flow and flow resistance, the additional specific fuel
consumption of pump is increased by 0.011g/KWh,
but the value is far less than the overall reduction of
additional specific fuel consumption of other
equipment. In conclusion, the contribution of the
OSC to the thermal efficiency of the unit is mainly
reflected in reducing the additional specific fuel
consumption of the boiler and less affecting the
reduction of the irreversible loss of the RH itself. The
reason is that the utilization of the superheat of the
steam at the heater inlet greatly increases the
temperature of feed water to bring it closer to the
optimal value of the system.
It can be seen from Table 1 that the superheat of
the second extraction and the fourth extraction has the
most potential for utilization. Therefore, when using
double steam coolers, it should be arranged at RH2
and RH4, respectively, with series and parallel
arrangement,
recorded as scheme 5 and scheme 6.
Table 4: Thermal efficiency comparison of installing
double OSCs.
Temperature
(℃)
Thermal
efficiency of
the unit
(%)
Specific fuel
consumption
of the unit
(g/kWh)
Base Case
304.503 50.979 266.826
Scheme 5 317.124 51.192 265.513
Scheme 6 312.857 51.158 265.742
In scheme 5, the main feed water passes through
RH1 and then enters into the double OSCs
respectively and then enters the boiler. In order to
reduce the thermal deviation at the outlet flow mixing
of double OSCs, the feed water mass flow into the
RH2 steam cooler is set to be 70% of the total flow.
In scheme 6, the inlet water supply of the double
OSCs comes from the outlet of their corresponding
RHs, respectively. According to the method of Ref.
(
XU Chuanpu, 1990), the feed water splitting
coefficient is chosen. It is calculated that when the
mass flow of RH2 steam cooler and RH4 cooler is
respectively 5.5% and 3% of the feed water mass
flow before shunting, the thermal efficiency of the
unit is the best. The comparison of thermal efficiency
between the two schemes and the Base Case is shown
in Table 4.Compared with the Base Case, the
temperature of feed water is increased by 12.621K,
the specific fuel consumption of the unit is reduced
by 1.313g/kWh, and the thermal efficiency of the unit
is improved by 0.213%. Because of the smaller mass
flow in the steam coolers in scheme 6, the
temperature of the feed water is less increased and the
reduction effect is lower than that of scheme 5.
Table 5 shows comparison of specific fuel
consumption of between the best single OSC
arrangement (scheme 2), the best double arrangement
(scheme 5) and the Base Case. Compared with the
single arrangement, when the double arrangement is
adopted, the superheat of the steam at the inlet of the
heater can be utilized to a greater extent. The
additional specific fuel consumption of the boiler,
turbine, condenser and RHs in the system is reduced.
And there is not much difference between the
additional specific fuel consumption of the pump
system. Therefore, the use of double series
arrangement is significantly better than single
arrangement.
Table 5: Distribution of fuel specific consumption in
different systems.
Specific fuel consumption
Base
Case
Scheme
2
Scheme
5
Additional specific
fuel
consumption(g/kWh)
Boiler 119.473 118.632 117.934
Turbine 6.738 6.733 6.861
Condenser 11.860 11.815 11.768
RHs 2.901 2.887 2.647
Pum
1.058 1.069 1.081
Specific fuel consumption of
the unit (g/kWh)
266.826 266.216 265.483
4 CONCLUSION
When using a single steam cooler, the best effect can
be obtained by arranging it at the RH2, which can
reduce specific fuel consumption by 0.619g/kWh.
When using double OSCs arrangement, arranged in
the RH2 and RH4 in series of the best way, can reduce
specific fuel consumption 1.313g/kWh.
The OSC enhances the thermal efficiency of the
unit mainly by reducing the additional specific fuel
consumption of the boiler, while contributing little to
reduce the additional specific fuel consumption of the
RH itself.
ACKNOWLEDGMENTS
This research is supported by the Special Funds of
the National Natural Science Foundation of China
(No. 51606066).