Saturday, October 18, 2008

classification of reciprocating compressor

Reciprocating Compressors can be classified as follows based on

I. Cylinder lubrication:
1. Lubricated type & Non-lubricated type
2. Dry running piston rings
3. Ringless or labyrinth type.

II. Cylinder cooling:
1. Air cooled
2. Water cooled
III Cylinder loading
1. Single acting
2. Double acting
IV Cylinder arrangement,
1. Vertical inline or V-type
2. Horizontal opposed balanced.


Lubricated Compressors :
Generally big reciprocating compressor cylinders are lubricated to avoid wear and tear of liner, piston rings, rider rings and stuffing box. Lubricants are injected in drops and are lost with the process gas.

Lubrication of cylinder reduces wear of parts, enhances life of parts and also reduces gas discharge temperature. It is necessary for the lubricant to be compatible with the process gas and down stream system. Generally recip compressors above 100 KW are lubricated type.

Non-Lubricated Type :
There are many services in which oil in any form in the compressed gas, is not acceptable such as instrument air compressor and some process gas compressors. In such services oil is not injected in the cylinders, instead wear parts are made of soft material with low co-efficient of friction such as PTFE,CFT etc. The wear rate of stuffing box packing, rider rings piston rings and cylinder/liner surface may be comparatively more.

Process requirements and gas to be compressed dictate whether to use a compressor with lubricated or non-lubricated cylinder. Some chemical processes do not permit use of lubricant in the system, due to quality problems, catalyst-poising etc.

Following factors must be considered,
1. Non-lubricated compressors cost more than lubricated compressors
2. Non-lubricated compressors require more power
3. Non-lubricated compressors require more maintenance, labyrinth compressors
being exception.


Air Cooled type :
This type of compressors has fins cast as part of the cylinder to dissipate some of the heat generated by the compression of the gas. In some compressors vanes are cast as part of the flywheel or sheave to act as fan to help remove the heat from the cylinder surface. This type of cylinder cooling is used in small portable compressors.

Water-cooled type:
Water-cooled compressors are most common in industry. It is impossible to sustain cooling with air in big compressors where heat generation is very high. Water jackets are cast as part of the cylinder. Water is circulated in the cylinder jacket from external source.

In some case the cylinder jacket temperature is to be maintained few degrees higher than the process gas temperature to avoid condensation during standstill, this is possible with water-cooled cylinders by maintaining the water temperature with use of heaters.


Single acting
In single acting cylinder compression takes place only on one side of the piston and valves are installed only on that side.

Double acting
In double acting cylinder compression takes place on both sides of the piston. When one side is in compression, the other is in suction.


Horizontal Type:
This type of arrangement is most common in industries. Multi cylinder reciprocating compressors with horizontal cylinders are very often designed as balanced opposed type. Balanced opposed frame is characterized by adjacent pair of crank 180 ° out of phase and separated by crank webs only. With this configuration inertia forces are balanced. The balanced-opposed design is separable frame.

Vertical Inline or V-type:
Vertical in line or V-type are used for small or moderate compression ratio and duty. Normally this type of compressors cylinder arrangement are non-lubricated type and occupy less space.

Labyrinth piston compressors
These are vertical type. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant.

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