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Tennessee Technology Center at Pulaski

Tennessee Technology Center at Pulaski. Superheat & Sub-cooling. A Technician’s Guide to HVACR Diagnostics. Introduction.

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Tennessee Technology Center at Pulaski

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  1. Tennessee Technology Centerat Pulaski

  2. Superheat & Sub-cooling A Technician’s Guide to HVACR Diagnostics

  3. Introduction • The ability to properly and accurately measure superheat and sub-cooling and interpret the results is very likely the single most important skill that you, as an HVACR service technician, can acquire • As you study this program, keep one very important thing in mind ……….

  4. No Test Is Valid If TheAir Flow Is Not Correct!!!

  5. Superheat • A vapor is said to be “superheated” when its temperature is higher than its saturation temperature at the same pressure • Sub-cooling occurs to a liquid when its temperature is below saturation for the same pressure

  6. Checking Superheat • Allow the system to operate for 15 –20 minutes to stabilize • Attach an accurate thermometer to the suction line near the sensing bulb on TEV systems or near the suction service valve on fixed restrictor systems

  7. Record the suction line temperature • Connect a manifold gauge set to the suction service valve and record the suction pressure • Use a temperature/pressure chart to obtain the saturation temperature for the suction gas at the pressure recorded • Subtract the saturated temperature from the actual suction line temperature – the result is the operating superheat

  8. Example 1 • Suction line temperature = 55 F • Suction pressure = 68.5 psig • 68.5 psig = 40 F

  9. 55 F – 40 F = 15 F superheat

  10. Let’s examine some common system problems and see how they affect superheat and sub-cooling

  11. High Superheat Possible causes and remedies for HIGH SUPERHEAT

  12. Excessive or HIGH superheat is an indication of insufficient refrigerant in the indoor coil for the heat load present This could be from insufficient refrigerant entering the coil or from an excessive heat load crossing the coil

  13. LOW REFRIGERANT CHARGE • If there is insufficient refrigerant in the indoor coil, all of the refrigerant will evaporate in the first few passes of the coil. • The excess sensible heat picked up by the refrigerant vapor causes a higher than normal suction gas temperature

  14. Discharge pressure will be lower than normal • Suction pressure will be lower than normal • Superheat will be higher than normal • Sub-cooling will be lower than normal • Current draw will be lower than normal

  15. Liquid Line Restriction • A restriction in the liquid line will not allow sufficient refrigerant to reach the evaporator coil • This causes many symptoms similar to a low refrigerant charge • Often there is a noticeable temperature change at the point of the restriction

  16. Suction pressure will be lower than normal • Discharge pressure will be normal to lower than normal • Superheat will be high • Sub-cooling will be high • Current draw will be low

  17. Evaporator Air Flow Too High • Excessive air flow reduces the latent capacity of the coil thus increasing the sensible heat load. • This additional sensible heat results in higher than normal suction gas temperatures and pressures

  18. Discharge pressure will be high • Suction pressure will be high • Superheat will be high • Sub-cooling will be lower than normal • Current draw will be higher than normal

  19. Excessive Load Conditions • Excessive indoor coil loads will cause a higher than normal heat content in the air crossing the coil • This excess heat will cause the refrigerant liquid to boil away sooner allowing the vapor to pick up additional superheat • Most commonly caused by internal gains such as an increase in occupancy load

  20. Discharge pressure will be higher than normal • Suction pressure will be higher than normal • Superheat will be high • Sub-cooling will be lower than normal • Current draw will be high

  21. Metering Device Not Feeding Properly • A restriction in a capillary tube, orifice or TEV will reduce the amount of liquid refrigerant entering the evaporator. • Symptoms are the same as for a liquid line restriction

  22. Discharge pressure will be lower than normal • Suction pressure will be lower than normal • Superheat will be higher than normal • Sub-cooling will be higher than normal • Current draw will be lower than normal

  23. Low Superheat Possible causes and remedies for Low SUPERHEAT

  24. LOW SUPERHEAT • Low superheat indicates an excess of liquid refrigerant in the evaporator coil • Liquid refrigerant is very likely entering the compressor • This results in reduced compressor life and possible imminent compressor failure

  25. Refrigerant Overcharge • An overcharge forces excessive refrigerant into the evaporator due to increased pressure differential • There is not enough heat present to completely vaporize the excess refrigerant • Compressor failure is likely

  26. Discharge pressure will be higher than normal • Suction pressure will be higher than normal • Superheat will be lower than normal • Sub-cooling will be higher than normal • Current draw will be higher than normal

  27. TEV Overfeeding • Many symptoms similar to an overcharge • Sensing bulb not insulated or not secured properly • Improperly sized valve • Wrong valve for the application

  28. Discharge pressure will be higher than normal • Suction pressure will be higher than normal • Superheat will be lower than normal • Sub-cooling will be lower than normal • Current draw will be higher than normal

  29. Low Evaporator Heat Load • Most common cause of low superheat • Low air volume (dirty coils, filters, restricted duct, etc.) • Reduces the heat available to vaporize the refrigerant • Liquid refrigerant may enter the compressor

  30. Discharge pressure will be lower than normal • Suction pressure will be lower than normal • Superheat will be lower than normal • Sub-cooling will be higher than normal • Current draw will be lower than normal

  31. Improper Metering Device • The wrong orifice • A capillary tube the wrong size (or that has been shortened) • An improperly sized TEV • Symptoms identical to device overfeeding

  32. Discharge pressure will be higher than normal • Suction pressure will be higher than normal • Superheat will be lower than normal • Sub-cooling will be lower than normal • Current draw will be higher than normal

  33. Equipment Oversized • When a system is greatly oversized there is not enough heat to vaporize the refrigerant present in the evaporator • Symptoms are similar to a low charge, except that a low charge will have a high superheat and run excessively • Oversized unit will likely short cycle and have a low superheat

  34. Discharge pressure will be lower than normal • Suction pressure will be lower than normal • Superheat will be lower than normal • Sub-cooling will be higher than normal • Current draw will be lower than normal

  35. Condenser Air Flow • Low condenser air flow or recycled condenser air will increase condensing temperature thus increasing condenser pressure • Increased pressure drop across the metering device results in a flooded evaporator

  36. Caused by: • Dirty coil • Bad motor or blade • Shrubs, bushes or other obstructions • Low overhangs • Other equipment too close

  37. Discharge pressure will be higher than normal • Suction pressure will be higher than normal • Superheat will be lower than normal • Sub-cooling will be lower than normal • Current draw will be higher than normal

  38. SUB-COOLING • A liquid is sub-cooled when its temperature is below saturation at the same pressure • Measuring sub-cooling is a good method of confirming your diagnosis based on other tests • TEV systems MUST be charged by sub-cooling in the absence of a known charge quantity

  39. Measuring Sub-cooling • Allow the system to operate for 15 –20 minutes to stabilize • Attach an accurate thermometer to the liquid line near the inlet of the metering device whenever possible • The condenser outlet may be used, but will be in error by the amount of liquid line pressure/temperature losses

  40. Record the liquid line temperature • Using a gauge manifold, obtain the liquid line pressure • Discharge pressure may be used, but allowances must be made for condenser pressure drop

  41. Using a temperature/pressure chart, convert the pressure reading to saturation temperature • Subtract the line temperature from the saturation temperature • The difference is operating sub-cooling • In the absence of manufacturer’s data, a sub-cooling reading of 10 degrees or more is usually acceptable

  42. Sub-cooling Losses • Long liquid lines • Liquid lines exposed to high ambient temperatures (un-insulated) • Low condenser air flow • Inadequate condenser size • Long vertical lifts

  43. Long Liquid Lines • Long liquid lines cause increased pressure drop due to friction losses • Use the shortest lines possible • Relocate equipment if necessary

  44. Liquid lines exposed to high ambient • High ambient increases liquid line temperature • Exposed liquid lines should be insulated • Heat exchangers or auxiliary sub-coolers may be considered.

  45. Low condenser air flow • Low condenser air flow reduces the condenser’s ability to reject heat • Causes increased condensing temperature • Clean condenser coils and clear any obstructions

  46. Inadequate condenser size • A condenser coil that is too small uses all of the available space for condensing leaving no room for sub-cooling • May be caused by the presence of NCG’s (non-condensable gasses)

  47. Long vertical lifts • The weight of the refrigerant in long vertical lifts causes a pressure drop • HCFC-22 looses about ½ PSIG for every foot of vertical rise • Reduce the lift, use a heat exchanger or artificial liquid line amplification

  48. Practice Exercises

  49. 2 ½ ton package gas unit 5 years old No previous service Not cooling enough odb = 95F Idb = 80F Iwb = 68F SP = 55 PSIG ST = 36F DP = 210 PSIG LT = 96F Current = low Exercise # 1

  50. LOW LOAD • Superheat is low • Sub-cooling is low to normal • Both pressures are low • Current draw is low

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