Page 350 - Steam Turbines Design, Applications, and Rerating

324 Chapter Fifteen

This example can also be carried further to determine the number of
stages in each section using the procedures outlined earlier. To do so
requires finding the energy available in each portion of the turbine.
The energy available (ΔH i ) to the inlet-to-extraction section is:

ΔH = 3413 Btu/kWh ÷ TSR (inlet-to-extraction)

= 3413 Btu/kWh ÷ 35.4 lb/kWh = 96.4 Btu/lb

V b =π(25 + 1)(4500)/720 = 511 ft/s

V j for an ideal Curtis stage would therefore be:

511
V b
V j = = = 2220 ft/s
0.23 0.23
V j for an ideal Rateau stage would be:

511
V b
V j = = = 1110 ft/s
0.46 0.46
With a ΔH i of 96.5 Btu/lb, V j for the inlet-to-extraction section with one
stage would be:

V j = 224 ΔH = 224 9 6.5 = 2200 ft/s
This is seen to be very close to the 2220 ft/s for an ideal Curtis stage.
We will therefore assume that the inlet-to-extraction section will con-
tain one 25-in Curtis stage.
Now for the extraction-to-exhaust section. To find the energy avail-
able to this section we need the temperature of the steam entering this
portion of the turbine (extraction steam temperature). The enthalpy of
this steam will be:
Inlet steam enthalpy −ΔH i (inlet-to-extraction)
×η (inlet-to-extraction)

= 1378 Btu/lb − 96.4 Btu/lb × 0.70 = 1378 Btu/lb − 67.5 Btu/lb
= 1310.5 Btu/lb
From the Mollier diagram, Fig. 14.1 at 250 psig and 1310.5 Btu/lb
the extraction steam temperature is found to be close to 590°F (say
600°F).