1-10
1.10 REFRIGERATION FLOW CYCLE
Figure 1-6. Refrigeration Cycle Diagram
1
2
3
4
5
6
7
8
9
11
12
13
14
16
18
19
20
10
15
1. Evaporator Coils
2. Inline Sight Glass
3. Low Pressure Service Port
4. Expansion Valves
5. Filter-Drier Outlet Valve
6. Filter-Drier
7. Filter-Drier Inlet Valve
8. Receiver
9. Condenser Coils
10. Discharge Service Valve
11. Suction Service Valve
12. Discharge Check Valve
13. Discharge Line Valve
14. Discharge Line
15. Suction Line
16. Unloader Pressure Port 2
17. Unloader Pressure Port 3
18. Condenser Fan Pressure Port
19. High Pressure Service Port
20. Liquid Line Solenoid Valve
21. Optional Suction Gas to
Liquid Line Heat Exchanger
21
OPTIONAL
17
The
refrigeration
system
contains
R-134a
refrigerant. The refrigeration cycle is the same for air
conditioning and heating (during reheat mode). The
refrigerant cycle is off during the vent only mode, only the
evaporator blowers operate to circulate air throughout
the bus.
The refrigeration cycle begins when the compressor
clutch is engaged. The compressor raises the pressure
and the temperature of the refrigerant and forces it into
the condenser tubes. The condenser fan circulates
surrounding air (which is at a lower temperature than the
refrigerant) over the outside of the condenser tubes.
Heat is transferred from the refrigerant (inside the
tubes) to the condenser air (flowing over the tubes). The
condenser tubes have fins designed to improve the
transfer of heat from the refrigerant gas to the air. This
removal of heat causes the refrigerant to liquefy. The
liquid refrigerant leaves the condenser and flows to the
receiver.
The receiver serves as a liquid refrigerant reservoir
so a constant supply of liquid is available to the
evaporator as needed and as a storage space when
pumping down the system. The receiver is equipped with
a sight glass to check for the correct refrigerant level.
The refrigerant leaves the receiver and flows through
the filter-drier inlet service valve to the filter-drier, which
contains an absorbent for keeping the refrigerant clean
and dry. The refrigerant flow continues through a
filter-drier outlet service valve.
The liquid refrigerant then flows through a normally
closed liquid line solenoid valve. When the compressor
clutch is engaged, 24-volts is also applied to the liquid line
solenoid valve coil opening the valve and allowing liquid
to flow.
The liquid refrigerant then flows through an
externally equalized thermostatic expansion valves,
which reduces the pressure and temperature of the liquid
and meters the flow of liquid refrigerant supplied to the
evaporator coil to maximum use of the evaporator heat
transfer surface.
The low pressure, low temperature liquid refrigerant
that flows into the evaporator tubes is colder than the air
circulating over the outside of the evaporator tubes by the
evaporator blowers. Heat is transferred from the
evaporator air circulating over the outside of tubes to the
liquid refrigerant flowing inside the tubes. The
evaporator tubes have aluminum fins to increase heat
transfer from the air to the refrigerant. The resultant
cooler air is then circulated throughout the interior of the
bus.
The transfer of heat from the air to the low
temperature liquid refrigerant in the evaporator causes
the liquid to vaporize. This low temperature, low
pressure vapor passes through the suction line back to the
compressor where the cycle is repeated.