Reciprocating Compressor

You can see reciprocating compressors all around you, at a dentist clinic (to run tiny tools), automobile garages, workshops, painting garages, etc. A reciprocating compressor is an economical way for them to get compressed air for running their tools/appliances.

A reciprocating compressor can be used to compress different gases, in different industries, however, for the sake of simplicity we will consider ‘reciprocating air compressor’ here to explain some of the concepts.

In this article, we will explain to you how reciprocating compressors works, different types of reciprocating compressors, valve types, major pros and cons of reciprocating compressors, and the applications of reciprocating compressors.

What is a Reciprocating Compressor?

A Reciprocating Compressor
A reciprocating compressor has a piston that moves up and down in a cylinder sucking air or gas at low/atmospheric pressure and pushing it out of the discharge valve at increased pressure. The piston is driven by a crankshaft either directly through a connecting rod or the crankshaft drives the crosshead through the connecting rod and the crosshead moves the piston through a piston rod.

The crankshaft may be driven by an electric motor or an internal combustion engine. Using an internal combustion engine makes the compressor portable.

The reciprocating compressors are called positive displacement compressors, since they discharge a constant amount of gas per stroke, at a fixed speed. There can be more than one piston/cylinder. The construction of the reciprocating compressor resembles the engine of your motorcycle or car.

Reciprocating compressors are preferred for high compression ratio (ratio of the gas pressure at discharge and suction valves), comparatively low flow rate, and dry gas.

How Do Reciprocating Compressors Work?

The construction of the reciprocating compressor is similar to the engine of your motorcycle or car. The capacity of the cylinder in a reciprocating compressor is inversely proportional to the absolute temperature of the gas at the inlet valve. The cooler the gas, the higher is the volume that enters the cylinder.

The position of the piston at its topmost position is called the “Top dead center (TDC)” and the position of the piston at its bottom-most position is called the “Bottom dead center (BDC)”.

Clearance Pocket: When the piston is at its TDC, there will be a clearance between the top of the piston and the inside surface of the cylinder head. The function of the clearance space in a reciprocating compressor is to avoid dashing the piston onto the cylinder head.

A reciprocating compressor has two valves at the top of the cylinder and the valves are in the closed status due to the spring force.  The valve allows the flow in one direction only.

The suction (inlet) valve allows the air to enter the cylinder and the discharge (outlet) valve allows the air out of the cylinder. Depending on the gas to be compressed the inlet line can have accessories including an air filter when the gas to be compressed is air.

How Reciprocating Compressor works

When the piston moves from TDC towards the BDC, the pressure in the cylinder falls below the atmospheric pressure, and due to the pressure difference across the suction valve, the suction valve overcomes the spring force and opens to allow the air into the cylinder. If the gas is air, it enters the cylinder at atmospheric pressure through an air filter.

The piston now moves from BDC towards the TDC, the air is slowly pressurized (the pressure of the gas will increase when you apply force to reduce its volume), the suction valve closes, and when the piston approaches the TDC, the pressure of the air/gas is sufficient to overcome the spring force of the discharge valve and hence, the discharge valve opens to allow the air/gas out of the cylinder at increased pressure. The cylinder becomes empty, the discharge valve closes, and the piston moves towards BDC.

The discharge valve is connected to the pipe and the compressed air or gas can either be used directly or stored in a tank for subsequent use.

Parts of a Reciprocating Compressor

The major parts of a reciprocating compressor are the cylinder, piston, piston rings, connecting rod, suction and discharge valves, and the crank-shaft. Let’s examine each of these items in detail.

Piston

You have seen the function of the piston in the above paragraphs. The piston is either driven directly by the crankshaft through a connecting rod or through a crosshead and piston rod.

The common material for the piston is cast iron. Smaller size pistons are made from solid cast iron, while bigger size pistons (more than 7ʺ diameter) are made hollow. Pistons made of carbon material are used for compressing gases where the presence of lubricating oil is not desired (like oxygen).

The piston is fitted with replaceable piston rings. During the working of the reciprocating compressor, the piston gets heated and expands, and hence sufficient clearance should be maintained between the cylinder and the piston.

Piston Rings

Piston rings are designed to provide a seal between the cylinder and the piston and to minimize leakage. The piston rings are normally made with a gap or they can be of several segmental pieces. The piston ring with a gap moves out and expands when it becomes hot.

Piston rings of materials like cast iron, bronze, Teflon, are used, and sometimes cast iron rings with Teflon or Babbitt rider band is also used, however, Teflon is preferred for applications where lubricant oil is not permitted (the complete piston ring can be of Teflon). Piston rings reduce or minimize the leakage of gas between the piston and the cylinder liner (or cylinder).

Cylinder

The cylinder is connected to the suction and discharge valve and allows suction and compression of the gas. Due to the compression of the gas, and continuous piston movement, heat is generated inside the cylinder and this limits the maximum pressure of the compressed gas. Your reciprocating compressor normally has a system to cool the cylinder.

The material of the cylinder depends on the operating pressure. Cast iron is the normal choice for pressures up to 1000 PSI, and nodular iron for 1000 to 1500 PSI. Cast steel and forged steel are used for pressures higher than 1500 PSI.

To increase the life of the cylinder, replaceable cylinder liners are press-fitted into the cylinder. However, fixing a cylinder liner diminishes the effect of jacket cooling and decrease the volume of the cylinder (and hence capacity).

Wear compatibility between the parts rubbing against each other viz. cylinder and piston rings, seal rings, etc. is an important factor for the selection of material.

Cylinder head and crank end head: The ends of your reciprocating compressor cylinder have cylinder heads and crank end head. The top cylinder head may have a liquid jacket for cooling off the cylinder.

Connecting Rod

This is normally made from a steel forging and connects the piston or the crosshead to the crankshaft.

Crosshead

If you have a smaller capacity reciprocating compressor, you can see that the piston is linked directly to the crankshaft by the connecting rod. However, this arrangement induces a sidewise force on the piston (due to the movement of the connecting rod). This sidewise force can be tolerated when your reciprocating compressor is small, however, in the case of a bigger size compressor, this force can cause high wear on the piston and increase overall friction inside the engine.

To overcome the above issues, a crosshead is introduced in between. A crosshead is a machined casting sliding between its guides; the crosshead is connected to the crankshaft through a connecting rod and converts the rotary motion of the crankshaft to reciprocation motion. The other end of the crosshead is connected to the piston (through the piston rod) and makes the piston reciprocate.

Crankshaft

The crankshaft of the compressor is made from steel forging. This is driven by an electric motor or an internal combustion engine and converts the rotary motion to the reciprocating motion. The crankshaft will have counterweights to keep it dynamically balanced due to the eccentric movement and runs in plain journal bearings. A flywheel is mounted on the other end of the crankshaft for providing mechanical advantage and also helps the manual movement of the piston (when the motor is detached) during maintenance work.

Compared to other types of compressors, reciprocating compressors run at relatively low speed, and the drive can be either through a belt or direct. There can be a variable speed drive controller between the electric motor and the reciprocating compressor.

Distance piece

The distance piece separates the cylinder of the reciprocating compressor from the compressor frame.

Valves

There are two types of valves in a reciprocating compressor, a suction valve and a discharge valve.

  • Suction (inlet) valve: Gas or air enters the cylinder through this valve.
  • Discharge (outlet) valve: Gas or air is discharged through this valve.

The suction and discharge valves are spring-loaded, one-way valves viz. the suction valve allows the gas into the cylinder and not the other way, and the discharge valve allows the compressed gas out of the cylinder only.

Compressor Lubrication

The reciprocating compressor has many moving parts viz. piston moving in the cylinder, piston connected to the crankshaft through a connecting rod for moving up and down, etc. and they all need lubrication. They are lubricated normally by the oil and can be done by splash lubrication or using an oil pump.

A reciprocating compressor with pressure lubrication normally has a higher or continuous duty cycle and the pump forces the oil into the connecting rod to lubricate all the key parts of the compressor. The lubricating oil helps to reduce the friction and generation of heat.

Other major parts of a reciprocating compressor are,

  1. Pipes: These are pipes connecting the discharge of the compressor to the point which utilizes the compressed gas or a receiver tank, which temporarily holds the compressed gas before it is consumed (example, receiver tank in a reciprocating air compressor).
  2. Pressure switch: This senses the cutoff pressure limits of the reciprocating compressor and sends a signal to the electrical circuit to cut off or restore the flow of electric current into the electric motor
  3. Unloader valve: The unloader valve ensures smooth restarting of the compressor.

How to Specify a Reciprocating Compressor?

When you want to purchase a reciprocating compressor, let us say for compressed air, here are the key things you must specify

  • The Horse Power (HP) of the motor
  • Pressure rating in PSI or Bar (PSI-pounds per square inch & 1 Bar=14.5 PSI)
  • The flow of the gas (can be in LPM (liters per minute) or CFM (cubic feet per minute) & 1 LPM=0.0353 CFT) and
  • Capacity of the storage tank if you need a storage tank connected to the compressor.

CFM/LPM is the volume of air delivered by the compressor at its discharge valve, and PSI/Bar is a measure of the force of the air.

For example; You can say “My requirement is for a reciprocating air compressor which delivers 10 CFM of air (volume) at 90 PSI (force of the air)

Duty Cycle

Duty cycle is another important factor you must consider when you look for a reciprocating compressor. The duty cycle indicates the time for which you can run the compressor continuously after which it should be left to cool for a fixed time. A reciprocating compressor can have a 100% duty cycle (you can run it continuously) or a variable duty cycle (example-duty cycle 70% means, in a one-hour cycle, you can run the compressor for 42 minutes followed by a cooling period of 18 minutes).

Reciprocating Compressor Types

Depending on the number of stages the reciprocating compressors can be classified into three.

  • Single stage compressors
  • Two-Stage compressors and
  • Multi-Stage Compressors

Another way to classify the compressor is according to the compression cycle. Under this we have,

  • Single acting compressor and
  • Double acting compressors.

Another variation is the diaphragm or membrane-type compressor which delivers compression without leakage.

A detailed explanation on the all these variants can be found at the next part of this guide; Types of Reciprocating Compressors.

Major Pros and Cons

Following are some of the main benefits of reciprocating compressors when compared to other types.

Advantages of Reciprocating Compressor

  • The initial investment for a reciprocating compressor is less compared to other types of compressors.
  • Reciprocating compressors are good for applications where the use is intermittent (not continuous).
  • Reciprocating air compressors can be made compact and portable, and you can connect them to your car battery and use it to inflate the flat tire.
  • The construction of a reciprocating compressor is simple and doing maintenance is comparatively easy.
  • Even though the number of parts is more in a reciprocating compressor, the spare parts are relatively cheap.
  • You have the option of reciprocating compressor run by an electric motor (suitable for a workshop or garage where grid power is available) or that run by an internal combustion engine (suitable for fieldwork where grid power is not available).
  • Reciprocating compressors are suitable for high-pressure use.

Disadvantages of Reciprocating Compressor

  • High level of vibration and noise, and you have to place it in a separate room with a good foundation.
  • The temperature of the compressed air at the discharge valve is high.
  • The maintenance downtime can be high since the number of wear parts is more.

Application of Reciprocating Compressors

  • A reciprocating compressor can compress a wide variety of gases viz. refrigerant, hydrogen, natural gas, etc., and finds its applications in different industries.
  • Reciprocating compressors are used to compress, (i) Oxygen, Hydrogen, Nitrogen, etc. for chemical processing Industries (ii) hydrocarbons in refineries, and petrochemical plants, and (iii) dry gases where a high compression ratio is required.
  • Reciprocating air compressors are used in a workshop to run pneumatic equipment viz. cutting wheels, grinding wheels, polishing wheels, fettling tools, drills, etc.
  • Reciprocating air compressors are used to run pneumatic tools like spanners or inflating tires in an automobile garage, and to run the spray gun in the painting garage.
  • Reciprocating compressors are used for compressing the refrigeration gas in a refrigeration system.
  • Reciprocating air compressors can be made compact and portable, and you can connect them to your car battery and use it to inflate the flat tire.
  • Many hobbyists prefer to have one reciprocating air compressor in their hobby shop or garage since it has multiple uses.

Conclusion

Reciprocating compressors are very versatile, useful, and do not pinch your pocket much. They are highly useful in your home garage as well as commercial workshops and factories.