering the box• Different compartments can be maintained

at different temperatures• Some units are equipped with ice and

water dispensers

WIRING AND CONTROLS

• Wiring diagrams permanently attached to the unit

• Diagrams can be of the pictorial or line variety

• Components controlled in a domestic refrigerator– Compressor– Fan motors– Heaters– Lights– Ice maker– Defrost components

Ice Maker

Water Valve Solenoid

Lamp

Energy Saver Mullion Heater

Thermostat

Defrost TimerM

M

Defrost Heater

Evaporator Fan Motor

Overload Compressor

C S

R Relay

Line Cord

COMPRESSOR CONTROLS

• Compressor is controlled by a line-voltage thermostat

• The thermostat– Closes on a rise in box temperature– Passes power to the compressor when closed– Has a remote bulb and is field adjustable– Does not have a numerical temperature scale– Remote bulb contains a volatile fluid that

pushes against the diaphragm in the thermostat

COMPRESSOR START CIRCUIT

• Designed to help the compressor start• The starting components give the

compressor additional starting torque• Circuit often consists of a relay and a start

capacitor• Some units use a positive temperature

coefficient– Known as the PTC– Resistance varies with temperature– As the temperature increases, so does the

resistance

Common Terminal

Start Terminal

Run Terminal

Compressor

Current Relay

Contacts

Coil

Overload

Thermostat

SWEAT PREVENTION HEATERS

• Designed to keep the refrigerator cabinet above the dew point temperature

• Cabinets will sweat more when the humidity level is high

• Small wire heaters are mounted within the cabinet walls

• Some units have manual switches to turn the heaters on and off

REFRIGERATOR FAN MOTORS

• Evaporator fan motor– Connected to small squirrel cage blowers– Runs all the time, except when the unit is in

defrost– Usually an open-type motor

• Condenser fan motor– Connected to a propeller-type fan– Usually a shaded pole motor– Located under the refrigerator at the back of the

unit

ICE MAKER OPERATION

• Located in the low temperature compartment• Water flow to the ice maker is solenoid controlled• Water flows to fill the ice maker tray• A timer gives the water ample time to freeze• The ice is then harvested by either twisting the tray

or by heating the tray to free the ice• Ice-making sequence is often time controlled

GAUGE CONNECTIONS

• Domestic appliances not equipped with service ports

• Installing gages is not always necessary• When to install gauges:

– When there is a leak– When repairing the refrigerant circuit– As a last resort

• Use process tubes for taking pressure readings

• Process tubes can be pinched off

LOW REFRIGERANT CHARGE

• If there is a low refrigerant charge, there is a leak

• Leak should be located and repaired• One method to determine if the charge is

correct– Turn unit off for about five minutes– Turn unit on while touching the outlet of the

evaporator– If the line gets cold for a short period of time,

the charge is most likely correct– If a low charge is suspected, gauges can be

installed

REFRIGERANT OVERCHARGE

• Forced-draft condensers are more efficient than natural-draft condensers

• Systems with forced-draft condensers operate with lower head pressures

• The head and suction pressures of the unit will be higher during a hot pull down (excessive load)

• If the compressor sweats around the suction line, there is most likely an overcharge

REFRIGERANT LEAKS

• Small refrigerant leaks affect unit’s performance

• Domestic refrigerators only hold a few ounces of refrigerant, so even a small leak will prevent the unit from operating effectively

• Leaks are best found in the shop• Small leaks are difficult to locate in the home• Very small leaks are found with high quality

leak detectors

EVAPORATOR LEAKS

• Soldering aluminum evaporators is not practical

• Aluminum evaporator leaks are repaired with epoxy

• The area around the leak should be cleaned well before attempting to make a repair

• Always follow the manufacturer’s instructions before attempting to make epoxy repairs

• Pulling a slight vacuum in the system while making an epoxy repair can help pull a small amount of epoxy into the leak, creating a stronger bond

CONDENSER LEAKS

• Refrigerator condensers are often made of steel

• Leaks usually occur at the ends of the coil where the coil connections are made to the rest of the system

• While operating, units lose refrigerant faster through high side leaks than low side leaks

• Solders with a high silver content are best for repairing leaks in steel tubing

REFRIGERANT PIPING LEAKS

• Interconnecting piping leaks can be very difficult to repair

• The evaporator may have to be removed • Often more economical to replace the entire

unit• Leaks within fiberglass walls are often

caused by electrolysis• If one leak is found, there are likely others• Flare unions can be used on

aluminum/copper joints

COMPRESSOR CHANGEOUT

• Use an exact replacement whenever possible• Identify all tubing connections before

removing the old compressor from the unit• The refrigerant should be recovered from the

unit• Remove piping connections from the

compressor using a tubing cutter or torch• Clean all tubing ends and compressor stubs• Add process tubes and a liquid line filter drier

SYSTEM EVACUATION• System evacuated after repairing and leak

checking• Removing Schrader pins (stems) and valve

depressors will speed the evacuation process• Moisture may be trapped under the compressor oil• Use full size gage connections on high and low

sides• Allow vacuum pump to operate for about 8 hours• Perform a triple evacuation whenever possible• Minimum vacuum should be about 500 microns

CAPILLARY TUBE REPAIR

• Capillary tubes can develop leaks when they rub against another surface

• Capillary tubes are delicate and must be handled carefully

• Capillary tubes should be cut with a file• Care should be taken to protect the bore• Capillary tube sections can be joined with a

section of larger tubing• Use as little filler material as possible when

brazing• Capillary tubes can be replaced if necessary

Leak in the Capillary tube

Use a file to cut through the capillary tube on both sides of the leak point. Be sure not to cut all the way through to the bore of the tubing!

Repeat the process on the underside of the capillary tube

Gently bend the tubing to break off the damaged section of tubing

Damaged section of tubing

Insert the ends of the capillary tube to be joined into a small section of larger tubing

Crimp the larger tubing around the two ends of the capillary tube, making certain

that the ends of the capillary tubes are not damaged

Make certain that the ends of the capillary tubes do not touch!

Pinched portion of copper tubing

When brazing, use as little

filler material as possible!

COMPRESSOR CAPACITY CHECK

• Before condemning the compressor, check– The door gaskets– Excessive load on the box– Leaking hot gas solenoid (defrost)– Improper condenser airflow

• Use manufacturer’s guidelines when evaluating the compressor

• Determine compressor amperage

UNIT SUMMARY

• Heat enters the refrigerator by conduction, convection and the introduction of warm food

• Warm food in the box raises the box temperature and the evaporator saturation temperature and pressure

• Evaporators can be natural or mechanical draft

• Evaporators can be either automatic or manual defrost

• Natural draft evaporators are defrosted manually

UNIT SUMMARY

• Compressors are often equipped with oil cooler lines

• Condensers are air cooled and can be natural or mechanical draft devices

• The metering device used on domestic refrigerators is the capillary tube

• Different compartments in the refrigerator can be maintained at different temperatures and humidities

UNIT SUMMARY

• The electrical components controlled in a domestic refrigerator are the compressor, fan motors, lights, ice makers, lights, heaters and defrost components

• The compressor thermostat closes on a drop in box temperature

• The compressor uses starting components to increase the starting torque of the compressor motor

• Gauges should be installed on process tubes and only when absolutely necessary

UNIT SUMMARY

• If there is a low refrigerant charge, there is a leak

• Evaporator leaks can be repaired with epoxy

• Condenser leaks are often repaired with high silver content brazing filler materials

• Capillary tubes can be repaired or replaced• Care must be taken to not block the bore of

the capillary tube during the repair process