Commercial Refrigeration
Commercial Refrigeration Service covers self-contained and remote systems: walk-in coolers and freezers, display cases, ice machines, and supermarket racks. NATE-style items test the refrigeration cycle, compressor/condenser/evaporator/metering-device service, defrost and controls, evacuation and charging, electrical diagnosis, and refrigerant safety.
Core rules
- Superheat is actual suction-line temperature minus saturated suction temperature at the same pressure. At the evaporator outlet, a typical TEV target is 6–12°F, but follow the manufacturer. Low superheat risks floodback; high superheat means a starved coil.
- Subcooling is saturated condensing temperature minus liquid-line temperature. It confirms liquid seal and charge on many systems; measure at the condenser outlet or the manufacturer's point.
- Defrost must terminate on temperature or pressure, not time alone. Verify the termination sensor, heater amp draw, fan delay, and drain.
- Evacuate to 500 microns and perform a decay test. Moisture and noncondensables cause acid, wax, and false pressure readings.
- Pressure controls: low-pressure cut-out protects against low charge; oil-pressure safety switches have a time delay and lock out the compressor.
Worked example
A low-temperature R-404A walk-in runs continuously. Suction pressure is 15 psig, superheat at the evaporator outlet is 30°F, the sight glass is clear, and subcooling is 10°F. What is the likely fault? A clear sight glass and normal subcooling make undercharge unlikely. High superheat means the TEV is not feeding enough refrigerant. Check the TEV inlet strainer, power-head bulb contact and charge, orifice, and any liquid-line restriction. If the bulb is loose, the valve closes down and superheat rises. Fix the bulb first, then recheck before adjusting the valve.
Traps
Do not charge by suction pressure alone. Do not adjust a TEV before checking bulb and strainer. Do not accept a defrost that terminates only on time. Do not ignore the oil-pressure safety delay. Do not vapor-charge a zeotropic blend; charge liquid.