TITLE.PM5

VAPOUR POWER CYCLES 583

dharm

\M-therm\Th12-3.pm5

h

s

4

2

1 3

Reheating

Sat. curve

T = const.

Condenser

pressure

(c) In a 15 MW steam power plant operating on ideal reheat cycle, steam enters the H.P.
turbine at 150 bar and 600°C. The condenser is maintained at a pressure of 0.1 bar. If the mois-
ture content at the exit of the L.P. turbine is 10.4%, determine :
(i) Reheat pressure ; (ii) Thermal efficiency ; (iii) Specific steam consumption ; and (iv) Rate
of pump work in kW. Assume steam to be reheated to the initial temperature.
(AMIE Summer, 1998)
Solution. (a) The erosion of the moving blades is caused by the presence of water particles
in (wet) steam in the L.P. stages. The water particles strike the leading surface of the blades. Such
impact, if sufficiently heavy, produces severe local stresses in the blade material causing the
surface metal to fail and flake off.
The erosion, if any, is more likely to occur in
the region where the steam is wettest, i.e., in the
last one or two stages of the turbine. Moreover, the
water droplets are concentrated in the outer parts of
the flow annuals where the velocity of impact is high-
est.
Erosion difficulties due to moisture in the
steam may be avoided by reheating (see Fig. 12.31).
The whole of steam is taken from the turbine at a
suitable point 2, and a further supply of heat is given
to it along 2-3 after which the steam is readmitted
to the turbine and expanded along 3-4 to condenser
pressure.
Erosion may also be reduced by using steam
traps in between the stages to separate moisture
from the steam.
(b) TTD means “Terminal temperature difference”. It is the difference between tempera-
tures of bled steam/condensate and the feed water at the two ends of the feed water heater.
The required temperature-path-line diagram of a closed feed water heater is shown in
Fig. 12.32.

Steam

Steam

Out

Feed water

in

Condensate

Feed water a

Path line

a, b : Ideal

a, b : Actual′

b

b′

Temp.

Fig. 12.32
(c) The cycle is shown on T-s and h-s diagrams in Figs. 12.33 and 12.34 respectively. The
following values are read from the Mollier diagram :
h 1 = 3580 kJ/kg, h 2 = 3140 kJ/kg, h 3 = 3675 kJ/kg, and h 4 = 2335 kJ/kg

Fig. 12.31