Compressors Power Machines N6

Definition of a Compressor 

A compressor is a machine that takes up air at a lower pressure (Usually at atmospheric pressure) at the induction stroke and compresses it to a higher pressure (compression stroke). The air is stored at this higher pressure.

A bicycle pump is a very good example of a compressor. This pump is used to inflate the tire of a bicycle. The inflation of an automobile is also done by a compressor. These compressors are motor driven compressors. This is, therefore, a criteria for classifying the compressor calculations (manually driven and motor driven compressors)

The Graph of a Compressor

One of the most fundamental procedures of compressor problems is the graphs  of compressors (PV Diagram). The numbering of the graphs in terms of the different processes is very fundamental.

Induction Process 1-2   Constant Pressure

Compression 2-3

Compression 3-4   Constant Pressure

Induction 4-1 

Classifications of Compressor Problems.

The criteria for classifying compressors are based on at least two or more of the following as stated below.  There is usually a relationship between one criteria and the other as stated in the formula. 

A very good example is in a multi-stage compressor  problem. Complete cooling of this compressor means that the work done in each stage is the same. The work done in incomplete intercooling is not the same.  

The Number of Revolutions of the motor per unit second 

(Motor Driven Compressors) 

A motor that is operating or rotating at 550 RPM drives the piston of compressor continuously in a reciprocating motion. The greater the number of RPM the greater the work done by the compressor.

Refer to the RPM component of the work done formula.  

Multi-stage Compressors

There are single stage, two stage and three stage compressors.

The PV graphs are as illustrated thus. (Refer to Fig )

The importance of the multistage compressors lies in the fact that the extraction of heat by the intercoolers improves the efficiency of the multistage compressors.

Complete inter cooling and Incomplete inter cooling

The work done for complete inter cooling is the same for each stage. Alternatively, the work done in stage one is equal to the work done in stage two.

The formula for calculating work done is:

Total WD= 3 X WD in one stage

This is not the case in the incomplete cooling. The WD are not equal to each other.

Refer to the formulas below.

Intermediate Pressure

The intermediate pressure of a two stage compressor is the pressure in between the two stages. It is lower than the final pressure and greater than the intake pressure. It is the pressure at which inter cooling takes place.

Intermediate pressures may also be applied to three stage compressors. In this case the intermediate pressures are the two pressures between the intake pressure and the final pressure.

Calculating the Volume of the Cylinder

The volume of the cylinder is as stated in the formula in fig........

The cross sectional area X stroke length= Volume of the cylinder

Calculating the Diameter of the Cylinder and The Stroke Length

Cross Sectional Area of the Cylinder=  pi(d)^2 

Volume of the cylinder = pi(d)^2 X SL

pi=  3.142

d= diameter of the cylinder

SL= Stroke Length 

The mass of air in kg/minute (intake)

The intermediate pressure of a two stage compre

The volume of air in meters cubed/minute or  meters cubed/second (intake)

The intermediate pressure of a two stage compre

The compression exponent or power

The intermediate pressure of a two stage compre

The clearance volume of the compressor.

The Volume V1 is usually calculated in terms of a number greater than 1 (in terms of the swept volume).

For example V1= 1.05 SV

 where CV=0.05SV

CV= Clearance Volume

SV= Swept Volume

Volumetric Efficiency of the compressor.

The formula for volumetric efficiency is:

Volumetric Efficiency = (Actual Amount of Air Taken In)/(Theoretical Amount of Air Taken in)

Volumetric Efficiency = (Actual Amount of Air Taken In)/(Theoretical Amount of Air Taken in)

Power Required to Drive The Compressor

The relationship between the Work done and the Power is the Work done divided by the time duration of the compressors working or doing positive work.

 Intake Pressure, Intake Temperature and the Final Pressure  

Power Required to Drive The Compressor