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 Post subject: Elements for Square Foot calculation in Watertube Boiler
PostPosted: Fri Jan 20, 2012 4:07 pm 
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Full Steam Ahead
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All,

I need some help. I have managed to confuse myself. (not much of a challenge really.......)

I know that in computing the square footage of a firetube boiler that you use the outside surface of the tubes since that is what the water is in contact with. In the case of a water tube boiler I assume that I use the inside surface area of the tubes .......... Is that correct ?

Also, I assume that I include the total available area in the watertubes and the mud drums, but how do I handle the downcomers and steam drum ? I am planning on running at a 50% fill on the steam drum. Does that mean that I only include half of the surface area ?

I look forward to your help.

jon


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Fri Jan 20, 2012 5:07 pm 
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Joined: Wed Dec 07, 2011 6:41 am
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Location: Lopez Island, Washington State, USA
Boat Name: S.L. Folly
I'm not an expert in water tube units but two things come to mind,

A: Run your numbers both ways on the tubes and see how much they differ.

B: If memory serves me right (Hah!) steam absorbs about half as much heat as water for a given area. You might try including the upper half of the steam drum at half of its area.

My experience it also that this is far from an exact science. A calculated variation of ten percent or even more has little real world usefulness. There is a heck of a difference in how much flame impingement there is on the various elements of any heat exchanger.

I have heard it said that decent circulation is as important as area in a fire tube boiler.

Let us know what you come up with.

Mike

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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Fri Jan 20, 2012 9:51 pm 
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The surface area of a boiler is relevant to the surface that most resists heat flow, which in the case of our boilers this is the fireside area.

The ASME Code directs that in the case of a VFT (firetube type boiler) this is the inside surface of the tubes. Inside diameter x Length x Number of tubes is the total tube surface area for a firetube boiler. Tubesheet area plus water leg areas are added to get the total heat transfer surface area. When sizing the relief valve, the ASME Code has much larger steam generation rate counted for boiler surface receiving radiant heat directly from the fire.

For a watertube boiler, it is the outside tube surface which is counted as boiler heating surface. Downcomers and mud drums are to be counted if they are receiving heat from the fire. In general the mud drum and steam space of the upper steam drum should not get direct fire heat, although many installations are arranged this way.

Boiling water has a very high heat transfer coefficient (5000) so the metal giving heat to boiling water is held almost exactly at boiling water temperature. Flowing liquid water is next, with heat transfer coefficients in the hundreds, followed by gasses (such as the hot flue gas from your fire) which has a heat transfer coefficient of around 10. For a superheater tube, the tube metal temperature is roughly half way between the hot flue gas temperature and the superheated steam temperature.


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Fri Jan 20, 2012 10:45 pm 
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Somehow I just knew Fred would have this all at his fingertips. I got fooled by the numbers for specific heat of steam v.s. water being by the pound rather than the volume. My bad.

My experience of full sized locomotive boilers matches Fred's words on radiant v.s convective heat transfer. You can help the convective transfer efficiency by using turbulators or what ever they are called in the flues in a fire tube but the flues are still more of a less wasteful way of getting rid of the combustion products than a critical part of the design. That's why I am always harping on the need for a water leg/mud drum on these units.

On horizontal fire tube boilers (read: locomotive or scotch) it really shows then it's time to clean. The hard deposits are almost all in the first ten percent of the length of the flues nearest the fire box. Where the gasses are still hot enough to do any good.

Oops! This was supposed to be all about water tube boilers. Off at right angles again.

Mike

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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sat Jan 21, 2012 5:07 am 
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On this same subject: I have heard several times that 1 square foot of water tube is equal to as much as 10 square foot of fire tube surface. Is this pretty accurate?


-Ron


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sat Jan 21, 2012 7:13 am 
DetroiTug wrote:
On this same subject: I have heard several times that 1 square foot of water tube is equal to as much as 10 square foot of fire tube surface. Is this pretty accurate?


-Ron


I believe that all depends on the design of the watertube and the rate of circulation. Monotubes and Lamonts have very high circulation rates and can do the work of firetube (and many watertube) boilers many times their size.


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sat Jan 21, 2012 3:20 pm 
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Location: Lopez Island, Washington State, USA
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I agree. Especially about it depending on the design. But also how hard you want to force them.

There aren't many large boilers that evaporated more water for their size than some of the last railroad locomotives but they had to be use extremely high rates of induced draft and an amazing amount of maintenance. The draft was so strong on the 4-8-4's that I have personally seen the entire bed of coal, six feet wide, almost twenty feet long, and several inches thick, suspended and pulsating on the air being sucked up through the grates! And by the time they got done replacing parts of the boiler one bit at a time, everything but the shell was new after ten years. It was considered normal to have a work-to-maintenance ratio about the same as a jet fighter. Cheap fuel, cheap labor and draconian size limits.

Our mostly natural draft boilers, that we expect to last essentially forever, live protected and quiet lives.

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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sat Jan 21, 2012 7:45 pm 
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Joined: Fri Nov 20, 2009 5:34 am
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The difference between watertube boilers and firetube boilers is closer to 2:1, certainly not 10:1!

Some boring history for those who are looking outside at snow, with no place to go on their steamboat, as I am:

A "Boiler Horsepower" is clearly defined as 33,475 British Thermal Units per hour energy delivered to the steam/water. This is equivalent to generating about 30 pounds per hour of steam at 70 PSIG.

A common early 1900's rule of thumb standard (USA) was 10 square feet heating surface per boiler horsepower, generally applied to the "Rating" of all boilers. When watertube boilers became popular about that time, they typically could be capable of being fired to higher outputs than industrial firetube boilers, but they were still "Rated" at 10 square feet surface area per boiler horsepower. Literature of that period would often state that watertube boilers were capable of exceeding their "Rating", and would typically state that a watertube boiler was capable of "170% of Rating, 200% of Rating, etc." Shortly after WWII, the small boiler manufacturers changed the standard to 5 square feet of heating surface per boiler horsepower, generally because more modern firing equipment allowed greater output per square foot surface area. This is the current "pseudo standard" in the USA, but clearly there are wide variations among boiler manufacturers. This was in normal industrial practice, and almost any boiler can be forced with a heavy fire to much higher output.

Perhaps the best perspective as to steam generating capacity of various heat absorbing surfaces in power boilers is the current ASME Boiler and Pressure Vessel Code. When determining the required safety valve capacity, the code requires consideration of the following rules:

Steam Generating Capacity in Pounds Per Hour (PPH) based on 1 square foot (non furnace) heating surface.

Hand Fired Solid Fuel Boilers – Firetube 5 PPH, Watertube, 6 PPH
Stoker Fired Boilers Firetube 7 PPH, Watertube, 8 PPH
Oil, Gas, PC Fired Boilers Firetube 8 PPH, Watertube, 10 PPH

Waterwall / Furnace Surface, receiving radiant heat from fire.

Hand Fired Solid Fuel Boilers – Firetube 8 PPH, Watertube, 8 PPH
Stoker Fired Boilers Firetube 10 PPH, Watertube, 12 PPH
Oil, Gas, PC Fired Boilers Firetube 14 PPH, Watertube, 16 PPH

From these numbers you can see that radiant heat absorbing surface carries much more evaporation potential than ordinary tube surface that does not “see” radiant heat from the fire. Note that one could easily build a firetube boiler with a big furnace that would out-perform a typical watertube boiler that has allot of ordinary (non-radiant) tube surface.


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sun Jan 22, 2012 1:12 am 
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Hi All,

thanks for the replies, they are very helpful. I have a design that I feel comfortable with, I am just not sure it will be cost effective to have it welded up. It will involve having a machine shop cut ~ 212 holes in various lengths of schedule 80 pipe and then having about 236 welds done.............

My materials cost should be around $1,500 or so. It is all extra heavy schedule 80 pipe and the enclosure etc will be made out of 1/8th inch steel. I have no idea what it will cost to have a machine shop do all of the drilling and then have it all welded.

Does anyone have any general opinions as to what I might be looking at for cost ? I am wondering whether I should just go with one of Lloyd Beckman's units. They are a known entity. The only issue is the cost. I would be looking at $6500 for HWT S48.

jon


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 Post subject: Re: Elements for Square Foot calculation in Watertube Boile
PostPosted: Sun Jan 22, 2012 1:59 am 
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Joined: Thu Feb 24, 2011 2:50 am
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I don't have any experience with Beckman's boilers, but I do have a lot of experience with thinking that I will save money on steam projects by building something myself or re-purposing some piece of equipment. What I have found is that unless you have both time and excellent tooling at your disposal, the custom made route is often more expensive, and the home-made route can set you back years. And from your description of your boiler design, I would hazard a guess that you'll spend at least 6 grand having it made.

Three years ago I thought I was saving money by buying a slightly used ASME code boiler and re-building it for solid fuel firing. By the time I was done I had a code boiler that only cost $2,500, but was too heavy, too tall, and too low output to be satisfactory for my boat. Now I'm building (and paying for) another boiler, and I still don't have a working boat! The lesson I take from this is that unless you have the tools, skills, and time to do it yourself efficiently, you will likely be better off (and will certainly be steaming sooner) if you buy components that work off the shelf.

Cheers,
Scott


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