Carrier 69NT40-561-201, Operation & Service Manual, Page: 26 / 117

T-364 2 − 9
2.5 REFRIGERATION CIRCUIT
2.5.1 Standard Operation
Starting at the compressor, (see Figure 2 − 6), upper
schematic) the suction gas is compressed to a higher
pressure and temperature.
The refrigerant gas flows through the discharge line and
continues into the air − cooled condenser. Air flowing
across the coil fins and tubes cools the gas to saturation
temperature. By removing latent heat, the gas con-
denses to a high pressure/high temperature liquid and
flows to the receiver, which stores the additional charge
necessary for low temperature operation.
The liquid refrigerant continues through the liquid line,
the filter drier (which keeps refrigerant clean and dry)
and the economizer (not active during standard opera-
tion) to the electronic expansion valve (EEV).
As the liquid refrigerant passes through the variable
orifice of the EEV, the pressure drops to suction
pressure. In this process some of the liquid vaporizes to
a gas (flash gas), removing heat from the remaining
liquid. The liquid exits as a low pressure, low
temperature, saturated mix. Heat is then absorbed from
the return air by the balance of the liquid, causing it to
vaporize in the evaporator coil. The vapor then flows
through the suction tube back to the compressor.
During the standard mode of operation, the normally
closed digital unloader valve (DUV) controls the system
refrigerant flow and capacity by loading and unloading
the compressor in frequent discrete time intervals. If the
system capacity has been decreased to the lowest al-
lowable capacity with the DUV, the unit will enter a trim
heat mode of operation, during which the controller will
pulse the evaporator heaters in sequence with the com-
pressor digital signal in order to absorb the excess capa-
city.
2.5.2 Economized Operation
In the economized mode, (see Figure 2 − 7) the frozen
and pull down capacity of the unit is increased by sub-
cooling the liquid refrigerant entering the electronic ex-
pansion valve. Overall efficiency is increased because
the gas leaving the economizer enters the compressor
at a higher pressure, therefore requiring less energy to
compress it to the required condensing conditions.
Liquid refrigerant for use in the economizer circuit is tak-
en from the main liquid line as it leaves the filter drier.
The flow is activated when the controller energizes the
economizer solenoid valve (ESV).
The liquid refrigerant flows through the ESV to the ex-
pansion valve internal passages, absorbing heat from
the liquid refrigerant flowing to the electronic expansion
valve. The resultant “medium” temperature/pressure
gas enters the compressor at the economizer port fit-
ting.
When the control air temperature falls to 2.0 C (3.6 F)
above set point, the DUV unloads the compressor’s
scroll and begins to reduce the capacity of the unit. Per-
centage of the unit capacity is accessed through code
select 01 (Cd01). For example, if Cd01 displays 70, it in-
dicates that the compressor is operating unloaded with
the DUV engaged 30% of the time.
2.5.3 Electronic Expansion Valve
The microprocessor controls the superheat leaving the
evaporator via the electronic expansion valve (EEV),
based on inputs from the evaporator pressure trans-
ducer (EPT). The microprocessor transmits electronic
pulses to the EEV stepper motor, which opens or closes
the valve orifice to maintain the superheat set point.