Measuring and Adjusting Operating Superheat

Measuring:

  1. Determine the suction pressure or saturation temperature at the evaporator outlet with an accurate gauge. On close-coupled installations, suction pressure may read at the compressor suction connection.
  2. Using a pressure temperature chart determine the saturation temperature at the observed suction pressure.
  3. Measure the temperature of the suction gas at the remote bulb location as follows:
  4. Clean area of the suction line at the bulb location and tape the
    thermocouple to the area cleaned.
  5. Insulate the thermocouple, read the temperature using an electronic thermometer.
  6. Subtract the saturation temperature determined in step 2 from the sensible measured in step 3. The difference is the superheat of suction gas at the evaporator outlet.

 

Preparation of refrigeration system for valve superheat adjustment:

  1. Requires operational head pressure and a solid column of liquid at the TEV inlet.
  • False load condenser if flooded head pressure control system is not utilized.
  1. Case or space must be under full load condition.
  • False load evaporator.
  1. Evaporator Pressure Regulators must be in a fully open position.
  • Use manual stem to open when available, or shut of the hot gas supply to the pilot of the ne energy efficient “BEPR(S),” or counterclockwise rotation of the regulator adjusting stem. (Ensure that there is minimum pressure drop thru the EPR.)
  1. On a multi-compressor system, extra precautions are required:
  2. Ensure constant suction pressure at design.
  3. Ensure constant discharge pressure at design.
  4. Ensure constant receiver outlet pressure at design.
  5. Control compressor cycling to match design suction pressures by isolating the automatic electronic/L.P. controls. Switch off the necessary compressors to maintain the design suction pressures.
  6. Isolate defrost control, especially on gas defrost systems to maintain discharge/receiver pressure.
  7. Isolate heat reclaim to ensure it doesn’t operate during superheat adjustments.
  8. Set TEV superheat
  • Adjusting an externally adjustable valve is accomplished without removing any lines from the valve, thus eliminating the need to pump the unit down. Removing the seal cap from the valve will expose the adjustment stem. Rotating the stem clockwise decreases refrigerant flow thru the valve and increases superheat. Rotating the stem counterclockwise increases the refrigerant flow thru the valve and lowers the superheat.
  1. Recheck superheat under low load condition.
  2. Re-adjust EPR pressure settings.
  3. Remove all false loads to the evaporator and condenser and reset all isolated controls.

 

GENERAL RULE OF THUMB-SUPERHEAT AT EVAPORATOR OUTLET

  1. High temperature: 10° to 12°F superheat (30°F evaporator and above)
  2. Medium temperature: 5° to 10°F superheat (0 to 30°F evaporator)
  3. Low temperature: 2° to 5°F superheat (below 0°F evaporator)