Question

Question 2 Water, at 350ºC, flows into a porous plug from a 10 MPa line. It exits at 1 bar. What is the exit temperature? Assume that the bulk kinetic energy of the stream is negligible and that the porous plug is sufficiently small as not to allow a significant rate of heat transfer.

Answer 1

Throttling through a porous plug is an isenthalpic process (enthalpy remains constant or H=0) as no work is done, no heat tranfer takes place (adiabatic process) and the kinetic energy is also constant.

So, h₁=h₂ (h=specific enthalpy)

p₁ v₁ + u₁ = p₂ v₂ + u₂ ...........(1)

p₁ = 100 bar    p₂ = 1 bar

C_p = 4.1868 kJ/⁰C (Heat Capacity of water)

u₁ = C_p T₁ u₂ = C_p T₂

100*10⁵v₁ + 4.1868*10³*350 = 10⁵*v₂ + 4.1868*10³*T₂ ..........(2)

Given the volumetric flow rates, the temperature of the water at exit can be calculated from equation 2.