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WHAT IS A COMPRESSOR?
A device used for pumping compressible fluids.
i.e. Air , gas and Steam
Compressor is a mechanical device that increases the pressure
by reducing volume.
Compressors are similar to pumps: both increase the pressure
on a fluid and both can transport the fluid through a pipe. As
gases are compressible, the compressor also reduces the volume
of a gas. Liquids are relatively incompressible, the main action
of a pump is to pressurize and transport liquids
Suction Pressure = Ps
Suction Temperature = Ts
Suction Velocity = Us
Discharge Pressure = Pd
Discharge Temperature = Td
Discharge Velocity = Ud
Compressor = Black Box to increase Gas Pressure
a) Principle of operation
b) Type of construction
d) Service duty
e) Lubricated/ non lubricated
Parts of centrifugal compressor
It is the housing of the compressor, in which all components were intacted.
There are two types of casing
1. Horizontal Split
2. Vertical Split
Centrifugal compressors with horizontal split casings typically permit internal
of around 700psi (70 bars) with flow rate of around 300,000 Nm3/hr at low
Vertical split or Barrel type compressor
Above 700psi horizontal split casing lose their sealing capability, resulting in an
increased in potential for the escape of process gas. Hence vertical casing is more
preferred for high pressure application.
Based on the nozzle arrangement:
It is the general arrangement of compressor. In which the casing consisting of two
nozzles, one is inlet and the other one is outlet. gas enters at the inlet nozzle passes
through one or more stages of compression and leave through the discharge nozzle in
to the piping system.
Impeller is the heart of the compressor. In which the mechanical energy (rotary motion)
of the rotor is converted in to kinetic energy of the process fluid (compressing fluid
There are three types of impellers
Semi open impeller
Open impellers have the vanes positioned radial direction and have no enclosing covers
on either front or back side.
Semi open impellers have the vanes positioned in a radial or backward leaning direction
and have a cover on the back side with extended to the periphery of the vanes.
Closed impellers have enclosed cover on both front and back side. this is the most
common type in large processing compressors. The blades are usually backward leaning
and in some special cases radial curved vanes
It is the stationary component of the centrifugal compressor. It consisting of two
parts one is the diffuser and the return guide vanes. Normally around 2/3 of
kinetic of gas is converted in to pressure energy in the impeller itself and the
remaining 1/3 of energy conversion is takes place in the diffuser as shown below.
After conversion of kinetic energy in to pressure energy in the
diffuser the gas pass through the return guide vanes. The function
of return guide vanes is to guide the gas to the next stage of
impeller with laminar flow.
Radial bearing of the centrifugal compressor are force lubricated. For the
ease of maintenance the bearing are horizontal split with replaceable
Babbitt lined pads. Number of pad in the radial bearing is depending
upon the speed and load condition of the compressor.
The main purpose of the bearing is to take the thrust load produced in
the rotor. Tilting pad is the most common type of bearing used in
centrifugal compressor. Flat land, tapper land , kingsbury and Michel
bearings are used by most of the manufacture , the selection of bearing
is depend up the thrust load produced by the rotor.
1.Liquid film seals
2.Labyrinth Type seals
4.Injection &Extraction seals
5.Dry gas seals
1. Liquid film seals:
This system is used when the gas medium is poisonous,
inflammable and expensive and no external leakage is
permitted. The principle of this sealing is that high
pressure oil is injected in between a set of close
clearance seal rings mounted on shaft ends. The seal
rings are free floating type with white metalled bore.
The oil leakage through LP side is recirculated through
the oil system and HP leakage is recovered by automatic
traps are re-used after separating dissolved gases.
This system is employed for gases like Air, Nitrogen. The amount of gas
leaking to ambient is reduced by selecting adequate number of
labyrinth fins. Some of the developments in a labyrinth type seals are
stepped type seals, honey comb type and turbine type.
In this type of seals the carbon-rings forming part of stator part
is kept in continuous contact with a rotating collar. The contact
surfaces are lubricated either by a cool buffer gas or
lubricating oil. These seals are very effective means of
providing shaft sealings.
For gases like oxygen, raw gas and other explosive, corrosive, media the
sealing has to be perfect and where sealing medium like oil cannot be
used, the above sealing system is selected. The principle of this sealing is
that vacuum is created by a ejector at the shaft ends which sucks the
gases and thus prevents any outward leakage of the contained gas.
Today the preferred choice is the dry-running, gas-lubricated seal over
conventional seals. A gas-lubricated mechanical seal necessarily needs a
gas supply system to ensure pressurizations of the seal with clean process
gas or external gas.
Dry gas seals
Single seal arrangement
Single seal is used for the compressors, which have lower sealing pressures
and where a small leakage of gas medium can be tolerated. The single seal is
particularly advantageous as they occupy less space at compressor ends.
A double arrangement is a sealing system, which reliably prevents product gas
escaping to the surroundings. It involves feeding a buffer gas, e.g. nitrogen,
between the seals at a higher pressure than the product pressure.