Carrier 69NT40-561-300, Operation And Service Manual

Page: 31 / 174

3–9 T-365
3.5 REFRIGERATION CIRCUIT
3.5.1 Standard Operation
Starting at the compressor, (see Figure 3.6 , 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. When operating
with the air-cooled condenser active, air flowing across the coil fins and tubes cools the gas to saturation tempera-
ture. By removing latent heat, the gas condenses 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 operation) 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 valves, digital loader valve (DLV) and digital unloader
valve (DUV), control the system refrigerant flow and capacity by loading and unloading the compressor in frequent
discrete time intervals. The DLV and DUV operate in opposition to each other such that when the DLV is closed the
DUV is open and vice versa. The valves cycle on a fixed duty cycle so that maximum capacity occurs when the
DLV is open 100% of the time and the DUV is open 0% and minimum capacity is when the DLV is open 0% of the
time and the DUV 100% of the time. If the system capacity has been decreased to the lowest allowable capacity,
the unit will enter a trim heat mode of operation, during which the controller will pulse the evaporator heaters in
sequence with the compressor digital signal in order to absorb the excess capacity.
3.5.2 Economized Operation
In the economized mode, (see Figure 3.7 ) the frozen and pull down capacity of the unit is increased by sub- cool-
ing the liquid refrigerant entering the electronic expansion 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 taken 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 economizer expansion 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 fitting.
3.5.3 Economizer Expansion Valve
The microprocessor controls the superheat leaving the economizer expansion valve (EXV). From the EXV the
refrigerant flows through the internal passages of the economizer heat exchanger, absorbing heat from the refriger-
ant flowing to the EEV. The resultant “medium” temperature/pressure gas enters the compressor at the economizer
port fitting.
Abnormal pressures/temperatures in
the high refrigerant side
Fusible Plug - Used on the Receiver 99°C = (210°F)
Rupture Disc - Used on the Water-
Cooled Condenser
35 kg/cm
2
= (500 psig)
Abnormally high discharge pressure High Pressure Switch (HPS) Opens at 25 kg/cm
2
(350 psig)
Table 3–2 Safety and Protective Devices (Continued)
UNSAFE CONDITION DEVICE DEVICE SETTING