Transcript Document

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HVACR318 – Refrigeration II
Domestic Refrigerators
Refrigeration Overview
• 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.
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Domestic Refrigerator
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Freezer Compartment of
Refrigerator
Freezer
Compartment
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Refrigerator/Freezer
Freezer
Compartment
Refrigerator
Compartment
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Refrigerator Diagram
The load on the
system
increases as
warm product is
added to the box
Refrigerator Diagram – Heat in Box
Heat is also
introduced to the box
when the door is
opened, and through
gaskets that seal the
box.
Heat infiltration
increases the
temperature of the air in
the box.
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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.
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Stamped-plate Evaporator
Freezer
compartment
Capillary Tube
Suction Line
Product in contact with the
evaporator coil
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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
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How NOT to Remove Frost on
Coil
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.
• They are located at the bottom of the unit
• They can have oil cooler lines.
• Refrigerant lines connected to the compressor
are often made of steel or copper
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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
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Back View of Domestic
Refrigerator
BACK OF
REFRIGERATOR
Natural-draft
condenser
Compressor
Suction line
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Defrost
Condensate
• Domestic refrigerators are low-temperature
appliances.
• Frost will accumulate on the evaporator coil;
the coil must be defrosted.
• The compressor and condenser help to
evaporate the water that accumulates during
defrost.
• Defrost water pan should be periodically
cleaned.
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Condenser Coil Diagram
Condenser
Coil
Condensate
Line
Condensate Pan
Discharge line run to bottom of
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.
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Compressor Oil Cooler Diagram
BACK OF
REFRIGERATOR
Oil Cooler Line
Chimney-type
condenser
Compressor
Suction line
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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.
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Capillary Tube Diagram
Suction Line
Capillary tube connected
to the suction line
Capillary tube run inside the
suction line
Capillary Tube
Capillary Tube Cross Section
Suction Line
Capillary Tube
High Temperature, High
Pressure Refrigerant
Suction Gas
Heat is transferred from the
refrigerant in the capillary
tube to the refrigerant in the
suction line
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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.
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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
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Wiring Diagram Sample
Water Valve Solenoid
Ice Maker
Lamp
Mullion Heater
Energy Saver
Thermostat
M
Defrost Timer
Defrost Heater
M
Evaporator Fan Motor
C
Line Cord Overload
Compressor
S
R
Relay
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.
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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.
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Compressor Start Circuitry
Current Relay
Coil
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Overload
Run Terminal
Thermostat
Compressor
Contacts
Start Terminal
Common Terminal
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.
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Refrigerator Fan
Motors
• Evaporator fan motor:
– Connected to small squirrel-cage blowers
– Runs continuously, 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
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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.
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Gauge Connections
• Domestic appliances not equipped with
service ports.
• Installing gauges 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.
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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.
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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.
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Refrigerant
Leaks
• 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
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Evaporator
Leaks
• Aluminum evaporator leaks are repaired with
epoxy; soldering aluminum evaporators is not
practical.
• Always follow the manufacturer’s
instructions before attempting to make epoxy
repairs.
• The area around the leak should be cleaned
well before attempting to make a repair.
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Evaporator Leaks
• 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.
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Refrigerant Piping
Leaks
• Interconnecting piping leaks can be very
difficult to repair.
• The evaporator may have to be removed.
• It is often more economical to replace the
entire unit.
• Leaks within fiberglass walls are often caused
by electrolysis.
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Refrigerant Piping Leaks
• Flare unions can be used on aluminum/copper
joints
• If one leak is found, there are likely others.
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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.
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Compressor Changeout
• 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.
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System
• Evacuate system after leak checking and
Evacuation
repairing.
• Removing Schrader pins (stems) and valve
depressors will speed the evacuation process.
• Moisture may be trapped under the compressor
oil.
• Use full size gauge connections on high and
low sides.
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System Evacuation
• Allow vacuum pump to operate for about 8
hours.
• Perform a triple evacuation whenever possible.
• Minimum vacuum should be about 500 microns.
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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.
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Capillary Tube Repair
• Use as little filler material as possible when
brazing.
• Capillary tubes can be replaced if necessary.
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Leak Repair, Step 1
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!
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Leak Repair, Step 2
Repeat the process on the underside of the capillary
tube.
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Leak Repair, Step 3
Gently bend the tubing to break off the damaged
section of tubing.
Damaged section of tubing
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Leak Repair, Step 4
Insert the ends of the capillary tube to be joined into a
small section of larger tubing
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Leak Repair, Step
5
Crimp the larger tubing around the two ends of the
capillary tube, making certain that the ends of the
capillary tubes are not damaged.
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Leak Repair, Step 6
Pinched portion of copper tubing
When brazing, use as
little filler material as
possible!
Make certain that the ends of the capillary tubes do
not touch!
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.
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Summary 1
• 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.
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Summary 2
• 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.
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Summary 3
• The electrical components controlled in a
domestic refrigerator are the compressor, fan
motors, lights, ice makers, 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.
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Summary 4
• 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.
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