Spirax Sarco Steam Jet Thermocompressor, Installation And Maintenance Instructions Manual

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IM-P493-01 EMM Issue 2 10
4. Operation
4.1 General information regarding operation
Steam under pressure is introduced to the Motive Nozzle, which is usually a converging /
diverging type. As the steam passes through the Nozzle, its velocity is greatly increased and
its pressure is reduced (pressure energy converted to velocity (Kinetic) energy). This results
in a high velocity jet of steam emerging from the Nozzle outlet.
By a principle of momentum transfer, energy is transferred from the high velocity jet to the
vapours present in the suction chamber or body of the Thermocompressor. This results in
the suction gases being entrained with the motive steam, in a direction towards the Diffuser.
In the converging section of the Diffuser, the motive steam and the suction vapours combine
and mix intimately.
The resulting uniform mixture of vapours is then decelerated in the diverging section of the
Diffuser, this is marked by a corresponding pressure rise as the velocity energy (Kinetic) is
transferred back to pressure energy, thus enabling the Thermocompressor to overcome the
specified backpressure.
To summarise, high pressure motive steam entrains lower pressure water vapour and, the
mixture is discharged at a pressure with a value lying between the suction and motive pressure.
In some operating circumstances, the required operating duty of the Thermocompressor
can be constantly changing. Thus the amount of motive steam actually required to make the
Thermocompressor perform is constantly changing too.
You will appreciate that the design must cater for the worst envisaged operating conditions,
which of course will correspond with the highest motive steam requirement. As a consequence
of this, when the Thermocompressor is only required to perform low duty (and therefore only
requiring a low amount of motive steam), if some form of steam flow regulating device is not
present, the Thermocompressor will actually be using much more motive steam than is actually
required to perform the duty. This can be a costly waste of motive steam.
It is in these circumstances that a steam flow regulating device is usually employed. This
comprises an Actuator, a Positioner and an Air Regulating Set, mounted directly onto the
Thermocompressor. The Actuator regulates the position of a spindle within the Steam Nozzle.
The operation of the Flow Regulating Device can be described as follows:-
A control signal is sensed by the Actuator, which then moves the spindle either into or out of
the Motive Steam Nozzle. In the region of the Motive Nozzle, the spindle is tapered so that
it has a varying cross-section. Thus, as the spindle moves in and out of the Motive Nozzle,
the cross-sectional area within the Nozzle that is available for the flow of steam is constantly
changing and, since the flow of steam is proportional to the cross-sectional area of the Nozzle,
this gives rise to a change in steam flow.
Since it is the cross-sectional area of the Nozzle that changes and not the motive steam
pressure, the design ensures that the velocity of the steam jet, as it emerges from the Nozzle,
is unchanged, which is a desirable feature for stable operation of the Thermocompressor.
If the Thermocompressor is supplied with the accessories mentioned above, more detailed
information regarding their operation will be provided within a supplementary documentation.
Please note that for clarity welds are not shown.
Fig. 2
Nozzle
Low pressure
water vapour inlet
Diffuser
High pressure
steam inlet
Intermediate pressure outlet
Diffuser