What are Compressors? Definition, Meaning, Types, Parts, Working

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What are Compressors? Definition, Meaning, Types, Parts, Working

The compressors are explained along with types, different components, basic working principles, etc.

Let’s get into the article!

What are Compressors? Definition & Meaning

The compressor is pretty much an important component in most industrial machines and even for some household applications.

From internal combustion engines to refrigerators the compressors are used and have a very important role in the operations.

They have a wide range of applications. Let’s know more about them in detail.

  • The compressor is basically, increases the pressure of the fluid by reducing its volume.
  • The word compression obviously means reducing the volume of fluid.
  • Later the fluid will be used for various applications or further processes.
  • The gases are compressible and the compressor makes the best use of that characteristics.
  • There is a vast variety of compressor types available right now. They are used according to the required applications.
what are compressors
Fig. 1 What are compressors?

The compressors and pumps are kind of similar but no worries. We will differentiate between these two in this article itself.

So, the compressors are mostly used in the stages. The later stage is smaller than the first one. To increase the discharge pressure the stages are used. The more stages more will be compression.

Let’s know about the wide range of compressor types.

Types of Compressors

The compressors are divided into two types. One is the positive displacement compressor that is mostly used and the other one is the dynamic compressor.

We will see the types and later each of them in detail.

Positive displacement

The positive displacement compressor is the compressor, that compresses the air by the displacement of a mechanical linkage reducing the volume.

In simple words, this compressor draws a discrete amount of volume of gas inside and forces them to exit the outlet of the compressor.

It is classified as follows,

  1. Reciprocating compressors
  • Diaphragm compressor
  • Double-acting compressor
  • Single-acting compressor

2. Rotary compressors

  • Lobe compressor
  • Scroll compressor
  • Liquid Ring
  • Screw compressor
  • Vane compressor
type of compressors
Type of compressors

Dynamic Compressors

The dynamic compressors work at a constant pressure. Though the performance of the dynamic compressors is affected by the external conditions.

In these compressors, the air is drawn between the blades with the help of rapidly rotating impellers. Later the gas is discharged via a diffuser.

These compressors are only having two types, depending on the main direction of the gas flow.

  1. Centrifugal compressor
  2. Axial compressor

Before going into the details of each compressor, let’s know about the general parts or components used in the compressor.

Parts of Compressors

The compressor parts are follows,

  • Pistons
  • Connecting rods
  • Actuators
  • Bushings
  • Gaskets & seals
  • Rotors
  • Valves
  • Motor
  • Intercoolers
  • Couplings, etc.

Compressor Parts Description


The pistons are the integral part of the reciprocating compressors.

  • The pistons create the air pressure with the aid of the piston and connecting rod movements.
  • Pistons reciprocate up and down.
  • For supporting the piston and guiding the piston the liner is available.
  • The piston rings are used wherever necessary according to the types and purpose.
parts of compressors
Parts of compressors

Connecting rods

The connecting rods attached to the piston and used to move piston up and down. They take the heavy workload and thus they are made by keeping durability in considerations.


The actuators are responsible for creating rotary and linear movements.

  • They force the air to come out from the outlet ports.
  • But if any leaks or deficiencies happens the actuators may experience some decrease in the output air force.


The bushings are used to create a space between the moving parts.

  • These are kinds of protection devices for the compressor.
  • Incorporated in the internal parts of the air compressor they provide protection from breakdowns.

Gaskets and Seals

These are protection devices for the compressors. The air in the compressor should not be leaked and they should tight and sealed completely to avoid any accidents or breakdowns.

These are provided to various parts of the compressor where leak protection should be implemented, Like,

  • valve plate heads,
  • crankcase,
  • intercoolers,
  • oil seals etc.


The rotors are the parts of the rotor compressors.

  • There are two interlocking arrangements of the two rotors that compress the air from the inlet valve.
  • In case of the rotary compressor, they are the one that compresses the air.


The vales are the controlling equipment’s for the operation of compressors.

  • The vales does the work like allowing air in and out, draining water, air flow regulation.
  • These are checked regularly because the operation depends on the valves, if the valves perform abnormally it will directly affect the compressor.


The motor is used to run the operation of compressed air. However, there may be other drivers that can be used, like –

  • Variable Frequency Drive.
  • Steam or Gas Turbines etc.


The intercoolers are used to increase the overall performance of the compressors. It can be used to set the stages in the compressor.


They prevent the leaking from the high-pressure systems.

Now its time to go into some brief details of each of the compressor types. Starting from the positive displacement compressor.

Positive Displacement Compressors

Reciprocating Compressors

Reciprocating compressors as the names suggests they are reciprocating nature with piston inclusion.

  • They use pistons and are driven by crankshafts. Reciprocating compressors can be either single or multistage.
  • They can be driven by electric motors or internal combustion engines.
  • They are available in smaller ranges like 5-40 horsepower that are commonly used in automotive applications.
  • Also, in wide ranges with the power of 1000 horsepower are used in large industries and petroleum applications. 

Reciprocating compressors are further classified based on the number of discharge strokes per crankshaft revolution.

  • Single Acting Reciprocating compressor: In this type, only one discharge stroke is completed in one crankshaft revolution.
  • Double – Acting Reciprocating Compressor: In this type, two discharge strokes are completed in one crankshaft revolution.

In case of air compressions, the double-acting multi-stage reciprocating compressors are best suited. But they are costlier than the rotary compressors.

compressors type reciprocating
Compressors type reciprocating

The reciprocating compressors are able to compress a wide range of gases, refrigerants. This characteristic of the reciprocating compressors enables them to be used in various applications by varying size, number of cylinders, etc.

Diaphragm compressors are the variant of reciprocating compressors. They have a flexible membrane that moves back and forth with the help of rod and crankshaft mechanisms. They are suited for pumping toxic and explosive gases.


  • They are small in size especially the single-acting ones.
  • These do not require any separate kind of lubricating system.
  • Simple and easy maintenance.
  • Multistage compressors are more efficient in operation.


  • The relatively high cost of compressions.
  • High space requirements
  • High noise

Rotary compressors

Rotary screw compressors

It has the two helical screws meshed with each other. The helical screws push the gas to move into smaller space and hence get compressed.

  • They are used for applications from 3 horsepower to 1200 horsepower.
  • If we talk a brief about it working the helical screws are used that are closely meshed with each other.
  • One is male and the other is female, by maintaining the precise alignment without any contact they compress the gas.
  • After forcing the gas through the compressor, the gas exits at the end of the screws.
compressor type rotary screw compressor
Compressor type rotary screw compressor Image: IndiaMART

They are generally used for larger industrial applications for the supply of compressed air. Mostly used where the demand for continuous air is required.


  • Rotary screw compressors have a long life compared to other compressors.
  • The demand capabilities are good.
  • They are quiet in operations.


  • High initial costs.
  • Requirements of special maintenance

Rotary vane compressors

Rotary vane compressors are equipped with the blades that are inserted in the radial slots available on the rotor. The rotor is mounted in kind of offset with the larger housing. When the rotors turns blades will slide in and out of the slots.

  • Hence because of the in and out of the blades, the increasing and decreasing volume is created.
  • That’s how the air gets compressed in the rotary vane compressors. Alongside, with the piston compressors, these are one of the oldest compressors.
rotary vane compressors
Rotary vane compressors


  • Vane compressors create good vacuum for suction.
  • These compressors work well for the continuous air supplies.
  • Longer life expectancy of vane compressors.
  • Suited for moderate pressures.


  • Vibrations are more.
  • Maintenance is high because of a greater number of moving parts.

Rolling piston

The rolling piston as the name suggests it has the piston included which makes a partition. The partition is made between the vane and rotor.

  • Hence the rolling piston will force the gas against the stationary vane and that’s how the air will get compressed.
  • These rolling pistons offer higher efficiencies due to less losses.
  • But the structure of rolling pistons if used for refrigerants it doesn’t allow the capacities beyond 5 tons.
compressor rolling piston type
Compressor rolling piston type


  • They are compact and light.
  • Machines parts are well balanced and less noisy.
  • Low initial costs.
  • Simple lubrication.


  • Discharge pressure per stage is low.
  • No flexibility in case of capacity and compression ratio.

Scroll compressors

The scroll compressors are also known as the spiral compressors because of its shape and structure. They are used in the air conditioning, as an automobile supercharger and as a vacuum pump.

  • It has two interleaving scrolls to pump and compress the fluids.
  • The geometry can be involute or Archimedean spiral.
  • One of the scrolls used is fixed and other orbits eccentrically without rotating.
  • Another method for producing the compression motion is co-rotating the scrolls, in synchronous motion, but with offset centers of rotation.
  • The relative motion is the same as if one were orbiting.
compressor type scroll
Compressor type scroll


  • They are quite in operations.
  • Simple design and fewer parts involved
  • Low maintenance requirements.
  • Oil-free designs.


  • Low capacity
  • Relatively they are expensive.
  • If one element fails you may need to buy a whole new element.
  • Compressed air may get very hot.

Dynamic Compressors

Centrifugal compressors

These are the sub-type of the dynamic compressors and are also known as radial compressors. They achieve the compression with the help kinetic energy addition to the flow of fluid with the aid of rotor or impeller.

Later the kinetic energy is converted to the potential energy by slowing the flow via a diffuser.

  • If we talk about its working, the fluid enters the compressor without any whirl or anything.
  • But when the flow passes through the centrifugal impeller, the impeller will force the flow to spin faster.
  • Later this kinetic energy is converted to potential energy in the form of pressure.
compressor types centrifugal
Compressor types centrifugal

They are used in wide range of applications like diesel engine turbochargers and superchargers. In the pipelines of natural gases to move the gas from production site to consumers of the gas.

In Air conditioning and refrigeration centrifugal compressors supply the compression in cycle of water chillers.


  • Low weight and easy to design
  • They are oil free in nature
  • High energy efficiency
  • Wide range of speed of rotations
  • They are reliable and have low maintenance


  • High compression rates are not suited
  • Sensitive to changes in gas composition
  • Vibration mountings are needed
  • Surging, stalling and choking problems

Axial Compressors

The second type of dynamic compressor is the axial compressors. These are the gas compressors that can continuously pressurize the gases. Axial compressor uses the arrays of fan-like air foils to compress the fluid.

  • There are two arrays of foils used in this compressor.
  • One is the stationery and another one is the rotating one.
  • The rotating airfoil can be called as blades or rotors and they will accelerate the fluid.
  • The other one will deaccelerate the fluid and will redirect the direction of fluid for the next stage.

Axial compressors has the high efficiencies but they are expensive and require high quality materials. They are mostly used in the large gas turbines, natural gas pumping stations and chemical plants.

Now we have seen both dynamic as well as positive displacement compressors.

compressor type axial
Compressor type axial Image: MAN Energy Solutions

Now let’s know the key differences between dynamic and positive displacement compressors.


  • Axial compressors have high peak efficiency.
  • Small frontal area for a given flow.
  • High ram efficiency.
  • Increased pressure rises due to the increased number of stages and negligible losses


  • Manufacturing is difficult.
  • High costs.
  • These are pretty heavy in weight.
  • High starting power requirements.

Compressor Capacity (FAD) Calculation

Compressor capacity is calculated in terms of FAD, that is Free Air Delivery and it is stated at actual conditions.

It is expressed, as Q

Q = (P2-P1)/P0 x V/T Nm3/Min


  • P0 = Atm pressure, kg/cm2
  • P1 = Innitial Pressure, kg/cm2
  • P2 = Final Pressure, kg/cm2
  • V = Storage Volume, m3
  • T = Time taken to build pressure.

A Nice ANIMATED VIDEO from Learnchannel!

Compressor Specification

A typical compressor specification shall consists the followings,

  • Type of compressors
  • Quantity
  • Capacity in FAD (Free air delivery)
  • Nos. of stages (as applicable)
  • Rated pressure
  • Maximum pressure
  • Shaft speed
  • Nominal power
  • Electrical power supply

Difference between Dynamic and Positive Displacement Compressors

There are a few differences between positive displacement and dynamic compressor.

Sr NoPositive displacement compressorDynamic compressor
1)The positive displacement compressors compress the air with the help of mechanical linkages, it physically reduces the volume and increases pressure.In the dynamic ones, the fluid is provided with velocity by the impeller, and then with the help of a diffuser pressure is increased.
2)It compresses the fixed packets of the fluid.It will compress the fluid in a continuous manner.
3)The velocity of fluid doesn’t require to be high.Velocity of fluid must be high.
4)PDP will give constant flow rate against variable output pressure.Dynamic will give a variable flow rate against variable output pressures.
5)The fluid will directly give the pressure energy.First fluid have the kinetic energy and then converted to pressure energy.

One more thing is to differentiate between the pump and compressor. Because of their similarities of work they often gets confused so to ease out your doubt here is the difference between the compressor and the pump.

Difference between Compressor and Pump

There are a few differences between compressor and pump.

Sr noCompressorPump
 1)Compressor increases the potential energy by pressuring in smaller volumes.The pump increases the kinetic energy of fluid which will further increase the pressure energy.
 2)Fluid used can be only gas.Any type of fluid can be used.
 3)The volume change is occurred.There is no volume change from inlet to outlet.
 4)Compressors are more expensive.They are cheaper than compressors.
 5)Compressors are available with the storage facility.Pumps don’t have any storage facility.
6)Compressible fluids are used.Normally incompressible fluids are used.
7)It doesn’t associate with the cavitation problem.It may associate with the cavitation problems.

Standards of Compressors

There are few standards used to design compressors, as follows,

  • ISO Standards: ISO-13707 and ISO-13631
  • API Std. 617: Axial & Centrifugal Compressor & Expander Compressor
  • API 618: Reciprocating compressor
  • API 619: Rotary compressor
  • API 681: Liquid ring compressor
  • API 672: Centrifugal air Compressor
  • API RP 688: Pulsation and Vibration Control.


So, these are the differences you should know about the compressors and the pump. There are lots of new advancements being made in compressor for increasing efficiencies and performances.

Further Study

Refer to our a few nice articles,



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