All gases have large spaces
among there molecules, these spaces could be enhanced further and decreased by
doing some work. Work done which is used for the enhancement of these spaces is
called expansion, and work done to minimize these spaces is called compression.
All gases while expansion produces cooling effect and while compression of
gases raises there temperature. Compression is a term used to force gases to
occupy smaller volumes by increasing their pressure. Volume of gases is interlinked
with temperature and pressure of the gas, which is cleared by 2 laws, govern
gas compressions.
·
Boyles Law
·
Charles Law
Centrifugal compressor are
machines in which velocity and pressure given to air or gas in a radial
direction by one or more impellers diffuser combination.
The centrifugal compressor may be
single stage or multistage. Multistage limits containing a series of impellers
on a single shaft rotating at high speed in a massive casing internal channel
lead from the discharge of one impeller to the in let of next these machines
compress enormous volumes of air or process gas up to 200000
ft3/min,(340000m3/hr).At the inlet to an outlet pressure of20atm, similar
capacity machines discharge at pressure up to several hundred atmospheres,
interstage cooling is needed on high pressure units.
WORKING PRINCIPLE:
In a centrifugal compressor the
mechanical energy is increased by centrifugal action. The gas enters the
suction eye of a high speed rotary element called the impeller which caries
radial vanes integrally cast in it. As the impeller rotates, the blades of the
impeller force the gas outward from the center the impeller to the outer rim of
impeller, the increase in velocity of the gas creates a flow pressure area at
the eye of the impeller. The gas at the outer rim of the impeller is forced in
to a passage way called a diffuser where the velocity decreases in the pressure
of the gas.
In a multistage compressor different sets
of impellers and diffusers the pressure of the gas in stages. Labryrinths are
installed between the impellers the prevents gas leakage between the stages,
each stage has its own casing drain to insure that no liquid is in the
compressor before starting.
To balance the axial thrust developed from the discharge end to the
suction end of machine, a balancing drum connected to the suction discharge end
of the machine to stop gas leakage along the shaft, a seal is formed with seal
oil.
