Terminology of Air Compressor...

  • Compressors are machines designed for compressing air or gas from an initial intake pressure to higher discharge pressure.
  • Reciprocating Compressors are those in which each compressing element consists of a piston moving back and forth in a cylinder.
  • Single Acting Compressors are those in which compression takes place on but one stroke per revolution in each compressing element.
  • Double Acting Compressors are those in which compression takes place on both strokes per revolution in each compressing element.
  • Single Stage compressors are those in which compression from initial to final pressure is complete in a single step or stage.
  • Multi Stage compressors are those in which compression from initial to final pressure is completed in two or more steps or stages.
  • Two Stage Compressors are those in which compression from initial to final pressure is completed in two steps or stages.
  • Inter Coolers are devices for removing the heat of compression of the air or gas between consecutive stages of multistage compressors.
  • After coolers are devices for removing the heat of compression of air or gas after compression is completed.
  • Moisture Separators are devices for collecting and removing moisture precipitated from the air or gas during the process of cooling.
  • Air Receivers are tanks into which the compressed air or gas is discharged from the compressor. Receivers help to eliminate pulsations in the discharge line and also act as storage capacity during intervals when the demand exceeds the capacity of the compressor. Use proper tank as furnished and /or recommended by manufacture. Do not reduce tank size or restrict discharge air line from compressor tanks.
  • Free Air is air at normal atmospheric conditions. Because the altitude, barometer and temperature vary at different locations and at different times.
  • Volumetric efficiency is the ratio of the actual capacity of the compressor to displacement and is expressed in percent.
  • Compressor Efficiency is the ratio of the theoretical horsepower to the actual indicated horsepower required to compress a definite amount of air or gas.

Compressor Type:

Positive Displacement (Intermittent Flow)
Continuous flow
  • Sliding Vane
  • Liquid Piston
  • Straight Lobe
  • Helical Lobe
  • Centrifugal
  • Axial Flow
  • Mixed Flow

Air Compressors are the heart of a compressed air system. This is the equipment used to compress the atomospheric air to a higher pressure. Which in tum can be utilized as a source of power to operate various machines.

The two major sections in which the Air Compressors can be divided are :

  • Positive Displacement Type
  • Dynamic Type

(A) A Positive displacement compressor is one in which successive volume of Air/ Gas is sucked into a closed space and elevated to a higher pressure. These types of Compressors are often called intermittent flow compressors.
Examples : Reciprocating (Piston Type) screw, Sliding Vane, Liquid Piston etc.

(B) A Dynamic Compressor is a fast moving rotary compressor in which the rapidly rotating element accelerates the gas as it passess through the element converting the velocity head into pressure partially in the rotors and partially in the stationary diffusers.
Examples: Centrifugal, axial flow etc.

The following are the 3 main types of compressors being used in Indian industry.

  • Reciprocating
  • Screw
  • Centrifugal

(1) Reciprocating Compressors (Positive Displacement)

This type of compressors are most common in the indian market today. These compressors are basically positive displacement machines in which the compressing & displacing element is a piston having reciprocating motion with the cylinders of the compressor.

Due to the rotational motion of the shaft the piston of the compressor get a linear back and forth motion which causes compression and discharge of the air/gas . The compressor may have single or multiple piston depending on the manufacturers design.

In general following are the ranges for Single and Multiple Reciprocating Compressors.

Pressure in PSIG Flow in CFM 150
10 to 200
225 to 500
525 to 1000
1100 to 1500
1525 to 2500

The Compressor cylinders of this compressor is lubricated with oil and hence the compressed air discharged from the compressor shall always contain traces of oil. This oil is harmful in many ways especially in food processing industry and pharmaceutical industry. There are two ways to overcome this problem. The first method is to filter the discharged air to remove the oil and the second method is to use special compressor with dry cylinders and Teflon /graphite piston rings. In these compressors there is no lubrication of the cylinders and hence no chance of oil carry over.

There are no economical substitutes for these compressors in the small range i.e, Upto 50 CFM. These compressors are available for air flows upto 20000 cfm and pressure as high as 60000 psig (approx. 4250 kgcm2G) This is the most versatile type of compressor & can be used for nearly all applications of modern industry.

The compression of Air / Gas and the continous sliding action of the piston within the cylinder causes heating of the cylinders.To reduce this heat which could damage the piston as well as liners,water is circulated in a jacket around the cylinders which can ensure proper cooling of the cylinders.

Multi Stage Compressor

Multistage compressors are used in place of single stage compressors, because if the compression is done in one stage, then the heat generated may be excessive and also material of construction would have to be of very high grade and possibly too expensive. Also power consumption of a single stage compressor would be higher.

In a Two Stage Air Compressor, the air/gas is compressed to a certain pressure which would be much lower than ultimately desired pressure in the first stage. This air is then passed through a heat exchanger called ‘INTER COLLER” , where the hot compressed air is cooled down, and then the air is further compressed to the final discharge pressure. In compressors where there are more than 2 stages of compression there shall be more than one intercooler. This discussion on multistaging is valid for all types of compressors.

2) Screw Compressors (Positive Displacement)

Screw compressor is basically a twin Helical lobe compressor, in which both lobes rotate simultaneously and compress the air/gas in between. These compressors are generally available upto an air flow of 20000 cfm. These compressors are not very well suited for high discharge pressures. These compressors also are available in both lubricated type and non-lubricated type. In case of lubricated type screw compressors the carry over of oil to the discharge is very high and to take care of this the manufactures supply mechanical oil separators and Air oil coolers as a standard accessory. These oil separators shall mechanically separate oil from the air and return the same to the compressor, even so the air leaving the system shall have oil content nearly equivalent to that in a lubricated reciprocating air compressor. The air/oil cooler which also is a standard accessory cools the mixture of air and oil. These coolers are of 2 types air cooled and water cooled. The air temperature after the air cooled cooler would be around 60o to 80oC while that after a water cooled unit would be around 40oC to 50oC. If the air/oil cooler is well designed it may eliminate the need for an aftercooler (shall be discussed later) it has been noted that screw compressors are better suited and found to be more efficient for relatively high air flows and low pressure.

The advantages of the screw compressors over reciprocating compressors, is the economy with gard to

  • Lesser space requirement
  • Less expensive foundation
  • Lower erection time
  • Lower power cost ( in many cases)
  • Lower maintenance cost

3) Centrifugal compressors (Dynamic)

These are compressors which are specifically meant for handling, very high flows and its economy is highest for large air flows. The centrifugal air compressors are best suited for requirements of continuous air flows in large quantities. These compressors can run continuously without stoppage for very long periods of time, and maintenance for the same is minimum. Also heat generated due to compression in these compressors is very less, as compared to other types of compressors.

These compressors can generate compressed air upto an air flow of 6,50,000 CFM and pressure higher than 5,500 PSIG.

These compressors are extremely sensitive to dirt and hence utmost care should be taken to provide dirt free air a the suction of the air compressors. These compressors are always oil free, i.e no oil shall every carry over into the system. An independent oil pump and oil cooler is provided for cooling of the lubricating oil.

The impellers of these compressors rotate at extremely. High speeds, which could be in a range of 20,0000 RPM to 1,00,000 RPM.

These compressors require lesser space as compared to an equivalent reciprocating compressor Power consumption also is lesser for these compressors.