xdxadmin
08-31-2004, 09:16 AM
Electric Defrost
When electric defrost systems are specified, electric defrost heaters in the fixtures are connected to the contactor in the control panel. Defrost is initiated by either time clocks, or the system electronic management system. Defrost termination may be determined by:
• Time
• Increase in evaporator pressure
• Temperature actuated by –
- Electronic sensor signaling the electronic
controller
- Solenoids in the time clocks connected to
thermostats at the evaporator.
- Thermostat causing Defrost Heater Circuit to open
- Air defrost continues
- Amperage sensor triggers Solenoids in the
timeclock
When employing this type of defrost system, when defrost is initiated, the compressors continue to run until pump-down is completed, usually a short time. Nonetheless, at the same time the compressors are pumping down, the electric defrost heaters come on. The electric feed to the refrigeration system control panel, or for that matter the sub-panel in the field, needs to be sized accordingly for this temporary combined load. Split or self-contained refrigeration systems should be equipped with isolation or auxiliary switches to prevent current to the defrost until the compressor is not running. This reduces circuit load, compressor load, and kW.
If the compressors run for a short period of time at the beginning of defrost, the defrost can occasionally be a little longer when a pump-down solenoid is used. In addition, the removal of liquid refrigerant from the inside of the evaporator coils to spread the heat a little more quickly in an even and uniform manner, can also cause the ends of the coil to warm slower, although liquid boil-off can sometimes keep the evaporator cooler for a longer period. Because these factors exist, it is important to carefully set and adjust the electric defrost time duration. Monitor defrosts to access the proper settings after referring to OEM recommendations.
When electric defrost systems are specified, electric defrost heaters in the fixtures are connected to the contactor in the control panel. Defrost is initiated by either time clocks, or the system electronic management system. Defrost termination may be determined by:
• Time
• Increase in evaporator pressure
• Temperature actuated by –
- Electronic sensor signaling the electronic
controller
- Solenoids in the time clocks connected to
thermostats at the evaporator.
- Thermostat causing Defrost Heater Circuit to open
- Air defrost continues
- Amperage sensor triggers Solenoids in the
timeclock
When employing this type of defrost system, when defrost is initiated, the compressors continue to run until pump-down is completed, usually a short time. Nonetheless, at the same time the compressors are pumping down, the electric defrost heaters come on. The electric feed to the refrigeration system control panel, or for that matter the sub-panel in the field, needs to be sized accordingly for this temporary combined load. Split or self-contained refrigeration systems should be equipped with isolation or auxiliary switches to prevent current to the defrost until the compressor is not running. This reduces circuit load, compressor load, and kW.
If the compressors run for a short period of time at the beginning of defrost, the defrost can occasionally be a little longer when a pump-down solenoid is used. In addition, the removal of liquid refrigerant from the inside of the evaporator coils to spread the heat a little more quickly in an even and uniform manner, can also cause the ends of the coil to warm slower, although liquid boil-off can sometimes keep the evaporator cooler for a longer period. Because these factors exist, it is important to carefully set and adjust the electric defrost time duration. Monitor defrosts to access the proper settings after referring to OEM recommendations.