Page 123 - Mechanical Engineer's Data Handbook

112 MECHANICAL ENGINEER’S DATA HANDBOOK

Fluid n IC

Air (n=y) 1.4 0.528
Initially dry saturated steam 1.135 0.577
Initially superheated steam 1.3 0.546

3.6 Steam plant

The simplest steam cycle of practical value is the second turbine the final dryness fraction is increased
Rankine cycle with dry saturated steam supplied by a with beneficial effects (e.g. reduced erosion of turbine
boiler to a power unit, e.g. a turbine, which exhausts to blades due to water droplets); in addition, there is a
a condenser where the condensed steam is pumped further reduction in specific steam consumption.
back into the boiler. Formulae are given for work In the ‘regenerative cycle’ efficiency is improved by
output, heat supplied, efficiency and specific steam bleeding off a proportion of the steam at an intermedi-
consumption. Higher efficiency is obtained if the steam ate pressure and mixing it with feed water pumped to
is initially superheated which also reduces specific the same pressure in a ‘feed heater’. Several feed
steam consumption and means smaller plant can be heaters may be used but these are of the ‘closed’ variety
used. If the steam is ‘reheated’ and passed through a to avoid the necessity for expensive pumps.

3.6. I Rankine cycle - dry saturated
steam at turbine inlet
From the T-s diagram:

(s2 -sf21
s2 =SI, x2 = ~
Sf,,
h,=hfz+xZh,g2
Work output W=(h, -h2)
Heat supplied Q=(hl -hf3)
Cycle efficiency q = W/Q (neglecting pump work)
Specific steam consumption
SSC = 36001 W kg kW - ‘h -
C
Note: if the turbine isentropic efficiency vi is allowed $= ssc
v)
for: W= (h, -h2)qi and expansion is to point 2’ on the
diagram.

3.6.2 Rankine cycle - with superheat

The method is the same as for dry saturated steam. The
graph shows the effect of superheat temperature on
efficiency and specific steam consumption. In this case
I
S h, is the enthalpy for superheated steam.