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SECTION 5 COMMERCIAL REFRIGERATION

SECTION 5 COMMERCIAL REFRIGERATION UNIT 29:TROUBLESHOOTING AND TYPICAL OPERATING CONDITIONS FOR COMMERCIAL REFRIGERATION. UNIT OBJECTIVES. After studying this unit, the reader should be able to.

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SECTION 5 COMMERCIAL REFRIGERATION

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  1. SECTION 5 COMMERCIAL REFRIGERATION UNIT 29:TROUBLESHOOTING AND TYPICAL OPERATING CONDITIONS FOR COMMERCIAL REFRIGERATION

  2. UNIT OBJECTIVES After studying this unit, the reader should be able to • List the typical operating temperatures and pressures for the low-pressure side of a refrigeration system for high, medium, and low temperatures. • List the typical operating pressures and temperatures for the high-pressure side of a system. • State how different refrigerants compare on the high-pressure and low-pressure sides of the system.

  3. UNIT OBJECTIVES After studying this unit, the reader should be able to • Diagnose an inefficient evaporator. • Diagnose an inefficient condenser. • Diagnose an inefficient compressor.

  4. Organized Troubleshooting • Have knowledge of how the system should operate • Know the product being refrigerated • Measure pressures, temperatures, amperages and voltages • Inspect the system for obvious problems • Defective components can trigger other problems • It is rare for more than one component to fail at any given time

  5. STORAGE REQUIREMENTS FOR COMMON FOOD ITEMS

  6. COIL-TO-AIR TEMPERATURE DIFFERENTIALS TO MAINTAIN DESIRED BOX HUMIDITY

  7. Troubleshooting High-Temp Applications • Boxes range from 45 to 60 degrees • Coils are 10 to 20 degrees cooler than the box • At the lowest temperature, the coil will be 25°F • 45°F - 20°F = 25°F • R-134a at 25 degrees is 22 psig • For R-134a, a suction pressure below 22 psig is considered to be low

  8. HIGH TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature BOX TEMPERATURE RANGES FROM 45°F to 60°F

  9. HIGH TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature 25°F FOR A BOX TEMPERATURE OF 45°F, THE COIL WILL BE A TEMPERATURE OF ABOUT 25°F

  10. HIGH TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature 22 psig 25°F FOR R-134a, THE PRESSURE AT 25°F WILL BE 22 PSIG

  11. Troubleshooting Medium-Temp Applications • Boxes range from 30 to 45 degrees • Coils are 10 to 20 degrees cooler than the box • At the lowest temperature, the coil will be 10°F • 30°F - 20°F = 10°F, R-134a at 10°F is 11.9 psig • During the off cycle, the highest suction pressure will be 40 psig (R-134a, 45°F box temperature) • After startup, the pressure will be about 22 psig (45 degrees – 20 degrees = 25 degrees)

  12. MEDIUM TEMPERATURE REFRIGERATION EVAPORATOR 11.9 psig 10°F BOX TEMPERATURE RANGES FROM 30°F to 45°F

  13. MEDIUM TEMPERATURE REFRIGERATION EVAPORATOR 11.9 psig 10°F FOR A BOX TEMPERATURE OF 30°F, THE COIL WILL BE A TEMPERATURE OF ABOUT 10°F

  14. MEDIUM TEMPERATURE REFRIGERATION EVAPORATOR 11.9 psig 10°F FOR R-134a, THE PRESSURE AT 10°F WILL BE 11.9 PSIG

  15. Troubleshooting Low-Temp Applications • Low temperature applications start at 5°F • Coil temperature is -15°F ( 5°F - 20°F = -15°F) • For R-134a, the highest suction is 0 psig at -15°F • Lowest suction is 14.7”Hg (-20°F - 20°F = -40°F) • With the compressor off at 5°F, suction pressure for R-134a is about 9.1 psig • Use the P/T chart to determine pressures for other refrigerants

  16. LOW TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature 14.7 “Hg -15°F HIHGEST BOX TEMPERATURE IS 5°F

  17. LOW TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature 14.7 “Hg -15°F FOR A BOX TEMPERATURE OF 5°F, THE COIL WILL BE A TEMPERATURE OF ABOUT -15°F

  18. LOW TEMPERATURE REFRIGERATION EVAPORATOR The evaporator coil is about 10°F to 20°F cooler than the box temperature 14.7 “Hg -15°F FOR R-134a, THE PRESSURE AT -15°F WILL BE 14.7”Hg

  19. Typical Air-Cooled Condenser Operating Conditions • Head pressure controls maintain the desired head pressure, which is about 105°F • Most condensers operate with condensing temperatures about 30°F higher than ambient • High efficiency condensers can operate with condensing temperatures as low as 10°F above ambient temperature

  20. R-134a 90°F Outside Air Temp. + 30°F Temp. Differential 120°F Condensing Temp. High side pressure is 184.6 psig

  21. R-12 90°F Outside Air Temp. + 30°F Temp. Differential 120°F Condensing Temp. High side pressure is 169 psig

  22. R-134a HIGH EFFICIENCY SYSTEM High side pressure is 124.2 psig 90°F Outside Air Temp. + 10°F Temp. Differential 100°F Condensing Temp.

  23. Typical Water-Cooled Condenser Operating Conditions • Wastewater systems require about 1.5 gpm/ton • Recirculating systems require about 3.0 gpm/ton • Water-regulating valves maintain desired pressure • R-12 system (head pressure of 220 psig, 145°F sat. temp) • Leaving water is 95°F • Refrigerant is condensing at a temperature 50°F higher than the leaving water • Condenser is likely dirty (high temperature differential)

  24. Condenser R-12 Compressor discharge 220 psig Liquid line to metering device For R-12, 220 psig = 145°F 145°F - 95°F = 50°F Condenser water in (90°F) 50° IS TOO HIGH! Condenser water out (95°F) CONDENSER TUBES MAY BE DIRTY!

  25. Typical Conditions for Recirculating Water Systems • Usually do not use water regulating valves • Water enters at 85°F and leaves at 95°F • Refrigerant normally condenses at a temperature that is 10°F higher than the leaving water temp. • Refrigerant normally condenses at about 105°F • Cooling tower water can often be cooled to a temperature within 7°F of the wet bulb temperature of the ambient air

  26. SIX TYPICAL PROBLEMS • Low refrigerant charge • Excess refrigerant charge • Inefficient evaporator • Inefficient condenser • Restriction in the refrigerant circuit • Inefficient compressor

  27. LOW REFRIGERANT CHARGE • Reduced system capacity • Low head pressure • Low suction pressure (except for AXV systems) • High superheat (Except for TXV systems) • Sight glass will have bubbles in it • Compressor will often run hot • Suction line will be warm

  28. REFRIGERANT OVERCHARGE • High operating pressures (Except AXV systems) • Capillary tube systems may have liquid floodback • Low superheat (Except for TXV systems) • High condenser subcooling • Sweating is a sign that liquid may be entering the compressor

  29. INEFFICIENT EVAPORATOR • System suction pressure will be low • Evaporator does not absorb heat properly • Caused by dirty or blocked coil, defective evaporator fan motor, defective expansion valve, or recirculating air • Evaporator saturation temperature should be no more than 20°F lower than the box temperature

  30. INEFFICIENT CONDENSER • High head pressure • Refrigerant cannot desuperheat, condense and subcool refrigerant effectively • Amount of cooling medium (air or water) must be sufficient • Condenser discharge air must not be permitted to recirculate through the coil

  31. REFRIGERANT FLOW RESTRICTIONS • Restrictions can be partial or full • A pressure drop is created at the restriction • Damaged or kinked tubing can cause a restriction • A temperature difference will exist across a liquid line restriction • Freezing moisture in the system can cause a restriction • Improperly set valves or controls can be at fault

  32. INEFFICIENT COMPRESSOR • Electrical problems are relatively easy to diagnose • Evaluating pumping problems are more difficult • Various tests can be performed on the compressor • Compressor Vacuum Test • Closed-Loop Compressor Running Bench Test • Closed-Loop Compressor Running Field Test • Compressor Running Test in the System

  33. COMPRESSOR VACUUM TEST • The compressor vacuum test is usually performed on a test bench with the compressor out of the system. • This test may be performed in the system when the system has service valves.

  34. CLOSED-LOOP COMPRESSOR RUNNING BENCH TEST • Doing a running bench test on the compressor can be accomplished by connecting a line from the discharge to the suction of the compressor and operating the compressor in a closed loop. • Hermetic compressors should operate at close to full-load current in the closed loop when design pressures are duplicated.

  35. UNIT SUMMARY • When troubleshooting, be organized! • Evaporator coils are typically 10 to 20 degrees cooler than the box temperature • Use the P/T chart to obtain operating temperatures • Head pressure controls maintain desired pressures • Standard efficiency condensers operate at temperatures that are about 30°F higher than ambient • Common system problems include overcharge, undercharge, restriction, inefficient compressor, condenser or evaporator

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