Compressors

Question 1

Describe, ideally, how a compressor works

Answer 1

At point 1, the cylinder has maximum volume as the piston is in the central position, with the gas a pressure ps. As the piston moves to decrease the cylinder volume, the gas inside compresses and the temperature and volume inncrease, assuming the process is ideal (isentropic. This means ideal gas equations can be used.

At point 2, the piston has decreased the volume to the point at which the gas is at the pressure pd. At this point the discharge valve will open. There will be no pressure loss across the valve. As the piston moves to further decrease the cylinder volume, the pressure remains constant as the gas is displaced into the discharge line.

At point 3, the volume of the cylinder has reached its minimum and the valve will close. The piston will then move back towards its original position and the gas that was remaining in the clearance volume will expand. The temperature and pressure will decrease.

At point 4, the gas has reached the suction pressure, ps, and the suction valve will open. As the piston moves to further increase the cylinder volume, gas is drawn in to the cylinder which maintains the pressure at a constant value. When the cylinder reaches its maximum volume, the piston will again be in the central position and the cycle will repeat.

Question 2

What type of compression is achieved with cooling?

Answer 2

Isothermal compression

Question 3

What kind of compression is achieved without cooling?

Answer 3

Adiabitic compression

Question 4

Give two examples of a positive displacement compressor and the condition under which they are used

Answer 4

1. Reciprocating
2. Rotary
3. Used for low flowrates (less than 85,000 m3/h)

Question 5

Give two examples of dynamic compressors and state the condition under which they are used

Answer 5

1. Axial
2. Centrifugal
3. Used for high flow rate (over 85,000 m3/h)

Question 6

What is compression ratio? What value should it usually be under?

Answer 6

The desired pressure divided by the current pressure. This value should usually be under 6, or 3 if you are using a centrifugal compressor.

Question 7

What fluids can drive the turbine for a compressor?

Answer 7

Hot gas or superheated steam.

Question 8

Draw a compressor driven by a turbine

Answer 8

Question 9

What does gamma represent in compression equations?

Answer 9

Cp/Cv

Question 10

What is polytropic compression and how is it measured?

Answer 10

It is a real situation somewhere between isothermal and adiabatic. It is measured using the factor N, which relies on the manufacturer measuring the pressures and densities in and out of the compressor.