# Modeling of a bypass valve in the two-phase structure

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Kai Franke on 29 Sep 2021
Answered: Yifeng Tang on 22 Jul 2022
I have now created a separate topic about it.
However, I have encountered a similar problem that I can't seem to solve currently.
I have created a heat pump circuit and would like to run this circuit in two ways (cooling/heating). The switching is done by two valves, which I have also modeled by orifices.
Now it is the case that a bypass next to an expansion valve (marked yellow) lets through a non-negligible mass flow despite being closed. This changes my simulation result. So how can I implement a true closed valve?
I tested the circuit without the TXV Bypass and it's running correctly.
The Evaporator Bypass is working fine. In heating mode the two-phase fluid flows from A to C and the connection A to B is closed. The leakage flow is negligible. The Evaporator Bypass and the TXV Bypass are modeled similarly. While the Evaporator Bypass contains two orifices the orifices for the TXV components are split (one orifice inside the Bypass and one inside the TXV External HX, but functionality is the same).
The situation is different for the TXV Bypass as you can see below. The mass flow through the bypass takes up almost 1/8 of the total mass flow, while opening fraction is zero throughout.
My guess is that the pressure differential along the TXV External HX (which is modelled as a thermostatic expansion valve component) influences the mass flow through the bypass. Any ideas how to stop fluid from running through the bypass? The boundary condition is that during the cooling phase the fluid flow must pass through this valve unaffected.
Thanks for your help! If there is any important information missing, please let me know!
Kai

Yifeng Tang on 22 Jul 2022
If "opening fraction is zero throughout" and the valve let through a significant fraction of the total mass flow, you have too much of a leakage. Check the leakage fraction value in the TXV block.
But why is the by-pass valve a TXV type of valve anyway? It looks like a TXV from the outside. Isn't it supposed to be a simple controlled orifice just to "by pass"?

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