On Fri, 16 Jan 2009 10:19:35 +0000, Adrian Stott <[email protected]> wrote:
>I've been puzzling over this since the thread started, so today when I >had to go into my forelocker for something else I had a look at my >accumulator while I was there. > >Mine does not have a diaphragm, and is not adjustable. It is just a >steel flask, containing (mostly) air. It is mounted with the >connection for the pipe from the water system at the bottom. The >air's compressability is what enables the pressure accumulation. > >This is exactly the same as, but much smaller than, one that was in a >house I owned. > >I have heard of one on a boat which was simply a champagne bottle >(sans champagne, of course). It worked fine. > >Why would a more complex (i.e. adjustable) one be needed? Apart from the comments made with regard to air dissolving in the water (without a diaphragm or bladder), consider this. if I remember my physics correctly (it has been a long long time!) Boyles Law states PV/T is constant. We can ignore T for a boat accumulator therefore Pressure x Volume will be constant. If the initial pressure is next to damn all for your system (water tank head only) lets assume it is 0.1bar assume the accumulator has a volume of 1000cc. When YOUR pump kicks out at 2 bar the air volume in the accumulator will be 1000/(2/0.1)= 50cc now assume water is drawn from the system until the cut in pressure of the pump is reached at say 1 bar. The air volume at this point will be 1000/(1/0.1) = 100 cc. The difference between 50cc and 100cc is the water displaced. Now do the same calculations based upon a diapragm type accumulator pressurised to 1 bar. Initial volume at 0.1 bar (water pressure) is 1000cc. When the pump kicks out at 2 bar the volume will change from 1000 x 1bar to 500 at 2 bar we now have 500cc of water retained in the accumulator. 10 times the amount in the unpressurised pot or bottle! Now lets do the same calculation for a higher precharge pressure of 1.5bar. At pump cut out (2bar water pressure) volume has changed from 1000cc at 1.5bar to 625cc at 2bar. We have therefore accumulated 375cc of water. Reduce the water pressure to 1bar and we actually had 0 water accumulated as we went past 1.25bar. Now do it for a lower charge pressure of .75bar. At the water pressure of 2 bar the volume has changed from 1000cc at .75bar to 375cc at 2 bar we have therefore accumulated 625cc of water. However when the pump cuts back in at 1 bar the volume will be 500cc. We have therefore only had 125cc of accumulated water that did anything for us. Sorry for a long post but you did ask ;-) I will probably be proven wrong but that is how I see it. Have a play with the numbers it makes a change from Sudoku! -- Richard
