30-11-2012, 11:48 AM
RECIPROCATING COMPRESSORS
Reciprocating-Compressor-II.ppt (Size: 1.42 MB / Downloads: 276)
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 reciprocating compressor.
Cylinder & Liner
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. A cylinder liner is usually counter bored near the ends of the outer ring travel i.e. counter bores are made just ahead of the points where the end piston rings stop and reverse direction. Shoulders may form in the liner where the ring’s travel stops unless counter bores are provided.
Piston
For low speed compressors (upto 330 rpm) and medium speed compressors (330-600 rpm), pistons are usually made of cast iron.
Upto 7” diameter cast iron pistons are made of solids. 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.
PISTON ROD AND PISTON
Generally, the piston rod is fastened to the piston by means of special nut that is prevented from unscrewing. The surface of the rod has 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 sealing of the compressed gas. The piston rod packing consists of series of cups each containing several seal rings side by side. The rings are built of three 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.
Piston Rings
Piston rings provide a seal that prevents or minimizes leakage through piston and 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 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 do not permit use of a lubricant.