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HVACR318 – Refrigeration II

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.

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HVACR318 – Refrigeration II

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  1. HVACR318 – Refrigeration II Domestic Refrigerators

  2. 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.

  3. Domestic Refrigerator

  4. Freezer Compartment Freezer Compartment of Refrigerator

  5. Refrigerator Compartment Refrigerator/Freezer Freezer Compartment

  6. Refrigerator Diagram The load on the system increases as warm product is added to the box

  7. 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.

  8. 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.

  9. Stamped-plate Evaporator Suction Line Capillary Tube Freezer compartment Product in contact with the evaporator coil

  10. 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

  11. How NOT to Remove Frost on Coil Frost on Coil NEVER USE SHARP OBJECTS TO REMOVE ICE FROM THE COIL

  12. 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

  13. 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

  14. Back View of Domestic Refrigerator BACK OF REFRIGERATOR Natural-draft condenser Compressor Suction line

  15. 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.

  16. Condenser Coil Diagram Condenser Coil Condensate Line Condensate Pan Discharge line run to bottom of condensate pan

  17. 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.

  18. Compressor Oil Cooler Diagram BACK OF REFRIGERATOR Oil Cooler Line Chimney-type condenser Compressor Suction line

  19. 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.

  20. Capillary Tube Capillary Tube Diagram Suction Line Capillary tube connected to the suction line Capillary tube run inside the suction line

  21. Capillary Tube Cross Section Capillary Tube Suction Line High Temperature, High Pressure Refrigerant Heat is transferred from the refrigerant in the capillary tube to the refrigerant in the suction line Suction Gas

  22. 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.

  23. 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

  24. Wiring Diagram Sample Water Valve Solenoid Ice Maker Lamp Mullion Heater Energy Saver Thermostat Defrost Timer M Defrost Heater Evaporator Fan Motor M C S Overload Line Cord Compressor R Relay

  25. 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.

  26. 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.

  27. Compressor Start Circuitry Current Relay Overload Coil Run Terminal Thermostat Compressor Contacts Start Terminal Common Terminal

  28. 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.

  29. 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

  30. 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.

  31. 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.

  32. 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.

  33. 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.

  34. 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

  35. 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.

  36. 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.

  37. 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.

  38. 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.

  39. Refrigerant Piping Leaks • Flare unions can be used on aluminum/copper joints • If one leak is found, there are likely others.

  40. 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.

  41. 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.

  42. System Evacuation • Evacuate system after leak checking and 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.

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

  44. 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.

  45. Capillary Tube Repair • Use as little filler material as possible when brazing. • Capillary tubes can be replaced if necessary.

  46. 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!

  47. Leak Repair, Step 2 Repeat the process on the underside of the capillary tube.

  48. Leak Repair, Step 3 Gently bend the tubing to break off the damaged section of tubing. Damaged section of tubing

  49. Leak Repair, Step 4 Insert the ends of the capillary tube to be joined into a small section of larger tubing

  50. 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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