Free Report On Combined Compressor System Design

Air compressors convert electric or chemical energy into potential energy stored in compressed gasses. Compressors are driven by electric motors or internal combustion engines. The pressurized gas is used to do work by releasing its kinetic energy during depressurization. In industries, pressurized gases are used for various purposes. These include powering tools such as nail guns and spray painters, braking systems in hydraulic, air-start systems among others.

The working principle of compressors

Compressors use pumps to compress gas into a confined space. A pump forces gas to flow against a pressure head until the pressure in a pressure vessel gets to a certain point. Pressure gauges on the inlet and outlet of the pressure reservoir automatically starts and stops the compressor. This means the compressor will start running when pressure levels fall below a certain point and stop running when pressure in the reservoir goes above a set point.

Adiabatic compression in compressors

Compressors operate on an adiabatic compression process. Compression can be expressed as a function of temperature and pressure. Gas compression can be expressed as:
PVn=Constant
This compression defined by the above equation is referred as polytropic. When n=1, the compression is isothermal. In adiabatic compression, n= γ=Cp/Cv, where γ is the heat capacity ratio of ambient air. In adiabatic compression, there is no heat loss to the environment.

Thus, compression in compressors is expressed as:

PVγ=Constant
The above relationship gives P against V compression curve in compressors when n= γ.

Calculation of the power generated by an adiabatic compressor

The power produced by an adiabatic compressor in kW can be given as:
P=γγ-1RT1Mm100η[P2P1γ-1γ-1]

Combination of five compressors

Compressors can be combined to operate in tandem. Only one machine operates initially and the others come online as compression power required increases past the capacity of one compressor. The compressed gas requirement is offset by the sum total of power output of the compressors until enough compressors are online to fully service the load.
A common reservoir is used to store the compressed gas from the combined compressors. The pressure sensor in the reservoir is set to actuate the compressors when pressure falls below a set point. When there is no compressed gas requirement, the pressure in the reservoir will be constant and hence no compressor will be running. When loads are connected to the system, the pressure starts declining and the first compressor goes online. If pressure continues declining, the second compressor goes online to fill the deficit of compressor one. The cycle continues until enough compressors are online to meet the load. When the loads are removed from the system, the reservoir pressure starts rising and some of the compressors go offline in order to balance the system.

Sample calculation for a combined compressor system

Assumptions: all the compressors are identical, the compression gas is air, and γ for air is 1.4.

The compressed gas requirement load can be evaluated using the formula below:

P=γγ-10.371Tq/η[Pγ-1γ-1]
Assume an instantaneous pressurized gas requirement of 10atm at a mass flow rate of 30,000g/s at 298K. The power requirement will be:
P=1.40.40.371*29823.184/0.8100.41.4-1=10,436.92kW
The power output from one compressor working at a maximum flow rate of 2,500g/s and output pressure of 10atm will be:
P=1.41.4-18.314*2980.029 2.5100*0.81011.4-11.4-1=4,437.09kW
10,436.92kW4,437.09kW=2.3
Therefore, three compressors will be activated to power the load while the other two remain at standby mode.