PROPERTIES OF PURE SUBSTANCES 79Dharm
\M-therm/th3-1.p65(i)The total heat supplied to feed water per hour to produce wet steam.
(ii)The total heat absorbed per hour in the superheater.
Take specific heat for superheated steam as 2.2 kJ/kg K.
Solution. Mass of steam generated, m = 1000 kg/h
Pressure of steam, p = 16 bar
Dryness fraction, x = 0.9
Temperature of superheated steam,
Tsup = 380 + 273 = 653 K
Temperature of feed water = 30°C
Specific heat of superheated steam, cps = 2.2 kJ/kg K.
At 16 bar. From steam tables,
ts = 201.4°C (Ts = 201.4 + 273 = 474.4 K) ;
hf = 858.6 kJ/kg ; hfg = 1933.2 kJ/kg
(i)Heat supplied to feed water per hour to produce wet steam is given by :
H = m [(hf + xhfg) – 1 × 4.18 × (30 – 0)]
= 1000 [(858.6 + 0.9 × 1933.2) – 4.18 × 30]
= 1000(858.6 + 1739.88 – 125.4)
= 2473.08 × 103 kJ. (Ans.)
(ii)Heat absorbed by superheater per hour
= m[(1 – x) hfg + cps (Tsup – Ts)]
= 1000[(1 – 0.9) × 1933.2 + 2.2 (653 – 474.4)]
= 1000(193.32 + 392.92)
= 586.24 × 103 kJ. (Ans.)
Example 3.8. Using steam tables, determine the mean specific heat for superheated steam :
(i)at 0.75 bar, between 100°C and 150°C ;
(ii)at 0.5 bar, between 300°C and 400°C.
Solution. (i) At 0.75 bar. From steam tables ;
At 100°C, hsup = 2679.4 kJ/kg
At 150°C, hsup = 2778.2 kJ/kg
∴ 2778.2 = 2679.4 + cps (150 – 100)i.e., cps = 2778 2 26794.. 50 − = 1.976. (Ans.)
(ii)At 0.5 bar. From steam tables ;
At 300°C, hsup = 3075.5 kJ/kg
At 400°C, hsup = 3278.9 kJ/kg
∴ 3278.9 = 3075.5 + cps (400 – 300)
i.e., cps = 32789 30755.. 100 − = 2.034. (Ans.)
☞ Example 3.9. A pressure cooker contains 1.5 kg of saturated steam at 5 bar. Find the
quantity of heat which must be rejected so as to reduce the quality to 60% dry. Determine the
pressure and temperature of the steam at the new state.