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.
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)