Refrigeration Circuit - Carrier 69NT40--541--200 TO 299 Operation And Service

2.5 REFRIGERATION CIRCUIT

Starting at the compressor, (see Figure 2-7, upper
schematic) the suction gas is compressed to a higher
pressure and temperature.
The gas flows through the discharge service valve into
the pressure regulator valve. During periods of low
ambient operation, the pressure regulator valve
modulates the flow of refrigerant to maintain a pre set
minimum discharge pressure. Refrigerant gas then
moves 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
temperature. 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.
When operating with the water cooled condenser active
(see Figure 2-7, lower schematic), the refrigerant gas
passes through the air cooled condenser and enters the
water cooled condenser shell. The water flowing inside
the tubing cools the gas to saturation temperature in the
same manner as the air passing over the air cooled
condenser. The refrigerant condenses on the outside of
the tubes and exits as a high temperature liquid. The
water cooled condenser also acts as a receiver, storing
excess refrigerant.
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. As the liquid refrigerant passes through the
variable orifice of the expansion valve, some of it
vaporizes into a gas (flash gas). Heat is 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 modulating valve to the compressor.
The thermostatic expansion valve is activated by the
bulb strapped to the suction line near the evaporator
T-317
outlet. The valve maintains a constant superheat at the
coil outlet regardless of load conditions.
During periods of low load, the suction modulating valve
decreases flow of refrigerant to the compressor. This
action balances the compressor capacity with the load
and prevents operation with low coil temperatures. In
this mode of operation, the quench valve will open as
required to provide sufficient liquid refrigerant flow into
the suction line for cooling of the compressor 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. When operating on the air cooled condenser,
the controller programming will operate the condenser
fan so as to attempt to maintain discharge pressures
above 130 psig 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 pres-
sure is greater than 200 psig OR the condenser fan
has been OFF for more than 60 seconds.
2 The condenser fan will stop if the condenser pres-
sure is less than 130 psig 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.
On systems fitted with a water pressure switch, the
condenser fan will be off when there is sufficient
pressure to open the switch. If water pressure drops
below the switch cut out setting, the condenser fan will
be automatically started. When operating a system
fitted with a condenser fan switch, the condenser fan will
be off when the switch is placed in the "O" position. The
condenser fan will be on when the switch is placed in the
"I" position.
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