Introduction to Defrost HVACR318 – Refrigeration Introduction to Defrost

Purpose of Defrost Systems Defrost systems are required in low temp and extremely low temperature applications to remove the ice that builds up on the evaporator coil. Ice is formed when warmer, humid air comes into contact with the below freezing conditions of an evaporator coil.

Type of Defrost Hot Gas Defrost Electric Defrost Uses a solenoid to re-route the discharge hot gas from the compressor to the evaporator bypassing the metering device and condenser. Electric Defrost Shuts down the compressor and the cooling system and energizes an electric heater.

Defrost Controls The controls required to run a defrost cycle are simple: Defrost Clock/Time Solenoid Valve

Defrost Timer Controls Initiates and terminates defrost cycle. Defrost frequency times can be changed by adding, moving, or installing more “set” pins. Try and keep the cycles “proportional” and keep in mind customers busy periods.

Defrost Time Controls The same controls are used for both hot gas and electric defrost systems. You do not need different timers as they are pretty much standard. Some contain a termination (clutch) solenoid to prematurely take the control out of defrost when needed.

Defrost Timer with Solenoid Timer Motor N 1 Source Source Copper band 3 2 Defrost Heater Or Hot gas Solenoid X 4 To Dtt To evaporator fan motor To refrigeration circuit (comp and cfm)

Defrost Timer with Solenoid Timer Motor X 3 Source Source N Copper band 1 2 Defrost Heater Or Hot gas Solenoid 4 To Dtt To evaporator fan motor To refrigeration circuit (comp and cfm)

Defrost Timer No Solenoid With X terminal Timer Motor N X Source Source 1 Defrost Heater Or Hot gas Solenoid 3 2 4 To evaporator fan motor To refrigeration circuit (comp and cfm)

Defrost Timer No Solenoid No X terminal Timer Motor N 1 Source Source 3 2 Defrost Heater Or Hot gas Solenoid 4 To evaporator fan motor To refrigeration circuit (comp and cfm)

Defrost Time Control Defrost Termination DTT Type DTT/High Limit Switch: Standard DTT which is a Defrost Thermostat, set at a high temperature to open when the ice has been melted or the coil reaches a specific temperature. The DTT de-energizes the heater.

Defrost Time Control Defrost Termination DTT Fan Delay Type Terminates and locks out the defrost cycle A Single Pole double Throw temperature actuated switch that runs the fan in refrigeration mode and will energize the release clutch solenoid (in the DTC) in the defrost mode.

Defrost Time Control When the control terminates the defrost cycle the EFM will be delayed until the compressor can make the evaporator cold enough to switch the control back to the EFM circuit. This control prevents a “Hot Pull Down” scenario on the compressor. Hot pull down means hot gas being returned to the inlet of the compressor.