Air Compressors: Types

Introduction Applications
Power Types
Selection
We thank www.about-air-compressors.com for permitting the use of information available therein for preparation of this article.

Types of compressors

Compressors are often classified into two types (viz. Positive-displacement or Dynamic-type), depending on how the mechanical elements act on the fluid to be compressed.

Positive-displacement compressors confine successive volumes of fluid within a closed space in which the pressure of the fluid is increased as the volume of the closed space is decreased.

Dynamic-type compressors use rotating vanes or impellers to impart velocity and pressure to the fluid.

The main types of gas compressors are illustrated below:

Types of Compressors
Compressors are also sometimes classified depending on the mechanical means used to produce compression of the fluid ; as reciprocating, rotary, jet, centrifugal, or axial-flow

The main types of compressors are discussed below:-

Positive Displacement Type

1. Reciprocating Compressors
2. Rotary Screw Compressors

Dynamic Type

1. Centrifugal Compressors
2. Axial-flow Compressors
3. Scroll Compressors

A) Positive Displacement Type:

1. Reciprocating Compressors: Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume.

Reciprocating compressors use pistons driven by a crankshaft. Reciprocating CompressorThey can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines.

(Image on right: Reciprocating Compressor: NASA public domain image)

Small reciprocating compressors from 5 to 30 horsepower (HP) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW) are commonly found in large industrial and petroleum applications.

Discharge pressures can range from low pressure to very high pressure (>18000 psi or 180 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, and more costly than comparable rotary units.

Household, home workshop, and smaller job site compressors are typically reciprocating compressors 1½ hp or less with an attached receiver tank.

Reciprocating compressors often have the lowest up-front cost, but the highest operating cost!

Single-stage and two-stage reciprocating compressors are commercially available.

Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig.
Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig.

Note that:

1 HP ~ 4 CFM at about 90 psi; and that 1 to 50 HP are typically for reciprocating units. Compressors 100 hp and above are typically Rotary Screw or Centrifugal Compressors.

Reciprocating compressors up to the 25 hp size are generally best priced in terms of purchase cost.

2.  Rotary Screw Compressors

A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism .

Rotary screw compressors use two meshed rotating positive-displacement Rotary Screw Compressorhelical screws to force the gas into a smaller space.

(Image on right: Ingersoll Rand R-series-R110 Rotary screw air compressor internal view: Licensed for public use with attribution to Ingersoll Rand)

They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers.These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable.

Their application can be from 3 horsepower (2.2 kW) to over 1,200 horsepower (890 kW) and from low pressure to moderately high pressure (>1,200 psi or 8.3 MPa).

B. Dynamic Type

Dynamic-type compressors use rotating vanes or impellers to impart velocity and pressure to the fluid.

1. Centrifugal Compressors: The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air.

The Centrifugal compressor achieves a pressure rise by adding kinetic energy / velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuser.

Imagine a simple case where flow passes through a straight pipe to enter centrifugal compressor. The simple flow is straight, uniform and has no swirl. As the flow continues to pass into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster.

According to a form of Euler’s fluid dynamics equation, known as “pump and turbine equation,” the energy input to the fluid is proportional to the flow’s local spinning velocity multiplied by the local impeller tangential velocity.

In many cases the flow leaving centrifugal impeller is near or above 1000 ft./s or approximately 300 m/s. It is at this point, in the simple case according to Bernoulli’s principle, where the flow passes into the stationary diffuser for the purpose of converting this velocity energy into pressure energy.
Turbocharger Centrifugal compressor

NASA: Cut-away view of a turbo-charger showing the centrifugal compressor (blue) on the right end of the rotor

A simple centrifugal compressor has four components: Inlet (typically a simple pipe) , Impeller/Rotor (It is the impeller’s rotating set of vanes (or blades) that gradually raises the energy of the working gas), Diffuser (Downstream of the impeller in the flow path, it is the diffuser’s responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity), and Collector (purpose is to gather the flow from the diffuser discharge annulus and deliver this flow to a downstream pipe).

2. Axial-flow Compressors:
Axial-flow compressors are dynamic rotating compressors that use arrays Caterpillar for handling logsof fan-like airfoils to progressively compress the working fluid.

(Image on left: Axial Flow Compressor: NASA public domain image)

They are used where there is a requirement for a high flow rate or a compact design. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials.

Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants.

3. Scroll Compressors:

A scroll compressor, also known as scroll pump and scroll vacuum pump, Scroll Compressoruses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases.

This type of compressor was used as the supercharger on Volkswagen G60 and G40 engines in the early 1990s.

(Image on right: Author Cacycle: under the terms of the GNU Free Documentation License)

Applications: Air conditioner compressor, Vacuum pump, Superchargers for automobile applications, e.g. Volkswagen’s G-Lader

Positive-displacement compressors are usually useful for supplying small amounts of a gas at relatively high pressures.

Centrifugal compressors are particularly suited for compressing large volumes of gas to moderate pressures.

Axial compressors are used for jet aircraft engines and gas turbines.

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We thank www.about-air-compressors.com for permitting the use of information available therein for preparation of this article.
References:
1. www.about-air-compressors.com
2. http://en.wikipedia.org/wiki/Gas_compressor

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