Regenerative
Ranking Cycle and Binary Vapour Cycle
The
regenerative Ranking cycle is so named because after emerging from the
condenser (possibly as a sub cooled liquid) the working fluid heated by steam
tapped from the hot portion of the cycle and fed in to Open Feed Water
Heater(OFWH). This increases the average temperature of heat addition which in
turn increases the thermodynamics efficiency of the cycle.
Figure
Binary
Vapour Cycle
Generally water is used a working fluid in vapour power
cycle as it is found to be better than any other fluid, but it is far from
being the ideal one. The binary cycle is an attempt to overcome some of the
shortcomings of water and to approach the ideal working fluid by using two
fluids. The most important desirable characteristics of the working fluid
suitable for vapour cycles are:
a .A high
critical temperature and a safe maximum pressure.
b. Low- triple point temperature
c. Condenser pressure is not too low.
d.high enthalpy of vaporization
e. High thermal conductivity
f. It must be readily available,
inexpensive, inert and non-toxic.
Figure:
Mercury-steam binary vapour cycle
Figure: T-S diagram for Hg-steam
binary vapour cycle.
Therefore it can be concluded that no single working
fluids may have desirable requirements of working fluid. Different working
fluids may have different attractive feature in them, but not all. In such
cases two vapour cycles operating on two different working fluids are put
together, one is high temperature region and the other in low temperature
region and the arrangement is called binary vapour cycle.
The layout of mercury-steam binary vapour cycle is shown
in figure. Along with the depiction of T-S diagram figure. Since mercury having
high critical temperature (898°C) and low critical pressure (180
bar) which makes a suitable working fluid will act as high temperature cycle
(toppling cycle) and steam cycle will act as low temperature cycle.
Here mercury vapour are generated in mercury boiler and
sent for expansion in mercury turbine and expanded fluid leaves turbine to
condenser. In condenser, the water is used for extracting heat from the mercury
so as to condensate it. The amount water entering mercury condenser. The
mercury condenser also act as steam boiler for super heating of heat liberated
during condensation of mercury is too large to evaporate the water entering of
seam an auxiliary boiler may be employed or superheating may be realized in the
mercury boiler itself. From the cycle,
Since
pump works are very small, it may be neglected. Work from Mercury Turbine, WHg
=mg (ha -hb )
Work from Steam Turbine, Wsteam =msteam (h1 -h2 )
Pump work, Wpump =mHg (hd -hc )+msteam (h4 -h3 )
Heat
supplied to the cycle, Qin =mHg (ha -hd )+msteam éë(h1 -h6 )+(h5 -h4 ) Heat rejected,Qout = msteam (h2 –h3)
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