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 :
i) The actual gas filling time for the vehicle
ii) The dwell time between vehicle or turn-around time for the vehicle
iii) 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 :
i) Inlet pressure
ii) No. of stages required
iii) Types of drive
iv) Discharge volume
v) per unit per consumption etc.
The CNG compressor may have various version such as :
- Online Compressor: It is used to compress gas directly from the pipeline and the discharge can be used to fill the CNG vehicles at CNG retail outlet. The capacity of such a compressor is selected based on the vehicle likely to be filled in that station and with the provision for expected future expansion. It may be equipped with a 1200 SCMH capacity compressor for compressing the natural gas from 49 bar to 225 bar. All categories of vehicles are refueled i.e. Bus, Car, LMV, LCV, and autos. Cascade filling facilities will not be available at the online station as it is in Mother station.
- Mother Compressor: The station at which the mother compressor operates is known as the mother station. Mother compressor is a high capacity compressor used to fill cascades for transportation of gas to a Daughter station or Daughter booster station or to other industrial or commercial customers. The mother station may also have CNG filling stations in addition to filling the cascades for other applications. The capacity of the mother compressor is generally large starting from 1200 SCMH.
- Daughter booster compressor: This is a small compressor which takes suction from cascades filled at mother stations. The size of the compressor starts from 200 SCMH and could be as high as 400 SCMH to 600 SCMH depending on the size of the inlet cascade and demand for CNG. The preferable version for such a booster compressor is variable stroke and variable suction pressure compressor. This helps the compressor to operate from the wide range of cascade pressure from 230 bar down to 30 bar. This provides a lesser weight of a gas to be transported back to the mother station and also gives better utilization of the gas brought from other stations to the daughter station. The CNG station at which such a compressor operates is called Daughter booster station. A loaded mobile cascade is always parked at the daughter booster station for unloading.