Rather than buying a bare compressor and assembling the associated components in the field, screw compressor packages are almost universally factory-assembled. In the first place, the packager selects properly sized components for the application. Furthermore, assembly in a factory environment will usually provide better control than a field assembly.
A screw compressor package is shown in Figure 5.33. The major components in a package, some of which are noted in Figure 5.33, are the compressor, its motor, coupling, the oil separator, and an oil pump and filter. The microprocessor controller is also installed at the factory so that all the control wiring can be installed by experienced technicians. The oil separator is classified as a vessel, so it is equipped with safety relief valves. The suction line before the compressor is equipped in sequence with the suction stop valve, a check valve, and, just before the compressor, a suction-line filter. At the refrigerant exit from the oil separator are found another check valve and the discharge stop valve.
The purpose of the suction and discharge check valves is to prevent backflow of refrigerant when the compressor shuts down. Backflow could cause rotors to spin backward and damage them. Also backflow may drive oil out of the compressor into the suction line. The engineer selecting the compressor package should be aware that the pressure drop through the suction and discharge stop valves is not usually acknowledged in the catalog ratings of the compressor.