Increasing lauter speed

[Seangoloid]

As the lauter progresses and sugars are carried into the boil kettle (or holding kettle, whichever you have) we've been able to speed up our draw-off speed due to the decreased visosity of the run-off. We obviously have a valve restricting flow for back-pressure, as well as a VFD for flow rate control. My question is this:
Is it better to use the VFD to increase speed towards the end of the lauter or the butterfly valve, and why?
We're simply looking to cut some time off our lauters and have been doing so by opening the "restriction" valve a little more as the lauter progresses. Just curious which one would be better and for what reason....

Cheers!

[schmogger]

What size batches are you doing and how long does your lauter take? Is your mash drying out before you stop the sparge?

[Seangoloid]

We're drawing off roughly 36 bbls each time, with no issues of drying out the mash... This usually takes about 90 minutes depending on grist load (i.e. what types of specialty malts are being used).
Obviously, we're running our darker beers a little slower and lighter ones faster due to the grind of the malt. Grind profile aside, I'm just wondering if it even matters which method of speed increase is used. Opening up the valve seems appropriate to balance out the back pressure caused by the weight of the wort in the kettle. However the VFD seems appropriate to avoid compacting the grain bed too much and sticking the mash....

[jwalts]

I'd use the VFD as much as possible so you subject your wort to the least amount of shear stress. Flow is the function of the overall pressure drop between the starting and ending points, so a constant flowrate will have the same impact on your grainbed regardless of which method you use.

Joe

[gitchegumee]

Ditto Joe. Well put. VFD.