TITLE.PM5

574 ENGINEERING THERMODYNAMICS

dharm

\M-therm\Th12-3.pm5

Solution. (i) The schematic arrangement including feed heating system, and T-s and h-s

diagrams of the process are shown in Figs. 12.21 and 12.22 respectively.

Fig. 12.21
(ii) Amounts of bled off. The enthalpies at various state points as read from h-s
diagram/steam tables, in kJ/kg, are :
h 1 = h 2 = 3460
h 3 ′ = 3050, and ∴ h 3 = 3460 – 0.85(3460 – 3050) = 3111.5
h 4 = 3585
h 5 ′ = 3140, and ∴ h 5 = 3585 – 0.85(3585 – 3140) = 3207
h 6 ′ = 2335, and ∴ h 6 = 3207 – 0.85 (3207 – 2335) = 2466
h 7 = 137.8 kJ/kg (hf at 0.05 bar)
h 8 = h 10 = 962 kJ/kg (hf at 25 bar)
and h 9 = 670.4 (hf at 6 bar).
Enthalpy balance for surface heater :
m 1 h 3 + h 9 = m 1 h 8 + h 10 , neglecting pump work

or m 1 =

hh

hh

10 9

38

−

−

=

962 670.4

3111.5 962

−
−
= 0.13566 kg
Enthalpy balance for contact heater :
m 2 h 5 + (1 – m 1 – m 2 )h 7 + m 1 h 8 = h 9 , neglecting pump work
or m 2 × 3207 + (1 – 0.13566 – m 2 ) × 137.8 + 0.13566 × 962 = 670.4
or m 2 = 0.1371 kg.