How can I cure my gravity circulation problem?

[Brambles]

This is confusing. 1 inch (25 mm) often has an OD of 28 mm.

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.

 

[gmartine]

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?

 

[Neville Hillyer]

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.

 

[John.g]

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?.

 

[gmartine]

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.

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.

 

[Neville Hillyer]

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?.

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.
[automerge]1589733757[/automerge]

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 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.

 

[John.g]

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?.

 

[Brambles]

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.

 

[SJB060685]

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.

Brambles any chance you could provide a link to this calculation method?

 

[Brambles]

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?