« on: December 17, 2010, 11:08:29 AM »
Um guys, it doesn't matter if there was a million feet of water column over the grain bed as long as there is no flow through the bed. The net downward force on the grain bed is the same as if there was 1 inch of water of the grain bed. Its when there is flow that the stress on the grain bed can go up some.
In this case, there is a concept called Effective Stress that applies here. This is a primary concept to geotechnical engineering, which is what my first Masters degree is in.
As was alluded to by the comments, we know that the pressure applied by the water increases as we go deeper under the water surface. But that water pressure doesn't just act on the top of the grain bed, it acts on each grain within the grain bed too. So to simplify, consider a single grain that is small so that the water pressure at the top of the grain is roughly the same as at the bottom of the grain.
Since water pressure always acts perpendicular to the surface to which it is applied and since there is equal upward facing surface area as downward facing surface area, the net effect is that the water force acting downward on a particle is the same as the water force acting upward on that particle when there is no flow.
The force imposed by flow is another matter and the depth of the water column over the bed can have an effect. As I mention above, the pressure on the top of the bed is roughly the same as on the bottom of the bed when there is no flow. To cause flow, you have to have a pressure gradient across the bed. For our tuns, we open a valve that is connected to the outside world where the water pressure is essentially zero. If we open the valve a crack, we can keep the pressure at the bottom of the bed only a little lower than the pressure at the top. But, if we open the valve all the way, we could get close to zero water pressure at the bottom of the bed. This is where the depth of the water column over the bed plays a part.
If I've got a million feet of water head over the grain bed and I make a mistake and open the valve too much, then I could be placing a 1,000,000' - 0' = 1,000,000 feet of head on the grain bed. But if I only have a foot of water over the grain bed, then the worst I could do is apply 1' - 0' = 1 foot of head on the grain bed.
So, its not really that putting a lot of water over your grain bed is bad. Its that you could place a lot of stress on the grain bed if you open the outlet valve too much and draw off wort too fast.
So its not the water's fault, its the operator's fault.
And regarding the issue with Batch spargers, they are draining all the wort from the bed. Recall that I mentioned that the water pressure acts on all surfaces. As we drain the bed, then a portion of the bed is above the liquid surface and instead being partially supported by the water pressure, that grain is applying all its soggy weight to the rest of the bed. That can certainly compress the bed and is a good reason why you should not perform batch sparging. Don't drain the bed until the final runoff.