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VAPOUR POWER CYCLES 585

dharm

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If we use steam as the working medium the temperature rise is accompanied by rise in
pressure and at critical temperature of 374.15°C the pressure is as high as 225 bar which will
create many difficulties in design, operation and control. It would be desirable to use some fluid
other than steam which has more desirable thermodynamic properties than water. An ideal fluid
for this purpose should have a very high critical temperature combined with low pressure. Mer-
cury, diphenyl oxide and similar compounds, aluminium bromide and zinc ammonium chloride
are fluids which possess the required properties in varying degrees. Mercury is the only working
fluid which has been successfully used in practice. It has high critical temperature (588.4°C) and
correspondingly low critical pressure (21 bar abs.). The mercury alone cannot be used as its
saturation temperature at atmospheric pressure is high (357°C). Hence binary vapour cycle is
generally used to increase the overall efficiency of the plant. Two fluids (mercury and water) are
used in cascade in the binary cycle for production of power.
The few more properties required for an ideal binary fluid used in high temperature limit
are listed below :

1. It should have high critical temperature at reasonably low pressure.


2. It should have high heat of vaporisation to keep the weight of fluid in the cycle to minimum.


3. Freezing temperature should be below room temperature.


4. It should have chemical stability through the working cycle.


5. It must be non-corrosive to the metals normally used in power plants.


6. It must have an ability to wet the metal surfaces to promote the heat transfer.


7. The vapour pressure at a desirable condensation temperature should be nearly atmos-
   pheric which will eliminate requirement of power for maintenance of vacuum in the
   condenser.


8. After expansion through the primemover the vapour should be nearly saturated so that
   a desirable heat transfer co-efficient can be obtained which will reduce the size of the
   condenser required.


9. It must be available in large quantities at reasonable cost.


10. It should not be toxic and, therefore, dangerous to human life.
    Although mercury does not have all the required properties, it is more favourable than any
    other fluid investigated. It is most stable under all operating conditions.
    Although, mercury does not cause any corrosion to metals, but it is extremely dangerous to
    human life, therefore, elaborate precautions must be taken to prevent the escape of vapour. The
    major disadvantage associated with mercury is that it does not wet surface of the metal and
    forms a serious resistance to heat flow. This difficulty can be considerably reduced by adding
    magnesium and titanium (2 parts in 100000 parts) in mercury.

Thermal properties of mercury :

Mercury fufills practically all the desirable thermodynamic properties stated above.

1. Its freezing point is – 3.3°C and boiling point is – 354.4°C at atmospheric pressure.


2. The pressure required when the temperature of vapour is 540°C is only 12.5 bar (app.)
   and, therefore, heavy construction is not required to get high initial temperature.


3. Its liquid saturation curve is very steep, approaching the isentropic of the Carnot cycle.


4. It has no corrosive or erosive effects upon metals commonly used in practice.


5. Its critical temperature is so far removed from any possible upper temperature limit
   with existing metals as to cause no trouble.