steam jet ejector - Main Press

Can a
steam jet ejector
fit your vacuum process?
by Frank Moerman, MSc., EHEDG Belgium and Nico Desanghere, MSc., Sterling SIHI
Vacuum is widely applied in the
chemical and food processing
industry, because it permits to
perform processes that cannot
otherwise be done under atmospheric
conditions.
The most well-known sub-atmospheric
application is vacuum distilation,
where vacuum is used to lower the
boiling point of a solvent or other chemical
compound in order to perform a separation
or purification of a high-boiling-point or
thermal sensitive product with minimal input
of heat. Vacuum processing is the solution
for the increasing high-purity requirements
for a growing number of materials in a large
variety of applications. The costs of rejected,
off-specification product and the rising energy
costs are the main incentives to apply vacuum
as a process aid.
Applications in the food and
chemical industry
Other vacuum applications in the chemical
industry are vacuum filtration, vacuum drying,
vacuum evaporation, evaporative cooling,
degassing, etc. Common vacuum applications
in the food industry are given in Table 1.
Keep up your vacuum with a
steam jet ejector
Frequently, food and chemical plants find
it less costly to obtain vacuum by means of
steam jet ejectors. Especially the chemical
industry makes largely use of steam jet ejectors
to generate the vacuum required in many
distillation processes. Table 2 gives an overview
of some advantages and disadvantages
of steam jet ejectors.
Steam jet ejectors (usually multi-stage) are
especially used for wet processes that require
vacuum levels ranging from 15 mbar down
to 0.1 mbar vacuum absolute. Water ring
pumps are not capable to generate such a deep
vacuum. Moreover, when steam jet ejectors
are used to produce the required vacuum for
distillation, evaporation or drying processes,
the same steam ejectors can act as condenser
for the water or solvent vapours drawn from
these processes along with the air. A cold wall
vapour condenser upstream of the steam jet
ejector may not be required, except where a
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PrOcess tecHNOlOGY
Fig. 1: fluctuation
of pressure and
velocity of the
steam/gas-stream
over its traject
through the steam
jet ejector.
very high degree of efficiency is required or
where recuperation of solvents is requested. In
contrast to steamjet ejectors, water ring pumps
may never act as a condenser in applications
where high amounts of solvent or water
vapours have to be condensed. In such a situation,
always a ‘barometric’ or shell-and-tube
condenser has to be installed upstream of that
water ring pump.
PUMPS & PROCESS MAGAZINE N° 69 - MAARt 2011 21
photo: Nitech

Application Vacuum Absolute
(mbar)
Function
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PrOcess tecHNOlOGY
‘Sous-vide’ cooking 50 - 250 To preserve the freshness and nutritional quality of the
minimal processed food.
Evisceration (poultry, fish, etc.) 100 - 150 To remove the bowels.
De-aeration of vegetable oil
50
To remove oxygen that may oxidize unsaturated fatty
acids.
Deodorization of vegetable oil
1 - 3
To strip compounds that affect flavour, odour, stability and
colour from the vegetable oil.
Fractional distillation of vegetable oil
Fractional distillation of essential oils
10
10
To separate fatty acids or their esters from edible oils.
Isolation of flavours and fragrances that otherwise may
decompose and polymerize in the presence of to much
heat.
Freezing drying 20 - 50 Applied to prolong the shelf life of food and to maintain
the basic nutrients in herbs, spices, coffee, fruit, vegetables,
etc.
UHT treatment - vacuum flash cooling of
milk
50 - 100 Milk is heated up to 140-145°C in as few as 3-5 s, contained
in a holding tube for a few seconds, and then fastly
cooled down to 75-80°C due to evaporative cooling as a
consequence of a sudden reduction in pressure.
Vacuum filtration (e.g. yeast) 300 - 600 On a rotary dum dryer a filter cake of yeast can be
sucked dry by means of vacuum.
Vacuum drying 20 - 50 (begin)
1 - 4 (end)
Water evaporates more quickly from food under vacuum.
By vacuum drying, food can become crispy, puffed and
may have a stable colour. Vacuum drying is biologically
desirable since some enzymes that cause oxidation of
food become active during normal air drying. These
enzymes do not appear to be active under vacuum drying
conditions. The speed and the fact that it happens at room
temperature guarantees that taste, colour and nutritional
value of the food are preserved. Also the fibers are fully
preserved, so after reconstitution with water, vacuum dried
fruit and vegetables will reproduce the original texture of
the fresh fruit and vegetables. The drying process can be
accelerated when assisted with micro-wave heating.
Vacuum evaporation of milk 50 To concentrate heat-sensitive products (milk, sugar juices,
etc.) that are prone to discolouration and formation of
cooking favours under the impact of heat.
De-aeration of water 50 Removal of air of the process water used in the preparation
of soft drinks, sparkling and mineral water.
Bottling of beer, soft drinks, mineral and
sparkling water
Vacuum packaging of food in plastic
bags
Modified atmospheric packaging 100
(to remove the air)
50 - 60 Removal of air (oxygen) and dust particles from the bottles
to fill.
100 To remove oxygen that may impair the nutritional quality
by oxidation and that may promote microbial growth of
spoiling bacteria and food pathogens.
In modified atmospheric packaging, the air is first removed
from the packaging by means of vacuum, to be finally
replaced by a modified atmosphere.
Table 1: vacuum applications in the food industry (Note: with vacuum absolute, we allude to the cacuum pressure measure relative to absolute perfect zero vacuum).
How a steam jet ejector
works
A steam jet ejector is in fact based on the
principles of a water aspirator, that produces
a vacuum by means of a venturi-effect and
which is the oldest known method of vacuum
generation. In a steam jet ejector, however, the
water is replaced by steam as motive fluid.
The motive steam fluid is expanded, after
passing through a motive nozzle, where the
pressure energy is transformed into kinetic
energy. This energy stream impinges with
and withdraws gases, air and water vapour
from an application where a sub-atmospheric
pressure (vacuum) must be established or
maintained. The steam vapour accelerates into
the inlet cone of the mixing nozzle. After passing
through the throat of the mixing nozzle,
into the diffuser, the kinetic energy of the
mixed vapour stream is gradually converted
back into potential energy, i.e. the medium is
PUMPS & PROCESS MAGAZINE N° 69 - MAARt 2011 23

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PrOcess tecHNOlOGY
Fig. 2: three-stage steam jet ejector with
two barometric intercondensers and one
barometric aftercondenser.
compressed to a higher discharge pressure.
Fig. 1 demonstrates how the pressure and
24
Steam-jet
type
1-stage
2-stage
3-stage
4-stage
Max.
absolute
vacuum
(mbar)
66
5-15
1-1.5
0.1-0.3
PUMPS & PROCESS MAGAZINE N° 69 - MAARt 2011
velocity of the steam/
gas-stream fluctuates
over its traject through
the steam jet ejector.
Basic
components
of a steam jet
ejector
A single stage steam
ejector can produce
only a limited vacuum
(cfr. Table 2). Multistage
steam ejectors
(Fig. 2) are used when
an application requires
a pressure lower than
what single-stage ejectors
can develop, as the
first can develop a greater suction pressure.
With more stages added to the system, the
pressure of the first stage becomes lesser
and lesser, generating a deeper vacuum. The
ejector which the entrained gases enter first,
is called the first stage and subsequent stages
Advantages Disadvantages
- High achievable vacuum
- High suction capacities and gas flow
- Controlable over a wide range of vacuum
and flow rates
- Excellent to handle condensable corrosive
and contaminated loads
- Excellent to handle liquid slugs and solid
particles
- Reliable and robust in arduous and
corrosive conditions
- Simple design
- Designed in many materials of construction
- Mountable in any position
- Low investment cost
- No moving parts, less failure risk
- Less susceptible to wear, and trouble-freeoperation
- Long life-span
- Simple repair & maintenance
- No heat emission
Table 2: advantages and disadvantages of steam jet ejectors.
are numbered in succession.
It is desirable to connect a condenser to
the discharge of each steam jet ejector to
bring all steam and condensable gases to
the liquid state, reducing the load to the succeeding
ejector stage and thus imposing on
subsequent stages the work of compressing
only those gases that are non-condensable.
The condensers so em-ployed are known as
intercondensers. A condenser connected to the
diffuser dis-charge of the final stage is known
as an aftercondenser, that is used to prevent the
discharge of motive steam and condensable
process vapours into atmosphere.
An intercondenser operates at pressures
less than atmospheric (under vacuum). It
is therefore necessary to provide means for
draining the mixture of condensing water
and condensed steam/condensable vapours
from a barometric intercondenser, or the
condensed steam/condensable vapours only
from a shell-and-tube inter-condenser. The
non-condensable vapours are withdrawn from
the top of each intercondenser by the vacuum
of the subsequent steam ejector. The after-
- Consumption of large amounts of steam as
pressurized motive fluid
- High energy consumption
- Low thermal efficiency
- Requires a steam infrastructure
- Need for high quality steam produced from
soft-demineralised water
- Steam must be dry or should have less than
2% moisture, because wet steam may cause
the ejector vacuum to break or fluctuate, and
can erode the nozzle and diffusers.
- Needs inter-condensers and after-condensed
and large amounts of cooling fluid to
condense the mixture of motive and process
vapour
- Contamination of the motive fluid
- Large amounts of contaminated steam condensate
(waste water)
- Load specific and very sensitive to variations
in process conditions and pressure
- Noisy, requires silencers or sound insulation

condenser operates at atmospheric
pressure and is provided
with a vent to finally allow air
and non-condensable gases to
escape in the atmosphere.
‘Barometric’
condensers
A ‘barometric condenser’ (also
called direct contact condenser)
is a vertical vessel where
withdrawn process vapours are
cooled and condensed by direct
contact with downward flowing
cold water injected into the top
of the vessel. Since the operating
pressure of the condenser
is sub-atmospheric (under
vacuum), collected condensate
(effluent cooling water and condensed steam/
vapours) must be continuously removed. That
condensate is normally dropped into a receiver
tank that is often vented to atmosphere
or a low pressure vent system. This creates
a situation where the condensate is under
vacuum in the condenser and is trying to
move toward the receiver tank that is under
positive pressure. To overcome this pressure
differential, the condenser must be located
higher than the receiver tank (the bottom of
the condenser should be at least 10.4 meters
above the ground) to create a tall ‘barometric
leg’ (10.4 m long pipe) in which a static
column of liquid balances the atmospheric
pressure. The condensate must flow by gravity
through this long sealed vertical tail pipe into
Fig. 3: if tail pipe must change in direction,
it should form at least a 45° angle
from the horizontal plane; the horizontal
piping (right drawing) is vulnerable to
gas accumulation.
a ‘hot-well’ (drainage basin provided with an
overflow or pump) or a sealed condensate tank
(provided with fluid-level control and condensate
pump). The ‘barometric leg’ allows the
effluent coolant and condensed vapours in the
barometric condenser to exit no matter what
its vacuum is, finally preventing the condenser
from flooding under normal operation.
In the receiver tank, the tail pipe must be
submerged enough (not less than 28 cm). If
this seal is broken, air will be drawn into the
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PrOcess tecHNOlOGY
tailpipe, increasing the risk
for flooding the condenser
and hence affecting the
performance of the downstream
vacuum source and
the upstream vacuum process.
The drainage lines or
tailpipes should be preferably
installed vertically.
Horizontal drain leg runs are
not recommended, because
they are susceptible to gas
pockets. The mixture of
cooling water plus condensed
steam/condensable
vapours always contain air
or other non-condensable
gases which cling to upper
pipe surfaces. All types of
pipe contain a certain amount of internal
roughness and, because of this, gases tend to
start clinging and building up in the smallest
crevice. In addition, every flanged joint has
a slight crack where a gasket is located, thus
permitting another place for gases to collect.
As these gases accumulate, they form tiny
bubbles, growing into larger ones that eventually
become big enough to partially or completely
block off piping at that point. In that
case, the condensate cannot flow downwards
and soon its level rises, flooding the condenser.
If piping changes direction, it must form at
least a 45° angle from the horizontal (Fig. 3).
With this amount of sloping, gases will either
slide back up the pipe or continue downward
with the thrust of the flowing water. The 45°
Fig. 4: (a) with barometric condensers, it is important to note that condensate is splashing down the barometric walls and could run
down the vapour inlet and back into the upstream vacuum process, unless the inlet is protected by a dam or series of elbows; (b)
Degradation of the absolute vacuum pressure of an upstream process due to the pooling of liquid in pockets located in the vapour
inlet or outlet piping. Pocket-free designs, however, may maintain the required absolute vacuum pressures.
PUMPS & PROCESS MAGAZINE N° 69 - MAARt 2011 25

end may only installed at no less than 5 pipe diameters away from the
condenser outlet flange. Where there is insufficient height to construct
a proper barometric leg, a low-level, condensate-removal system can
be added. This consists of a receiver equipped with a level controller
and a condensate pump. As a condensate pump removes condensate at
a constant rate, a mechanical level controller opens and closes a valve,
to control the flow of cooling water to the condenser. However, if either
pump or controller fails, there is a risk of flooding the vacuum system.
Direct-contact condensers are easily to design, relatively inexpensive,
and make multi-stage steam jet ejector designs less vulnerable to
damage or fouling resulting from carryover of entrained solids. The
major disadvantage of direct contact condensers is the large quantity
of water that passes through once and goes to disposal, increasing the
cost of wastewater and the environmental impact.
Shell-and-tube condensers
Surface-type condensers (that can be provided with supplementary
mechanical refrigeration) are more complex and more expensive;
but the amount of waste water to be treated decreases and valuable
compounds can be recycled. In the case that a shell-and-tube surface
condenser is used, this condenser must also be installed to allow for
complete condensate drainage. The condensate may not flood the
lower tubes of the condenser, otherwise they will not be able to remove
heat effectively.
Measures to protect the upstream process
With barometric condensers, it is important to note that condensate is
splashing down the barometric walls and could run down the vapour
inlet and back into the vacuum process equipment, unless the inlet is
protected by a dam or series of elbows (Fig. 4a).
Condensable vapours flowing in the inlet or outlet vapour pipeline will
naturally condense since the pipe is usually cooler than the saturation
temperature of the vapour it contains. Vapour piping entering and
leaving a barometric condenser (or a shell-and-tube condenser) may
not contain any pockets (Fig. 4b) where this liquid can accumulate.
The liquid pooling in these pockets, will completely seal off the line,
which finally results in a downgrading of the vacuum. The absolute
vacuum pressure up-stream of a pocket will rise dramatically.