by Ron Walker
In this application, rather than controlling the cooler 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 which 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. This is also an off-cycle defrost.
In this application, rather than controlling the cooler 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 which 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. This is also an off-cycle defrost.