Cejas Cooling & Construction

Between Pressure and Pulse

Between Pressure and Pulse: Diagnostics in the Cold Trenches

When refrigeration systems go off the rails, it’s not always fireworks. Sometimes it’s the soft hiss of pressure equalizing, the whisper of a compressor gasping for life, or the eerie silence of a freezer that should be humming. This is where real technicians earn their stripes—not in the install, but in the unglamorous hours spent reading pressures like scripture and catching faults that hide in plain sight.

Troubleshooting High-, Medium-, and Low-Temperature Systems

Each system dances to its own beat:

  • High-temp systems (coolers, beverage units) run evaporator temps around 35°F to 45°F. Suction pressure typically 60–70 psig with R-134a.
  • Medium-temp systems (deli, dairy) hold 20°F to 30°F evap temps. Suction pressure is lower—maybe 40–50 psig with R-404A.
  • Low-temp systems (freezers, ice cream)? Welcome to the subzero club. Evap temps at -10°F to -20°F. Suction pressure often below 20 psig.

Each has its own rhythm, and if you don’t know the song, you won’t hear when it’s out of tune.

Air-Cooled Equipment Operating Conditions

  • Condenser Temp Split: Expect 25°F to 30°F above ambient air temp
  • Subcooling: Typically 10°F to 15°F
  • Discharge Pressure: Proportional to ambient; high temps = higher head pressure
  • Common Issue: Dirty condenser coil = high head pressure = inefficient system

Water-Cooled Equipment Operating Conditions

  • Condenser Water Outlet Temp: 10°F to 12°F above inlet temp
  • Head Pressure: More stable due to consistent water temps
  • Subcooling: Still around 10°F–15°F
  • Red Flags: Scale buildup or poor water flow will spike head pressure and tank performance

Symptoms of an Incorrect Charge

  • Undercharged:
    • Low suction pressure
    • High superheat
    • Bubbles in sight glass
    • Evaporator starved and sweating
  • Overcharged:
    • High suction and head pressure
    • Flooded condenser
    • Poor subcooling
    • Compressor running hot

Symptoms of a System with a Restriction

  • At the Liquid Line:
    • High subcooling
    • Starved evaporator
    • Frost after the restriction
  • Cap Tube Issues:
    • Low suction pressure
    • Flash gas noise
  • TXV Bulb Malfunction:
    • Erratic superheat
    • Hunting pressures

Symptoms of an Inefficient Compressor

  • Won’t reach proper head pressure
  • Suction pressure won’t drop under load
  • Amp draw is below rated spec
  • Poor temperature pull-down
  • Constant run time with low cooling effect

Testing Compressor Pumping Capacity

Method One: Gauges & Isolation

  • Close the suction service valve
  • Let the unit run and observe how deep it pulls vacuum
  • A good compressor should pull well below atmospheric pressure (under 0 psig)

Method Two: Pump-Down Test

  • Shut off the liquid line
  • Let the unit run into a vacuum
  • Watch the low-side gauge: if it doesn’t drop or rebounds quickly, you’ve got worn valves or mechanical issues

Diagnosing and Repairing Common System Faults

  • TXV Not Feeding Properly:
    • Check bulb placement
    • Ensure line is insulated
    • Test with known load
  • Contaminated Refrigerant:
    • Use a refrigerant identifier
    • Recover, flush, replace
  • Short Cycling:
    • Check pressure controls
    • Evaluate refrigerant charge
    • Verify airflow and sensor placement
  • Frost Patterns:
    • Uniform frost = good
    • Patchy or back-to-compressor frost = mischarge, airflow, or restriction

Refrigeration diagnosis isn’t about chasing ghosts—it’s about reading the quiet signals. You look, you listen, you let the system talk. Every psi, every amp, every degree tells a story. Miss the signs and you’re just swapping parts. Catch them early, and you become the guy who saves the day before the beer gets warm.