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1. Steam jet ejectors
Introduction:- Steam jetejectors offer a simple,reliable,low-
cost way to produce vacuum.They are especiallyeffectivein the
chemicalindustry where an on-site supply of the high-pressure
motivegas is available.
Ejectors are considered an alternative to mechanical vacuum pumps for
a number of reasons:
1. No source of power is required other than the motivegas;
2. Because they have no moving parts, they are reliablevacuumproducers;
3. They are easy to install, operateand maintain.
Ejector Design
Very simply, an ejector is a pumping device. It has no moving
parts. Instead, it uses a fluid or gas as a motive force. Very often,
the motive fluid is steam and the device is called a “steam jet
ejector.” Basic ejector components are the steam chest, nozzle,
suction, throat, diffuser and they discharge (Fig. 1).

2. Thermo Compressors:- Thermo compressors are
ejectors applied to recompressing spent steam and
process fluids. In the compressors recycle waste steam
and low-pressure water vapor discharge, reducing energy
consumption by 30% or more.
Vacuum Producers:- Ejector-based systems are particularly
appropriate as primary vacuum producers, particularly where
motive steam is almost always available.They are applied in
processes such as crystallization, deaeration, drying, cooling,
high vacuum distillation and deodorization.
Ejector systems range from the simple, single-ejector stage to
very complex systems with as many as six ejectors in

3. combination with inter condensers.
Ejectors are available in either single-nozzle or multiple-
nozzle designs. Single-nozzle units are also available with
an automatic spindle for special applications. The single-
nozzle, fixed orifice is the simplest type of ejector.
Multiple-nozzle ejectors are more efficient. They usually
save as much as 10% to 20% in motive steam,compared
with single-nozzle units designed for the same conditions.
In a spindle-operatedejector, the nozzle and tapered
spindle work together similar to a needle valve. In this
case, the spindle is essentially changing the dimensions of

4. the ejector’s internalities is, creating a new single-point
design. For this reason, the spindle-operatedejector
provides high off-design efficiency, which is very handy if
operating conditions must vary.
Multistaged ejectors often promote economy by
including intercondensers between some of the
stages to reduce the load to the following stages.
Sometimes precondensers, booster condensers and
aftercondensers are also used.
Condensers
Condensers may be barometric or surface. The size
and type of condenser selected is a function of air-
vapor ratios, cooling water temperatures available,
steam and water costs, and contaminants in the
suction vapor.
Barometric condensers cost less to buy and install.
They have many advantages. However , users should
be aware that the barometric condenser is a direct-
contact design. The cooling water is mixed directly

5. with the vapor to be condensed. If there are any
environmental considerations concerning the process
fluid, it should not be mixed with cooling water.
The shell-and-tubecondenser keeps cooling water
separate from the process fluid. No contamination can
occur; thus, the condenser water is cooled and reused. On
the other hand, the shell-and-tubedesign may require
more maintenance due to the possibility of scale or solids
buildup on the condenser tubes.
Ejector Operation: In operation, a high-pressure
motive gas enters the steam chest at low velocity
and expands through the converging-diverging
nozzle.This results in a decrease in pressure and an
increase in velocity.Meanwhile, the suction fluid
enters at the suction inlet. The motive fluid , which
is now at high velocity,entrains the suction fluid

6. and combines with it.
The two fluids are then recompressed through the
diffuser. Potential energy is converted to kinetic
energy; thus,velocity increases and pressure
decreases. The mixture reaches its maximum
velocity and lowest pressure at the Venturi throat
(Fig. 2). The fluid then is charges at an intermediate
pressure, which is higher than the inlet suction fluid
pressure, but much lower than the inlet motivefluid
pressure.

7. Optimum Efficiency: An ejector represents a single-
point design. That is, its optimum efficiency exists at a
single set of conditions. The specifications for which
it is designed represent the maximum capacity built
into the unit.
Ejector designs are classified as critical or non-
critical. Critical flow means the fluid velocity in the
diffuser throat is sonic. In non-critical units, the fluid
velocity is subsonic.
Ejectors designed in the critical range are sensitive to
off-design operating conditions. Suction capacity

8. cannot be increased. In fact, it is actually lowered by
increasing the motive fluid pressure. Because the
nozzle is a fixed orifice, any change in the motive fluid
pressure will be accompanied by a proportionate
change in the quantity of motive fluid.
Changes are more gradual in non-critical design units
but suction capacity still cannot be increased. The
best solution is to be sure that service conditions are
specified correctly.
Specifying Ejectors
While the ejector itself can be quite simple, specifying
the optimum system to meet specific needs is not
simple. Important parameters involved in ejector
sizing and staging include pressure of motive gas,
required discharge pressure, suction pressure and
relative mass flow rates of motive fluid to suction
fluid.

9. For instance, most ejectors use steam as the motive
fluid. The quality of the motive steam affects the
operation of the unit. The usual requirement is for dry,
saturated high-pressure steam.
In operation, it is very important to maintain the
design quality of steam. If the quality of the steam is
low, suction pressure and capacity will decrease,
especially in multistage designs.
Excessive steam superheat can also adversely affect
the suction capacity of an ejector. It decreases the
energy level ratio, and the increase in specific volume
tends to choke the diffuser.
Ejectors can generally be applied on a variety of
processes as long as the proper conditions exist: the
motive fluid pressure drop is large enough to develop
high velocities in the nozzles, the difference between
suction and discharge pressures is not excessive and

10. the suction fluid flow is small compared with the
motive fluid.
Advantages and Disadvantages:
Some advantage in using an ejector system instead other
vacuum and refrigeration system such as mechanical vacuum
pumps and refrigerators.
 Ejectors could be built from a variety of materials.
 Ejectors can be incorporated into harsh environment.
 No moving parts to adjust or repair.
 High reliability, because of the fact there are no moving
parts.
 Easy installation, many ejector systems are compact and
could be installed easily into an existing system
Applications include:
 Out-gassing steel at low pressure (few microns absolute)
 Turbine Condenser Vacuum
 Pneumatic Conveying
 Pneumatic Pumping
 Pump Printing