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| Illustration 1 | g00358258 |
Refrigerant Flow Through Air Conditioning System (1) Compressor (2) Condenser Coil (3) Receiver-Dryer (4) Expansion Valve (5) External Equalizer Line (6) Evaporator Coil (A) Air conditioner refrigerant as a high pressure, high temperature vapor (B) Air conditioner refrigerant as a high pressure, high temperature liquid (C) Air conditioner refrigerant as a low pressure and low temperature liquid (D) Air conditioner refrigerant as a low pressure and low temperature vapor | |
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| Illustration 2 | g00645819 |
Expansion Valve in the open position (typical example) (4) Expansion valve (5) External equalizer line (7) Capillary tube (8) Diaphragm (9) Ball and rod (10) Metered restriction (11) Spring (12) Inlet (13) Outlet | |
The expansion valve (4) converts the liquid refrigerant from a high temperature and a high pressure liquid to a low pressure and a low temperature liquid. The expansion valve (4) provides a metered restriction (10). The expansion valve (4) allows the high pressure liquid to be reduced to a low pressure liquid. When the pressure of the refrigerant is reduced, the boiling temperature of the refrigerant will also be reduced. The refrigerant will enter the evaporator coil (6). Heat from the cab will be absorbed by the refrigerant.
The reduced liquid pressure will lower the boiling temperature of the refrigerant. The heat will be absorbed by the liquid R-12. The heat will change the liquid R-12 into a vapor in evaporator coil (6). The boiling temperature of R-12 at 0.0 kPa (0.0 psi) is approximately -30 °C (-22 °F). In the evaporator coil of a normal operating system, the pressure will be approximately 103 kPa (15 psi) to 207 kPa (30 psi). The boiling temperature of R-12 at 103 kPa (15 psi) to 207 kPa (30 psi) is raised approximately -11.7 °C (11 °F) to 0 °C (32 °F). This is cold enough to remove heat from the air in the cab without freezing the evaporator coil.
The liquid refrigerant with high pressure and high temperature enters the expansion valve (4) at the inlet (12). Only a small amount of the liquid refrigerant is allowed to flow through the metered restriction (10) and into the outlet (13). The size of the metered restriction (10) is controlled by a ball and rod (9). The spring (11) moves the ball and rod (9) up the expansion valve. This makes the restriction (10) smaller. The diaphragm (8) moves the ball and rod (9) down the expansion valve. This makes the restriction (10) larger. A temperature sensing bulb with a capillary tube (7) is connected to the evaporator coil outlet pipe. The temperature sensing bulb measures the temperature at the outlet of the evaporator coil. If the temperature increases, the pressure in the capillary tube (7) increases. The pressure in the capillary tube (7) pushes down on the diaphragm (8). The diaphragm pushes down on the ball and rod (9). When the ball and rod are pushed down by the diaphragm the size of the restriction (10) increases. The pressure that pushes on the diaphragm allows more liquid refrigerant to flow into the evaporator coil (6). When the temperature decreases, the pressure decreases on the diaphragm (8). The spring (11) pushes up the ball and rod (9) and this makes the restriction (10) smaller. As a result, less liquid refrigerant flows into the evaporator coil (6) .
The external equalizer line (5) connects the low pressure from the outlet of the evaporator coil (6) to the underside of the expansion valve (4) and the diaphragm (8). The pressure from the external equalizer line pushes up against the bottom of the diaphragm. The pressure that is created by the temperature sensing bulb in the capillary tube (7) pushes down the top of the diaphragm (8). The pressures on each side of the diaphragm act against each other. The variation of pressure on each side of the diaphragm will help to regulate the refrigerant flow by moving the ball and rod (9) up and down. Regulating the refrigerant flow will prevent the flooding of the evaporator coil (6) with refrigerant.