picklefactory Posted September 16, 2010 Posted September 16, 2010 Hello folks, first post here and hope I'm not being cheeky, but looking for a scientific answer to a domestic question. I have a domestic plumbing problem, 'Get a plumber' I hear you say'..... well, I have and he's stumped too. I'm an engineer by trade, and I'm sure there will be a scientific answer, but not my field. OK, here's the problem. I've enclosed an image of a hot water cylinder that is heated via a heat exchanger coil through which flows hot water from the boiler, ie the water in the cylinder is not directly heated itself. The water flows through the coil using convection, or so I'm told, and it is NOT pumped. Apparently the lighter hot water rises, the cooler water sinks and creates the flow around the system. Now due to a new cylinder being fitted, I believe there is air trapped within the coil restricting the flow. There is a vent at a high point on the inlet flow pipe, but my initial question is (And probably daft), will any trapped air ALWAYS rise through the water in the pipe, even against the flow, or could it get 'stuck'? I think it should rise, surely bubbles would always come to the highest point and should escape through the vent. Can anybody suggest another reason why the flow would not occur? Blockage is the obvious one, but as all the pipework is brand new, that has been ruled out. If it is possible for air to get trapped, how would I encourage it to move? Would applying heat at any point in the circuit have any effect? Apologies if this is not the sort of thing normally posted here....... but thought it worth a go. Cheers
swansont Posted September 16, 2010 Posted September 16, 2010 It's possible. Bubbles are going to be weakly stuck via adhesion to the surface of the pipe. If the angle of the coil is shallow enough, the buoyancy force might not be enough to make it rise, and I suspect that a large enough bubble could prevent convection from starting. Banging on the pipes might reduce the adhesion and let the bubble move to the top, if there is no flow to oppose it.
picklefactory Posted September 16, 2010 Author Posted September 16, 2010 Thanks swansont, I can certainly give that a try. Would heating the pipe where it exits the cylinder (Lower end of the coil) have any benefit, I'm kinda thinking if I can get the convection going the other way, it might 'push' the bubble towards the vent? I've been trying to clear the air with heating on up to now, so that would be trying to push air against any flow of water. I may have better luck with cold water. Cheers
between3and26characterslon Posted September 16, 2010 Posted September 16, 2010 Are there any additives in the water ie. something to reduce the surface tension of the water and break up any bubbles Also won't you need to have a reasonable height of water in the vent so that when the heated water rises it can't just rise into the vent. The less dense hot water will just find the highest point but the height of water above it will push it round the system.
ewmon Posted September 16, 2010 Posted September 16, 2010 (edited) From an engineering standpoint: I can't see why the water would naturally circulate to the extent that it would be effective. Water needs a lot of heat to warm up. I think a pump would help immensely. Regarding bubbles, I'd be more concerned with bubbles sticking to the pipes in the furnace/boiler where they can create hot spots, causing the pipes to fatigue and fail (in addition to driving down the efficiency of heat transfer). It looks like counterflow would work best, so the diagram looks okay with the hot water flow into the top of the cylinder and cooler water exiting near the bottom. Edited September 16, 2010 by ewmon
picklefactory Posted September 16, 2010 Author Posted September 16, 2010 between3and26characterslon Thanks Re-additives, that is something I hadn't considered. It is common to include an additive to primarily reduce corrosion, but I wonder if it has any effect on surface tension? I haven't really shown the entire vent set up, sorry, that was a quick 3D model I knocked up for illustration. The vent shown has a thumscrew at it's top for manually venting, so water cannot escape there, but there is also a further vent pipe that branches off from that one and then travels up and into the header tank (Where the 'Top up' comes from). That is high enough that the water wouldn't rise that far. ewmon Thank also. this system is simply a replacement of what was existing, and worked fine, just with new components, so I won't be adding a pump although I agree it would be better and should also force any air out. The water circulating (Or not in my case!!!) comes directly from the boiler below, so is receiving a lot of heat. The representation of the coil is a bit of a guess, as that is hidden inside the cylinder, so I don't really know it's size, helical pitch etc, I used a little artistic license for illustration only.
Mr Skeptic Posted September 16, 2010 Posted September 16, 2010 Depending on what type of connection you have, if you loosen the connection back to the tank, eventually it will have water leaking out. Unscrew it enough, and it should start leaking but not come off. The flow of water should be fast enough (now based on the water pressure rather than convention), to force any bubbles through. This will be messy and even more messy if the pipe comes off completely. Also dangerous if the water coming out is hot. And it won't work if the water is not under pressure. Actually, if your water source is the part it says top up rather than somewhere in the tank, it would work even better if you can let the water out of the vent instead. And so long as there is a steady inclination upwards towards the vent, just vibrating the pipe should let the bubbles up toward the vent.
picklefactory Posted September 17, 2010 Author Posted September 17, 2010 (edited) Mr Skeptic Thanks for that. Regarding pressure, there isn't really much. The top up pipe feeds the system from a header tank, so there is only the pressure generated by the height of that tank (Approx 2mtrs and fed through 15mm pipe), once the system is filled and the air removed, it is purely convection that creates the flow around it, at least that is how it was explained to me. So based on that, my thought is that slackening the lower connection will only release water that will be continually replaced from the top up pipe adjacent to it, and whatever pressure the header tank gives, will just push water towards the coil and probably not allow any bubbles to move in that direction. In fact, now that I think about it, I wonder if the top up pipe is in the wrong place. Any pressure from there can go in either direction where it joins the circuit, so that could also be fighting the convection current. That top up pipe was not in that position originally, but on a separate spur that fed across into the circuit in the correct direction of flow. Hmmm, I will ask the plumber about that when he is back. Edited September 17, 2010 by picklefactory
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