BINARY VAPOUR CYCLES

No single fluid can meet allthe requirements as mentioned above. Although in the overall evaluation, water is better• than any other working fluid. at high temperatures, however, there arc a few better fluids. and notable among them are: (41) diphenyl ether. (CC.H5hO. (b) aluminium bromide AlBrJ and (e) liquid metals like mercury. sodium, potassium and so on. Among these. only mercury has actually been used in practice. Diphcnyl ether could be considered but it has not yet been used because like most organic substances, it decomposes gradually at high temperatures. A luminium bromide is a possibility and yet to be considered. 

As at pressure of 12 bar. the saturation temperatures for water, aluminium bromide, and mercury arc I ~p 0(" 4X2.5°(' and 560°(', I c-pcctivcly. The highest cyclic temperature consistent with the best available material for use in powerplant is about 560°C. Therefore, mercury is a better working fluid in the high temperature range because at 560°C, its vaporization pressure is relatively low. Its critical pressure and temperature are IOXO bar and 1460 cc, respectively, 

But in the low temperature range. mercury is unsuitable because its saturation pressure becomes exceedingly low. and it would bc impractical to maintain such a high vacuum in the condenser. At 30°C. the saturation pressure of mercury is only 2.7 x 10 4 em Hg. Its specific volume at sutJh a low pressure is very large, and it would bc difficult to accommodate such a large volume flow. 

For this reason. to take advantage of the beneficial features of mercury in the high temperature range and to get rid of its deleterious effects in the low temperature range, mercury vapour leaving the mercury turbine is condensed at a higher-temperature and pressure, and the heat released during the condensation of mercury is utilized in evaporating v. atcr to form steam to operate on a conventional turbine. 

Thus. in the binary (or two fluid) cycle. two cycles with different working fluids arc coupled in series, the heat rejected by one being utilized in the other .