4 replies to this topic

[mvanrijnbach]

Hi,

 

I'm trying to calculate pressure reduction in a steam heat exchanger due to condensation.

If I look at the website of a well-known control valve supplier they mention that since the volume of condensate is much smaller than vapor the pressure in the steam space is reduced . If condensation is complete it is even possible to create a vacuum. However, if I try to verify this with the momentum balance equation for refrigerant flow through a horizontal pipe I cannot demonstrate this effect (for example Eq. 2.2 in the thesis of He, "Dynamic modeling and multivariable control of vapor compression cycles in air conditioning systems"). In steady-state you have that the pressure difference is equal to two terms: the viscous pressure drop and pressure changes due to density changes. I expected the last term (equal to (mdot/A)^2/rho with rho the total density, mdot the mass flow and A the pipe cross-section area) to demonstrate the pressure reduction due to condensation, but it is only a very small contribution. Am I overlooking a term in the momentum balance? Maybe somebody can give me a good reference.

Thanks,

 

Michiel van Rijnbach.

 

[Pilesar]

The pressure of steam used as the heating medium in a shell and tube heat exchanger is in steady state when the steam flow into the exchanger is balanced by the volume of steam condensing. The volume of steam condensed is closely related to the heat transfer duty, heat transfer area, chest pressure, temperature approach, heat transfer coefficient. Typically, there is negligible steam pressure drop inside the shell side of the exchanger as long as the exchanger has plenty of open area for the steam to travel. Any significant steam pressure drop is found in the inlet nozzle, inlet piping, inlet control valve -- primarily the inlet control valve. I am confused how your question concerning steam relates to refrigerant flow through a horizontal pipe.

[breizh]

Hi,

To support your work.

https://docs.lib.pur...1&context=iracc

Breizh

[mvanrijnbach]

Hi Pilesar,

thanks for your reply. I meant that the steam (which is a refrigerant as well) is travelling through the tubes and the medium to be heated would be on the shell side. If in steady state the steam mass flow is equal to the condensation rate is it not possible for this steady state value to be different than the pressure at a position before the condensation starts? This is what I'm trying to demonstrate, but I guess I am not using the correct equation for this.

Best regards, Michiel.

[mvanrijnbach]

Hi Breizh,

yes, I found this article before as well, but it is referring to a lot of derivations from other articles I don't have access to. I'll have a look at it again to see if I can find the answers to my questions.

Best regards, Michiel.