Question

Steam at 18 bar is throttled to 1 bar and the temperature after throttling is found to be 150°C. Calculate the initial dryness fraction of the steam.

Answer 1

Pressure of steam before throttling, p₁ = 18 bar 

Pressure of steam after throttling = 1 bar 

Temperature after throttling = 150°C 

Initial dryness fraction, x₁ : 

From superheat tables at 1 bar and 150°C, we have

h₂ = 2776.4 kJ/kg 

Then for throttling, h₁ = h₂ = 2776.4 

But h₁ = h_f1 + x₁h_fg1

At 18 bar : h_f = 884.6 kJ/kg, h_fg = 1910.3 kJ/kg 

∴ 2776.4 = 884.6 + x₁ × 1910.3

x₁ = \(\cfrac{2776.4-884.6}{1910.3}\) = 0.99

Initial, dryness fraction = 0.99.

The process is shown on a p-v diagram in. States 1 and 2 are fixed, but the intermediate states are indeterminate ; the process must be drawn dotted, as shown. No work is done during the process, and the area under the line 1-2 is not equal to work done.