A52x Series Electronic Refrigeration Controller with Adaptive Defrost Technical Bulletin
7
A
defrost duration
is the time period that the refrigeration system is in defrost mode and the system is melting ice
off of the evaporator coil. The
defrost duration
may include Drip Time. A
defrost interval
is defined as the time
between the start of a defrost cycle and the start of the next defrost cycle.
Figure 6: Defrost Intervals and Defrost Durations
Defrost Types
The A52x Controller provides defrost control for off-cycle defrost systems on medium temperature refrigeration
applications of 2C° to 7C° or 35F° to 45F°. The A52x Controller also provides defrost control for electric heat or hot
gas defrost systems on low temperature refrigeration applications of < 2°C or 35°F.
Off-Cycle Defrost
Off-Cycle (passive) Defrost does not use the defrost relay. Off-Cycle Defrost is the defrost type commonly used on
medium temperature refrigeration applications. Off-Cycle Defrost is not used on low temperature applications.
During Off-Cycle Defrost, refrigerant flow to the evaporator is interrupted. The evaporator fans remain on to move
air over the evaporator coil and melt any accumulated frost or ice. During Off-Cycle Defrost, the compressor relay
remains off for the defined defrost duration. Off-Cycle Defrost uses Timed Defrost Termination only. Off-Cycle
Defrost does not use an evaporator sensor or switch, and temperature defrost termination cannot be set up.
Electric Defrost
Electric Defrost uses electric resistive heating elements controlled by the defrost relay to melt the ice that
accumulates on the evaporator coil during normal cooling operation. The electric resistive heating elements are
typically embedded in or positioned near to the evaporator coil and drain pan. Electric Defrost is often used on low
temperature refrigeration applications. During Electric Defrost, the compressor relay remains off for the defrost
duration, the defrost relay is on, and the evaporator fan relay or relays are usually off. Electric Defrost cycles use
either temperature-based defrost termination or time-based defrost termination.
Hot Gas Defrost
Hot Gas Defrost systems use a bypass valve controlled by the defrost relay and extra refrigerant piping to
temporarily reroute the hot gas discharge from the compressor through the evaporator coil. The re-routed hot gas
melts the accumulated ice on the evaporator coil. Hot Gas Defrost is used on low and very low temperature
refrigeration applications. During the defrost duration, the compressor relay is on, the defrost relay is on, and the
evaporator fan relay or relays are usually off. Hot Gas Defrost cycles typically use temperature-based defrost
termination but may be set up for time-based defrost termination.
Defrost Termination Types
The A52x Controller can terminate the defrost durations based on time or temperature. Temperature termination
applications use a temperature sensor (Sn2) or a temperature switch mounted on the evaporator coil. On
applications that use temperature-based termination by sensor or switch, the maximum defrost duration value is
also defined in minutes. The defrost duration value terminates any defrost durations that do not reach the defined
defrost termination temperature.
