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Steam Stationary Engines, Traction Engines, Steam Boats

Boiler Design and Materials


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  #1  
Old 12-07-2013, 01:16:09 AM
Jk29 Jk29 is offline
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Default Boiler Design and Materials

Hi All,

My name is James. I'm a newbie to these forms. I'm currently planning the construction of propane fueled water-tube boiler. I'm looking to achieve a steady 40-60 psi of steam pressure. Currently I have a conceptual idea of what I would like to build (drawn out below). I'm looking for guidance on the proper materials to use that would ensure a safety factor of around 2!

I was thinking of a steel cylinder of 1/8'' - 1/4'' think with welded end caps. 12'' STD schedule 40 steel pipe has a working pressure around 370psi and a burst over 2000 psi, so I figure that would be more than I need. For the "water tubes" I was going to look into getting my hands on some copper tubing or stainless steal tubing.

I see a lot of copper being used in the "steam-engine building" community. Which I'm sure has something to do with it's stellar Thermal Conductivity. Would it be devastating to use stainless on an application this small? Wouldn't it just take longer to get up to pressure?

I look forward to paging though this forum. Thanks for your knowledge!

Any suggested resources/plans/tutorials/literature would me more than appreciated!

Thanks,
JK
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Old 12-07-2013, 02:59:40 AM
Mike McKnight Mike McKnight is offline
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Default Re: Boiler Design and Materials

James,
You hit the nail on the head of why folks use copper for small tubes, since it has such a great heat transfer rate.

You want to stay away from stainless, as it will be prone to cracking.

I'm sure others can point to publications and websites that will give you some pointers.

Welcome aboard, and hope you enjoy yourself here. There's lots of knowledge to be gained on this board, though we're a crotchety bunch from time to time! The main thing we're concerned about is SAFETY.

Mike M
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Old 12-07-2013, 09:01:50 AM
Joe K Joe K is offline
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Default Re: Boiler Design and Materials

JK, your sketch is an old design not far off from Babcock & Wilcox designs of the 1860s (See their book, Steam - Its Generation and Use for a pocket history of the B&W boiler.)

Instead I might suggest a boiler in the "roberts" form - which can be built largely of welded pipe fittings. Including the steam liberating space (drum) which can be a large diameter pipe, but doesn't have to be. Some have built a roberts type boiler using fire extinguisher casings as the steam drum.

You can google "roberts boiler" and come up with MANY variants to this design going from the extremely basic to the extremely sublime. One constructor did it using compression fittings!

Heh. Talk about copper. Here is a website of a man who built the entire roberts boiler from copper dvw pipe. http://aldunlop.com/boiler2011/DunlopBoiler2011.html

Anyway, welcome to the club. And don't EVER forget that you're building a bomb and only your intellect and quality materials is keeping that bomb from going off. Best not to scrimp on either of these.

Read "The Secret of the Machines" by Rudyard Kipling.

And be humble...

Joe K
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Old 12-07-2013, 03:09:38 PM
Jk29 Jk29 is offline
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Default Re: Boiler Design and Materials

Thanks for the guidance. Safety is my #1 concern.


The Roberts boiler looks like a sweet design. I think that I can work something like that out.

I've never worked with copper before so I was thinking a using a steel weld connection/female compression fitting to attach to the cylinder (pic attached). Then using compression fittings on the copper tubing.

Does the number of copper down tubes matter or is it more of a function of the surface area of those tubes? For example could I have only 2 copper down tubes on each side of the cylinder and have them tightly coil around the inside of the fire box. Or would it be better to have 3 copper down tubes on each side of the cylinder with less intricate, surface-area-grabbing coils?

Does the diameter of the water down tubes play a role in the design? Do they have to have a volume larger than that of the sum of the volume of the copper tubing?

Thanks! You guys are awesome!

JK
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Old 12-07-2013, 03:42:00 PM
Joe K Joe K is offline
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Default Re: Boiler Design and Materials

I think you mean the "flow area" of the downcomers versus the flow areas of the liberating surface.

Those roberts that I have seen included a "U" shaped lower header. At the forward (firedoor) side two "downcomers" would be extended from the drum. At the back side (opposite the firedoor) a single larger downcomer would feed the other side of the U shaped header in its center, flow splitting to each of the two sides.

I have considered that the area of this downcomer COULD be made the diameter of the drum. I.e. the drum connection of this downcomer COULD be an elbow the diameter of the drum. But I don't think this much area is necessary.

As to exactly how much area for downcomers, I don't think you need to equal the area of the upcomers as part of the flow area for the upcomers tends to be blocked by the formation of steam. You might be able to assume the specific volume of steam given a certain liberating rate (figure 5 square feet per boiler horsepower of 34lb steam/hr/hp) at your chosen pressure/saturation temperature. Thus you have the rate of liberation, the area blocked by liberation (can be worked back into tube diameter/length) and the volume of liberation traveling in parallel in the upcomer. The rest of the water circulates in a circle around that. And the area OTHER than steam can be made equal in the downcomers.

Gosh, it's been a while since I've thought about these things. I hope I've said it plain.

Or you can "reverse engineer" from some others work which they have pictured on the Internet. One instance I've seen includes welded fitting and the hobbiest chose to do the welding with full argon shielding inside the boiler (a nice refinement to assure grape free insides - but not usually done even in professional boiler construction.)

The use of compression fittings HAS been done before. Take a look at Beckman's Boat Shop who has provided MANY boat boilers constructed along these lines. He pre-makes the tube platens on a form and uses compression fittings to make the connections to drum and headers. http://www.steamboating.net/page7.html



Joe K
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Old 12-07-2013, 03:52:52 PM
DianneB DianneB is offline
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Default Re: Boiler Design and Materials

In designing a boiler the usual starting point is to determine the volume of steam required and the pressure. Knowing the volume and pressure, you can determine the required evaporative capacity of the boiler and therefore the heating surface. (There is a general capacity specified for each of various boiler types.)

As to the design and construction of a steam boiler, the ASME boiler code is by far the best guide - you can't go wrong complying with ASME! The ASME codes are very expensive but you can probably locate someone who has a copy.

For small boilers you might refer to "Model Boilers & Boiler Making" by Harris. It is a good reference work for producing small boilers that work well.
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Old 12-08-2013, 12:03:18 AM
Jk29 Jk29 is offline
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Default Re: Boiler Design and Materials

Quote:
Originally Posted by Joe K View Post
I think you mean the "flow area" of the downcomers versus the flow areas of the liberating surface.

Those roberts that I have seen included a "U" shaped lower header. At the forward (firedoor) side two "downcomers" would be extended from the drum. At the back side (opposite the firedoor) a single larger downcomer would feed the other side of the U shaped header in its center, flow splitting to each of the two sides.

I have considered that the area of this downcomer COULD be made the diameter of the drum. I.e. the drum connection of this downcomer COULD be an elbow the diameter of the drum. But I don't think this much area is necessary.

As to exactly how much area for downcomers, I don't think you need to equal the area of the upcomers as part of the flow area for the upcomers tends to be blocked by the formation of steam. You might be able to assume the specific volume of steam given a certain liberating rate (figure 5 square feet per boiler horsepower of 34lb steam/hr/hp) at your chosen pressure/saturation temperature. Thus you have the rate of liberation, the area blocked by liberation (can be worked back into tube diameter/length) and the volume of liberation traveling in parallel in the upcomer. The rest of the water circulates in a circle around that. And the area OTHER than steam can be made equal in the downcomers.

Gosh, it's been a while since I've thought about these things. I hope I've said it plain.

Or you can "reverse engineer" from some others work which they have pictured on the Internet. One instance I've seen includes welded fitting and the hobbiest chose to do the welding with full argon shielding inside the boiler (a nice refinement to assure grape free insides - but not usually done even in professional boiler construction.)

The use of compression fittings HAS been done before. Take a look at Beckman's Boat Shop who has provided MANY boat boilers constructed along these lines. He pre-makes the tube platens on a form and uses compression fittings to make the connections to drum and headers. http://www.steamboating.net/page7.html



Joe K
Wow Mr. Beckman's boiler looks pretty awesome. I wish he had some more pictures of it on his site.

I think my best course of action is to "reverse engineer" the Roberts water tube boilers I've seen online with robust materials.



Quote:
Originally Posted by DianneB View Post
In designing a boiler the usual starting point is to determine the volume of steam required and the pressure. Knowing the volume and pressure, you can determine the required evaporative capacity of the boiler and therefore the heating surface. (There is a general capacity specified for each of various boiler types.)

As to the design and construction of a steam boiler, the ASME boiler code is by far the best guide - you can't go wrong complying with ASME! The ASME codes are very expensive but you can probably locate someone who has a copy.

For small boilers you might refer to "Model Boilers & Boiler Making" by Harris. It is a good reference work for producing small boilers that work well.
Thanks for the references!

I'm thinking I will want to build either a 4-5HP steam engine. Some of the models I've seen in that range have around a 4in piston, 6in stroke, and operate efficiently at ~300rpm and ~80-90psi.

So wouldn't my evaporation Rate be ~784.8 cubic feet per hour: (See attached image)

Thanks!
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Old 12-08-2013, 01:09:24 AM
Joe K Joe K is offline
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Default Re: Boiler Design and Materials

A rough rule of thumb for a throttling governed engine is diameter of cylinder squared divided by four equals horsepower. So for a 4" cylinder you're talking about 4 hp. Given that your engine is not "square" (i.e. stroke equal to bore) then you might expect a bit more.

Firetube boilers in this size are typically 10 square feet heating surface per boiler horsepower. My 24" shell "Ames" boiler of 1926 has about 50 square feet including the firebox - which just about matches my 4x5 Sears Roebuck steam engine.

Water tube boilers are typically rated at about 5 square foot per boiler horsepower (hence the number I cited above) I can't account for the difference except that watertube boilers generally have a closer "intermingling" of the flue gas and conduction surface.

Water tube boilers also have a higher overload capacity than fire tube - but are less forgiving of interruptions in water supply (You may have barely time to start the injector should your main mechanical feed pump fail in service.) But the nice thing about watertube is that they're "home constructable" compared to a riveted or welded firetube boiler - and watertube failures tend to be less disasterous.

Anyway, you sound like you've been doing your homework and you're off to a good start.

Joe K
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Old 12-08-2013, 03:34:05 AM
Jk29 Jk29 is offline
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Default Re: Boiler Design and Materials

Thanks again for helping out Joe!

My next question is in the attached picture to this post.
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Old 12-08-2013, 07:46:00 AM
DianneB DianneB is offline
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Default Re: Boiler Design and Materials

You are correct about the heating surface being the copper tube in the firebox but the area of the tube exposed to the heat is (2)x(pi)x(OD of tube)x(length of tube). If OD and length are in inches, divide by 144 to get heating surface in square feet.

---------- Post added at 05:40 AM ---------- Previous post was at 05:37 AM ----------

P.S. You should not have horizontal runs in water tubes or you will trap steam at the high points and burn through your tubes. Water tubes should have a marked slope to ensure steam can peculate to the top and water flow freely down to replace it. I would suggest water tube slopes should be 45 degrees or more.

---------- Post added at 05:43 AM ---------- Previous post was at 05:40 AM ----------

Since your boiler is big enough to require inspection in some jurisdictions, you might want to check on the acceptability of copper tubing and copper tube in the firebox. I have seen it done in models but not in a certified boiler and I don't remember any mention of copper in the firebox in the ASME code.

---------- Post added at 05:46 AM ---------- Previous post was at 05:43 AM ----------

(I thought we had an "EDIT" button but I don't see one! I meant "xopper tube and copper tube fittings"!)
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