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nh3simman
18-10-2008, 11:48 AM
Hi,

Recently, I had the opportunity to witness the testing of a brazed plate heat exchanger.

It was a test of a water to water heat exchanger in order to determine the Nusselt number coefficients.

This is not my normal area of work so I was surprised to see the that the range of water flows was limited to a range of Reynolds number Re = 250 to 3000.

When I asked about this, the researcher told me that by Re=300 you have fully turbulent flow.

But the Moody diagram for a rough pipe shows the transition from laminar to turbulent at around Re=2300. By using the correct hydraulic diameter, the same Moody diagram applies equally to rectangular ducts.

So my question is this:
Is there a recognized Moody diagram that is used for plate heat exchangers or should the researcher select a more appropriate hydraulic diameter.

Andy
18-10-2008, 02:38 PM
Hi,

Recently, I had the opportunity to witness the testing of a brazed plate heat exchanger.

It was a test of a water to water heat exchanger in order to determine the Nusselt number coefficients.

This is not my normal area of work so I was surprised to see the that the range of water flows was limited to a range of Reynolds number Re = 250 to 3000.

When I asked about this, the researcher told me that by Re=300 you have fully turbulent flow.

But the Moody diagram for a rough pipe shows the transition from laminar to turbulent at around Re=2300. By using the correct hydraulic diameter, the same Moody diagram applies equally to rectangular ducts.

So my question is this:
Is there a recognized Moody diagram that is used for plate heat exchangers or should the researcher select a more appropriate hydraulic diameter.

Hi nh3simman,

not my area, but I would say the design of the heat exchanger, not the roughness of the surface creates the turbulent flow. This would have been found by testing mainly.

Kind Regards Andy:)

US Iceman
18-10-2008, 04:05 PM
My guess is this: due to the various plate configurations they have found ways to lessen the boundary layer effects, which in principle would allow lower Reynolds numbers in what might be equivalent to the turbulent region above Re=2300.

But as I said, it's just a guess...

nh3simman
25-10-2008, 03:29 PM
due to the various plate configurations they have found ways to lessen the boundary layer effects

We know that the Moody diagram was developed for a round pipe with various levels of roughness.

But, by choosing the correct "equivalent diameter", I can apply the same Moody diagram to a rectangular duct. And there is no guesswork or approximation here.

So my real question is based on exactly what you have expressed. If the transition to turbulent depends on the configuration, then there is no reference for comparison. Every heat exchanger will have to have it's own Moody diagram and this won't be of much use.

Thanks, after writing this, I'm now convinced that the researcher is not using the correct equivalent diameter.

US Iceman
25-10-2008, 03:37 PM
Let me know the outcome of this. It would be interesting to learn the real story behind this.

Somewhere in the past I heard an explanation that seemed to imply the various plate configurations were used for specific flow ranges and fluid properties. What I posted earlier was my thought on why they would do this.