FILTERS & STRAINERS - EJECTORS AND EDUCTORS PDF Print E-mail
Written by Norrie   
Tuesday, 11 May 2010 19:22
Article Index
FILTERS & STRAINERS
FILTERS
STRAINERS
STEAM TRAPS
MECHANICAL STEAM TRAP
THERMOSTATIC STEAM TRAP
THERMO-DYNAMIC STEAM TRAP
EJECTOR SYSTEMS - INTRODUCTION
EJECTORS AND EDUCTORS
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EJECTORS & EDUCTORS

Where process equipment is to be operated under vacuum, a popular method of creating the vacuum is by the use of Ejector Systems. An ejector uses the Bernoulli Principle of creating vacuum in that a high pressure fluid (often steam) is passed through a 'Venturi' tube. The narrow section of the tube causes an increase in the fluid velocity which produces a pressure drop in the system.

A 'suction' line from the equipment to be put under vacuum is connected into the ejector and, depending on the operation to be performed, gases and maybe liquids will be pulled out by the ejector action.

In a vacuum distillation unit, steam jet ejectors are used to pull the overhead vapours from the column through the overhead condenser. The process of condensation of vapour also causes a vacuum to be created. The ejectors improve and maintain the vacuum by removing the uncondensible gases. (See Figure : 31)

STEAM EJECTOR SYSTEM

Figure : 31

In a condensing steam turbine unit, the same principle is used to maintain a high vacuum in the exhaust steam surface condenser. Its purpose is to obtain maximum power from the steam driving the turbine.

In this system, the steam exhaust is first condensed in the surface condenser by cooling water coils. The process of condensation causes a very large decrease in volume - (changing high pressure, superheated steam to water decreases volume by about 1800 times - i.e. 1800 ft3 of steam will condense to 1 ft3 of water). This huge volume decrease creates a vacuum.

However, in such systems, uncondensible gases are also present and, if they aren't removed, they will build up and destroy the vacuum. To remove these gases, steam ejectors and ejector condensers are used. The gases are pulled from the surface condenser by a set of steam ejectors.

The ejector steam is then condensed in a separate exchanger which again helps to maintain the vacuum. The condensed steam (water) is piped away from the ejector condenser to the surface condenser.

Again however, the uncondensibles will tend to build up in the ejector condenser. These are pulled out by a second stage ejector system and the process repeated. The gases are allowed to build up in the second ejector condenser until they reach a pressure which will open a check valve and go to atmosphere. (See Figure : 32)

An 'EDUCTOR' is similar in operation to the ejector and can use any high pressure fluid as its motive force. They are used to cause a flow of air through a tank or vessel inside of which men are to work. An eductor can also be used to 'pump out' liquids from pits and vessels.

SURFACE CONDENSER

Figure : 32