Air compressor:

An air compressor is such a device which is used to compress the air and to raise its pressure. It sucks the air from atmosphere and delivers it at same temperature but at high pressure.

Types of compressor:

There are mainly two types of compressor.

1.      Rotary air compressor

2.      Reciprocating air compressor

Rotary air compressor:

            It is such type of air compressor where air is entrapped between two sets of engaging surface and the pressure of air is increased by squeezing action or back flow of air. It is used for large discharge at low pressure.

Types of rotary air compressor:

Ø      Roots blower compressor

Ø      Vane blower compressor

Ø      Centrifugal blower compressor

In Ashuganj power station, there are two service compressors and two control compressors are used for unit ST-1 and ST-2. Here service compressors are rotary type and control compressors are reciprocating type air compressors.

Reciprocating air compressor:

            It is such type of air compressor in which the pressure of the air is increased in its cylinder with the help of a moving piston. It is suitable for low discharge of air at very high speed.

There are many types of reciprocating air compressor. It may be single action or double actions compressors. In Ashuganj power station, V-type double actions reciprocating air compressors are used for unit ST-3, ST-4 and ST-5.

V-type reciprocating air compressor:

V-type reciprocating air compressor has a spring loaded body and base to resist the vibration. It consists of a low pressure cylinder (LPC) and a high pressure cylinder (HPC). There is an intercooler and after cooler beside of these two cylinders. It decreases the temperature but not pressure. Every cylinder has four suctions and four discharge valves. For safety operations there are some valves such as:

Ø      Lube-oil valve

Ø      Air valve

Ø      Manual valve

Ø      Motor valve etc.

Working principle:

1.      When piston moves downward, air is sucked by suction valves at low pressure and room temperature.

2.      When piston moves upward, air is pressurized and the temperature becomes high.

3.      Then this pressurized air should be passes to HP cylinder for more pressure.

4.      But it is not possible to get pressurized air at high temperature. So to be cool intercooler is used before going to HP cylinder. Here the temperature is maintained about 66⁰C.

5.      In HP cylinder temperature raises at 185⁰C and pressure maximum 9.2 bar.

6.      But we need high pressurized air, not high tempered air.

7.      So to cool the air of 185⁰C after cooler is used. Here the temperature is maintained at 90⁰C.

8.      Finally high pressurized air is obtained and passed to reservoir.

Observations:

Ø      Compression starts at 8.5 bar

Ø      Compression stops at 9.2 bar

Ø      No load condition at maximum pressure (9.2 bars).

Ø      Half load condition at 9bar to 9.2 bars.

Ø      Full load condition at 8.5 bars to 8.9 bars.