are employed in the industry in numerous, unique and even sometimes bizarre
ways. They can be used singly or in stages to create a wide range of vacuum
conditions, or they can be operated as transfer and mixing pumps. The ejectors have the following advantages over other kinds of pumps:
• Rugged and simple construction
• Capability of handling enormous volumes
of gases in relatively small sizes of equipment
• Less maintenance requirements
• Simple operation
MAKES THEM WORK
ejectors operate on a common principle. The single stage ejector, in its
simplest form, consists of an actuating nozzle, suction chamber and a diffuser.
The actuating fluid, which may be a gas, vapor or liquid, is expanded from its
initial pressure to a pressure equal to that of the secondary fluid. In the
process of being expanded, the actuating fluid is accelerated from its initial
entrance velocity, which is negligibly small, to a high velocity. In the
suction chamber, the actuating fluid induces a region of low pressure-high
velocity flow which causes the secondary fluid to become entrained and mixed
with the actuating fluid. During the mixing process, the actuating fluid is
retarded and the secondary fluid is accelerated. As the mixture enters the diffuser,
it is compressed to the exit pressure by rapid deceleration. The purpose of the
ejector is to transport and compress a weight of induced fluid from the suction
pressure to the exit pressure. By staging ejectors it is possible to obtain a
very large range of suction pressures from atmospheric down to as low as one
micron of mercury absolute.
multistage ejectors, it is usually advantageous to condense the steam from each
stage in a water-cooled inter condenser so as to reduce the load to the succeeding
stage. This reduces the size and steam consumption of the succeeding stages and
results in a much more efficient ejector system. Of course, the steam
condensing must take place at a pressure above that corresponding to the
saturation pressure of the cooling water.
inter condenser, however, increases the initial cost of an ejector. It must be
determined, therefore, whether or not the additional expense of an inter
condenser is worth the savings that will be forthcoming in steam economy. Small
ejectors for pilot plants, laboratory use or intermittent service may not
warrant inter condensing. As many as 4 or 5 non-condensing stages may be
justified under some circumstances.
it is desired to reclaim condensate, it is good practice to use a condenser on
the final stage of an ejector system which discharges to atmospheric pressure.
An after condenser. As it is commonly called, will serve to silence any noise
that may issue from the discharge of the final stage. An after-condenser also
eliminates the nuisance or objection of discharging steam in a confined area.
After-condensing does not increase the efficiency of an ejector, but it may
increase the efficiency of the steam generating cycle by reclaiming condensate
or preheating boiler feed water.
for ejectors are available in either surface type or direct contact type
(barometric or jet condensers). Direct contact condensers have the following
• They cost less than a surface type
designed for equal service.
• They seldom or may never require
problems with corrosion are usually minimized in a direct contact condenser
since the corrosive media from the ejector is diluted by the condensing water.
Condensable vapors of relatively high vapor pressure which are partially
soluble in water, such as ammonia, can be more effectively condensed in direct
contact condensers because of the diluting effect of the cooling water.
condensers, on the other hand, are advantageous for the following reasons:
do not mix the cooling water with the condensate thus permitting recovery of
the condensate which may be suitable for boiler feed water.
height limitations require the use of a condensate pump only a relatively small
pump is required as compared to a low level direct contact condenser where the
pump must handle both the condensate and the cooling water.
the condensate contains a corrosive, poisonous or radioactive substance,
special provisions may be necessary for disposing of the condensate which can
be kept to a minimum by the use of a surface type condenser.
ejectors have a fixed capacity curve . In this type of ejector the capacity is
a function of the absolute pressure at the suction inlet. Increasing the steam
pressure above the design pressure will not increase the capacity of the
ejector: as a matter of fact, it will actually decrease the capacity because of
the choking effect of the excess steam in the diffuser throat.
are most sensitive to changes in discharge pressure. If the discharge pressure
on an ejector exceeds its maximum stable discharge pressure, the operation will
become unstable and the capacity will no longer be a function of the absolute
pressure. Stabile operation can be attained either by increasing the steam flow
or by decreasing the discharge pressure.
stable operation, the steam pressure for most ejectors must be above a certain
level. This lower boundary will have two magnitudes depending upon whether the
steam pressure is approaching the transition point from the unstable side or
from the stable side.
is the connection through which the high pressure motive steam supply is
This provides a plenum chamber
with the appropriate connections for the suction inlet, diffuser and steam
nozzle. This part can sometimes be eliminated by incorporating the diffuser
connection and steam nozzle connection in the vessel which is to be evacuated.
Frequently more compact designs and savings in cost can result from such
This is the heart of an
ejector since it converts the energy of pressure to velocity and directs the
flow of motive steam into the diffuser.
provides a correctly shaped introductory section and converging diffuser
section to handle the high velocity flow of fluids. It is in this section that
entrainment and mixing of the motive and load fluids is completed and the
energy of supersonic velocity is converted to pressure.
is the transition piece between the converging supersonic inlet diffuser and
the diverging subsonic outlet diffuser.
provides a correctly shaped diverging diffuser section for completing the
conversion of velocity to pressure. After the fluid flow has passed through the
throat of the diffuser, the flow is essentially subsonic. The outlet diffuser
section further reduces the fluid velocity to a reasonable level so as to
convert practically all the velocity energy to pressure energy.
steam pressure is being increased from a region of unstable operation, the
point at which the ejector first becomes stable is called the motive steam
pickup pressure. The pickup pressure is a direct function of the discharge
pressure. At the higher discharge pressure, the ejector will regain its
stability once the motive steam pressure is increased to the pickup pressure;
but the absolute pressure for a particular load may be increased slightly from
what it was at the lower discharge pressure.
every discharge pressure in an ejector there is also a minimum steam flow below
which the operation will be unstable.
steam pressure is being decreased from a region of stable operation, the point
at which the ejector becomes unstable is called its motive steam break
pressure. The motive steam break pressure is below the motive steam pickup
pressure for any given discharge pressure and load. For this reason, the
ejector operating with steam pressure between the break and pickup points may
be stable or unstable depending on the direction of the steam pressure change.
terms “break” and “pickup” pressures are also used in reference to the
discharge pressure of an ejector for the pressures at which ejector operation
becomes unstable and stable, respectively. These critical discharge pressures
are a function of the steam pressure and
ejector stages have no motive steam break and pickup pressures because of the
low ratio of discharge pressure to suction pressure over which they operate, or
because they are designed to eliminate this characteristic. In these ejectors
the capacity varies directly with steam pressure over certain operating limits.
Design. If only one load and vacuum are required for a particular application,
single and multistage ejectors can be designed specifically for one condition.
This saves steam.
however, single-point design ejectors are not always stable at very light loads
or at loads slightly in excess of design. An ejector of this design is not
necessarily undesirable if the ejector always operates at the exact design
conditions. This, of course, depends on whether or not it’s possible to
determine accurately the load on the ejector before hand.
designs can often result in substantial steam and water savings in large
systems. However, it is usually not possible to determine exact operating
conditions prior to design. For this reason single-point designs are not in
Design. Occasionally an ejector must operate alternately at two or more
conditions of load and vacuum. In this instance the ejector must be designed
for the most difficult conditions (or the conditions that call for the largest
ejector). The other conditions will then fall within the performance curve of
the larger ejector.
ejector of this type is sometimes considerably oversized for some of the
required conditions in order to achieve the most economical design from the
standpoint of initial cost. If operational economy is important at each of the
conditions, it may be desirable to use two separate ejectors to achieve
efficiency at both operating points.
possible in some applications to provide an ejector for two or more different
operating conditions—with maximum efficiency at each point—by providing a steam
nozzle or diffuser designed for each condition. In changing operations from one
condition to the other, it is necessary to shut down the system long enough to
change the nozzle or the diffuser. Often, substantial steam savings can be
realized in this way, thus avoiding the cost of two ejector
Designs of this kind have found applications in the recompression boosters for
evaporators and large ejectors for high-altitude wind tunnels.
certain applications an ejector is required to meet a specific design curve.
Then sometimes considerably more steam is used than for a single-point design
to produce the desired characteristic curve. At some point in the curve the
ejector is, of course, relatively efficient and at either side of this
high-efficiency point the ejector is relatively inefficient.
Give Versatility. It is possible to meet a large variety of operating
conditions economically with multistage ejectors by operating only some of the
stages at a time.
ejectors have at least as many different performance curves as they have
stages. For a particular stage to operate, all the succeeding stages must, of
course, be operating. Practically all points within the envelope formed by
these curves can be reached by the ejector.
Thus, the ejector can cover an entire area of possible operating conditions.
and seven stages of compression have lengthened the range of operation of steam
ejectors down to absolute pressures as low as 1 micron of Hg (0.001 mm. Hg.
Commercial designs are available and should often be used in place of other
kinds of vacuum pumps.
field of vacuum processing, steam jet ejectors have been most widely used.
Steam jet ejectors are ideal for use on all kinds of stills, vacuum deaerators,
evaporators, crystallizers, oil deodorizers, steam vacuum refrigeration, flash
coolers, condensers, vacuum pan dryers, dehydrators, vacuum impregnators,
freeze dryers, vacuum filters and more recently on stream degassing of metals
and vacuum melting of metals. Steam jet ejectors offer operational and
economical advantages over a very large range of pumping capacities. Corrosive
applications are easily handled providing a suitable material is available that
can withstand the corrosive medium in question with reasonable structural
strength at the temperatures to be encountered by the ejector system.
effect of water temperature is more critical on ejectors designed for low
absolute pressures. For example; in a 4-stage ejector, the increase in capacity
for 65 F. water over 85 F. water for a particular steam consumption will be
greater at 1 mm. Hg. Abs. than at 4 mm.
pressures higher than 100 psig will permit designing for a larger capacity for
a particular steam consumption. A greater benefit from high steam pressures can
be realized in 1- and 2-stage ejectors than in other designs.
benefit from high-steam pressures becomes less as the absolute pressure for
which the ejector is designed decreases. Single-stage ejectors designed for
absolute pressures lower than 200 mm. Hg Abs. cannot operate efficiently on
steam pressures below 25 psig. However, initial stages of multistage ejectors
can often be designed to operate efficiently on steam pressures below 1 psig.
not uncommon to use an extra stage for an ejector designed to operate on steam
pressures as low as 15 psig.
very important that the steam used to motivate ejectors be at least
dry-saturated steam. Small amounts of moisture can be removed successfully by
using a good, properly sized steam separator which will remove 98 to 99 percent
of the moisture entering the separator.
jet ejectors are available that can economically handle a nominal amount of air
leakage in vacuum equipment along with a large condensable vapor load. Water
pressures as low as 10 to 20 psig will sustain a moderate vacuum while water
pressures of 40 psig and higher will efficiently sustain a vacuum in the range
of 4.0 inches to 1.0 inch Hg. Abs. pressure with a single stage ejector
depending on the load to the ejector and the temperature of the motivating
water operated ejector combined with a steam jet ejector can yield a very
compact, efficient and relatively inexpensive vacuum pumping system. The water
jet ejector serves both as an inter condenser and a final stage to the ejector
system. In instances where
non-condensable entering the ejector system are small compared to the
condensable vapors, say 10 percent by weight non condensable or less, the water
jet ejector can take the place of a two stage ejector in maintaining a suction
pressure of 3 inches to 2 inches Hg. Abs. to which the steam jet ejector can
discharge. The result is a low priced and efficient vacuum pumping system. In
Europe, an interesting equivalent to the water ejector is a centrifugal water
jet ejector which combines a centrifugal pump with the ejector principle and
which re circulates the motive water handling air and vapor loads in a single
piece of equipment. It is likely, however, that the combination of a separate
centrifugal pump and a separate water jet ejector would be a more competitive
pumping system than the European centrifugal ejector pump.
recent years steam vacuum refrigeration systems have become more popular than
ever. They produce chilled water in the range of 40 F to 60 F for cooling
process heat exchangers, air conditioning and the multitude of other
applications where chilled water is required. The booster steam jet ejector
draws off flash vapors and discharges them to the barometric condenser. The
non-condensed materials are drawn from the barometric condenser
more stages of steam ejectors and an inter condenser. Another type of
refrigeration system uses a surface condenser to replace the barometric
condenser. Since the latter system does not mix the condensate with the cooling
water, the condensate may be pumped from a condenser draw-off leg.
addition to cooling water, flash cooling and concentrating of liquid solutions
are two other applications for the steam vacuum refrigeration system. These
systems are extremely simple in operation and rugged in construction. With
simple automatic controls, they can function at excellent efficiency at all
times of the year at all loads. Over-loading the system does not harm the
equipment. The system will produce more refrigeration capacity at over-loads.
But at a higher temperature than that for which it was designed. Most large
capacity steam vacuum refrigeration units are installed outside and require no
protection from the weather other than a protective coat of weather resistant
paint. When indoor installations are required, the component parts can be
arranged in almost any fashion to suit the particular space requirements. Steam
vacuum refrigeration efficiently utilizes steam at pressures as low as 2 psig
and sometimes lower.
PUMPING AND MIXING
jet ejectors can be designed to pump liquids and even finely divided solids for
pneumatic conveying systems. In the latter service, they are becoming widely
used to transfer fluidized catalyst with ease and a minimum of loss.
jet ejectors can also pump liquids with good efficiencies as can steam jet
ejectors. In any case, the motive liquid pressure must be sufficiently higher
than the suction and discharge pressures of the ejector to yield an economical
flow ratio of motive liquid to load liquid. In addition to liquid pumping,
liquid operated ejectors can be used to agitate, mix or meter liquid solutions.
operated ejectors are popularly used for fume scrubbing. A large volume of gas
and vapor can be entrained at drafts from a fraction of an inch of water to
about 10 inches of water and higher with good water economy. In these
applications, obnoxious, corrosive or poisonous fumes can be sucked out of an
enclosed area by means of a simple water jet ejector.
addition to vacuum pumping, ejectors have found many applications for compressing
fluids to pressures above atmospheric. Steam jet thermo compressors are a very
efficient means of boosting low-pressure waste steam to a pressure where it can
serve a useful purpose. High-pressure motive steam which might otherwise be
throttled for use in an evaporator or heat exchanger can be used to actuate a
steam jet thermo compressor. The thermo compressor can serves two purposes:
throttles the high-pressure steam to the desired pressure and
compresses low-pressure waste steam to a pressure where it is again useful.
jet air compressors are useful for supplying compressed air in an explosion
hazardous area where electrical equipment would have to be of explosion proof
construction and relatively expensive. Compressed air at 20 psig for pneumatic
controls is a typical application for steam jet air compressors.
often essential that an ejector be actuated by a gas or vapor other than steam.
Excellent examples of this are the gas actuated ejectors used to compress low
pressure manufactured gas from atmospheric pressure to the 15 or 20 psig
pressure required for distribution mains. High pressure natural gas at about
100 psig or higher is used to actuate the ejector. The ejector serves to
compress the manufactured gas
the high pressure natural gas and
blend the two gases to the correct proportion.