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Advantages
The principal advantages of steam-jet ejectors over
other types of vacuum producing units are:
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Low capital costs
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No moving parts hence
maintenance free
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Compact construction hence
easy installation
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Reliable operation and long
working life
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Corrosion / erosion resistant
as it can be constructed in a wide range of
materials
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High vacuum performance at
high suction loads
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Can be installed at remote or
inaccessible locations
Apart from single stage ejectors, multi stage vacuum
systems are used to create vacuum for process
applications. Water vapour is generally used as a
motive agent, but in special cases process vapours
may be used too. When using multi stage steam jet
vacuum systems it is possible to achieve absolute
suction pressures down to 0.01 mbar.
Chem Process steam-jet ejectors can be staged from
single through five stages. A detailed analysis of
initial costs and steam consumption should be made
before making a decision as to the stages and type
required to meet the specific requirements.
Chem Process engineers should be consulted for their
recommendations, which are based on their extensive
experience in various applications. Chem Process
offers a size ideally suited for individual
requirements. Newer designs provide far greater
capacities than available before and the smallest
unit covers a range, which previously required two
or more ejectors. The usual method adopted for
staging is to use a vapour condenser between the
stages. The size and types of condenser used is a
function of the air-vapour ratios, cooling water
temperatures and utility costs.
Non-Condensing Multi Stage Vacuum Systems
Non-condensing type has the first stage ejector
discharging directly into the suction of the second
stage ejector without employing condensers. The
non-condensing design has the advantage of lower
purchase and installation costs. In corrosive
environments non-condensing ejectors are
significantly more reliable than the condensing
systems. Motive steam requirements for non-condensible
ejectors are much higher than motive steam
requirements for multi stage ejectors.
Condensing Multi Stage Vacuum Systems
Multi stage steam jet vacuum systems employs
intercondensers between ejector stages to condense
steam and vapour of preceeding stages, and cooling
air and non-condensible vapour to reduce load to
succeeding stages. The intercondensers may be direct
contact type or surface type arranged barometrically
or at a low level. Ejector systems with direct
contact type condenser is considerably cheaper than
a system with surface condenser. Vacuum system with
surface condenser is suitable where the cooling
water is not to be contaminated with process vapour.
There are two basic types of condensers - surface
and direct contact.
Direct Contact Condensers / Mixing Condensers
Advantages
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Lower capital costs
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Lower installation costs
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Smaller terminal difference
allows operating at lower absolute pressure,
hence less water required for a given vacuum
condition
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Less floor area required
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No fouling, hence little or
no maintenance required
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Trouble-free operation
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Can be fabricated with
corrosion resistant materials, rubber or any
other suitable lining
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Open barometric discharge
provides safe operation without an atmospheric
relief valve
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Separation of process and
cooling water flows possible
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