Carrier 69NT40-541-505, Operation And Service Manual, Page: 29 / 143

3–9 T-368
3.5 REFRIGERATION CIRCUIT
Starting at the compressor (see Figure 3.6 ), the
suction gas is compressed to a higher pressure and
temperature.
The gas flows out the compressor through the dis-
charge service valve. Refrigerant gas then moves into
the air-cooled condenser, where 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
service valve, the filter-drier (which keeps refrigerant
clean and dry), and a heat exchanger (that increases
subcooling of the liquid) to the thermostatic expansion
valve (TXV).
As the liquid refrigerant passes through the variable
orifice of the TXV, the pressure drops to suction pres-
sure. In this process some of the liquid vaporizes to a
gas (flash gas), removing heat from the remaining liq-
uid. The liquid exits as a low pressure, low tempera-
ture, 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.
The TXV is activated by the bulb strapped to the suc-
tion line near the evaporator outlet. The valve main-
tains a constant superheat at the coil outlet regardless
of load conditions.
The TXV is a mechanical device that regulates the flow
of liquid to the evaporator coil in order to maintain a rel-
atively constant degree of superheat in the gas leaving
the evaporator regardless of suction pressure.
The flow of liquid to the evaporator is regulated by a
variable orifice which opens to increase refrigerant flow
(decrease superheat), or closes to decrease refrigerant
flow (increase superheat). The variable orifice is con-
trolled by the temperature sensing bulb which is
strapped to the suction line near the evaporator outlet.
During periods of low load, the suction modulating
valve (SMV) decreases flow of refrigerant to the com-
pressor. This action balances the compressor capacity
with the load and prevents operation with low coil tem-
peratures. In this mode of operation, the quench valve
will open as required to provide sufficient liquid refriger-
ant flow into the suction line for cooling of the compres-
sor motor. The quench valve senses refrigerant
condition entering the compressor and modulates the
flow to prevent entrance of liquid into the compressor.
The refrigeration system is also fitted with a condenser
pressure transducer, which feeds information to the
controller. The controller programming will operate the
condenser fan so as to attempt to maintain discharge
pressures above 130psig in low ambients. At ambients
below 27°C (80°F), the condenser fan will cycle on and
off depending on condenser pressure and operating
times.
1. The condenser fan will start if the condenser
pressure is greater than 200psig OR the con-
denser fan has been OFF for more than 60 sec-
onds.
2. The condenser fan will stop if the condenser pres-
sure is less than 130psig AND the condenser fan
has been running for at least 30 seconds.
At ambients above 27°C (80°F), condenser pressure
control is disabled and the condenser fan runs
continuously.
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