77.
A cogeneration facility generates electricity and supplies steam to a chemical process plant, as
shown in the figure. The following data are known:
Main steam supply 2,500 psia, 1,000°F
Feedwater supply to boiler, FW 3,000 psia, 400°F
Extraction steam supply 300 psia, 500°F
Extracted steam to closed feedwater heater leaves the heater as saturated liquid at 300 psia.
Extraction steam is used to heat feedwater in the closed feedwater heater. Condensate leaves the
heater as saturated liquid. The ratio of steam (lbm of steam per lbm of feedwater) needed to heat
the feedwater from 110°F to 400°F is most nearly:
? A. 0.10
? B. 0.22
? C. 0.34
? D. 0.62
77.
THERMAL AND FLUID SYSTEMS SOLUTIONS
Consider the feedwater heater.
$h_a$ (T = 500°F, P = 300 psia) = 1,257.5 Btu/lbm
$h_b = h_f$ (P = 300 psia) = 394.1 Btu/lbm
$h_c$ = 378.3 Btu/lbm
$h_d$ = 85.8 Btu/lbm
Energy balance:
$m_{steam} (h_a - h_b) = m_{feedwater} (h_c - h_d)$
$\frac{m_{steam}}{m_{feedwater}} = \frac{(h_c - h_d)}{(h_a - h_b)}$
$= \frac{378.3 - 85.8}{1,257.5 - 394.1}$
= 0.34 lbm of steam
lbm of feedwater
THE CORRECT ANSWER IS: C
