Refrigeration Systems
Know the compressors, controls and defrost methods used in commercial coolers and freezers, what normal readings look like, and how to maintain them.
Table of contents
A walk-in cooler is not just a small air conditioner. It runs year-round, holds food at temperatures that leave no room for error, and it has to melt its own frost. It has parts a home system does not: receivers, sight glasses, defrost heaters and pressure controls. This guide covers the equipment you find in restaurants, grocery stores and cold storage, and how to keep it running.
What You Will Be Able to Do
- Match box temperature to the expected evaporator temperature.
- Identify the compressors and system parts on commercial equipment.
- Explain what the common controls and safeties do.
- Compare the defrost methods and solve defrost faults.
- Set up a maintenance schedule.
Temperature Ranges
| Application | Box temperature | Typical evaporator temperature |
|---|---|---|
| Medium-temperature cooler | 34 to 38°F | About 10 to 15°F below the box |
| Low-temperature freezer | -10 to 0°F | About 10°F below the box |
| Display case | Varies by product | Set by the manufacturer |
A walk-in held at 35°F usually has a coil temperature of about 20 to 25°F. On a medium-temperature R-404A system, that puts suction pressure around 55 to 62 psig, with head pressure around 275 to 325 psig on a 95°F day. Those numbers are starting points. Check the equipment data for the job.
Target subcooling at the condenser outlet is usually 10 to 15°F. Superheat at the evaporator outlet is often 8 to 12°F, with total superheat at the compressor around 20 to 30°F. A measurement at the coil outlet shows you liquid is fully boiled off, which protects the compressor from slugging.
Compressors
| Type | How it works | Where you find it |
|---|---|---|
| Hermetic reciprocating | Pistons, welded shell, not field repairable | Small reach-ins and freezers |
| Semi-hermetic reciprocating | Pistons, bolted housing, valves can be serviced | Medium commercial systems |
| Open drive | Separate motor, belt or coupling | Large industrial and ammonia systems |
| Scroll | Two interlocking spirals, one fixed and one orbiting | Medium-temperature systems and most AC |
| Screw | Two helical rotors, slide valve for capacity | Supermarket racks, cold storage |
| Centrifugal | Spinning impeller | Very large chillers |
A scroll compressor compresses in a smooth, continuous motion with few moving parts. Reciprocating compressors can use cylinder unloading to reduce capacity at lighter loads.
Parts a Home System May Not Have
Receiver. A storage tank for liquid refrigerant on the high side. It holds the extra charge and lets refrigerant move as the load changes. Pump-down stores the charge here.
Filter-drier. Sits in the liquid line and removes moisture and particles. A temperature drop across it points to a restriction. Replace it whenever the system is opened.
Sight glass. A small window in the liquid line that shows the refrigerant's condition. Clear liquid is normal, and steady bubbles suggest a problem. A moisture indicator shows if water is in the system.
Solenoid valve. An electric valve in the liquid line, often used for pump-down.
Accumulator. Catches liquid before it reaches the compressor on some systems.
Oil separator. Returns oil to the compressor on larger systems, especially racks.
Crankcase heater. Keeps refrigerant from settling in compressor oil during the off cycle.
Metering devices. A thermostatic expansion valve (TXV) uses a sensing bulb on the suction line to control superheat. Electronic expansion valves are more precise. Capillary tubes are used in small sealed equipment.
Controls and Safeties
| Control | What it does |
|---|---|
| High-pressure cutout | Shuts the compressor off at excessive head pressure |
| Low-pressure control | Shuts off the compressor on low suction, and is used for pump-down |
| Oil pressure safety | Protects compressors with oil pumps |
| Defrost timer or controller | Starts and stops defrost |
| Head pressure control | Holds condensing pressure up in cold weather |
Head pressure can fall too low in cold weather, which starves the metering device. Fan cycling and flooded condenser valves keep it within range. High head pressure means the condenser is not rejecting heat well. The cause is usually a dirty coil, a failed fan, too much refrigerant or air in the system.
Pump-Down Cycle
Many systems pump down when the thermostat is satisfied. The liquid line solenoid closes, the compressor pulls refrigerant out of the evaporator into the condenser and receiver, and the low-pressure control stops the compressor. This keeps liquid out of the compressor during the off cycle.
Walk-In Cooler and Freezer Basics
- Door gaskets, sweeps and closers control load. A door that does not seal brings in warm, moist air and heavy frost.
- Frame heaters on freezer doors stop the door from freezing shut.
- Evaporators hang from the ceiling, with drains that need heat and a slope in a freezer.
- Product placement must allow airflow around the coil.
- Keep the condensing unit clean and out of hot, stale air.
Defrost
Frost on a coil insulates it and blocks air, so capacity falls. Every system that runs below freezing needs a defrost plan.
| Method | How it works | Where it is used |
|---|---|---|
| Off-cycle | Compressor stops, fans keep moving box air over the coil | Medium-temperature boxes above 32°F |
| Electric | Heaters warm the coil with compressor and fans off | Most freezers and low-temperature cases |
| Hot gas | Discharge gas is sent through the coil | Larger commercial and industrial systems |
Electric defrost usually runs 2 to 6 times in 24 hours. It ends when a termination thermostat reaches a set point, often 50 to 55°F, or when a time limit runs out. A drain pan heater stops the drain from refreezing.
Defrost faults:
| Symptom | Likely cause | What to check |
|---|---|---|
| Coil packed with ice | Failed heater, timer or termination control | Heater continuity, control operation |
| Box warms too much during defrost | Defrost too long | Duration, termination setting |
| Water on the floor | Plugged or frozen drain | Drain line, drain heater |
| Ice on the fan blades | Fans running too soon after defrost | Fan delay setting |
Common System Problems
| Symptom | Likely cause | Next step |
|---|---|---|
| High head pressure, high subcooling, hot compressor | Dirty condenser coil | Clean the coil, inside out |
| High head pressure, normal subcooling, cool weather | Air in the system | Recover, evacuate, weigh in |
| Low suction, frost at a point in the liquid line | Restriction, often at a drier or TXV | Check for a temperature drop across the part |
| Low suction, high superheat, low subcooling | Low charge | Find the leak before adding refrigerant |
| Suction line sweating or frosting at the compressor | Liquid returning, low superheat | Check TXV, bulb, airflow, overfeeding |
| High suction, low head pressure, poor cooling | Compressor valve failure | Check amps and a pump-down test |
Choosing and Handling Refrigerants
Commercial refrigeration is moving away from R-404A, which has a GWP of 3,922.
| Refrigerant | Notes |
|---|---|
| R-404A, R-507A | Older HFC blends with very high GWP |
| R-448A, R-449A | Lower-GWP replacements, about 1,400 |
| R-134a | Medium-temperature systems, GWP 1,430 |
| R-290 (propane) | Small self-contained units, A3 flammable, strict charge limits |
| R-744 (carbon dioxide) | GWP 1, can run at 1,400 psig or more |
| R-717 (ammonia) | GWP 0, B2L, large industrial plants |
New equipment rules vary by type and date. For example, stand-alone retail units have had a limit of 150 GWP since 2025, and supermarket systems have a limit of 1,400 GWP from 2027, with lower limits planned for 2032. Check EPA's current list, because dates have moved.
Carbon dioxide systems need gauges, hoses and training built for their pressure. Ammonia is toxic, with an OSHA limit of 50 ppm over 8 hours and an immediately dangerous level of 300 ppm. Ammonia work is specialized, and you need formal training.
Warning: A commercial system can hold a large charge in a small room. Ventilate before you enter a suspected leak, and respect refrigerant alarms. Recover refrigerant before any brazing.
Maintenance Schedule
Monthly
- Check pressures, temperatures and box temperature logs.
- Inspect the condenser for dirt.
- Confirm condenser fan operation.
- Listen for noise and vibration.
- Look for oil stains.
Quarterly
- Clean the condenser coil.
- Clean the evaporator coil and drain pan.
- Check defrost operation, including heaters, drain and termination.
- Inspect and tighten electrical connections.
- Check compressor oil level on semi-hermetic and open-drive units.
- Test high-pressure, low-pressure and oil safety controls.
Annually
- Review the full system's performance.
- Check all joints for leaks.
- Run an insulation resistance test on compressor windings with a megohmmeter.
- Service fan motors and bearings.
- Review refrigerant records.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Treating a walk-in like a home AC | Wrong targets and wrong diagnosis |
| Ignoring door gaskets | Heavy frost, and a long run time |
| Skipping defrost checks | Ice-packed coils and lost product |
| Topping off without finding a leak | Lost refrigerant and leak rule problems |
| Using R-410A gauges on a CO2 system | Gauge failure at very high pressure |
Check Your Understanding
A walk-in holds 35°F. The coil reads about 5°F. What does that suggest? The coil is running colder than the usual 20 to 25°F, so check for low charge, low airflow or a restriction.
An R-404A cooler has a suction pressure of about 60 psig, and box temperature is on target. Is that a problem? No. That is normal for a medium-temperature system.
A freezer's coil is packed solid with ice. Where do you start? Test the heater, the defrost control and the termination switch.
On a cool morning, head pressure is high but subcooling looks normal. What does that point to? Air or other non-condensable gas in the system.
Key Takeaways
- A medium-temperature coil runs about 10 to 15°F below box temperature.
- A receiver stores liquid, a drier cleans it and a sight glass shows its condition.
- High head pressure means the condenser cannot reject heat.
- Frost insulates the coil, so defrost is part of normal operation.
- New refrigerants bring new pressures and hazards, so train before you touch them.
- Follow a monthly, quarterly and annual maintenance plan.