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AIR EVACUATION SYSTEM
COMPARISON IN ALL STAGES
RHITESH KUMAR GUPTA
Inside the shell, a central or side portion longitudinally is
separated by an outer shield except at the bottom. This partition is
called the Air zone.
All the gases released in the condenser due to cooling are taken
out via these air zone tubes.
From a suitable portion of this air zone inside the shell an air vent
pipe is taken out and brought out of the shell for connection to an
air extraction device.
Two types of ejector:
Main Ejector (two in no.)
Steam is used as motive medium in air ejector.
All ejectors have its steam source from Aux PRDS Header.
VACUUM PUMP AND WORKING PRINCIPLE
Positive displacement pump:
A positive displacement pump makes a fluid move by trapping
a fixed amount and forcing (displacing) that trapped volume
into the discharge pipe.
Some positive displacement pumps use an expanding cavity on
the suction side and a decreasing cavity on the discharge side
The Liquid Ring Vacuum Pump:
A Liquid Ring Vacuum Pump consists of a multi bladed, rotating
element (impeller), electricity positioned within a cylindrical
The compression of the gas occurs in one or two impeller stages .
Incompressible fluid, called service liquid, is introduced to the
Its heat capacity (specific that) and the boiling point (vapour
pressure) being two of the most important specific characteristics
Other factors such as the solubility of process components into
the service liquid and service liquid viscosity is also important.
The impeller sits between two end plates (port plates) which have
shaped holes cut into them called ports.
As the impeller rotates, a ring of liquid is formed inside the pump
casing from centrifugal force.
This action draws the gaseous stream into the pump through the
In two stage LRPV, the second stage is volumetrically smaller
doing the final compression of the gas.
The gas then exits the pump usually at atmospheric pressure
along with the service liquid.
The gas enters the pump chamber A-B via suction port. The gas is
trapped between two impeller
vane. The impeller rotate
eccentrically in relation to the centre line of the liquid ring that, by
centrifugal force, assumes the shape of impeller casing. The
progressive change of volume between the two vanes, the impeller
hub and the liquid ring first creates a vacuum and then a
compression of the gas in the B-C area till the gas is discharged,
together with a portion of the liquid, through the discharge port
C-D. The lost liquid must be replenished.
Cavitations is a phenomenon which occurs in specific fluid
environments and is an important factor when using this type of
The vaporization of the service liquid sets up the structures of
cavitation in the LRVP, but the damage is caused when the vapour
bubbles collapse, not when they form. When the collapse occurs, a
high velocity micro-jet of water tears away at the metallic surfaces
of the pump internals.
The damage is recognized as a series of craters or holes in a
continuous pattern on an LRVP impeller.
Also, damage can occur on the suction and discharge ports in
either stage of the LRVP.
Vacuum Pump Specification of St.-2 & 4
Vacuum Pump & Motor:
Edwards pump ltd. UK
NO. OF PUMPS
NO. OF STAGE
WATER, 15° C
WATER, 15° C
Single-Stage vs. Two-Stage Pumps:
The above describes the cycle of a single-stage pump; so in one
revolution, we have suction (pulling a vacuum), and compression
(back to atmosphere). Some manufacturers single stage pumps can
operate up to -0.89 ksc,
A two-stage pump is just two of these operating in series, so the
discharge of the 1st stage goes into the suction port of the 2nd
Two-stage pumps have better efficiency at higher vacuum
levels (higher than -0.79 ksc) than a single stage pump.
A two-stage pump is also a much better choice if you are
handling solvents at higher vacuum levels (higher than -0.79
ksc). This has to do with the effect of sealant temperature rise
being spread across two stages and its relationship to the sealant
No. of stages
Use of vacuum ejector
Used in single stage vacuum P/P
Comes in operation when (gas temp. –sealant temp) < 7 c
Sucks gas form separator tank and supply it to vacuum P/P I/L
port to prevent cavitations without affecting the air I/L line
Use of Anti - Cavitation line
Used in two stage vacuum P/P.
Connected to second stage second stage I/L (first stage dish.) to