Hybrid Boiler Design

Hello, I've been pondering designs for boilers for a while, and while having a hard time coming up with a design that is both easy to build and suitable for my needs, I came up with a couple ideas that I was curious if anyone had some input on. Weighing the differences between firetube and watertube boilers, there are distinct advantages and disadvantages to each.

While firetube boilers possess a large steam reservoir, they are hard to build safe and efficient. While water tubes/monotubes are much easier to build safely and much more efficient, they are complicated in that its best to have an automated feed pump. They also cannot store a large amount of steam.

I would like to build a steam car that is fueled by wood or other solid fuels, and due to the dynamic nature of the steam usage a water tube boiler wouldn't have the storage capacity to compensate for the lack of instant altering of the wood burning rate. While it is possible to control a wood fire to a certain extent by altering the amount of forced air feeding the fire, It would be best to have a boiler to have a large reserve capacity to smooth out the relatively constant output of the wood fire versus the dynamic nature of the steam requirements. Due to the fact that I would be using wood and a boiler with a large capacity, I accept that fact that my cold start time will be significant, and I am willing to ignore that aspect for the time bieng.

I have come up with a couple designs that I believe would be easy and practical to build yet fulfill my requirements. I have drawn a picture with 2 variants of the same basic design. The design is to have a large cylinder to fulfill the reserve capacity requirements. this cylinder will be easy to coat with a significant amount of insulation to keep heat from escaping. The cylinder itself will not be heated, but like a watertube boiler, it will have tubes going into the firebox area which will be heated to boil the water.

This will allow the boiler to be easily constructed since it can use a prefabricated pressure vessel because it does not have to be perforated for the insertion of firetubes, thus maintaining structural integrity. Instead only a small number of holes will be drilled in the sides/ends.

The 2 designs vary slightly in that design 1 uses a large coil of soft copper tubing in a way similar to a monotube boiler. Design 2 uses several larger pipes that hold and heat the water similar to a more traditional water tube boiler.

I was curious about any ideas, suggestions or input on this design idea. I also have a few questions about further design considerations. First of all, with either design, would it be preferable to have the water line somewhere below the top of the tubes, such that the upper parts of the tubes would be above the water line and contain steam; or would I be able to have the water line completely above the tubes and have the tubes completely filled with water? Second, with design 1, would natural action of the water be sufficient to keep the monotube supplied with water, or would there have to be forced circulation? I am trying to keep this as simple and easy to construct safely as possible.

Have you considered heating with a gasifier, they are much more responsive then an open flame fire.
They were used extensively to run gasoline engines during WWII in Europe.

[www.gengas.nu]

edge.



Edited 1 time(s). Last edit at 07/19/2010 08:04PM by edge.

Hi I have extensive experience with solid fuel boilers, I understand your concerns relative to storage capacity, if you can get your burner opimized by using wood or wood chunks, you will achieve a steady state rather easily with a primative burner.

Quote
Zimirken
Second, with design 1, would natural action of the water be sufficient to keep the monotube supplied with water, or would there have to be forced circulation?

No, the convective (natural) circulation will be insufficient, you must use forced circulation. Your storage capacity comes from the (hopefully insulated) unfired pressure vessel [in that case]. Its a good idea to look into code for that section of the boiler, :ie wall thickness per pressure capacity and volume and you need to figure flow needed, for the safety relief valve used (cv factor).

If you go with the monotube heatexchanger arrangement, you may be able to scroll ( progressively increase thickness of the tube dia towards the direction of the burner source) the tubing, at any rate use schedule 80 to 160 not hardware store copper water pipe. If I use soft copper tube its always over sch 80. This works out well if you have to braze something to it such as a stay or fins. Remember its not possible to force water flow "down" or towards the burner using natural convection. However in some cases the discharge of the monotube must be above the water level, to prevent sparging.

Use 8 gallon's per minuite (flow) or greater (for a single pass monotube) for a 75 ft section of tubing, regardless of diameter.

Your going to have to figure what pressure your going to run, Im thinking a max of 250psi for such an arrangement. Also very important, the safety valve exhaust must be ducted to a safe place.

Jeremy

-edit- you dont have to use a positive displacement pump that's needed with a "pure monotube design" with what your describing, you could use a "LaMont style" circulation pump, instead of being a positive displacment type, like a pressure washer pump, its of a centrifugal design like a pool pump, with such pumps you looking to provide flow in gpm not pressure.-



Edited 1 time(s). Last edit at 07/19/2010 09:27PM by Jeremy Holmes.

Judging by your response I've decided that it would be easier to use the second design instead of a monotube-like configuration. And yes, I talked about adding extensive insulation to the main tank. with a large diameter water tube design, would it be better to have the water line above or below the upper return on the water tubes? i.e. would it be better to have the top of the tubes contain steam or would they be able to be full of water?

Hybrid Boiler Design?
Take hybrid out of the equation.
I won’t tell you how to build your boiler or what type to build, what I will tell you are some design facts that should sway the direction you go in your design.
There are basically three types of designs you could go with.
1. Natural circulation boilers
2. Forced circulation boilers.
3. Monotube boilers.
With solid fuel firing you would probably want to stick with the natural circulation type. Although Herreshoff coil boiler was not a Monotube but a forced circulation solid fuel fired boiler long before Lamont was born. It would require a constant circulation pump under boiler pressure.

Natural circulation type basically falls in two types, the fire tube and the water tube.

The fire tube is the very least efficient of any of the boiler designs. Yes I have built them and use them in my Stanley. It goes with the car.

The water tube can be very efficient depending on its design.
A tube 90 degrees to the flow of gasses is four times more efficient then a tube in the same direction as the gases.
Ninety percent of the work (heat transfer into the water) takes place around the combustion chamber. This is the largest area of differential between the cold (water side) and the heat of combustion.
Short tubes are better then long tubes. No tube should be horizontal; they should be at least seven degrees from horizontal to facilitate national circulation.
Tubes should be arranged to cause turbulent flow to the gasses. (Combustion gases after the combustion chamber).

One of the most efficient boilers ever built of the national circulation type was the Herreshoff three drum. 300 PSI and built with no welding.

In that case, would you have any suggestions as to the configuration of the pipes in the firebox? Perhaps staggered arrays of horizontal pipes perhaps an inch or so in diameter?

In this case Im going to have to yeild to Rolly's extensive experience, he has posted some photos of natural circulation boilers for his stanley that he has built, that (I feel) would work great for a solid fuel burner. Since the last board upgrade ive lost track of where these pictures are. Both types will work forced/natural circulation. When I use my burner to produce live steam, I use forced circulation, this is mainly a personal preference, since ive burned tubes out before. Also steel tubing will work aswell, instead of the copper.

Jeremy

I was thinking something along the lines of this, most likely made from steel pipe threaded/soldered/brazed/welded. Although browsing the images of the 3 drum boilers that general configuration might be workable provided some way could be determined to increase the capacity somewhat. Would it be plausible to simply increase the size of the lower drums or possibly the upper drum to increase the reserve capacity? On another note, where would one source the construction materials, i.e would it be reasonable to use discarded compressed welding gas cylinders or something similar for the drums or is there a relatively inexpensive source of prefabricated pressure vessels? Or is it common for one to solder/braze/weld their own cylinders? Fabrication labor/time is no object except when a job must be outsourced out from the home tinkerer.

On an unrelated note, I'm curious how you would register a steam car for road legal status? I tired searching for the answer but I could not seem to find it if there is a particular thread that explains this.

Boilers are fired pressure vessel, I have posted data sheets of all the types of steel approved by the ASME code.
Do a search of this site for boiler related questions.
Pipe and tubing are mostly A-178 boiler tube or A-106 pipe. A-106 comes off the shelf up to 48 inches D.
Plate used should be SA-516 grade 60 or 70-T.
I am attaching a PDF drawing of some of the weld approved weld details.

Hello all

Zimirken's quest to generate steam has promoted me to revisit the tunnel configuration. This type generator is a 6 foot long tube about 9 inches in diameter. It is mounted beneath the vehicle so as not to consume premium space. In lieu of pancake coils connected in series, it has multiple helix flow paths that spiral from the outer surface to an inner header. Combustion gases flow in opposite directions creating a counter flow arrangement. Ram air is ducted through the condenser, relieving blower power. Not only are multiple flow path generators more efficient, they also exhibit significant reduction in pump power. I am trying to put some construction details together. Has anyone else considered this approach to steam generation?

jerry

jandp Wrote:
-------------------------------------------------------
> Hello all
>
> Zimirken's quest to generate steam has promoted me
> to revisit the tunnel configuration. This type
> generator is a 6 foot long tube about 9 inches in
> diameter. It is mounted beneath the vehicle so as
> not to consume premium space. In lieu of pancake
> coils connected in series, it has multiple helix
> flow paths that spiral from the outer surface to
> an inner header. Combustion gases flow in
> opposite directions creating a counter flow
> arrangement. Ram air is ducted through the
> condenser, relieving blower power. Not only are
> multiple flow path generators more efficient, they
> also exhibit significant reduction in pump power.
> I am trying to put some construction details
> together. Has anyone else considered this
> approach to steam generation?
>
> jerry


Would you happen to have a picture or diagram describing this? I'm having a hard time visualizing it.

Zimirkin

No photos yet. I am making a cardboard/wood cross sectional model to illustrate how the headers would be integrated with the tube banks. It has 8 paths in a series arrangement as opposed to parallel paths. Four headers are required: one to two, two to four: four to eight, eight to a phase separator. A single path from the phase separator comprises the superheater. Combustion gases enter the center of the generator tube and spiral outward through 3.5 turns. No insulation is necessary since combustion gases cool to managable level on the last turn. However, a heat shield does protect the last turn from environmental conditions. Suraface area depends upon the length of the generator core. A 30 inch core length yields about 45 square feet. This type generator probably exceeds your performance requirement. It is intended for modern production applications. Also, it is more difficult to make and the geometry is a nightmare. Never the less, I do intend to document details for construction during the pending cold witner months.

Good luck with your own steam generation project.

jerry

All files from this thread

File Name	File Size		Posted by	Date
boiler.jpg	26.9 KB	open | download	zimirken	07/19/2010	Read message
boiler.jpg	28.6 KB	open | download	zimirken	07/20/2010	Read message
Welding code\'s.pdf	7 KB	open | download	Rolly	07/20/2010	Read message