Water pressure and flow restricton - does size matter?

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I'm trying to figure out how to best filter some liquids. A 5 gal bucket is filled with some filter medium (say sand on top 1/2 and fine charcoal on bottom 1/2). The liquid is slowly added at the top and gravity is used and vacuum may be used at some point with a complete seal along the bottom of the bucket.

Now what I'm wondering is about the number and size of holes in the bottom of the bucket. I've done this with 2 smaller holes (diameter of a pencil) and there was a steady drip from each hole, about 1 a second. I used a different bucket and moved the holes from the extreme edges to 1/3 of the way from the edge so there were 2 holes directly across from each other - same size as before and another in the center and then 3 small slits about 1/2 inch long by 2mm across around the edges. This time flow was about 1/5 the rate as before.

The reduction of flow was the opposite of what I was expecting and was wondering if there may be some reason for this which I didn't understand such as there being the same pressure in the vessel but spread out over more holes which would decrease the pressure at each hole. Since the surface of the filter material wants to hold onto the water, this decrease of pressure makes a big difference in flow.

Is that a reasonable conclusion or am I experiencing something else I have to figure out.
 

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Nidum
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You are forcing the draining liquid to take paths which are mostly sideways through the lower layers of filtering material rather than straight down and out .

Depending on the placement of your limited number of drain holes these paths could be of different lengths and present different levels of flow resistance .

There may be additional problems caused by flow being active in some holes and sluggish in others thus causing varying levels of clogging .

In any case your total drain hole flow area is probably insufficient .

Bottom plate of a filter should ideally be a uniform mesh of many generous sized holes .

Often useful to have a layer of very coarse granular material at lowest level of a filter .
 
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Nidum
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Try sketching the likely flow paths for different arrangements of drain holes .
 
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sophiecentaur
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Ideally, there would be no bottom at all in the bucket so that each part of the filter material would work with the same flow rate. Something at the bottom is needed, of course, which has to be strong enough to hold the load and the holes need to be small enough so that the filter medium can support itself. That's all obvious stuff and if you look at colanders and sieves, used in cooking, their construction is mostly holes. So why not copy existing designs? Is there some extra force or load involved that isn't there in the kitchen?
Often useful to have a layer of very coarse granular material at lowest level of a filter
A good idea and that's how commercial, high volume filters work.
Also, a coarse, pre-filter can allow the main filter to do its job better by allowing better flow of the liquid into it.
 

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