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How can I cure my gravity circulation problem?
- Thread starter Neville Hillyer
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View the thread, titled "How can I cure my gravity circulation problem?" which is posted in Central Heating Forum on UK Plumbers Forums.
The two designs I looked at, that were prepared for an indirect gravity hot water cylinder for solid fuel Rayburn ovens. They both had 28mm internal diameter coils specified extending over the lower two thirds of the cylinder.
I only read the OP's first post and not much else, would the fact that the new cylinder is made from stainless steel rather than copper have a stalling effect being experienced because of the less efficient thermal transfer or are the coils made from the same material?
The earlier copper cylinders were 100% copper and the new cylinder is 100% stainless steel. If you view conductivity on a log scale between rubber and metal, glass is somewhere in the middle and all metals are very close together at the end. Any difference between copper and steel conductivity is negligible in this application.
Above posts are interesting but while copper should give a faster warm up than stainless for the same coil area, using stainless should not cause stalling.
Is there any way that the coil has been installed somewhat like the attached even though the flow/return connections are shown at 600mm apart in your drawing.
Also, if you don't me asking, what do you intend to do to resolve your problem as you have lived with it for a year or so?.
Is there any way that the coil has been installed somewhat like the attached even though the flow/return connections are shown at 600mm apart in your drawing.
Also, if you don't me asking, what do you intend to do to resolve your problem as you have lived with it for a year or so?.
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If the rate of heat transfer is critical to how a thermosyphon works and yours appears particularly sensitive, irrespective of the scale compared to others if you then used a material that was subsequently 1/20 less efficient at doing so it isn't a negligible factor.
I saw a report of copper being faster but this was for identical metal thicknesses. Stainless cylinders are normally designed to withstand specified pressures which results in lighter and thinner cylinders and coils. In my case I am not concerned with speed. I have ample storage for my preferred relatively low primary temperatures.
As far as I could tell my coil approximates to a normal spiral between the connections. It is however supported by stainless wire and thin straps which, combined with the very flexible coil, caused it to 'sing' when moved empty just like moving a box of very light springs.
As a retired design engineer I am determined to do all I can to understand what is happening. Next I may try isolating the coil, disconnecting the primary from the boiler and power flushing each pipe in turn from roof to kitchen.
The attached schematic has been updated for: pipe colours and sizes, cylinder and pump isolation valves, non-return valve and pressure relief valve.
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I am afraid I misunderstood. I thought your remarks were about energy efficiency rather than speed. You could have a point but I suspect this is not the cause of my problem.
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Is the ~ 0.5M horizontal section of pipe hot right up to the coil inlet?.
You might also consider isolating the coil and install a 600 mm (spool) piece of pipe between the flow&return and see if you get thermosyphon circulation.
At the end of the day, if all else fails would you consider a pumped solution?.
You might also consider isolating the coil and install a 600 mm (spool) piece of pipe between the flow&return and see if you get thermosyphon circulation.
At the end of the day, if all else fails would you consider a pumped solution?.
Neville,
I think that you have hit the nail on the head by looking at the calculations from first principles - I have never needed to calculate the pressure differential required to drive the thermosyphon effect before. It is a bit of an iterative process, but with water at 60 degrees C ( for the density) and a required ( my estimate) velocity of 0.5m/second the pressure differential to drive the flow 1meter is:
22mm inside diameter 1.00 inch head
28mm inside diameter 0.15 inch head
Thereafter for a 7m coil multiply the above differential by 7.
If you reduce the required velocity to 0.25m/second the equivalent pressure differential needed drops by a factor of 4.
At 0.125m/second the differential pressure required drops by a factor of 16.
There are rough estimates in the above for the friction factors and Reynolds Number. However, I was surprised by the significant influence that internal diameter has, and presumably in the longer term corrosion and silting of the flow pipework.
I think that you have hit the nail on the head by looking at the calculations from first principles - I have never needed to calculate the pressure differential required to drive the thermosyphon effect before. It is a bit of an iterative process, but with water at 60 degrees C ( for the density) and a required ( my estimate) velocity of 0.5m/second the pressure differential to drive the flow 1meter is:
22mm inside diameter 1.00 inch head
28mm inside diameter 0.15 inch head
Thereafter for a 7m coil multiply the above differential by 7.
If you reduce the required velocity to 0.25m/second the equivalent pressure differential needed drops by a factor of 4.
At 0.125m/second the differential pressure required drops by a factor of 16.
There are rough estimates in the above for the friction factors and Reynolds Number. However, I was surprised by the significant influence that internal diameter has, and presumably in the longer term corrosion and silting of the flow pipework.
SJB
You don’t normally need it for general plumbing - the results are normally tabulated into charts such as the ones that give optimum flow rates for different diameter pipes.
If you want to calculate the values from scratch use the Darcey Weisbach equation:
P= f * v2 * p/2*L/D
P = pressure
f = friction factor
v2= velocity of the fluid squared
p = density of the fluid
L = pipe length
D= internal pipe diameter
f is determined (estimated) by calculating the Reynolds Number (Re)
Re = V * D *(p/vi)
vi is the viscosity of the fluid.
V is velocity of the fluid.
Published tables then give you the friction factor ( f )for a specific Reynolds Number (Re) to input into the first equation.
It is an iterative process so you need to do it a couple of times to be sure that the answer starts to be consistent.
I am unclear as to what the minimum velocity is for a gravity hot water system to be effective - but I would guess that the minimum flow rate that allows the boiler to operate properly is a key design criteria for the tank to match.
Does this make sense?
You don’t normally need it for general plumbing - the results are normally tabulated into charts such as the ones that give optimum flow rates for different diameter pipes.
If you want to calculate the values from scratch use the Darcey Weisbach equation:
P= f * v2 * p/2*L/D
P = pressure
f = friction factor
v2= velocity of the fluid squared
p = density of the fluid
L = pipe length
D= internal pipe diameter
f is determined (estimated) by calculating the Reynolds Number (Re)
Re = V * D *(p/vi)
vi is the viscosity of the fluid.
V is velocity of the fluid.
Published tables then give you the friction factor ( f )for a specific Reynolds Number (Re) to input into the first equation.
It is an iterative process so you need to do it a couple of times to be sure that the answer starts to be consistent.
I am unclear as to what the minimum velocity is for a gravity hot water system to be effective - but I would guess that the minimum flow rate that allows the boiler to operate properly is a key design criteria for the tank to match.
Does this make sense?
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Yes makes perfect sense.
Like you said its not really needed to go into such detail but I'm the sort of person who doesn't like not knowing something exactly. If something like this pops up I will go and have a read through and mock up some rough figures just to get a feel for it
Like you said its not really needed to go into such detail but I'm the sort of person who doesn't like not knowing something exactly. If something like this pops up I will go and have a read through and mock up some rough figures just to get a feel for it
The flow is about 40cm and the return is about 50cm to the tank connections.
Not as easy as it sounds but is is possible. However, I already have excellent thermosyphon circulation through the bathroom radiator. An easier trial would be to temporarily join the two pipes at the small header tank.
As indicated I would be very reluctant to abandon gravity circulation especially as it has worked well for 50 years.
I don't think joining the pipes at the header tank is a good idea as there will be virtually no static head at this point but as you stated there is excellent circulation through the rad and one might expect the flow pipe to be hot almost right up to the coil connection due to thermosyphon circulation within the pipe itself.
I get the gist of what is being discussed above but it must be remembered that the gravity driving force available is only determined by the flow/return temps and the head available.
If one realistically accepts a max head of 2.5M and a flow/return temp 75/15C with a cold cylinder and 75/60C when hot, then the max circulating force is 0.061M/2.42ins and the min is 0.021M/0.84ins. A pumped system with a differential head of 3M will give a flow rate factor of X12 in the first case and X7 in the second case and that is one and the main reason that pumped systems are now almost universal. I do have a vague memory (50 years) of some gravity driven heat exchanger with a huge finned coil.
If one realistically accepts a max head of 2.5M and a flow/return temp 75/15C with a cold cylinder and 75/60C when hot, then the max circulating force is 0.061M/2.42ins and the min is 0.021M/0.84ins. A pumped system with a differential head of 3M will give a flow rate factor of X12 in the first case and X7 in the second case and that is one and the main reason that pumped systems are now almost universal. I do have a vague memory (50 years) of some gravity driven heat exchanger with a huge finned coil.
No you misunderstood me. I said that was a typo to your reply where I mistakenly said 100 n/m2 equates to 1 bar. As I said above 1 bar = 100,000 Pa or should have read 100,000 n/m2. It was late and I missed out some noughts.
Brambles, can you please give me your calculated flow and resistance for the installed coil, preferably with the resistance in M.
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My own basic calcs would indicate that 5.8M of 22mm ID pipe should flow ~ 7.5 LPM @ 0.061M head which should satisfy the requirements except that the corrugations are having a huge effect but even if they do then there should be some reduced level of performance.
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My own basic calcs would indicate that 5.8M of 22mm ID pipe should flow ~ 7.5 LPM @ 0.061M head which should satisfy the requirements except that the corrugations are having a huge effect but even if they do then there should be some reduced level of performance.
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I would prefer a better source than Amazon but it appears that corrugated pipe is specified in the old British way ie by internal diameter. The following is an example from: https://www.amazon.co.uk/Corrugated-Stainless-Steel-Pipe-DN25/dp/B07B2JVJFX
Nominal width: DN25
Wall thickness: 0.20 mm
Inner diameter: 25.5 mm
Outer diameter: 31.8 mm
Bending radius: 39 mm
Operating pressure: 10 bar
This appears to be compatible with my specification to Telford and my 1 inch (28mm) pipes. What was not clear to me was the use of corrugated pipes to replace my original standard coil.
I had hoped that somebody would say they had some experience of the use of corrugated coils in a gravity system.
I am trying to compile a full list of possible reasons for the replacement cylinder not getting any heat. Here is my list so far:
1 - Partial or total blockage which allows full heat to bathroom radiator but no heat to cylinder.
2 - Incorrect or imprudent connections to cylinder.
3 - Partial blockage inside cylinder which still allows it to be flushed.
4 - New cylinder fails to self-clear air locks like all earlier cylinders did.
5 - Corrugations inhibit upward air flow in the face of static or slow downward water flow.
6 - Slight slopes on 'horizontal' pipes more critical than previously.
I suspect that if the cylinder fails to self-clear air locks then it will be a continuing source of trouble in the future. My experience is that large old vented systems can slowly suck in small quantities of air at any time.
In an ideal system any air or dissolved oxygen within the system is usually expelled to minimal amounts within a few months, thereafter any remaining is only enough to slightly corrode any ferrous materials. Air should ideally not be drawn in, unless you drain down for maintenance or a pump is sucking it in somewhere.
You say that you flushed all sections of the pipework, if this didn't include the coil then you should do this. If the coil is clear then IMO, your options are to either fit a cylinder/coil like your previous one or at least one with a self supporting non corrugated coil, the other option is one that I suggested before, install a pump (like the one I suggested) on the flow side, this pump cannot, as I thought, be set down to 0.1M head but can be set to a PP (proportional pressure) setting of 0.5M, at this setting if still no flow (for whatever reason) it will ramp down to a minimum of 0.25M at zero flow which can be read off in the form of the power consumption, this, again IMO, may give you a big stick to beat some body with and, at the very worst, provide you with a spare circulating pump.
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Came across a few readings that a neighbour gave me a few years ago from his standard hot water cylinder which he converted to fully pumped from gravity when he changed his boiler. If his readings were accurate then he seems to have been getting ~ 1.5 LPM at 50C continuously in gravity mode only with 19mm ID flow&return.
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Many thanks to all who have contributed to this thread and given such helpful advice.
Having considered all the evidence and advice my view is that it may be most profitable to initially investigate a permanent 15mm link between the mains and the cold feed from the small header tank. With suitably placed isolation valves this could provide mains flushing as and when required.
I plan to:
1 - Isolate the bathroom radiator, cylinder and pump to flush around the gravity circuit.
2 - Open the cylinder coil valves and hope to get a sufficient reverse flush up the coil to remove any air locks.
Since I will be doing this while positioned near the small header tank I will be able to see:
1 - Any debris.
2 - Air - this will require a short temporary hose or pipe to take the vent below water level.
The above may take a while but I plan to keep this thread updated.
Having considered all the evidence and advice my view is that it may be most profitable to initially investigate a permanent 15mm link between the mains and the cold feed from the small header tank. With suitably placed isolation valves this could provide mains flushing as and when required.
I plan to:
1 - Isolate the bathroom radiator, cylinder and pump to flush around the gravity circuit.
2 - Open the cylinder coil valves and hope to get a sufficient reverse flush up the coil to remove any air locks.
Since I will be doing this while positioned near the small header tank I will be able to see:
1 - Any debris.
2 - Air - this will require a short temporary hose or pipe to take the vent below water level.
The above may take a while but I plan to keep this thread updated.
My apologies and again to get to 20 characters!
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Ok, so for 22mm ID at 7.5lpm, that will equate to a velocity of around 0.33m/second.
If you run the two equations and consider the coil length is 8m. The minimum head required for the thermosyphon to operate at that velocity is 2.4 inches of water or 0.061m of water.
This may be on the low side depending on the friction factor if the pipe coil is corrugated on the inside.
The above only calculates the force required to drive the flow at 7.5lpm through the coil - it does not allow for the interconnecting pipework.
As stated earlier, for a 28mm dia coil at the same velocity, the force required is significantly lower
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