The purpose of the compressor is to compress the natural gas from 10-14 bar of pressure to 250 bars. This may be achieved in two or three stages of compression. In CNG station we use reciprocating compressor which may be either Motor-driven or Engine driven diesel preferably natural gas.
The CNG compressor alone covers more than half of full station costs. It may be called “Heart of any CNG Station”. So, its selection and sizing is a very important process in both ways Technical as well as Commercial. The actual sizing and number of compressor for the CNG Stations is the function of the following :
- The actual gas filling time for the vehicle
- The dwell time between vehicle or turn-around time for the vehicle
- The storage capacity provided in the cascades for the system
Compressor are available for capacities varying from 110 SCMH to 1200 SCMH and more for suction & discharge pressure of 12 bar to 255 bar.
Compressor selection mainly depend on the following points :
- Inlet pressure
- No. of stages required
- Types of drive
- Discharge volume
- per unit per consumption etc.
1. Positive Displacement Compressors
a) Reciprocating Compressors
- Reciprocating compressors are positive displacement compressors and are used in CNG industry. It operates at a fixed volume.
- In reciprocating compressor, there is a possibility of multiple services on one compressor frame. In the multiple stage frames, different cylinders may be dedicated to different gas pressures.
- The compressor mainly used in CNG is multistage in nature.
Reciprocating compressors can be single acting as well as double acting. In a compressor, there are four stages to compression. In single acting compressors, the backward stroke of the piston causes a suction which pulls in gas through the inlet valve. The suction action keeps the discharges valve closed. On the forward stroke, the positive pressure generated by the piston, closes the inlet valve and opens the discharge valve. In double acting compressors, the cylinder has inlet and outlet ports at each end of the cylinder. As the piston moves forward gas is drawn into the cylinder at the back end while, at the front end, gas is discharged. When the piston direction is reversed, the sequence is reversed.
b) Rotary Compressors
Rotary compressors in many configurations are used in gas distribution industry. Rotary compressors compress gases with the help of lobes, screws and vanes into smaller volumes. These compressors have a simpler design and do not have too many moving parts which make them easier to maintain. However they have high rotational speeds and shorter life expectancy as compared to other designs.
Rotary screw compressors are very widely used in gas industry. This type of compressor operates with two helical rotors rotating towards each other. The counter clockwise motion of the screws forces the flowing gas to become trapped in the central cavity. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws. A lobe type compressor has two kidney-bean shaped impellers, which are used to trap and transfer gas. The two impellers move in opposite directions and trap gas between the lobe cavities. As they rotate, gas gets compressed and is discharged through the discharge port. These compressors are designed to have a constant volume discharge pressure and constant speed.
Other rotary compressor such as vane type and scroll type are also used in the industry in some regions.
2. Centrifugal compressor
Centrifugal compressors are used in gas industry across the world. The basic components of a centrifugal compressor are Impellers, Inducers, Vanes, Volute, Suction eye, Discharge lines, diffuser plates, seals, shaft, casing, suction vane tip & discharge vane tips.
Air is drawn into the center of a rotating impeller with radial blades and is pushed toward the center by centrifugal force. This radial movement of air results in a pressure rise and the generation of kinetic energy. Before the air is led into the center of the impeller, the kinetic energy is also converted into pressure by passing through a diffuser and volute. Each stage takes up a part of the overall pressure rise of the compressor unit. Depending on the pressure required for the application, a number of stages can be arranged in a series to achieve a higher pressure. This type of multi-stage application is often used in the oil and gas and process industries.