he Anatomy of a Reciprocating Compressor: Understanding the Key Parts

Blueprint for Reciprocating Compressor Components

Compressors are work-absorbing machines used to boost the pressure of the fluid at the expense of work done on the fluid. These machines are made up of dozens of compressor components and are used for various applications requiring high-pressure air and across several industries. This include:

  • Driving pneumatic tools
  • Spray painting
  • Material handling for the transfer of material
  • Surface cleaning
  • Chemical industry, etc.

Several compressors exist, but one of the most common is the reciprocating compressor.

What is a Reciprocating Compressor?

Reciprocating compressors are positive-displacement machines that use a piston to compress gas and deliver it to its final destination at high pressure. Nearly every industrial facility you know owns a reciprocating compressor.

Reciprocating compressors are comprised of several components that play significant roles that support the equipment in one way or another. Some of the duties of compressor components include

  • Managing heat
  • Boosting the machine’s durability
  • Aiding rotation
  • Narrowing gas flow
  • Enabling other parts of the machine to perform their tasks efficiently

So, which components of reciprocating compressors are a big deal? This is what you are about to find out.

The Major Reciprocating Compressor Components

The primary reciprocating compressor components give the equipment the expected level of functionality. Here are the components, arranged in no particular order:

  • Frame
  • Valve
  • Cylinders
  • Piston
  • Piston rings
  • Crankshaft
  • Crosshead
  • Connecting rod
  • Bearings
  • Distance piece

Let’s take a brief look at all compressor components individually and their distinct contribution to the overall functionality of a reciprocating compressor.


Most reciprocating compressors come with heavy, rugged frames that enclose every component of the machine, including the crosshead guide and cylinder.

The frame – also known as the ‘crankcase’ – has a rectangular or square shape. Its primary role is to support the crankshaft.

Some compressors – e.g., integral or separable–are designed slightly differently. The engine-power cylinders of integral compressors come fitted to the frame, and the same crankshaft drives them.


Valves are crucial to the optimal operation of a reciprocating compressor. The primary function of a compressor valve is to permit the unrestricted flow of gas in the right direction while preventing those attempting to flow in an undesired or unspecified direction.

Valves are positioned at opposing – and operating – ends of the cylinder. One end features inlet valves, while the other showcases two discharge valves.

Every reciprocating compressor has a ring valve, a poppet valve, and a plate valve. Each of these configurations serves a unique purpose.


Large, low-pressure cylinders are composed of cast iron. They are those pressure vessels containing the gas to be compressed. They are usually detachable from the compressor frame. A distance piece connects the cylinder to the frame.

On the other hand, a small high-pressure cylinder is made from steel and is often directly attached to the compressor’s main body.

The #1 function of a reciprocating compressor cylinder is to bring down or minimize the machine’s temperature during compression cycles which often generates lots of heat.

Compressor cylinders are designed to provide support to suction as well as the discharge valve plates. They often feature replaceable sleeves or liners that don’t slip from the cylinder’s surface. They help ensure the easy replacement of damaged or worn cylinders.


A reciprocating compressor’s modus operandi relies heavily on the piston. This is because the piston is the primary component that compresses air during operation.

As a result, the piston must be weighty, compatible with the gas to be compressed, and have strength. It is also responsible for moving energy from the crankcase to the gas already contained within the cylinder.

This is to prevent the refrigerant from leaking through the gap. Due to this, piston rings cover this component, sealing the gap between the cylinder walls and the piston.

Pistons in reciprocating compressors are composed of either cast iron or aluminum. During operation, the piston in a reciprocating compressor moves upwards and downwards within the cylinder. This movement gives rise to the suction as well as compression of the refrigerant.

Piston rings

Piston rings are seals wound on the piston. They come in direct contact with the cylinder wall as the piston moves up and down during operation.

The constant movement of the piston during operation necessitates the regular changing of the piston rings. This is due to the considerable friction when the piston is in motion.

Using a rider ring or wear band as an extra piston ring is sometimes necessary. The primary function of the rider ring in a reciprocating compressor is to significantly minimize the probability of severe wear and tear between the piston and cylinder.

Different types of rings can be employed for the piston, and they could be made from metallic or non-metallic materials. However, they must be softer than the wall of the cylinder.

Most metallic rings are made from cast iron or bronze and are actively used for lubricating services. On the other hand, most non-metallic rings are fabricated from fluorocarbon compounds. As a result, they are usually employed for non-lubricated service and weigh far less than their metallic counterparts. As a result, they have enjoyed wider adoption in reciprocating compressors.


This is the main shaft in a reciprocating compressor. The other shaft in this machine is the motor shaft.

The crankshaft revolves around the frame axis while driving the piston rod, connecting rod, and piston. Most crankshafts are manufactured from forged steel and usually operate in machines above 200 horsepower.


The primary duty of the crosshead is to enable the insertion of the piston into the cylinder bore. The crosshead allows a reciprocating compressor to employ a narrow piston, guaranteeing higher efficiency and enabling longer strokes.

Connecting rod

A connecting rod links the piston and crankshaft in a reciprocating compressor. The material used in making connecting rods depends significantly on the mechanical device’s power. Most connecting rods are made from ductile iron material or forged steel.

For instance, connecting rods made from malleable iron material are used in reciprocating compressors operating around 150kW. On the other hand, connecting rods that operate above 200 horsepower or 150kW are forged steel.

A piston pin connects one side of the rod to the piston, while the connecting cap rod connects the other side to the crankshaft. Without this connection, the crankshaft’s rotary motion would be impossible.


Bearings come in different sizes and function differently, depending on their positioning on a reciprocating compressor. The primary duty of bearings is to ensure every other component in the reciprocating compressor is appropriately positioned axially and radially.

Major bearings are fixed in the compressor frame to ensure the crankshaft remains well-fitted. In addition, a crank pin bearing is appropriately positioned between the crankshaft and connecting rod.

Wrist pin and crosshead bearings are also crucial parts of a reciprocating compressor.

Distance piece

This component separates the frame from the compressor cylinder. It comes in 3 unique arrangements:

  • A single design involves a structure in which the distance between the cylinder packing and diaphragm is fully extended to prevent any part of the rod from entering the cylinder stuffing box and crankcase.
  • A double design: No part of the rod enters the compartment in this design. However, the crankcase is positioned adjacent to the gas cylinder.
  • An extra-long compartment arrangement: This design helps separate the piston rod part that enters the crankcase. The other part of the rod, i.e., the part that enters the cylinder, sends its lubrication to that long-distance piece.

As a result of this development, the lubricant will not be able to enter the cylinder to contaminate the gas.


These are the primary components of reciprocating compressors. Each component contributes to the machine’s overall functionality and works conjointly for optimal performance.

Bear in mind that different manufacturers produce reciprocating compressors. But they operate using the same positive-displacement principle.

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