Carrier 69NT40-541-306, Операция и обслуживание

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T-354 3–10
3.5 REFRIGERATION CIRCUIT
Starting at the compressor (see Figure 3.7 , upper schematic), the suction gas is compressed to a higher pressure
and temperature.
The gas flows through the discharge service valve into the pressure regulating valve. During periods of low ambi-
ent operation, the discharge pressure regulating valve modulates the flow of refrigerant to maintain a pre -set mini-
mum 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 3.7 , 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 con-
denser. 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 (TXV).
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 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 suc-
tion line for cooling of the compressor motor. The quench valve senses refrigerant condition entering the compres-
sor 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 control-
ler. When operating on the air-cooled condenser, 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 pres sure is greater than 200psig 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 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.
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.