Properties of Steam (Wet, Dry, Superheated)
1 / 10
At 4 bar, the saturation temperature is 143.6°C. Superheated steam at 4 bar and 200°C has an entropy of 7.127 kJ/kg·K. Compared to dry saturated steam at 4 bar (sg = 6.895 kJ/kg·K), the entropy is:
Lower
Equal
Higher
Cannot be determined
Superheated steam at a higher temperature (200°C > 143.6°C) has more disorder, so its entropy (7.127 kJ/kg·K) is higher than that of dry saturated steam (6.895 kJ/kg·K) at the same pressure.
2 / 10
Dry saturated steam at 6 bar (sg = 6.760 kJ/kg·K) is expanded to 1 bar (sg = 7.671 kJ/kg·K) isentropically. The final state is:
Dry saturated steam
Wet steam
Superheated steam
Saturated liquid
Isentropic expansion means s_initial = s_final = 6.760 kJ/kg·K. At 1 bar, s_g = 7.671 kJ/kg·K > 6.760, and s_f = 1.302 kJ/kg·K < 6.760, so the steam is wet. Dryness fraction x = (s – s_f)/(s_g – s_f) = (6.760 – 1.302)/(7.671 – 1.302) ≈ 0.86.
3 / 10
The specific volume of wet steam at 8 bar with a dryness fraction of 0.85 is (vf = 0.001115 m³/kg, vg = 0.2404 m³/kg):
0.2043 m³/kg
0.2054 m³/kg
0.001115 m³/kg
0.2404 m³/kg
For wet steam, v = vf + x (vg – vf). Given x = 0.85, vf = 0.001115 m³/kg, vg = 0.2404 m³/kg:
v = 0.001115 + 0.85 × (0.2404 – 0.001115) = 0.001115 + 0.85 × 0.239285 = 0.001115 + 0.203392 = 0.2043 m³/kg
4 / 10
Superheated steam at 15 bar and 350°C has an enthalpy of 3037.6 kJ/kg. If it is cooled at constant pressure to the saturation temperature (198.3°C), the enthalpy change is (hg at 15 bar = 2794.0 kJ/kg):
243.6 kJ/kg
3037.6 kJ/kg
-2794.0 kJ/kg
-243.6 kJ/kg
Cooling to saturation temperature at 15 bar results in dry saturated steam (h_g = 2794.0 kJ/kg). Enthalpy change Δh = h_final – h_initial = 2794.0 – 3037.6 = -243.6 kJ/kg.
5 / 10
At 2 bar, the enthalpy of saturated liquid (hf) is 504.7 kJ/kg, and the enthalpy of vaporization (hfg) is 2202.6 kJ/kg. The enthalpy of wet steam with a quality of 0.95 is:
2597.15 kJ/kg
2707.3 kJ/kg
2202.6 kJ/kg
504.7 kJ/kg
For wet steam, h = hf + x hfg. Given x = 0.95, hf = 504.7 kJ/kg, hfg = 2202.6 kJ/kg:
h = 504.7 + 0.95 × 2202.6 = 504.7 + 2092.47 = 2597.15 kJ/kg.
6 / 10
The quality of steam is relevant for:
Superheated steam only
Dry saturated steam only
Wet steam only
Both wet and superheated steam
The dryness fraction (quality) applies only to wet steam, which is a mixture of liquid and vapor. Dry saturated steam has x = 1, and superheated steam has no liquid phase, so quality is not defined.
7 / 10
Superheated steam at 10 bar and 300°C has a specific volume of 0.2328 m³/kg. Compared to dry saturated steam at 10 bar (vg = 0.1944 m³/kg), the superheated steam is:
More dense
Less dense
Equally dense
Density ρ = 1/v. For superheated steam, v = 0.2328 m³/kg, so ρ = 1/0.2328 ≈ 4.30 kg/m³. For dry saturated steam, v_g = 0.1944 m³/kg, so ρ = 1/0.1944 ≈ 5.14 kg/m³. Since ρ_superheated < ρ_saturated, superheated steam is less dense.
8 / 10
Dry saturated steam at 10 bar has an entropy (sg) of 6.586 kJ/kg·K. If the steam is wet with a dryness fraction of 0.9, and the entropy of saturated liquid (sf) is 2.138 kJ/kg·K, the entropy of the wet steam is:
6.142 kJ/kg·K
5.931 kJ/kg·K
4.793 kJ/kg·K
6.586 kJ/kg·K
For wet steam, s = sf + x (sg – sf). Given x = 0.9, sf = 2.138 kJ/kg·K, sg = 6.586 kJ/kg·K:
s = 2.138 + 0.9 × (6.586 – 2.138) = 2.138 + 0.9 × 4.448 = 2.138 + 4.0032 = 5.931 kJ/kg·K.
9 / 10
At 5 bar, the saturation temperature of steam is approximately 151.8°C. The enthalpy of dry saturated steam (h_g) is 2748.7 kJ/kg, and the enthalpy of saturated liquid (h_f) is 639.7 kJ/kg. The enthalpy of wet steam with a dryness fraction of 0.8 is:
2198.96 kJ/kg
2358.46 kJ/kg
2039.46 kJ/kg
2748.7 kJ/kg
For wet steam, h = hf + x (hg – hf). Given x = 0.8, hf = 639.7 kJ/kg, hg = 2748.7 kJ/kg:
h = 639.7 + 0.8 × (2748.7 – 639.7) = 639.7 + 0.8 × 2109 = 639.7 + 1687.2 = 2198.96 kJ/kg.
10 / 10
The dryness fraction (quality) of wet steam is defined as:
The ratio of the mass of liquid to the total mass of steam
The ratio of the mass of dry vapor to the total mass of steam
The ratio of the mass of dry vapor to the mass of liquid
The ratio of the total mass to the mass of liquid
The dryness fraction (x) of wet steam is x = mv / (mv + mf), where mv is the mass of dry vapor and mf is the mass of liquid in the mixture.
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