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Warning!!! The following articles are NOT plans for building a model boiler. There are no drawings or engineered specifications posted on these pages for a reason. These articles only serve as a record of my experience in building a vertical boiler. Please DO NOT try to use these articles as a guide to build you own boiler. They simply aren’t written for that purpose. Thank you...
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Posted on March 28, 2011
Hello again folks!!! I have been guilty of taking a few months off to relax this year but now I am ready to get back to building :0) I have chosen to start out my year with my boiler project that will be used to power my 6CI Steam Engine. Ever since I built the engine I have wanted a boiler big enough to power the large 6CI engine and possibly do some useful work. Unfortunately I have found very little info on the web about a boiler of this size which leads me to believe that there are not too many of them out there. I plan to build a vertical boiler with the aid of a book called “Model Boilers & Boilermaking” by K.N. Harris (eighth impression - 1980). I will use the book as a guide to understanding the means and methods for building a “safe” boiler and save myself a lot of time avoiding trial and error methods. More important than that I want be to be sure that I am following an accepted guideline for building a safe boiler!!!
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Now it should go without saying that operating any pressure vessel has a certain amount of risk involved. This especially pertains to steam boilers where the pressurized contents is scalding hot!!! Just like my work with experimental turbines, this stuff has to be treated with the utmost respect or you will get hurt (or killed)!!! I plan to do my best to follow an accepted method for calculating my boiler specifications and not stray for safety sake. There are numerous laws and codes (vary from state to state) that govern the construction and use of boilers in general. This is to prevent any accidents that may happen from a poorly designed or built boiler. In California I believe that the Department of Industrial Relations has code articles posted that cover model boilers. However, most model boilers (hobby use only) that are smaller than 16” inside shell diameter, have less than 5 cubic feet gross volume, operate at less than 100 PSI and have less than 20 square feet of heating surface are exempt from these codes. It is important to know where you stand on these codes and local laws before building and operating a boiler in your state. This is especially true when demonstrating a model boiler in a public venue. I don’t mean to be an alarmist but I cannot emphasize enough on the fact that IF YOU DON’T KNOW WHAT YOU ARE DOING YOU SHOULD NOT BE BUILDING OR OPERATING ANY TYPE OF STEAM BOILER!!!!!!! Now I probably don’t have to explain the failure modes of a poorly built or maintained boiler to a mechanically apt individual. Although I think it is worth mentioning to the curious that you can “Google” the term “Boiler Explosion” and see some examples of full scale failures. Luckily there are very few examples of model boilers hurting people. The model engineering community as a whole is careful to educate those seriously interested in the hobby and this is why (in my opinion) there aren’t more reportable accidents to stain the hobby’s reputation. No matter how you want to look at it it is better to err on the side of caution when working with model boilers so please know what you are doing before before getting involved in the hobby. Luckily I have the book “Model Boilers & Boilermaking” by K.N. Harris (seen below) to reference which will make my job of designing the boiler much easier. Although the Harris book is somewhat dated in it’s methods of construction it still is an invaluable source of information to the model engineer. Note: The Harris book is out of print but there are sources that offer reprints of the book (Camdenmin.Co.Uk). You can also download a PDF version of the book for free at Ebookbrowse.
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Now that I have all of that serious stuff out way I would like to get started with my project!!! My goal for this build is to build a boiler that can supply enough steam to run my 6CI engine at about 160 RPM under a moderate load. This means that I will have to build a fairly large boiler compared to the majority of model boilers made today. The 6CI engine has a 1.5” diameter piston with a stoke of 2.5” which can really use up a great deal of steam volume in a short period of time. To calculate my steam usage I will need to know how much steam will be needed per minute as well as at what pressure. I know that the 6CI engine has a cylinder volume of 4.42 cubic inches or “cu in” (per stroke). This means that the engine will use up about 8.84 cu in per every revolution. This is helpful info but I will need to know what steam pressure will be needed at that volume. To find my pressure value I hooked up an air pressure regulator to my engine (from my shop air) and used different pressure settings to see if it would drive the engine while hooked to the load of my generator. Surprisingly enough it took very little pressure to run the engine at 160 RPM. In fact it took less than 20 PSI to run the generator (with the throttle valve wide open). However if I tried to regulate the RPM with the engine’s needle valve I was forced to increase the air pressure to above 50 PSI. This has lead me to the decision that I want my boiler to operate at around 80 PSI but be metered to around 20 PSI at the engine for better throttle control. Higher boiler pressure will also increase the overall boiler efficiency. However I must calculate my steam usage at the lower pressure as this will determine the actual water evaporation rate needed to run the engine. As it stands the 6CI will need about 1,414 cu in of steam (8.84 cu in X 160 RPM) at 20 PSI to run the generator at full power. This means that the boiler will need to evaporate about 1.95 cu in of water per minute (726 cu in of steam @ 20 PSI per 1 cu in of water evaporated: 1,414/726 = 1.95). Note: calculations based on the “Properties Of Saturated Steam” chart on page 181 of the K.N. Harris book). According to the “Model Boilers & Boilermaking” book (of which I will refer to as the “Harris Book” from now on) a model boilers power is measured by it’s ability to evaporate a certain amount of water with a given size heating surface in a fixed period of time. Most model boilers are capable of evaporating a minimum of 1 cu in of water per 100 sq in of heating surface or per minute. Some boiler designs will exceed this figure but for my calculations I will figure it on the previous values. Now you might ask what the term Heating Surface or “HS” is. The heating surface is the area of the boiler that the water is in contact with that is heated by the fire or the hot combustion gasses created in the firebox. In my case the HS needed to boil 1.95 cu in of water per minute is around 195 square inches. However I don’t want to limit myself to this volume of steam as I may want to use the engine’s full power potential later on to drive other machines. For this reason I am going to effectively double my boilers evaporative capacity design to 400 sq in of heating surface so I won’t be wishing I built a bigger boiler later on. Now this brings me to the type of boiler I plan to build. Since I wish to make the boiler as compact as I can I plan to build a vertical boiler with a “fire tube” design. This type of boiler has a high evaporative capacity for it’s size and will be perfect for mounting next to the engine much like the “steam donkey” engine which was once used for logging.
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I am planning on using copper for my boiler building material as it is widely considered the best material to make a model boiler out of. Copper is extremely resistant to corrosion and can be annealed and formed with ease unlike mild steel. Although copper has less tensile strength over steel it will last for decades of hard use and can easily last for a lifetime (if built correctly). I hope to build this boiler strong enough so that my grand kids kids will get to see it work :0) WARNING!!! THE CALCULATIONS AND VALUES POSTED IN THIS ARTICLE ARE NOT TO BE USED AS A GUIDE IN BUILDING A STEAM BOILER!!! The specifications in this article are only to be used as a reference to this article and do not translate to any other boiler project. I must insist that anyone that wishes to build a boiler do their own research and use a properly written textbook like the Harris Book to design a safe boiler. To make the boiler shell (outer cylinder of the pressure vessel) I am using a piece of solid drawn (seamless) type “L” copper tubing (seen below). This tubing is 6.125” in OD and has a wall thickness of .140”. According to the Harris book (page 31) it is satisfactory to have a seamless boiler shell at 5.845” ID x 0.094” wall operating at a working pressure of 100 PSI. This is calculated by multiplying the the working pressure (P) by the internal diameter of the shell in inches (D). You then divide this value by two times the derated tensile strength of the material being used (t). T is equal to the thickness of the boiler shell in inches:
(P X D) ÷ (2t)= T
The normal tensile strength of copper is around 25,000 pounds per square inch which in this case is derated by the factor of 8 times for a safety margin (3,125 X 8 = 25,000 PSI). This means that the boiler can handle 8 times the stress that would be applied to it under normal operating conditions. Knowing these values I can then plug them into the equation:
(100 x 5.845) ÷ (2 X 3125) = T ................... 584.5 ÷ 6250 = 0.094” thick
The boiler shell I am going to use would effectively handle a working pressure of 150 PSI. In fact, the manufacturers listed burst pressure of this type of tubing is at around 2,690 PSI !!!! I will have no trouble trusting this tubing at 80 PSI. Of course copper starts to lose it’s strength at elevated temperatures so it is important to keep it within it’s operating temperature.
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For the end plates of the boiler I am using 1/8” (.125”) thick pure copper plate (seen above) which will be thick enough to withstand the 8 times safety factor I spoke of earlier. This copper plate will be formed and then riveted into the ends of the boiler shell for maximum strength. I will then be adding a set of boiler tubes to the end plates which will create the majority of the boiler’s heating surface (seen below). These tubes will be made from 1/2” type “L” copper tubing as well (wall thickness of 0.040”). Note: the fire tubes do not need to be as thick as the boiler shell as they are subjected to a compressive state as opposed to being in a expansive state. Since I am planning to expand the ends of the tube or “swage” them into place I will be using 0.040” tubing (type “L”). This heavier walled tubing will withstand the tubing expander without cracking.
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To get started I will need to size the boiler shell for my particular application. In this case I want the boiler to have around 400+ sq in of heating surface. To figure out how long I will need the boiler shell to be I must first figure out how many “fire tubes” will be installed into the end plates. After consulting the Harris book on tube size and spacing I came up with 31 fire tubes which will be spaced about 0.205” apart (0.830” center to center). This will just about fill up the boiler shell with fire tubes which will maximize the heating surface area. Since my boiler will contain about 2/3 water I will need to calculate the tubes contact with the water at around 66%. In addition to the fire tubes I will have the bottom end plate’s surface area to calculate for total HS. The OD of the fire tubes is 0.625” which will be the surface I will calculate for HS area. Each tube will have 1.96 sq in of HS per linear inch which will require about 204 linear inches of tubes in contact with the water. If I divide 204 with 31 tubes I get 6.58” of tubing in contact with the water. Since this measurement is 66% of the total tube length I can multiply it by 100/66 X (1.52) and get 10” on the nose !!! So that will be the “internal” length of my boiler shell with added length for the riveted flange and mounting holes (total of 12”). The added HS of the end plate (26.82 sq in - fire tubes @ 9.51 sq in total area = 19.31 sq in HS) will just be added to the 400 sq in HS for around 420 sq in HS total.
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Now that I know how long my boiler shell will be I can cut it on my bandsaw to a length of 12”. To assist me in making a straight cut I am using a vinyl strip to wrap around the tubing which when wrapped tightly will produce a straight line to cut on.....
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Now that I have a straight line to cut on I can mark the vinyl edge and move to the bandsaw.
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Now I think that it is worth mentioning that copper can be a tricky metal to cut or thread as it has the tendency to stick to the bit and clog the teeth. I have found that a good cutting oil can help prevent bits from sticking or clogging......
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In this case I decided to cut the tube “dry” as I don’t have a chip pan to catch the oil. It worked fine without jamming up my saw blade this time :0)
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I now have my boiler shell cut to 12” long. I even have some spare material for a second boiler if I want.....
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I cleaned up the ends of the shell with a file and squared them up as best I could. I then used the flat surface of my workbench in conjunction with a large metal square to check the vertical shells 90° angle to the bench (not shown). By rotating the shell tube and checking it with the square in various spots I was able to identify “high” spots on the ends and shave them down for perfectly square ends.
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The next step was to cut out my 1/8” copper end plates. I had ordered a 12” X 12” piece of ultra-conductive copper alloy 101 which is an oxygen-free electronic copper. This stuff is 99.99% pure copper and would be ideal for welding if I was planning to join the end plates that way. Since I am planning to rivet the end plates into the boiler shell I will need to make provisions for a “cupped” edge on the plates that will allow me a surface to drill and rivet to the boiler shell. To form the end plates I will need to make a former “plug” out of hardwood that will allow me to hammer the end plates into the “cup” shape I need. This will require extra material on the outer edge of the plate to fold over and make the cupped edge. To calculate the “overhang” I must first calculate the plug size. In this case the shells ID is 5.850” of which 2 X 1/8” will be burned up by the thickness of the plate. This leaves me 5.850 - (2 X 0.125) = 5.6” as the size of the plug former. I want to add 5/8” of an overhand to the 5.6” diameter disc which is 6.85” overall diameter so I proceeded to scribe out the two discs on the plate as seen below....
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Using my band saw I cut out the two discs allowing room for final shaping on the belt sander.
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After the discs were trimmed up on the sander they were scribed to reflect the plug center as well as the layout for the central circle of flue tubes. This will help out later on when I go to lay out the drill pattern for the tubes...
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I now was able to drill a 3/16” center hole in the plates that will help center the plates on the former plug.
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I pulled out some 3/4” thick mahogany to make my former plug and backing block out of. I penciled up two circle cut outs on the wood for the plug and the backer as seen below. The plug is 5.6” in diameter and the backing block is smaller at about 5.1”....
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The smell of cutting hardwood is pretty cool :0)
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Since the plug former needs to be perfectly round I decided to turn it on my lathe to ensure my end plates will fit the first try.
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I mounted the plug to a faceplate that will attach to the lathe spindle.
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Turning the block was easily done this way and my 5.6” diameter was cut perfectly.
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I added a 1/8” quarter round edge to the plug so the copper sheet can wrap around the edge leaving a small radius to help promote strength.
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I drilled out the center of the backer block to accommodate a mounting screw which runs through the hole on the copper plate. This will keep the assembly together while it is out of the vise as well as center the plate on the plug every time it is placed back into the former (after each annealing).
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I will now have to anneal the plates to relieve them of their temper. This will make them soft enough to hammer into shape around the former plug.
Please join me again for the continuation of the vertical boiler project!!!! Till then stay creative my friends!!!!!
Don R. Giandomenico
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