A breakdown of all compressor types is shown in the figure above, with the two main categories being positive displacement and dynamic compressors.
A positive displacement compressor is one in which the volumetric flow rate remains constant regardless of the system’s load. The characteristic that distinguishes a positive displacement compressor from dynamic compressors is that a fixed amount of fluid is drawn into the chamber. As it progresses from the suction to the discharge end, the fluid’s volume is reduced and compressed to a higher pressure.
Additionally, the geometry and operating principle enable screw compressors to compress a controlled amount of liquid refrigerant and inject oil without damaging the compressor.
Unlike positive displacement compressors, dynamic compressors accelerate the fluid to a high velocity, converting kinetic energy into pressure energy as the flow passes through a diffuser. As a result, the volumetric flow rate depends on the system’s pressure ratio and operating conditions, making dynamic compressors more sensitive to variations in system demand. Dynamic compressors operate based on high-speed aerodynamic principles. Allowing liquid droplets into the impeller can cause blade erosion, vibration, and imbalance, as well as a loss of aerodynamic efficiency.
Positive displacement compressor characteristics:
Constant flow rate
High Pressure
High efficiency at low, medium & high speed operation (1,200 – 6,600 rpm)
Characteristics of dynamic compressors:
Variable flow rate
Variable pressure
Higher speed operation (Range 3,000 – 50,000rpm)
What is a single screw compressor?
A single screw compressor is a positive displacement compressor utilising 1 screw rotor for compression.
A single screw compressor comprises of the following components:
Suction cover
Motor housing
Main casing
Discharge cover or oil separator
External or integral inverter
Let's unravel the purpose each member serves:
The suction cover is the compressor’s inlet, where a strainer is installed to filter any foreign material. The suction cover operates under low-temperature and low-pressure conditions. Additionally, the third (suction) bearing is housed in the suction cover, providing access during maintenance.
The motor housing, as the name suggests, encloses the motor and is fastened between the suction cover and the main case. The main motor types used in these compressors are permanent magnet and induction motors, with speeds ranging from 20 Hz to 110 Hz. Additionally, it should be noted that the larger the compressor, the lower the maximum operating speed.
In the case of a one-piece main case, the motor housing is integrated into the casting of the main case. The main case is the primary component of the compressor, as it houses all the rotating compression parts. The screw rotors, gate rotors, variable volume ratio system, discharge section, and center bearings are all located within the main case.
The oil separator is a device that separates the oil from the refrigerant and functions as an oil sump. The oil then passes through an oil filter and is circulated back to the main case and suction cover, where it lubricates the bearings and seals the screw rotor. If the system includes an external oil separator, the compressor is equipped with a discharge cover, which is a much smaller housing. The efficiency of external oil separators is approximately 99.5%, and the choice between an external or integrated oil separator depends on the application.
Finally, in the past, compressors operated without an inverter, usually at a fixed speed of 50 Hz (3,000 rpm). Nowadays, inverter technology has become more affordable, and an integral or external VFD is likely to be used. The selection depends on the application and system imitations. Finally, a VFD offers many advantages, which will be discussed in a later section.
