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Question Merit d4000 still 'pumping' issue - a possible mitigation.

11 months 2 weeks ago #41594 by AndyG
After the thread concerning water pumping from the D4000 and nobody having a solution, I had a ponder.
There are two interrelated issues:
1) The water 'bumps' in the boiler causing a pressure wave front and
2) The wave front covers the level device causing hot water to directly flow to waste and can allow some short-lived siphoning action to waste. This results in cooling of the boiler fluid as the water is replaced with 'cold'.
This action is both scary and reduces the efficiency of the still to, in our case, production of 2l per hour .
I cannot solve issue (1) however I have attempted to mitigate (2) as follows using a technician's bodge :-)

Slit a length of plastic overflow pipe to make it squeeze up to about 3mm diameter smaller than the inside of the inlet to the boiler (heat from a heat gun will soften it but keep it round). Chop most of the bottom 30mm off, just leaving a 'foot' then drill several 3mm holes above that, towards the lower end of the tube and only on one side. Slide on a couple of o-rings (from a convenient o-ring kit!) that just squeeze the tube together and allow the assembly to be just pushed with very little effort into the still inlet tube - the fit should be snug so the tube will not rotate. The top o-ring should be above the water inlet pipe, the lower one below the waste outlet and the small holes in between, pointing away from the inlet/waste pipes. This makes a plastic baffle tube inside the glass inlet tube so flow from inlet to boiler to waste is open but restricted between the main boiler and the inlet/waste tubes. There are enough holes so there is no danger of them blocking up and the thickness of the o-rings prevent the body of the plastic tube closing off the waste pipe exit point. If I can post a picture I will, however imagine a holey tube with two o-rings and a spigot to hold it off the bottom.

In use the inlet water flows slowly, either around the baffle, through the holes to the boiler and maintains level or down the outside of the baffle to waste as normal. However, if the boiler bumps, the rapid pulse of water means the majority of the hot water travels straight up the plastic tube then back down into the boiler - the leakage across the baffle is minimal and thus the boiler water level changes only slightly and is returned to normal by only a small quantity of cooler inlet water flowing through the baffle. The important bit is that the baffle significantly reduces the expulsion of water directly to waste and the chance of any siphoning action is similarly reduced.

I know it's a first, short test but this afternoon our production rate has gone from 2l/hr to 4.5l per hour and both distillate and waste flows are much smoother. It is solving a problem that should not need to be solved but for a few minutes work and almost not cost it may be worth it ... for some.

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11 months 2 weeks ago #41595 by AndyG
This is the baffle tube modification. The balance is getting safe and effective cold water flow through the baffle and to waste but limit the hot water pumping out. In this test unit the holes are 3.5mm, the o-rings are R14 (21mm i.d. x 2.5mm section) which guarantees there will always be about a 2mm gap between the wall of the baffle and the wall of the inlet/waste tube assembly.
I put more holes than required above the normal water line to ensure there will always be safe passage for water in or out - I'm now thinking about how many are actually needed and it may be that fewer but larger holes below the water line would be a better idea.

Before anyone asks, the bung has a very large bore hole with a large cut-off funnel mounted in it. It was there to safely contain boiling water when the boiler inevitably bumped and it's staying just in case! There should not be a seal in that position. We put a loose pot lid on at one point to stop the spitting and it worked ... but the boiler bumped hard, the air in the inlet assembly was vented then the lid sat back in position, sealing onto the tube which was now full of water ... at which point the boiler started to self-siphon out to waste! A simple copper clip over the side of the glass tube to prevent the lid sealing stopped that. Do not think using a simple bung with a small hole in it would be ok - if the boiler pumps hard it would be like putting your finger over the end of a hose pipe ...

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The following user(s) said Thank You: Leigh Preece

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