Thermo King 098203, Руководство по обслуживанию

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Operating Theory
134
When the defrost mode is terminated:
• 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 MP-4000 controller pulses the Compressor Digital Control solenoid valve between open and closed
positions. This provides precise cooling capacity control. No pump down function or warm gas bypass
control is used in conjunction with the Compressor Digital Control valve.
Figure 104: Compressor Digital Control Solenoid 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.
AXA0428