Reciprocating Air Compressors – All You Need To Know

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Reciprocating Compressors

Reciprocating compressors are normally used in the production facilities. it is used in oil refineries, petrochemical plants, gas pipelines and many other industries. So, understanding what reciprocating compressors are and how they work is very crucial.

So let’s talk about first, what are reciprocating air compressors?

Reciprocating air compressors are also known as piston compressors. These are the positive displacement devices that suck air in the chamber and uses a reciprocating piston to compressed air.

The reciprocating device is also known as a positive displacement device. Because in it firstly air sucks in the chamber, then to achieve air compression chamber area decrease, there is a piston in it which uses to decrease area by the reciprocating motion.

There are many types of compressors used in various industries. These are the following kinds of gases are compressed with the help of reciprocating air compressor.

Oxygen, hydrogen, etc. for the chemical processing
Air for the compressed instruments and tools air systems
Little hydrocarbon fraction in the refining
Many gases for transmission or storage
Other applications

In industry mainly two types of air compressor use first are intermittent flow, and another one is a continuous flow. Intermittent flow includes rotary and reciprocating types. Continuous flow includes axial and centrifugal flow types.

These compressors use at a place where the fluid of process is comparatively dry, and higher compression ratios need per phase without higher flow rates.

How does a reciprocating compressor work?

In the reciprocating air compressor, when the piston stirs towards BDC, firstly air sucks within the chamber from the atmosphere. Then piston stir towards TDC, air compression starts, it remains to continue and pressure starts increases in the chamber.

When air pressure raises to the design level of reciprocating compressor, to open the discharge value reciprocating air compression pulls it. Then in this way compressed air is transferred to a storage tank.

The reciprocated air compressor is powered by gas/diesel engines, or by an electric motor. Let’s explore the above-described steps one by one to get a better idea about the working of a reciprocating air compressor.

The first step was power-on of air compressor when reciprocating air compressor got powered its electric motor begins rotating which consequently rotates crankshaft which attached to it.
Then piston begins moving up and down within the cylinder.
When piston stir downwards (towards the BDC), air enters from the atmosphere within the cylinder chamber.
And when piston stir upwards (towards the TDC), air compression begins, and air pressure starts increasing in the cylinder chamber.
When air pressure reaches the limit of the cylinder chamber, or its pressure becomes more significant than the pressure of the discharge valve, then to open the discharge valve reciprocating air compression pulls it.
Then in this way compressed air transfer to a storage tank, where this compressed air is ready to use for work.

Basic Design of a Reciprocating Compressor

In this section, we will take a look at the basic design of the reciprocating compressor. Let’s take a look at some primary components of the reciprocating compressor.

Piston

A piston is used for the reciprocating movements in the chamber of the cylinder, and for the air compression.

Crankshaft

The crankshaft connects with the shaft of the electric motor. Then crankshaft delivers its rotatory movement towards the piston.

Connection Rod

Connection rod connects the crankshaft and piston.

Cylinder

A cylinder is a chamber, where the air is compressed.

Discharge value

The discharge valve is used to discharge compressed air to the storage tank.

Suction value

Suction valve is used to suck air when the piston stirs to BDC.

Different kinds of Reciprocating Air Compressor

Below are the different kinds of reciprocating air compressors.

Single-stage reciprocating air compressor
Two-stage reciprocating air compressor
Single-acting compressor
Double acting reciprocating compressor

Single-stage reciprocating air compressor

In the single-stage reciprocating air compressor, air compression takes place within the single cylinder. In the first stage, air sucks from the atmosphere and then in the second stage air compression takes place which transfers to the storage tank.

In this compressor, all compression takes place in the single cylinder. The closing and opening of its check valves depend on the distinction in pressure. If it’s mechanical valves use for discharge and suction then their functionality is controlled by the cams.

In a single-stage compressor, when the piston moves towards the top, air weight will be equal to zero within the cylinder. At this point, you should neglect the clearance volume. While when the piston begins moving downwards, air pressure within the cylinder drops down atmospheric pressure and the suction inlet/valve opened. Air is pinched within the cylinder through the suction filter part. This working is called the suction stroke.

Contrary, when the piston moves toward upwards, the air compresses within the cylinder, and the inlet valve closed when air pressure reached the atmospheric pressure.

Additionally, air compression follows when the piston moves to the peak of a stroke. Unless air pressure within the cylinder becomes more significant than the air pressure within the receiver, this is the compression stroke of the reciprocating compressor. At the last of this stroke delivery/discharge valve opens and the air is transferred to the receiver.

Two-stage reciprocating air compressor

This is another type of reciprocating air compressor in its air compression takes place within two phases.

In the first phase, air compression takes place to some limit in the one cylinder; then for further compression, the air is delivered to the second cylinder. Then, at last, compressed air is cumulated in the tank.

A two-stage reciprocating compressor or double stage reciprocating compressors have the two-cylinder. One cylinder is called a low-pressure cylinder, and another one is called as the high-pressure cylinder.

While piston will be in the low-pressure cylinder, it will be at its outer dead center and the weight of the air within the cylinder will be equal to zero.

When piston stir towards the inner dead center, the air pressure drops down the atmospheric pressure and suction valves become open, because of distinction in pressure.

Air is pinched within the cylinder through the suction filter part. This air is more compressed by the piston & the pressure outside and inside the cylinder remain equal, at this section suction valves become closed.

So, in single stage compressor air is compressed only one time. While in double stage, the air compression rate is twice the single-stage compressor.

Single-acting compressor

A reciprocating compression can be either a single acting or double acting compressor. In the single-acting, only one side of the piston uses for air compression. While another side of the piston is connected with the crankcase, and it is not used for air compression.

So, in it, working fluids can work on only one piston side. A single-acting compressor cylinder depends on the springs, loads, momentum of the flywheel, other cylinders, to pull back piston in another direction. This compressor is found in many types of reciprocating engines. Single action compressor is used in many internal combustion engines such as diesel engines, petrol, and it is also used in internal combustion engines such as the steam engine, Stirling engine. You will also see its use in hydraulic rams and pumps.

Single-acting is the hydraulic fluid that enters with the port at the one side of the cylinder, which expands the rod with the means of the difference in the area. There is the external gravity or force which returns the piston rod.

The single-acting compressor is conventional, and a world most used a compressor. It is mostly used for pneumatic systems in trucks, refrigerators and the multi-stage for the higher pressure. Most of the single-acting compressors are splash oil greased, while some are dry which run on PTFE to make the air oil-free.

Double acting reciprocating compressor

A double-acting reciprocating compressor is a cylinder where working fluids work on both sides of the piston. If you want to connect the piston with the external mechanism like with the crankshaft, there must be a hole at one side of the cylinder for the piston rod. This hole is filtered with the stuffing box or gland to prevent flit of working fluid.

These cylinders commonly used in the steam engines, however, it is not used in other types of engines. There are many pneumatic and hydraulic cylinder which has the port at both side; it is supplied with the hydraulic fluid for extension and retraction of piston. The double-acting cylinder uses when there is no force available to retract the piston, or where higher power is needed in both travel directions.

This compressor has the crosshead that creates the pure strait motions after crankshaft and the connection rod. This compressor is mostly used in the industry, where is a need for it in a very heavy amount, and process compressors are from 45kW.

A double-acting compressor is like a double-acting pump. It has the following components:

Cylinder
Two delivery valve & two suction valves
Crank & crankcase
Piston, piston rod, and the connecting rod
One inlet and outlet port

Reciprocating gas compressor

A reciprocating gas compressor is the positive ejection compressor that uses the pistons operated by the crankshaft to transfer the gases at the higher pressure. Intake gas goes into the suction divers; then it goes to the compression cylinder. In the compression cylinder, it compresses by the piston in the reciprocating movement by the crankshaft, and then this gas is discharged.

A reciprocating gas compressor is used in the gas pipeline, oil refineries, chemical plants, refrigerator plants, natural gas action plants. It is also used in the bluster of plastic bottles which is made up of polyethylene terephthalate.

Diaphragm reciprocating compressor

Diaphragm reciprocating compressor matches with the single-acting reciprocating compressor. The main difference between both is instead of piston movement within the cylinder; piston moves the diaphragm which contracts and expands.

If we talk about compressed air, then you will see diaphragm pumps in very less amount. It can be found outside the pumping water.

Working Principle of the Diaphragm reciprocating compressor

A diaphragm reciprocating compressor is like a displacement compressor that runs by the electric motor. The diaphragm compressor has two chambers in every cylinder such as the gas chamber and hydraulic oil chamber. When the machine starts, the electric motor operates crankshaft which makes rotational movements.

Crankshaft connects with the piston through a connecting rod; it can deliver the crankshaft rotational movement to the reciprocating motion of the piston.

While piston pushes down, hydraulic oil flows back to the compressor cylinder. The distinction in pressure creates the descending elastic deformation in diaphragms. The capacity of the gas chamber will become more substantial, and the valve inlet will open itself. In this way, gas sucks in.

While the con rod pulls piston to the upside, the piston will pull hydraulic oil, to create diaphragms in the ascending elastic deformation. The capacity of the gas chamber will be lessened, and gas compressed more. The discharge valve of gas opens itself when gas pressure is reached to a specific limit.

If we describe it in a single process, due to the constantly elastic deformation in the diaphragm, gas sucks itself, compress & discharge constantly. So, this is known as the Working Principle of the Diaphragm reciprocating compressor.

Uses of reciprocating air compressor

Below are some of the uses of reciprocating air compressors

A reciprocating air compressor is used in many places. Such as you will find air compressors in gas stations, in the desktop computer, in the HVAC system, etc.
This is also used to provide compressed air to increase air pressure in vehicle tires.
A reciprocating air compressor is using in many industries such as in chemical plants, natural gas plants, oil refineries.
It is also playing a significant role in refrigerator technology.
You will also find reciprocating compressors in some air compressed applications such as automotive workshops, dental surgeries, and agriculture.

Further Studies

Reciprocating Compressors

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