Tube square footage

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Tube square footage

Post by jpigg55 » Tue Feb 16, 2021 7:13 pm

I've been searching for an older post concerning the "Thumb Rule" for piping/tubing square feet per desired HP with no luck. Wondered if someone could post info here or post the link to the thread ?
A couple things I don't recall being pointed out in the thread discussion:
1. Does material thermal conductivity effect this ? i.e. stainless steel tubing vs copper tubing, etc.
2. Is this based on tube ID or OD and is there a difference between fire tube vs water tube ?
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Re: Tube square footage

Post by Lopez Mike » Thu Feb 18, 2021 1:48 am

Is this for a fire tube or a water tube boiler?

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Re: Tube square footage

Post by barts » Thu Feb 18, 2021 4:09 am

jpigg55 wrote:
Tue Feb 16, 2021 7:13 pm
I've been searching for an older post concerning the "Thumb Rule" for piping/tubing square feet per desired HP with no luck. Wondered if someone could post info here or post the link to the thread ?
A couple things I don't recall being pointed out in the thread discussion:
1. Does material thermal conductivity effect this ? i.e. stainless steel tubing vs copper tubing, etc.
2. Is this based on tube ID or OD and is there a difference between fire tube vs water tube ?
1) Metal thermal conductivity really don't matter for our boilers. Both the gas and water film coefficients are far more of a hindrance for heat conduction than the metal. The faster the flow of either combustion products or water over the tubes, the better the heat transfer. You really want turbulent flow, which can be difficult to arrange in a firetube boiler w/o additional means to force circulation.
2) The difference in water circulation rates is why the rule of thumb for firetube boilers is 10 square feet per engine hp, and that for water tube boilers is 5 sq feet per hp; both these numbers assume a single expansion engine in good condition and the usual 120-200 psi steaming pressures. Adding a significant economizer can help these numbers somewhat.

The effect of fluid velocity on heat transfer is quite marked; the amazing performance of small monotubes or Lamont pumped circulation boilers is due to the very high heat transfer rates obtained with high flow rates. There's no need for all that complexity in our boats, however. Note that water tube designs such as the B&W or Worthington boilers steam well because they have rapid circulation which can become quite vigorous once a significant portion of the water in the tubes turns to steam, and the shorter tubes allow for different circulation rates if the firing is uneven. And those of us with ample firetube boilers enjoy the relaxed steaming that is possible when you know that you always have time to clear the pump checks while underway w/o running the boiler low on water. Note that circulation in vertical firetubes tends to be better than horizontal ones like Scotch boilers.

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Re: Tube square footage

Post by fredrosse » Thu Feb 18, 2021 2:31 pm

"Thumb Rule" for piping/tubing square feet per desired HP with no luck. Wondered if someone could post info here or post the link to the thread ?

ANS: The FAQ section of this forum has technical simple answers that explain all of this, sizing the engine, sizing the boiler, etc.

"A couple things I don't recall being pointed out in the thread discussion:

1. Does material thermal conductivity effect this ? i.e. stainless steel tubing vs copper tubing, etc."

ANS: The thermal conductivity of the boiler tubing has virtually nothing to do with boiler performance with respect to heat transfer. Using high thermal conductivity copper tubes, compared to steel boiler tubes, results in reduced performance with that is typically well under 1%. The copper tubes however are far lower strength, and become very low strength with elevated temperatures. The US Boiler Code (ASME) prohibits any copper alloy in boilers and their attachments above 406F (208C).

Also please note, the ASME Boiler Code absolutely prohibits stainless steel for any wetted part of a boiler. That is a serious safety issue and invites failure, potential catastrophic failure, of pressure parts of a boiler.

"2. Is this based on tube ID or OD and is there a difference between fire tube vs water tube ?"

ANS: The dominant resistance to heat transfer in a boiler is occurring between the hot combustion gasses and the boiler metal tube surface. The thermal resistance of the metal tubing is entirely trivial. The thermal resistance between the metal tubes and the boiling water is more significant, but so small as to also be virtually trivial. Since the hot gas to metal thermal resistance dominates, that surface area is what matters, and what should be considered. For firetube boilers, use the inside tube surface area. For watertube boilers, use the outside tube surface area
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Re: Tube square footage

Post by jpigg55 » Mon Feb 22, 2021 4:07 pm

My intent was for a water tube boiler, but couldn't remember what the original thread I'd read concerned. I was trying to get a "General" idea of the length of tubing that would be required.
As for stainless steel, thanks for the info concerning ASME code. The idea of using SS came from a "How it's Made" video about the Cyclone steam engine: https://www.youtube.com/watch?v=6NPpelLCIkk
I figured SS would be stronger and more corrosive resistant than copper tubing.

My idea is for a simple, small rotary vane steam engine to run a 1" semi-trash pump. The concept being for a light, portable engine & pump unit running at a stable load at fairly high RPM. The idea came from this: https://www.youtube.com/watch?v=IBdLRw34DqM
The original pump engine was a 79cc 4-stroke in the 2.5 HP range or less. The original engine is no longer functional and would cost more to repair or replace than a new one. While I did purchase a new one, I like to repurpose stuff if I can.
I know rotary vane engines are less efficient, but my hope is to offset this somewhat by making it multi-stage to increase power and efficiency.

Thanks for the "Thumb Rule" info.
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Re: Tube square footage

Post by barts » Mon Feb 22, 2021 5:08 pm

The cyclone "boiler" is a monotube steam generator - a rather different beast than a watertube boiler.

I'd stay away from rotary vane motors unless someone with a big boiler and a bigger fuel budget is supplying the steam. Note that lubrication of the vanes is important at higher rpm due to centrifugal loading, so you have oily exhaust steam to get rid of as well.

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Re: Tube square footage

Post by DetroiTug » Mon Feb 22, 2021 6:01 pm

As Bart aptly points out its simply a monotube generator. Or in the case of the Cyclone engine an individual generator coil for each cylinder.

I have no dog in the race, but I wouldn't be getting any ideas from that particular engine, it's been proven to be an inefficient inferior design bolstered by many false claims. The fact it made it to "How it's made" was a real disservice to the show and anyone watching.

Huge long story with many "twists".

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Re: Tube square footage

Post by Lopez Mike » Tue Feb 23, 2021 6:53 am

I'll get beat up for this but here goes.

There are a bunch of wonderful applications for small steam. Boats, some cars. Those sort of things.

But what hasn't worked well is trying to displace I.C. engines from applications where it isn't just esthetics but other factors. A trash pump is needed very often in an emergency. There is a flooding situation and you have to get the water out of there now. And the pump must be portable. Read, light and damage resistant. In such a situation there is little time to build a fire and sort out problems and the like. It's one of things we don't talk about here but all of us have spent an amazing amount of time sorting out glitches. Also more time than we might admit to fussing with our power plants even after we have solved most of the larger issues.

The idea of messing around with a small steam plant in an emergency is probably not a good idea. In your power range you can buy a small gasoline engine for peanuts. Sort out the adaptation problems in a jiffy, and have lots of time and money left over for hobby steam stuff.

I just looked at the Harbor Freight web site and they will sell you a 3 h.p. engine for $130. You can't buy the steel for your boiler for that much less build any sort of engine. Maybe build a flash boiler and make a steam ejector. Really get that storm water going!

O.K., I'll go put my helmet on and take my punishment.

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Re: Tube square footage

Post by fredrosse » Tue Feb 23, 2021 2:39 pm

"My idea is for a simple, small rotary vane steam engine"

Rotary type steam engines were common "pipe dreams" and many inventors had them made, throughout the later part of the 1800s, and even occasionally more recently. Some were even manufactured, however virtually all failed to meet the performance of ordinary piston steam engines. Too hard to maintain reasonable seal, short lifetime, and they need excessive lubrication, and consume excessive steam.

Even in the age of steam driven torpedoes of WWI (not needing long life, lubrication not an issue) they were rejected, in favor of reciprocating piston engines. Rotary engines are technically inefficient to drive a torpedo for a couple of minutes.
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Re: Tube square footage

Post by fredrosse » Tue Feb 23, 2021 2:46 pm

"I just looked at the Harbor Freight web site and they will sell you a 3 h.p. engine for $130."

In Pennsylvania they sell a 212cc 6 horsepower OHV Gasoline engine at $99 on sale. I have bought a few over the years, and they have always performed very well.

Small steam plants, perhaps similar to the Meryweather portables of about 100 years ago, would only be practical in a place with no hydrocarbon fuels. That was typically England many years ago, where there was plenty of coal, and very little oil based fuels.
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