Tubing for boilers - Rigid or Coils?

[GaryK]

I am researching building a boiler. Everything is still up in the air, just trying to gather facts.

Looking at some boiler designs I see two basic types. I know there are more but I'm looking at these two.

1. Straight tubes through the boiler body.
2. Bent tubes attached on the outside of the boiler body.

Looking at the straight tubes it would seem best to use rigid straight tubes.

But what about the bent tubes. Should they also start out straight and annealed and bent and generally worked to create the bends. Or will soft bendable coils work.

Is there a drawback to using coiled copper and is there a special type that needs to be used?

Assistance is appreciated.

Gary

 

[enfieldbullet]

i can predict greater heat losses with the tubing on the outside.

i think copper could be used with success but you have to consider this is a pressure vessel and everything has to take this into consideration.

if the strength of the copper tubing is in accordance with the design i don't see a drawback to using coiled copper besides price.

 

[Dave Sohlstrom]

Gary

I think you are talking about two different boiler types.

The first you talk about is a fire tube boiler, where the heat goes through the tubes and the water is on the outside of the tubes. They are built in both horizontal and vertical models. They are fairly easy to build.

The second type is a water tube boiler where the heat is on the outside of the tubes and the water on the inside. This type of boiler is most common as a horizontal boiler with a steam drum and a mud drum. Mud drum is generally smaller in size than the steam drum. Mud drums are directly below the steam drum or below and to one side of the steam drum.

Water tube boilers are harder to build and when you have one in steam need to be watched closer because they can generate more steam and need a good supply of water.

If you do not have a copy of " Model Boilers and Boilermaking" by K. N. Harris you should see if you can find a copy. Lots of good info.

Dave

 

[GaryK]

Gary

I think you are talking about two different boiler types.

The first you talk about is a fire tube boiler, where the heat goes through the tubes and the water is on the outside of the tubes. They are built in both horizontal and vertical models. They are fairly easy to build.

The second type is a water tube boiler where the heat is on the outside of the tubes and the water on the inside. This type of boiler is most common as a horizontal boiler with a steam drum and a mud drum. Mud drum is generally smaller in size than the steam drum. Mud drums are directly below the steam drum or below and to one side of the steam drum.

Water tube boilers are harder to build and when you have one in steam need to be watched closer because they can generate more steam and need a good supply of water.

If you do not have a copy of " Model Boilers and Boilermaking" by K. N. Harris you should see if you can find a copy. Lots of good info.

Dave

Thanks for your reply. I understand all that you say in your post. Basically what I am asking is: Can coiled copper tubing be used as a replacement for straight rigid tubing? Are there trade offs? If the wall thickness for a given size is the same are they interchangeable?

I have Harris's book and I see nothing in there about coiled copper tubing.

Gary

 

[Lakc]

Thanks for your reply. I understand all that you say in your post. Basically what I am asking is: Can coiled copper tubing be used as a replacement for straight rigid tubing? Are there trade offs? If the wall thickness for a given size is the same are they interchangeable?

I have Harris's book and I see nothing in there about coiled copper tubing.

Gary

Coiled tubing is generally annealed, bends easily, and is only what you have on hand when wanting straight tubing.
Straight tubing is generally hard or half hard, quite stiff, wont bend well without annealing, and, you guessed it, the only thing your likely to have on hand when needing bent tubing.

If they are both pure copper and of suitable wall thickness I cant see any reason they cant interchange, I suspect it all becomes annealed in use eventually.

 

[Dave Sohlstrom]

You can use Hard copper tubing by annealing it. You can use coiled or annealed tubing by anchoring one end and stretching it just a little to work harden it.

Dave

 

[SandyC]

Hi Gary,

For internal tubes in a firetube boilers you should use Hard Drawn Straight tubes since they are subject to EXTERNAL PRESSURE and the STRESS Calculations for these are only accurate if the tube/s are perfectly round in section, straight and undamaged.
You should not use STRAIGHTENED coiled tubing for these, since the coiling will almost certainly have deformed the cross section into an UN-ROUND state.
Stretching this type of tube , as has been suggested, will straighten it somewhat, but it will never remove any out of roundness and also could in fact produce sections where the wall thickness is dissimilar.
Out of round, or wall thickness changes will render such tubes subject to collapse under pressure...at a much lower pressure than the calculations would produce... Always use STRAIGHT HARD DRAWN and undamaged tubing for these.
The same applies to the centre flu of a marine type boiler.

Coiled tubing can be used (straightened or otherwise) for water tubes, either internal (cross flu) or external (such as on a Yarrow type) since these are subject to INTERNAL PRESSURE and will have a much higher HOOP STRESS.

Hope this helps.

Best regards.

SandyC

 

[Dave Sohlstrom]

I stand corrected thanks Sandy.

Dave

 

[chrsbrbnk]

The first step is deciding between fire tube or water tube might depend on if its going to be a historical model or freelance.

 

[KBC]

Hi fellas,
I have been building boilers for many years , not as many as Sandy and I have no intention of contradicting him but have used coiled central heating copper tubing straightened out for the fire tubes in vertical return flue boilers.

I agree whole heartedly with Sandys statement.

"Out of round, or wall thickness changes will render such tubes subject to collapse under pressure...at a much lower pressure than the calculations would produce... Always use STRAIGHT HARD DRAWN and undamaged tubing for these".

The problem arrises when you try to get values of the safe working pressures of copper tubing especially if it's the external pressure on the tube, I have searched with no luck.

My latest boiler has a 3" dia copper tube which by calculation from K.N. HARRIS'S book with a safety factor of 8 will give a S.W.P. 134.78 p.s.i

If you use the same calcs for a 3/8" copper tube used for the vertical tubes it will give a S.W.P of 525.7 p.s.i internally and as my boiler is only tested to 120 p.s.i. hydraulic to work at 60 p.s.i.
Bearing in mind that there will be a much lower pressure on the external dia of the tubes the problem is that no figures can be obtained on what the drop will be and a boiler working at 60 p.s.i has enough of a safety factor to take the pressure drop and has never given me any trouble.

I straighten the tube as much as possible and then with soft jaws in my vice gently squeeze the tube as I rotate it to keep the ovality out of the tube and prevent dents , if dents appear I scrap the tube and use another bit.
So far I have never had any trouble with this form of construction although I have been tempted to hydraulic test to destruction but never had the heart to destroy the boiler that I have made.

First set of pics are for a Vertical fire tube boiler 3" dia with 3/8" fire tubes fired with a ring burner..

Also a Horizontal Scotch return flue boiler 4.75" dia with 5/16" dia return tubes.

George.

 

[SandyC]

Hi Guys,

KBC, those are very nice boilers you have built there, I assume they are to your own design.

Bearing in mind that there will be a much lower pressure on the external dia of the tubes the problem is that no figures can be obtained on what the drop will be

Not so I am afraid... the pressure on the outside of the firetubes or centre flu is the same as the pressure on the rest of the boiler... there is NO drop in pressure involved.
60psi is 60psi... and would apply to all exposed internal surfaces within the boiler pressure vessel, either INTERNAL PRESSURE (outer shell or water tubes) or EXTERNAL PRESSURE (Firetubes or centre flu).

Superheaters are subject to the same INTERNAL PRESSURE as the main boiler, but are subject to much higher temperatures. Superheaters do not increase pressure... only the steam temperature.

As you already know, Harris, Evans, Farmer, Tubal Caine et al... all provide the necessary formulae for INTERNAL PRESSURE, But... they are a bit short on info when it comes to flu and firetube calculations for EXTERNAL PRESSURE, but they do give lists of typical sizes.

When it comes to designing your own boilers then it is necessary to provide the necessary calculations in order to satisfy the boiler testers that the design is safe. If you cannot provide these then the tester can refuse to test your boiler.

Not necessarily a problem for home use only, but vital if you need to use your boiler in a public place... so: -

The correct formulae for tubes subject to EXTERNAL PRESSURE are: -

For wall thickness: -

T = (PD / (2S+P)) + 0.005D

And for max safe pressure this would be: -

P = S [(2T - 0.01D) / (D - (T - 0.005D))]

Where D = Outside dia.

and S = Maximum allowable stress value of the design material at design temperature.

For copper @ temperatures below 450deg F it is reasonable/acceptable to use the following for S: -

Note… Annealed Copper has a typical Tensile Strength of 25000 PSI so divided by 8 (safety Factor) gives 3125 PSI.

which is the correct figure for use with INTERNAL PRESSURE calculations.

To obtain the correct figure for the EXTERNAL PRESSURE calculations above; this should be further divided by 3.5... Hence: -

S = 3125 / 3.5 = 892.857psi.

These calculations assume tubes are solid drawn, truly round and straight, with uniform wall thickness and no dents or other damage.

Hope this is of some help to you all.

Best regards.

Sandy

PS... I cannot remember where I eventually found the above formulae, it was a long time ago, but I am sure it was in an early edition of the boiler design codes from either the USA or possibly the Australian boiler design codes... these being the FULL issues, not the latest revised codes.

They have served me well for more years than I care to admit.::shrug:

 

[KBC]

Hi Guys,

KBC, those are very nice boilers you have built there, I assume they are to your own design.



Not so I am afraid... the pressure on the outside of the firetubes or centre flu is the same as the pressure on the rest of the boiler... there is NO drop in pressure involved.
60psi is 60psi... and would apply to all exposed internal surfaces within the boiler pressure vessel, either INTERNAL PRESSURE (outer shell or water tubes) or EXTERNAL PRESSURE (Firetubes or centre flu).

Superheaters are subject to the same INTERNAL PRESSURE as the main boiler, but are subject to much higher temperatures. Superheaters do not increase pressure... only the steam temperature.

As you already know, Harris, Evans, Farmer, Tubal Caine et al... all provide the necessary formulae for INTERNAL PRESSURE, But... they are a bit short on info when it comes to flu and firetube calculations for EXTERNAL PRESSURE, but they do give lists of typical sizes.

When it comes to designing your own boilers then it is necessary to provide the necessary calculations in order to satisfy the boiler testers that the design is safe. If you cannot provide these then the tester can refuse to test your boiler.

Not necessarily a problem for home use only, but vital if you need to use your boiler in a public place... so: -

The correct formulae for tubes subject to EXTERNAL PRESSURE are: -

For wall thickness: -

T = (PD / (2S+P)) + 0.005D

And for max safe pressure this would be: -

P = S [(2T - 0.01D) / (D - (T - 0.005D))]

Where D = Outside dia.

and S = Maximum allowable stress value of the design material at design temperature.

For copper @ temperatures below 450deg F it is reasonable/acceptable to use the following for S: -

Note… Annealed Copper has a typical Tensile Strength of 25000 PSI so divided by 8 (safety Factor) gives 3125 PSI.

which is the correct figure for use with INTERNAL PRESSURE calculations.

To obtain the correct figure for the EXTERNAL PRESSURE calculations above; this should be further divided by 3.5... Hence: -

S = 3125 / 3.5 = 892.857psi.

These calculations assume tubes are solid drawn, truly round and straight, with uniform wall thickness and no dents or other damage.

Hope this is of some help to you all.

Best regards.

Sandy

PS... I cannot remember where I eventually found the above formulae, it was a long time ago, but I am sure it was in an early edition of the boiler design codes from either the USA or possibly the Australian boiler design codes... these being the FULL issues, not the latest revised codes.

They have served me well for more years than I care to admit.::shrug:

Hi Sandy,
I miss quoted here
Quote:
Bearing in mind that there will be a much lower pressure on the external dia of the tubes the problem is that no figures can be obtained on what the drop will be.

What I meant to say in agreement with your statement about the lower external collapsing pressure on the fire tubes than the internal but as my boiler was working at 60 psi there was such a great difference to that of the internal 525.7psi of a 3/8" dia tube I didn't see any danger, so basically the 60 psi WP of the boiler was insufficient to cause the external dia of the vertical tube to collapse.

At least I think that was what I was trying to say !!!

Thanks for the calcs as I couldn't find any reference to them anywhere, I shall digest them tomorrow.

George.

 

[SandyC]

Hi George,

No offence meant by my last post... I thought that was what you were trying to say since you have made quite a few boilers, however, I felt it needed clarification for any members not quite so up to speed with the technical side of boilermaking.

I know what you mean about finding such info... I searched high and low for quite some time before I came accross them... quite by accident actually... I was actually looking for the calculations for parabolic formed end plates, rather than hemi-spherical, when I came accross them.

Keep happy.

Best regards.

Sandy.

 

[fcheslop]

There is a thread with some more information . A Scotch Marine Boiler by Doubletop.
I think Martin Evans showed the calc in one of his books but not certain.
Cheers from another copper basher.

 

[Lakc]

I wonder why they treat compressive stress different then tension, as all the digging I have done shows the values the same? I am missing something here.

 

[KBC]

Hi George,

No offence meant by my last post... I thought that was what you were trying to say since you have made quite a few boilers, however, I felt it needed clarification for any members not quite so up to speed with the technical side of boilermaking.

I know what you mean about finding such info... I searched high and low for quite some time before I came accross them... quite by accident actually... I was actually looking for the calculations for parabolic formed end plates, rather than hemi-spherical, when I came accross them.

Keep happy.

Best regards.

Sandy.

Hi Sandy,
No offence taken, although I have made many boilers to all sorts of designs using what material I have on hand I am always open to glean information from those that can provide calcs in order to further my knowledge and allow me to make safe working boilers.

Using your calcs for the external pressure on the 3/8" dia vertical tubes with a WP of 60 p.s.i to find the wall thickness which I work out at .0126" thk.

To work out the max pressure for a wall thickness on a 3/8" tube of .027" thk I work this out at 128 psi. S.W.P.

Assuming that I have interpreted the formuli and calculated correctly this gives me twice the working pressure that my boiler can take which is just about the limit of my 3" dia shell.
As long as when I straighten out the 3/8" dia copper pipe with no dings or dents in it I am happy with that.
Of course as I operate out with public places and club venues no certificate is required.
What would be the position if I wanted to operate say on club open days would these calcs be acceptable to a boiler inspector, bearing in mind that I am the sole appointed inspector at GRMBC as the new rules allow although we have other named inspectors who would test my boiler.

Perhaps you could run your eye over these figures and thanks for taking the time to post the Calcs.

What is the saying round the edge of the £2 coin( WE CAN ONLY SEE FORWARD BY STANDING ON THE SHOULDERS OF GIANTS )

George.

 

[SandyC]

Hi Guys,

Sorry if all the math stuff is boring but it must be done.

George... I make the thickness required = 0.01406" for 3/8" OD tube @60psi.

For pressure 128psi is correct for 0.027" (22swg) wall thickness @ 3/8" OD... so plenty of additional margin
to allow for any minor surface scratches/blemishes.

In general, I always calculate the minimum thickness required and then choose a tube at least 1 gauge larger than the closest standard gauge... assuming it was easily available... else I would go up to the next higher available gauge.

E.G if the calculations calls for 0.021" (just under 24swg @ 0.022") I could choose to use 0.024" (23swg) but, since this is less likely to be easily available, I would go for 0.027" (22swg) which is readily available.

The only exception would be if the wall thickness was critical for some other reason (such as, the resulting clear internal bore becoming too small)... in which case I would go with the 24swg (0.022") but this would be a very rare case.

The main reason for choosing at least 1 standard gauge higher than the calculations suggest is to allow plenty of working pressure margin... probably overkill... but at least I can sleep easy knowing there is extra safety built in.

The formulae are totally acceptable for any boiler inspector as proof of design stress... so you would have no problems on that score.

As you rightly say... you cannot test and certify your own boilers for use in public, but you can, and must, provide the necessary calculations for any other approved tester.

??? WHAT IS A £2 coin????... you must be rich!!!!!

Keep happy.

Best regards.

Sandy.

 

[KBC]

Hi Sandy,
I have double checked the formula and I can't get the wall thickness to .0146 " which doesn't really mater as I use whatever tube I can lay my hands on and domestic C/Heating copper tube x 3/8" dia is .027" thk so I can then prove the W.P. by using this Formula.

Copper at .0146" is casket material and far to thin for any available tube so I will stick with the 23 swg tube.

£2 coins are those round things that you now have to put 50p to them to buy a cup of coffee.!!!!!

My wife and I had an interesting afternoon in Cameron House at Balloch on Loch Lomond, we had been given a present of afternoon tea which was very nice.

Looking on line later at the tariff for B&B, now there is a thing that would make your eyes water !!!!

Lets you see how the Hoy-Paloy live.

George.

Pics of the completed test boiler, I have made a ceramic burner to your design which can support a bigger jet in the gas line ,so I still have to try it.