TITLE.PM5

VAPOUR POWER CYCLES 587

dharm

\M-therm\Th12-3.pm5

Total work done in the binary cycle is given by

Wt = m Whg + Ws ...(12.17)

∴ Overall efficiency of the binary cycle is given by

η =

Work done

Heat supplied

=

W

h

t

t

=

mW W

mh

hg s

hg

+

1

...(12.18)

Mercury

cycle

Steam

cycle

T

s

Fig. 12.36. Binary vapour cycle on T-s diagram.

Thermal efficiency of the mercury cycle is given by

ηhg =

mW

mh

hg

hg 1

=

W

h

hg

hg 1

=

hh

h

hg hg

hg

12

1

−

= 1 –

h

h

hg

hg

2

1

...(12.19)

=

mh h

mh

hg s

hg

1

1

−

= 1 –^1

m

. h
h

s

hg 1

...(12.20)

Heat lost by mercury vapour = Heat gained by steam

∴ mhhg 2 = 1 × hs ...(12.21)

Substituting the value of m from eqn. (12.21) into eqn. (12.20), we get

ηhg = 1 –

h

h

hg

hg

2

1

...(12.22)

The thermal efficiency of the steam cycle is given by

ηs =

W

h

s

s

=

hh

h

ss

s

12

1

−

=

hh

mh

ss

hg

12

2

−

...(12.23)