Evaluate total heat of steam:
10 kg of wet saturated steam at 15 bar pressure is superheated to the temperature of 290°C at constant pressure. Find heat needed and the total heat of steam. The dryness fraction of steam is 0.85.
Sol.: From steam table, we get following data:
|
Absolute pressure (P)
bar
|
Saturation
temperature (t) ºC
|
Specific enthalpy kJ/kg
|
|
Water (hf)
|
Latent heat (hfg)
|
|
15
|
198.3
|
844.6
|
1947
|
Total heat of 10 kg wet saturated steam
=10 × 2499.55 = 24995.5 kJ .......ANS
Total heat of 1 kg superheated steam can be given by Hsup = hf + hfg + Cp(tsup - ts) kJ
= 844.6 + 1947 + 2.1 × (290 - 198.3) kJ
= 2984.17 kJ .......ANS
Total heat of 10 kg superheated steam =10 × 2984.17 = 29841.7 kJ .......ANS
= hf + hfg + Cp(tsup - ts) - (hf + xhfg)
= hfg + Cp(tsup - ts) - xhfg
=1947 + 2.1 x (290 - 198.3) - 0.85 x 1947 kJ = 484.62 kJ
Heat which is required to convert 10 kg wet saturated steam into 10 kg superheated steam
=10 × 484.62 = 4846.2 kJ .......ANS
Total heat of 1 kg wet saturated steam can be given by Hwet = hf + xhfg kJ
= 844.6 + 0.85 × 1947 kJ = 2499.55 kJ
Heat which is required to convert 1 kg wet saturated steam into 1 kg superheated steam
= Hsup - Hwet,
here Hsup = enthalpy of 1 kg superheated steam = hf + hfg + Cp(tsup - ts) kJ
Hwet = enthalpy of 1 kg wet saturated steam = hf + xhfg kJ
Heat which is required to convert 1 kg wet saturated steam into 1 kg superheated steam