SURFACE CONDENSER, STEAM-JET EJECTOR SYSTEM FIG. 2
condenser AC IC
Steam IC drain-loop seal
ejectors for noncondensable removal,
connected directly to the sides of the
surface condenser (Fig 2). The condenser includes an air-cooling section,
where up to 30% of the available heat-transfer area is devoted solely to minimizing the load on the steam ejectors.
Condensing as much water vapor as
possible from the stream entering the
steam ejectors is important because
water vapor increases loading on the
The ejectors themselves work by
expanding motive steam through a
specially designed nozzle (similar to
those on the exhaust of rocket engines)
designed to maximize the steam’s exit
velocity. This isentropic expansion results in a local low-pressure area generated around the nozzle into which
gases from the surface condenser naturally flow because of pressure differential (from high to low pressure).
In a properly working ejector, the
motive steam accelerates these gases from the surface condenser to supersonic velocities so that the combined gases
enter the converging part of the ejector diffuser. In this converging section, supersonic gases increase in pressure as the
cross-sectional area drops. As the converging section ends,
the supersonic gases slow to the speed of sound in the throat
section where a sudden sonic shock occurs as the gases transition to subsonic velocities. Then, in the diverging section
of the ejector, the cross-sectional area increases, enabling
pressure of subsonic gases to increase as they leave the ejector. This is how the ejector functions as a vapor compressor.
If the ejector isn’t working properly, however, and gases
aren’t traveling at design velocities across the different areas
of the ejector, ejector performance declines substantially.
Vapors from the condenser’s first-stage ejector are routed
to an IC designed to condense as much water vapor from
the gases as possible. This serves to unload the second-stage
ejectors, while condensed water is returned to the surface
condenser. Because the IC operates at a higher pressure than
the surface condenser, condensed liquid must pass through
a loop seal before returning to the surface condenser to prevent vapors from simply blowing through and recycling
around to the first-stage ejector’s inlet.
The IC also includes its own air-cooling section to help
minimize the water-vapor content of the stream entering the
smaller second-stage ejectors.
Second-stage ejectors work on the same principle as first-
stage ejectors with the exception that they discharge into an
AC operating slightly above atmospheric pressure. This en-
ables any gases in the AC enough pressure to flow freely into
the atmosphere without requiring further compression. Con-
densate generated in the AC typically drains back into the sur-
face condenser as well, except that instead of a loop seal, a
steam trap prevents vapor blowby into the surface condenser.
While this installation uses two stages of ejectors, more
stages are possible depending on process requirements. Also
unique to this installation is the combined IC-AC, which is
a single exchanger separated by an internal division plate
(Fig. 2). In the Norco plant, condensate generated in the AC
is routed to an open drain rather than being routed back to
the surface condenser. The AC’s remaining noncondensable
gases are simply vented to atmosphere.
Field investigations, step solutions
Operating data showed the surface condenser was performing poorly relative to its design, and performance was gradually worsening. Since the associated turbine was running at
its maximum steam flow, the surface condenser’s declining
performance was limiting the amount of work achievable by
each pound of steam entering, making the turbine unable
to deliver its maximum power output and restricting overall unit performance. Specifically, the second-stage ejectors
were operating above their designed suction pressure.
Field measurements executed with the original equipment manufacturer eliminated typical causes for poor ejector performance, including high noncondensable inleakage,
poor motive quality, and poor cooling-water supply. The initial theory was that both second-stage ejectors were in some
way damaged, so replacement ejectors were ordered. Following installation of the new ejectors, however, operating data