|Reciprocating compressor controls air output by unloading cylinders.|
A reciprocating compressor is probably the best known and the most widely used of all compressors. The displacing action of the piston, together with the inlet valve or valves, causes a quantity of gas to enter the cylinder where it is compressed and discharged in turn. The action of the discharge valve is such that it prevents the backflow of gas from the discharge line into the compressor during the next intake cycle.
Reciprocating compressors control air output by unloading cylinders. The pistons operate against very little air-pressure resistance in this mode and therefore very little energy is wasted. For this reason, reciprocating compressors are typically efficient at part-load operation.
Figure 9: Reciprocating compressor
There are four types of reciprocating compressors, namely:
- Multi stage
- Single stage
All these compressors are driven by electric motors or internal combustion engines. For a single stage compressor; air is drawn in from the initial suction pressure, it is compressed to its final discharge pressure and released in one step.
A multi-stage compressor compresses air partly; the air is cooled outside the cylinder then returned to be compressed further with a second stage and possibly a third or fourth stage depending on the final pressure requirement.
Single and double acting compressors
On each side of the cylinder are two valves that open or close, depending on whether the air is compressed or discharged. In figure 43(B) the valve below opens for the compressed air to escape while one of the valves on the suction side opens to allow for incoming air. The air that came in from above will then be compressed on the next compression stroke.
Figure 10: A single-acting (A) and double-acting compressor (B)
Typically, large industrial reciprocating air compressors are double-acting. Reciprocating compressors are mostly multi-stage compressors.
Components of reciprocating compressors
The components of a reciprocating compressor responsible for compressing the gas or air are shown in the figure below.
Figure 11: A double-acting compressor
A cylinder is the space within which a piston travels. It is always filled with air to be compressed.
Figure 12: An example of a cylinder
A piston is a sliding plug that fits closely inside the bore of a cylinder. Its purpose is to exert a force on a fluid inside the cylinder.
Figure 13: An example of a compressor piston
The cross-head is a beam that connects the piston rod to the connecting rod of a reciprocating compressor. It slides on parallel guides and moves a connecting rod back and forth.
Figure 14: Example of a cross-head
A connecting rod transmits motion or power from the crankshaft to the piston.
Figure 15: Examples of connecting rods
The crankshaft receives reciprocating motion from the pistons and converts it to rotary motion. Both the crankshaft and the connecting rods transform the pistons’ reciprocating motion into rotary motion.
Figure 16: Crankshaft
The valve controls the inlet and outlet of compressed air in the cylinder. Also, internal relief valves are installed to relieve air if high pressure builds up.
Operating principles of reciprocating compressors
During the downward stroke, discharge valves are forced shut. The gas is then sucked into the cylinder by the suction pressure. During the upward stroke, the suction valves are forced shut and the gas is moved into the discharge port.
|Example – bicycle pump|
A bicycle pump is an example of a simple positive displacement compressor. The piston, secured to a handle by a long rod, has a cup shaped leather face opening downward when the pump is in use. Downward motion of the piston causes sufficient initial pressure to open the cup and produce a tight seal between the leather and the seal wall. Air is forced through a non-return valve into the tyre. The upstroke of the piston creates a partial vacuum inside the cylinder, permitting atmospheric air to flow past the leather cup and fill the cylinder so that the cycle may be repeated. The flexible cup-leather actually serves as a non-return valve because it opens to admit air into the cylinder (on the up-stroke) but prevents the escape of air from the cylinder (on the down-stroke) as air pressure forces the edges on the leather tight against the inside of the cylinder (see the following figure).
Figure 17: A cross section of a bicycle pump
Advantages and disadvantages of reciprocating compressors
- High efficiency when the compressor is new and after overhauls.
- Possibility of very high pressures
- Low initial cost
- Easy to install
- No oil carry-over.
- Requires high maintenance
- Fixed speed
- Creates noisy atmosphere
- Damping requirement
- More moving parts
- Efficiency drops off between overhauls
- High cost.
Applications of reciprocating compressors
Reciprocating compressors of power ranges between 5 and 30 HP (Horse power) are used for automotive work especially for intermittent duties. Compressors of greater power such as 1000 HP are meant for large industrial applications. They are also used for:
- Wellhead gas compression.
- Natural gas distribution.
- Plastic bottle blowing.
- Fuel gas boosting.
- Vehicle refuelling.