The capacity control valve (figure 6-8) is located in the compressor crankcase cover. The valve is actuated by oil pressure from the main oil pump, and its function is to admit or relieve oil to or from the individual unloader power elements, depending on suction or crankcase pressure. When the compressor is at rest the two cylinders equipped with the unloader element are unloaded and remain unloaded until the com-pressor is started and the oil pressure reaches nor-mal operating pressure.

As the high pressure oil from the pump enters chamber A of the capacity control valve and passes through an orifice in the top of the piston to chamber B, it forces the piston to the end of its stroke against spring A. When the piston of the valve is forced against spring A, the circular grooves which form chamber A are put in con-tact with the unloader connections. This admits high pressure oil to the unloader cylinder, actuating the unloader mechanism.

To control the oil pressure from the capacity control valve, a capacity control regulating needle valve is installed. It is connected to the crankcase and has an oil connecting line to chamber B of the capacity control valve. As the crankcase, or suction pressure pulls down slightly below the setting of the regulating valve, the regulator needle valve opens and relieves the oil pressure from chamber B of the capacity con-trol valve. This permits spring A to push the capacity control piston one step toward chamber B, uncovering the unloader connection nearest the end of the capacity control valve. This action relieves the oil pressure from the power element and allows the power element spring to rotate the cam rings and unload the cylinder.

If the suction pressure continues to drop, the regulator needle valve relieves more oil pressure and more cylinders become unloaded. On the other hand, if the heat load increases, the suction pressure increases, causing the regulating needle valve to close and more cylinders to become loaded.

MAINTENANCE PRECAUTIONS

If a compressor cannot be pumped down and is damaged to the extent that it has to be opened for repairs, it is necessary first to close the suction and discharge valves and then following all safety precautions, to allow all the refrigerant in the compressor to vent to the atmosphere through a drain plug.

When it becomes necessary to remove, replace, or repair any internal parts of the compressor, observe the following precautions:

1. Carefully disassemble and inspect while removing all parts, noting their correct relative position so that errors will not be made when reassembling.

2. Inspect all parts that become accessible after the removal of those parts requiring repair or replacement.

3. Make certain that all parts and surfaces are free of dirt and moisture.

4. Apply clean compressor oil freely to all bearing and rubbing surfaces of the parts being replaced or reinstalled.

5. If the compressor is not equipped with an oil pump, make certain that the oil dipper on the lower connecting rod is in the correct position for dipping oil when the unit is in operation.

6. Position the ends of the piston rings so that alternate joints come on the opposite side of the piston.

7. Take care not to score the gasket surfaces.

8. Renew all gaskets.

9. Clean the crankcase and renew the oil following correct procedures.