Positive Displacement Compressors

Positive displacement type compressors are those who mechanically displace a fixed volume of gas into a reduced volume.

A compressor that confines successive volumes of fluid within a closed space in which the pressure of the fluid is increased as the volume of the closed space is decreased. 

The constant volume can be delivered by compressor when operated at fixed speed.

The discharge pressure is determined by the system load conditions.

If the consumption of gas is more than discharge pressure falls below the cut-off pressure & compressor works on load condition.

CLASSIFICATION OF PD COMPRESSOR

  1. Reciprocating
  2. Rotary

The rotary compressors are further divided into:

  1. Screw
  2. Vane
  3. Lobe
  4. Liquid Ring compressors.
  5. Scroll compressors.

4.0 ROTARY SCREW COMPRESSORS

A screw compressor is a positive displacement volume reduction device.

An oil flooded twin screw compressor consists of male and female rotors mounted on bearings to fix their position in a rotor housing which holds the rotors in close tolerance with intersecting cylindrical bores.

The rotors basic shape is a screw thread, with varying numbers of lobes on the male and female rotors.

The driving device is (Prime mover) is generally connected to the male rotor with the male rotor driving the female rotor through a film of oil.

As the rotors revolve, the air/gas is forced into a decreasing inter-lobe cavity until it reaches the discharge port.

In lubricated units, the male rotor drives the female and oil is injected into the cylinder serving as both a lubricant and coolant, and also as an oil seal to reduce back slippage.

On non-lubricated types, timing gears are used to drive the rotors and multi-staging is necessary to prevent gas temperatures from going too high.

            Rotary screw compressors are compact and smooth running with limited vibration.

4.1 Oil-free compressor

In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal.

They usually have lower maximum discharge pressure capability.

However, multi-stage oil-free compressors, where the air is compressed by several

            sets of screws, can achieve pressures of over 150 psig, and output volume of over

2000 cubic feet per minute (measured at 60 °C and atmospheric pressure).

Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing.

4.2 Oil-flooded compressor

In an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge.

The oil is separated from the discharge stream, then cooled, filtered and recycled.

Condensate and drop out of the air stream are removed from the compressed air system via condensate management equipment.

Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured at 60 °C and atmospheric pressure).

4.2a Atlas Copco Air and Oil System

4.2b Air Flow

Air drawn through filters and unloaders is compressed in compressor elements’

Compressed air and oil are discharged through check valves to air receiver / oil separators where oil is separated from the compressed air. Air is blown through the minimum pressure valve to air coolers. The cooled air is discharged through condensate traps and out towards the air net. Check valves prevent back flow of compressed air. Minimum pressure valve prevents the receiver pressure from dropping below a minimum pressure. This valve has a built-in check valve.

4.2c Oil System

Air pressure forces the oil from receiver through oil coolers, filters and oil stop valve to compressor elements and the lubrication points. Oil stop valve prevents the compressor elements from flooding with oil when compressor is stopped. By-pass Valve, by-passes oil coolers when starting the compressor from a clod condition, so ensuring rapid warming of the oil to normal working temperature.  In air receiver, most of the oil is removed from the air centrifugally. Almost all of the remaining oil is removed by the separator element.

4.3 Vilter Single Screw Compressors.

The Vilter Single Screw Compressor is a positive displacement, capacity and volume controlled, oil flooded, rotary compressor which uses a single main screw intermeshed by two opposing gate rotors.

Gas compression occurs when the individual fingers of each gate rotor sweep through the grooves, or flutes, of the main screw as the screw rotates.

As the main screw rotates, the gate rotor is also driven, causing the gate rotor tooth to sweep the groove in the main screw. This sweeping action reduces the volume of the groove ahead of the gate rotor tooth and causes the trapped gas and oil to be compressed in the reduced volume. As the main screw continues to rotate, the gate rotor tooth continues to reduce the groove volume to a minimum, thus compressing the trapped gas to a maximum pressure.

A labyrinth seal arrangement prevents the compressed gas from leaking past the end of the screw.

As the gate rotor tooth reaches the end of the groove, the groove rotates to a position that lines up with the discharge port in the compressor housing and the gas/oil mixture is discharged from the screw at high pressure. This completes the compression cycle for a single flute of the main screw.

Once the gas is swept from the main screw flute through the discharge port, it passes into the discharge manifold of the compressor.

From the discharge manifold, the gas/oil exits the compressor housing.

4.4 ROTARY VANE COMPRESSOR

Rotary vane compressors consist of an off-centre drive shaft that rotates a housing for a number of vanes. As the off-centre housing rotates, the vanes slide in and out of their housing slots to keep contact with the outer wall of the compressor. Thus, a series of decreasing volumes is created by the rotating vanes. Rotary vane compressors are, with piston compressors, one of the oldest of compressor technologies.

With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2bar) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about13 bar in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.

4.5 ROTARY LOBE COMPRESSOR

Straight lobe compressor or blowers, as they commonly called, are low pressure machines. They impart energy to the gas being compressed by way of an input shaft moving a single or multiple rotating elements. They do not use inlet and discharge valves. Rotors are timed by a set or timing gears.

Lobe compressor

As the lobe impellers rotate, gas is trapped between the lobe impellers and the compressor case where the gas is pressurized through the rotation of lobes and then discharged.

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