alternating flow through heat exchanger

[virk]

Who can help me?

We use a plate heat exchanger for to change heat between two fluids similar to water. Hot side always enters with constant temperature and constant flow.
Cold side enters with constant temperature, but changing flow due to control valve which controls level somewhere. So not the average flow is pumped through the HE but a flow oscillating around this average flow.

No I see that the outlet temperature of the constant flow medium exceeds design conditions enourmously. Does anyone have an equation for this behaviour.
The mass of the exchanger itself seems to be of large importance, because it works as a accumulator and stables the outlet temperature.

Equation would be helpful, for f.e. flow like sin-curve.

Kind regards

Virk

 

[rmw]

I don't think there will be any equation readily available for your situation.

What you can do, however, is give a set of different flow conditions, say some given percentage above and below the average flow to your PHE OEM, and have them run these conditions for your Hx. This should give you some estimates for what to expect at various flows along your sine wave, and you can extrapolate in between.

What you have happening is that the velocity through the cold side is constantly changing, which is changing the HTC of that side, and it is not linear. PHE's hate changing flows. They depend on velocity to get their advantage, and when the flow falls off, they "die" rapidly.

You may want to look at process changes that would enable you to keep the flow, and hence the velocity up on both sides of the heater. (recycle flow, etc.)

rmw