Defrost mode
When ice builds up on the ambient coils, an automatic defrost cycle is initiated with the air-source
coil working as a condenser. Each of the refrigerant circuits are defrosted separately. When in
defrost mode, the circuit operating in heat pump mode is in balance with the circuit operating in
defrost (cooling). Heat output is, at worst case, net zero during defrost period.
Advanced defrost logic is employed to reduce defrost cycle time and system impact. This includes
the following capabilities:
• Defrost based on the refrigerant pressure.
• Subcooling circuits are designed to prevent ice build-up at the bottom of the coil.
• Defrost staging logic that cycles circuits through defrost sequentially, minimizing deviation
from setpoint during defrost. No more than half of the total number of circuits can defrost at
the same time. In the most extreme case, the system provides net zero heating output for the
defrost period.
• The defrost generates water, which collects in the unit drain pan below the coils and channels
out the bottom of the unit frame through large diameter flexible tubing. The drain pan and
tubing are freeze protected by the heat generated inside the unit cabinet. It is possible to
include a drain under each unit to collect condensate water and move it away from the working
area surrounding the unit.
Figure 3: Ambient heat exchanger
25YMAE Air-Cooled Inverter Scroll Chiller and Heat Pump
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