In a reciprocating compressor, a volume of gas is drawn into a cylinder; it is trapped, and compressed by piston and then discharged into the discharge line. The cylinder valves control the flow of gas through the cylinder; these valves act as check valves. There are two types of a reciprocating compressors. Fig. 1 shows the classification of Compressors.
Reciprocating Compressor Types
- Single – Acting compressor: It is a compressor that has one discharge per revolution of the crankshaft.
- Double – Acting Compressor: It is a compressor that completes two discharge strokes per revolutions of the crankshaft. Most heavy-duty compressors are double-acting.
Fig. 2 Shows typical configuration of Single and double acting reciprocating compressor.
Construction of Reciprocating Compressors
Construction of Reciprocating compressors can be divided into two main area:
- Gas end.
- Power end.
Parts of the reciprocating compressor (Fig. 3)
- Piston & piston rod.
- Suction valves.
- Discharge valves.
- Piston rod Packing
- Suction and discharge gas jacket
Cylinder & Liner
The piston reciprocates inside a cylinder. To provide for reduced reconditioning cost, the cylinder may be fitted with a liner or sleeve. A cylinder or liner usually wears at the points where the piston rings rub against it. Because of the weight of the piston, wear is usually greater at the bottom of a horizontal cylinder.
The ends of the cylinder are equipped with removable heads, these heads may contain water/liquid jacket for cooling. One end is called head-end head and other crank-end head. The crank-end contains packing (a set of metallic packing rings) to prevent gas leakage around the piston rod.
- The piston moves forward and backward to suck and compress the gas. It pushes the gas in the discharge pipe during the compression stroke.
- For low-speed compressors (up to 330 rpm) and medium speed compressors (330-600 rpm), pistons are usually made of cast iron.
- Up to 7” diameter cast iron pistons are made of solid bars. Those of more than 7” diameters are usually hollow (to reduce cost).
- Carbon pistons are sometimes used for compressing oxygen and other gases that must be kept free of lubricant.
Clearance in Piston and Cylinder
As the compressor reaches operating temperature, the piston and rod expand more than the liner/cylinder does. In order to prevent seizure adequate clearance should be provided. Similarly, end clearance is also important.
A cold piston is usually installed with one-third of its end clearance on the crank end and two-third of its end clearance on the head end.
Piston Rings (Fig. 4)
Piston rings provide a seal that prevents or minimizes leakage through piston and cylinder liner. Metal piston rings are made either in one piece, with a gap or in several segments. Gaps in the rings allow them to move out or expand as the compressor reaches operating temperature. Rings of the heavy piston are sometimes given bronze, Babbitt or Teflon expanders or riders. Lubrication is a must for metallic rings. Teflon rings with Teflon rider bands are sometimes used to support the piston when the gas does not permit the use of a lubricant.
Piston Rod and Piston (Fig. 5)
The piston rod is fastened to the piston by means of special nut that is prevented from unscrewing. The surface of the rod has a suitable degree of finish designed to minimize wear on the sealing areas as much as possible. The piston is provided with grooves for piston rings and rider rings.
Piston rod packing
Piston rod packing ensures the sealing of the compressed gas. The piston rod packing consists of a series of cups each containing several seal rings side by side. The rings are built of multiple sectors, held together by a spring installed in the groove running around the outside of the ring.
The entire set of cups is held in place by stud bolts. Inside channels are there for cooling, gas recovery and lubrication of the piston rod packing.
An arrangement of scraper rings serves to keep the oil, entrained by piston rod, from leaking out of the crankcase. The oil scraped is returned to the crankcase reservoir.
Valves (Suction and Discharge valves): allow gas to enter into the piston during suction stroke and allow gas to go out into the discharge line during the compression stroke.
There are normally three types of valves, these are
- Plate valve.
- Channel valve.
- Poppet valve.
Parts of reciprocating compressors that assist in transferring power and converting rotary motion into reciprocating motion are grouped in this category.
The crankcase (Fig. 6) supports the crankshaft. All bearing supports are bored under setup conditions to ensure perfect alignment. The crankcase is provided with easily removable covers on the top for inspection and maintenance. The bottom of the crankcase serves as the oil reservoir. The main pump for lubrication of the crank mechanism is placed on the shield mounted on the side opposite the coupling and is driven by the compressor.
Crankshaft receives the power from Driver and transfers it to the piston. The crankshaft is built in a single piece. On the inside of the shaft are holes for passage and distribution of lube oil.
The main bearings are built in two halves, made of steel, with an inner coating of antifriction metal.
Connecting rod (Fig. 7) connect crank shaft with the piston rod.
The connecting rod has two bearings. The big end bearing is built in two halves. It is made of metal with the inner coating of antifriction metal. The connecting rod small end bearing is build of steel, with an inner coating of antifriction metal. A hole runs through the connecting rod for its entire length, to allow passage of oil from the big end to the small end bush.
Crosshead fastens the piston rod to the connecting rod. The sliding surfaces of crossheads are coated with antifriction metal like Babbit shoe. That permits it to slide back and forth within the crosshead guides. The shoes have channels for the distribution of lube oil. The lubrication is obtained under pressure; it comes out from the two guides of the crosshead slide body.
The connection between connecting rod and crosshead is realized by means of a gudgeon pin. The piston rod is connected to the crosshead by a nut.
The distance piece is used to separate the Gas end and Power End of Reciprocating compressor.
API 618 defines 4 types of distance piece which can be used based on criticality of service.
- Type A – Short single compartment (Where oil carryover to piston packing is acceptable)
- Type B – Long Single Compartment (Where oil carryover to piston packing is not acceptable)
- Type C – Long-long two-compartment (For critical services like Oxygen and Hydrogen)
- Type D – Long-short two-compartment (For Process gas services)
The distance piece is provided with drain and vent arrangement and if required continuously purge with buffer gas.
Pulsation Dampeners /Bottles
Pulsation bottles provided at suction and discharge to the reciprocating compressor, to keep the pulsation within the desired limit.
Pulsation study carried out to decide the minimum volume of pulsation bottles.
Lubricants reduce friction and therefore wear between moving compressor parts. The lubricant also serves as a coolant. Fig. 8 shows a typical Lubrication System.
Generally, two types of systems are used to lubricate the positive displacement compressors.
- SPLASH SYSTEM
- FORCED FEED LUBRICATION
It is used in older machines. The level is maintained in the crankcase. Oil is splashed up by the rotation of the crank and the counterweight into the collecting ring. Centrifugal force throws the oil outward through an oil passage to the crank pin.
Forced FEED Lubrication
A pump is used to feed the oil. Oil is pumped under pressure to the required parts. Following are the main parts of system
Reciprocating Compressor Capacity Control Method
- By Recirculation
- By VSD
- By Valve Un-loader
- By Volume clearance pocket
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