TK 60275-4-MM-EN
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• The Heat and Defrost indicators turn off and the solid state is de-energized. The controller starts the compressor to
pre-cool the evaporator coil. The condenser fan starts if required.
The controller pre-cools the evaporator coil to the supply air temperature (or for 3 minutes maximum) to minimize heat
energy release into the container. The controller then starts the evaporator fans.
Compressor Digital Control Valve
The Compressor Digital Control valve is normally closed. The normally closed position provides full cooling capacity.
When the controller energizes, it opens the Compressor Digital Control valve. Refrigerant gas flows from the digital port
of the compressor back to the suction line. This disengages the compressor 100 percent and temporarily reduces the
compressor pumping capability.
The controller uses a proportional-integral derivative (PID) algorithm to provide accurate temperature control. This is in
direct response to load demand. However, instead of generating a voltage signal to position a suction line modulation
valve to regulate cooling capacity, the algorithm establishes a pulse width signal to cycle the Compressor Digital
Control valve open and closed on a duty cycle. The percent ON time (compressor pumping time) in the duty cycle
equals the cooling capacity percent required to meet the current load demand.
Remember that the percent ON time defines the time the compressor is engaged. The compressor is engaged
(pumping) when the Compressor Digital Control valve is closed (OFF). Therefore, a duty cycle of 100 percent means the
compressor is pumping 100 percent of the time and the Compressor Digital Control valve is ON (open) 0 percent of the
time. A 60 percent duty cycle means the compressor is pumping 60 percent of the time and the Compressor Digital
Control valve is ON (open) 40 percent of the time.
Economizer System
A vapor injection line tee is located in the liquid line between the filter drier/in-line filter and the economizer heat
exchanger. A vapor injection valve controls refrigerant flow through the vapor injection line to the economizer
expansion valve. When this normally closed valve is energized (open), a portion of liquid refrigerant flows through the
economizer expansion valve and evaporates in the inner coiled tube of the economizer. This cools the rest of the liquid
refrigerant that flows past the tee and through the economizer to the evaporator coil.
The economizer suction gas continues through the vapor injection circuit and returns to the intermediate suction port of
the scroll compressor. Injecting the economizer suction gas into the compressor downstream from the suction port
prevents the gas from affecting the suction pressure or cooling capacity of the evaporator coil. However, the
economizer suction gas adds its heat and volume to the condenser side of the refrigeration system, increasing the
discharge pressure.
Because the economizer system increases system cooling capacity, the vapor injection valve is energized (open)
continuously when the compressor duty cycle (ON time) is 100 percent (Full Cool). High compressor discharge
temperature may cause the vapor injection valve to energize (open) but only while the Compressor Digital Control valve
is not energized (closed).
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