FIELD OF INVENTION
The invention relates to a system for removing air from an exhaust condenser of
a steam turbine in steam-water cycled operated power plants.
BACKGROUND OF THE INVENTION
In a steam operated power plant the water is used as working fluid. The water is
compressed corresponding to the operating pressure of boiler and then heated in
the boiler to form high pressure steam. This steam is expanded in the turbine to
form mechanical energy. The exhaust steam from the turbine is condensed back
into water in a condenser. The expansion of the steam in the last stages of LP
turbine is below the atmospheric pressure, and hence there is ingress of air from
these low pressure areas into the condenser. The accumulation of the air in the
condenser reduces the thermal efficiency of the power plant. Lower the absolute
pressure, higher is the efficiency of the power plant.
The existing technologies for the evacuation of air from the exhaust steam
condenser in a steam operated power plant, can generally be categorized as
under:
1. Water ring pump along with an air operated ejector: A water ring
vacuum pump is capable of operating in moist air. These pumps are
capable of achieving a higher pumping capacity when operating at more
than 200mbar of absolute pressure. As the vacuum gradually reduces
below 200mbar absolute, the pumping capacity of the pumps declines
sharply. Moreover, the churning of water by the impeller increases the
temperature of the water by at least 6-9 degree centigrade. This
phenomenon increases the temperature of the water flowing out of the
water ring pump constitutes a governing temperature for the vacuum. The
water vapour pressure corresponding to the pump outlet temperature
limits the vacuum that can be achieved in an exhaust steam condenser.
2. Steam operated Ejector: A steam ejector may be a single stage or
multiple stage ejector with provisions for inter cooling. The jet velocities
in the ejector are supersonic and erode the venturi in a short period .
After such erosion, the efficiency of the ejector decreases and the ejector
can no longer produce specified vacuum. Also, if the air load increases,
the vacuum creating efficiency of the ejector drops sharply. In general,
the ejector in the context of the existing technology, is an inefficient
device and consumes a large quantity of steam, which excess steam
otherwise could be used for production of electricity.
Thus, the prior art devices consume a larger quantity of energy to create the
vacuum, and fail to achieve the desired vacuum in the exhaust steam condenser.
Since the vacuum created by these devices is substantially lower than a
calculated designed value, a substantial loss of efficiency in generation of
electricity is inevitable.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a system for air evacuation
of the turbine exhaust steam condenser in a steam operated power plant.
Another object of the invention is to propose a system for evacuation of the
turbine exhaust steam condenser in a steam operated power plant, which
exhibits improved vacuum creating efficiency.
A further object of the invention is to propose a system for evacuation of the
turbine exhaust steam condenser in a steam operated power plant, whose
pumping speed does not decrease appreciably with the improvement in vacuum.
A still further object of the invention is to propose a system for evacuation of
the turbine exhaust steam condenser in a steam operated power plant, whose,
pumping capability is less dependent on the temperature of air being evacuated.
SUMMARY OF THE INVENTION
The inventive system is designed for the removal of air from the steam turbine
exhaust condenser of a steam operated power plant.
The system is connected to the air evacuation line of the steam turbine exhaust
condenser. The system comprises a positive displacement pump known as roots
pump, and a single or multistage water ring vacuum pump, operating in tandem.
The roots pump acts as the primary pump and creates a vacuum between 0.05
mbar and 200mbar. The exhaust of this pump is sucked by the single or multiple
stage water ring vacuum pump acting as the backing vacuum pumps. Thus, both
the pumps operate at the optimum range of their designed parameters and
achieve results substantially near to the calculated designed value.
The targeted absolute pressure is the water vapour pressure of the cooling water
or in case of air cooled condenser the temperature of air going into the
condenser to condense the low pressure steam. Lower the pressure in the
condenser, higher is the efficiency of the power plant.
The invented system is suitable for improving efficiency of all types of closed
steam-water cycled power plants, independent of the fuel used to generate
steam.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.l- Shows a schematic diagram of the system according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure-1, the existing vacuum system is removed or isolated from
the condenser (3) by the use of a valve (1). The roots pump (2) is a single stage
or multiple stage and is connected to a pipe (3a) connecting a condenser (3).
The exhaust or the outlet of the single stage or multiple stage roots pump (2) is
connected to a single or multiple stage water ring vacuum pump (4a). The water
ring pump (4a) has means (5,6) for inlet of cooling water and separation of
water from air at an outlet (7). A flow control valve (8) is provided to control the
temperature of water in the water ring pump by controlling the seal water flow.
Figure-1 further shows the existing pump (4) provided with the roots pump (2).
The invention adapts two different types of pumps (2,4a) to form a system for
the removal of air from the steam turbine exhaust condenser (3) of a steam
operated power plant. The first pump (2) that evacuates the air from the
condenser (3) is a positive displacement pump called as mechanical blower or
roots pump. This pump (2) operates at high vacuum, but cannot exhaust to
atmosphere, hence the exhaust of this pump (2) is evacuated by another pump
(4a) commonly known as water ring pump or any other type of backing vacuum
pump.
The degree of vacuum achieved governs the efficiency of a thermal power plant.
For each milibar of improvement of vacuum in the condenser (3) of the steam
operated power plant, there is small increase in the efficiency of the total plant.
The ultimate vacuum in any condenser (3) that can be achieved is equivalent to
the water vapour pressure at the temperature of the water in the air cooling
zone, inside the condenser (3).
The primary pump (2) is a positive displacement mechanical blower and is also
called roots pump. This pump (2) can achieve very low vacuum, and the
limitation will be the pumping capacity of the pump (2) due to size or due to the
water vapour pressure corresponding to the temperature of water in the air
cooling zone of the condenser (3). This pump (2) compresses the air to the
pressure where the backing pump (4) can take over.
The backing pump (4a) used is water ring type single or multiple stage vacuum
pump. This pump (4a) uses water as the working fluid with partial recirculation.
At the pump inlet (5), a small vessel is provided (9) for mixing of the seal water
and the outlet from the roots pump (2) for air to be evacuated. Alternatively, if
there can be provided a separate inlet for seal water in the pump (4a), then the
small vessel (9) is not required. In case of such an optional provision, the outlet
of the roots pump (2) can be directly connected to the inlet (5) of the backing
pump (4a). At the outlet of the backing pump (4a) there is an air-water
separator (6). This separator (6) extracts seal water from the air water mixture
at the outlet of the pump (4a), for recirculation. Excess water flows out from the
outlet (7) of the separator (6). The cold water inlet (5) is also connected to the
separator (6) to keep the temperature of the recirculation seal water within
limits. The flow control valve (8) controls the flow of water into the pump (4a)
to achieve the desired backing vacuum.
The backing pump (4a) being used along with the roots pump (2) is a much
smaller in size and lower powered, as compared to the prior art water ring
vacuum pump and air ejector used directly to evacuate the condenser.
The complete system is able to achieve the vacuum which is very near to the
water vapour pressure of the cooling water. This leads to increased efficiency of
the power plant.
We Claim:
1. A system for removing air from an exhaust condenser (3) of a steam
turbine in steam-water cycled operated power plants, comprising:
- at least one single stage or multiple stage roots pump (2)
connected to a pipe (3a) connecting the exhaust condenser (3) of
the turbine, the roots pump (2) creating a vacuum by evacuating
air from the condenser (3);
- at least one single stage or multiple stage water ring vacuum pump
(4a) connected to an outlet of said at least one single stage or
multiple stages roots pump (2), the roots pump (2) and the water
ring vacuum pump (4a) operating in tandem, the vacuum pump
(4a) sucking the exhaust air from the roots pump (2) and
operating by using water as the working fluid with partial
recirculation, the pump (4a) creating an absolute pressure
substantially the same as that of the water vapour pressure of the
cooing water or air being returned back from the system to the
condenser (3) to condense the low pressure steam.
2. The system as claimed in claim 1, wherein the roots pump (2) is enabled
to create a vacuum between 0.05 mbar and 200 mbar, and wherein the
roots pump (2) is a positive displacement pump.
3. The system as claimed in claim 1, wherein the water ring vacuum pump
(4a) acts as a backing pump.
4. The system as claimed in claim 1 or 3, wherein the backing pump (4a)
comprises an inlet (5) for ingress of cold water including a vessel (9) to
allow mixing of seal water and the evacuated air from the roots pump (2).
5. The system as claimed in claim 1 or 3 or 4, wherein the backing pump
(4a) is provided with an air-water separator (6) which extracts seal water
from the air-water mixture at the outlet of the backing pump (4a) for
recirculation.
6. The system as claimed in any of the preceding claims, wherein the cold
water inlet (5) is directly connected to the separator (6).
7. The system as claimed in any of the preceding claims, wherein the
separator (6) comprises an outlet (7) for allowing egress of the excess
water.
8. A system for removing air from an exhaust condenser of a steam turbine in
steam-water cycled operated power plants as substantially described and
illustrated herein with reference to the accompanying drawings.

The invention relates to a system for removing air from an exhaust condenser
(3) of a steam turbine in steam-water cycled operated power plants, comprising:
at least one single stage or multiple stage roots pump (2) connected to a pipe
(3a) connecting the exhaust condenser (3) of the turbine, the roots pump (2)
creating a vacuum by evacuating air from the condenser (3); at least one single
stage or multiple stage water ring vacuum pump (4a) connected to an outlet of
said at least one single stage or multiple stages roots pump (2), the roots pump
(2) and the water ring vacuum pump (4a) operating in tandem, the vacuum
pump (4a) sucking the exhaust air from the roots pump (2) and operating by
using water as the working fluid with partial recirculation, the pump (4a) creating
an absolute pressure substantially the same as that of the water vapour pressure
of the cooing water or air being returned back from the system to the condenser
(3) to condense the low pressure steam.