Binary Vapour Cycle
Table of Contents
Binary Vapour Cycle
What is Binary Vapour Cycle
In the Carnot cycle, maximum efficiency is attained.
The total heat should be supplied at constant temperature T1 and rejected at T2 in order to improve the actual engine’s efficiency, or to approach the carnot cycle efficiency. Only wet vapour, not superheated vapour, can be used to accomplish this.
Only the higher temperature T1 affects thermal efficiency because the natural sink to which heat is rejected sets the lower temperature T2. T1 should therefore be as high as it can be. Working fluids with desirable thermodynamic characteristics, such as high critical temperature and low critical pressure, include mercury, diphenyl ether (C6 H5)2 O, aluminum bromide, and ammonium chloride. Among the aforementioned fluids, mercury has a desirable low critical pressure (21 bar) and high critical temperature (588.4 °C). However, since mercury’s saturation temperature at atmospheric pressure is high (357°C), we are unable to use it alone. To increase thermal efficiency, a binary vapour cycle using mercury and water as the two fluids is used.
The mercury vapour that is expelled from the mercury turbine condenses, and the heat generated by this condensation is used to warm and evaporate the feed water into steam, which is then expanded in the steam turbine to generate work. Also producing work is the mercury turbine. Thus, the binary vapour cycle uses two fluids, water and mercury. In this cycle, water makes use of the heat that mercury rejects. Fig. 1.92 displays a schematic representation of the binary vapour cycle. A T.S. diagram is also displayed.
See the illustration. Mercury cycle and water (steam) cycle make up the binary vapour cycle.
Mercury Cycle (Topping Cycle)
An easy Rankine cycle is the mercury cycle (a, b, c, d). In this cycle, work is generated while mercury is expanded in the mercury turbine (processes a–b). In the mercury condenser (steam generator), where heat is released to heat and evaporate the feed water into steam, the mercury that has just left the mercury turbine condenses (processes b and c). The mercury generator is supplied with the condensed mercury (processes C and D). Mercury cycle is finished as a result.
Steam Cycle (Bottoming Cycle):
As previously mentioned, the heat rejected in the mercury condenser heats and evaporates the feed water into steam (processes 5–6). This steam is heated further in the superheater (process 6–1) by outside sources, resulting in superheated steam. In steps 1-2 and 2-4, the steam turbine expands the superheated steam before condensing it in steps 2-4. Thus, the condensate (feed water) is pumped (steps three through four), heated in the economizer (steps four through five), and then sent to the mercury condenser (or steam generator), concluding the steam cycle.
Qs = m(ha-hd)+1(h1-h6)+(h6-h4)
Qr =h2 – h3
WT = m(ha – hb)+ 1(h1 – h2)
Wp = m(hd – hc)+1(h4 – h3)
Heat rejected by mercury = Heat absorbed by water to become steam.