problems pumping through HX

[cervezale]

Cooling 500L of hot wort through a Thermaline T4 U-16, 2 stage plate heat exchanger. Design flow rate is 3gpm. Cooling is not a problem. The problem is that I need to run my brewhouse pump (centrifugal, open 4" impeller) at full speed (1.5 hp, 1750 rpm, 60Hz) in order to get anything resembling 3gpm at wort out port. Basically, 25gpm going in to HX and 4gpm coming out. I have a VFD on pump and have worked through frequencies but massive pressure loss exists throughout range. HX rated pressure loss at 3gpm is 4psi.
I have also tried other pumps (1 and 1.5hp, 1750 and 3500 rpm, 60Hz) and impellers (3-4"), even connected different suction tanks to increase NPSH but problem continues wether I'm pumping hot wort or cool water.
Pump curves are unavailable for pump/motor combination. Can pump simply be too powerful and create so much turbulence in HX that flow basically ceases? Why isn't VFD solving this? Do I need a much smaller pump only for this transfer which can flow at 3-5gpm but with a higher pressure?
I'm concerned about hot side aeration as impeller beats up wort and generally frustrated about unknown issue.
I'm in rural Guatemala, Central America so this forum is my best hope for an answer.
Thanks

[CPESystems]

Probably a not a pump problem just physics and a small heat exchanger

I don't have the details on your heat exchanger but I am sure Thermaline can confirm this for you. If the designed pressure drop is 4 PSI at 3 GPM it is probably 10 PSI at 4 GPM. You also don't say what kind of pump you have but a standard 4" sanitary pump at 1750 RPM will generally only create 8 to 10 PSI maximum.

You could try putting a pressure gauge on the inlet of the heat exchanger to confirm this. I would suspect that if you adjust the VFD to give you 4 PSI at the inlet you will get 3 GPM of flow, and if you speed the pump up to give you 10 PSI you will get 4 - 5 GPM.

Don't forget this is at the outlet of the heat exchanger (as if it was flowing straight into a bucket), if you have a hose going to a fermenter and a lift in to the fermenter that is adding to the pump head pressure as well, reducing your flow.

You could try programming your VFD to have a maximum output HZ of 75 from the default of 60 HZ, this will increase the RPM to 2250 and give you a bit more pressure.

You could purchase a 3 GPM a minute pump, but I would not think it is worth while. If you have a decent quality pump it wont aerate too much and a small centrifugal pump would be worse than what you have. So you would have to get a positive displacement pump, like a peristaltic or rotary pump and both are pretty expensive.

I hope this help.

[cervezale]

Thanks for the feed back. I will run a few of the suggested trials and come back.

[mmussen]

Another possibility I can think of is if your HX has some gunk in it. I put together a brewery from used equipment and the HX didn't allow any real flow through it until it had been fully taken apart and properly cleaned. I was amazed at the amount of gunk hiding between the plates.