The refrigeration cycle for medium temperature walk-in coolers is exactly the same as residential air conditioning systems. However, the operation of the physical components is slightly different.
- The compressor is energized by a potential relay.
- The condenser fan is wired in parallel with the compressor and runs whenever the compressor is energized.
- The system may or may not have a contactor to provide power to the compressor.
- The evaporator fan(s) run continuously circulating cool air through the cooler whether or not the compressor is running.
- Unlike a residential air conditioning system, mild frost may be present on the evaporator coil.
- Mild frost is not a concern as the evaporator fan(s) continue to run when the compressor is in the off cycle.
- Circulating 35 to 39°F air over the evaporator coil. This, above freezing, air will melt the frost on the coil during the off cycle.
Basic Operation of Temperature Controlled Medium Temperature Walk-in Coolers
Commercial refrigeration system is controlled by a close on rise in temperature thermostat. You will find either a mechanical type with a sensing bulb or a digital thermostat with a thermistor. Most of the walk-in coolers that require repair will be older and may have been retrofitted with a digital thermostat.
Most walk-in cooler thermostats are set to 35°F. Like residential air conditioning thermostats, as the temperature rises, the thermostat closes, applying power to the compressor which is energized by a current relay. The condenser fan is wired in parallel with the compressor and runs whenever the compressor is energized. The system may or may not have a contactor to provide power to the compressor.
As the box reaches temperature, the thermostat opens, de-energizing the compressor and condenser fan. REMEMBER – the evaporator fan(s) run continuously circulating cool air through the cooler whether or not the compressor is running.
This is called an off-cycle defrost system.
Basic Operation of Pressure Controlled Medium Temperature Walk-in Coolers
In this application, rather than controlling the commercial refrigeration temperature with a thermostat, is is controlled by a low pressure cut-out switch. The low pressure cut-out switch is connected to the suction side of the compressor and is set to cut-out (or turn off the compressor) at 15-20°F below the cooler box temperature (35°F).
As the temperature in the cooler begins to rise so does the suction side pressure. Once the pressure reaches the cut-in point the pressure switch closes, applying power to the compressor
This is also an off-cycle defrost system.
Basic Operation of Automatic Pump Down Medium Temperature Walk-in Coolers
This type of system is usually found where the condensing unit is located outside and the outdoor temperatures could drop below the cooler temperature.
In this system there is both a solenoid valve and a low pressure cut-out switch. The cooling thermostat (mechanical or digital) controls a solenoid valve connected to the liquid line inside the cooler. The solenoid valve is usually located in on the back side of the evaporator.
The low pressure cut-out switch is used to energize and de-energize the compressor.
The thermostat is set to de-energize power to the liquid line solenoid at 35°F cooler temperature. When the solenoid is de-energized, it stops the flow of liquid refrigerant to the expansion valve and the evaporator coil.
The compressor will continue to run pulling the refrigerant out of evaporator coil, suction line, and part of the liquid line. All of the refrigerant is pumped into and stored in the condenser and receiver. The compressor will continue to run, pulling refrigerant out of the cooler until the suction pressure reaches the low pressure cut-out switch setting which is usually set at 2 psig.
Once the cut-out pressure is reached the low pressure cut-out switch de-energizes the compressor. The evaporator fan(s) run continuously circulating cool air throughout the cooler.
During the off-cycle, the Commercial refrigeration temperature will gradually increase to the cut-in temperature of the thermostat (usually 38-39°F). When the thermostat closes it energizes the liquid line solenoid. This releases the stored refrigerant into the expansion valve, coil, and suction line. Once the suction line pressure reaches the cut-in temperature of the low pressure cut-out switch, the compressor will start and the cooling cycle will begin.
This is called an off-cycle defrost system with an automatic pump down cycle.