Pressure drop along an EEV? 2

[heinasirkka]

Dear forum users,

I have a theoretical question. We have a cycle where the condensation pressure (temperature) is too high and for reducing it I'll buy an electric expansion valve. It needs to cause a certain pressure drop. In the datasheets the pressure drop and flow rate are correlated, but only for when the valve is at fully opened position (so the k_vs is given). Is the k_v value proportional to the position? Can we say then if the valve is 50% closed, then the k_v = k_vs/2? I'm having problems in understanding the relationship between the valve position and the pressure drop. I'd be very thankful for insights.

 

[Lnewqban]

Each valve is different regarding how pressure and flow change with position.
The manufacturer should be able to give you that information.

Please, see this:

http://www.engineeringtoolbox.com/control-valves-flow-characteristics-d_485.html

"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

 

[DrRTU]

The rating of EXVs are listed at full flow rate and most mfgs do not provide tables of valve position and performance. The ideal text book valve has a 1 to 1 ratio over the throttling range but this never found. Most vlvs have a stretched out S type graphic and your throttling range is on the gama (flat section) of the curve. This linear section is needed to allow for an actuator to control the process. An oversized vlv will exhibit low dp but no throttling range. It’s a big trade off. You will need to select at max conditions then test. The control loop that feeds the EXV will most likely need to a non linear signal to make up for the valve response. Don't forget your approach condition will be changing.

http://sporlanonline.com/literature-and-educational-materials/electronic-controls/

 

[heinasirkka]

Thank you all for the answers and the links you've provided. I'm looking to them and will also contact with the manufacturers for more detail.