# Questions about making stainless steel boiler

I am currently working on a project (or actually returning to a project) and need to have a heat exchanger boiler made. The hx boiler will be relatively small, a 10" long cylinder around 4" in diameter. One end can be capped, but the other should have a flange so I can work inside the unit. Inside, I will have a coil of copper tubing (heat exchange) and various other sensors/ports. I don't have access to a full machine shop to fabricate such a boiler myself so I am contacting a number of companies. My question really is in regards to the thickness of the steel required for the boiler. The boiler, at max pressure, will only be at 2.5 bar. Additionally, the material must be food-safe; the main boiler will contain water that will be used in food preparation. Given these working parameters, of what type and how thick should the stainless steel be? I will attach various rough sketches of the system for extra information. Thanks in advance for the help!

• Not a direct answer, just some source material to look into: ASME has multiple heat exchanger designs and specifications that you should follow when designing and building such a vessel. Another publication that could be interesting is McCabe's unit Operations of Chemical Engineering, 2004, McGraw Hill. The book covers the design of shell and tube heat exchangers, among other things. Lastly, do not put notes on top of a technical drawing, it is bad practice.
– BOB
Mar 28 '16 at 14:53
• As for type of stainless steel, what fluid will be inside the boiler? Don't forget that depending on material there can be a galvanic couple between the copper and steel, and corrosion can occur without proper isolation. Mar 28 '16 at 15:11
• Thank you both for your comments! Bob, I will check out those materials, thanks! Starrise, oops, I forgot to say (will edit op). The boiler will contain water that will be used to steam milk (potable). Mar 28 '16 at 15:14
• Does milk pass through or come in contact with the copper heat exchanger? If so you may consider changing the material out for a food-safe stainless steel. Copper may pose a health hazard and I'm not sure copper alloys are considered food safe. Copper can leach in acidic media, and milk has pH between 6.5-6.7, making it very slightly acidic. You might consider also contacting the FDA (or whatever regulatory body is equivalent) for more information on what is and isn't safe. Mar 28 '16 at 18:08
• Milk does not actually pass through anything. Only water. Mar 29 '16 at 15:27

1. Based upon your local jurisdiction, they may require a permit, which requires a PE Stamped calculation for the work you are requesting, since the internal pressure exceeds 1 bar (the limit into which it goes into ASME BPVC Section VIII). See for example in California - anything which generates pressures over 15 psig requires a permit.

2. As a PE who does design these vessels for a living, usually I use a pretty simple set of formulas to get a good estimate. Treat the top, shell, and bottom as separate entities

• The shell can be treated using cylinder stress rules. Typically use 20,000 psi as the maximum allowed stress (= 36,000 psi yield with 1.8 Factor of Safety)

• The bottom (and for estimation purposes, the top) can be treated using a simple supported plate. Maximum deflection should be limited to 0.5% and the stress should be 20,000 psi. Don't forget that those openings create stress concentrators - which in itself is a big subject in the pressure vessel design world. For estimation, you can take the principle of replacing the material removed - that is double the thickness around the hole for an area equal to one hole radius outwards. From a cross section then, the material is the same.

• You may find that other bottom head designs fit better, especially for reducing stress. Ellipsoidal heads are almost always the same thickness as the shell. Hemispherical heads follow simple cylinder stress rules and are half that thickness. Torispherical heads are usually double, and flat heads are typically four - eight times as thick. That's why fire extinguishers are usually rounded bottoms on a skirt.

• The joints between the bottom and shell, as well as the joint between the top and shell, need to be doubled due to the bending boundary layer. This layer exists for $4\sqrt{R*t}$ from the actual joint. The way to avoid this is to double the pressure vessel thickness from each of the above numbers for a uniform vessel thickness. Again, you probably have to get a permit, and the PE who stamps the design will need to review these area especially

3. For material selection I would recommend 304 Stainless steel - it is very common, easy to afford, and is considered biosafe per ASTM F899

• Consider adding something about selecting the materials for food-safe applications. I'm not an expert, but this article seems to indicate that AISI 316, 304, and 430 are preferred for food-safe applications, in order of decreasing corrosion resistance. The OP may wish to consult with someone whose expertise centers around food-safe materials. Mar 28 '16 at 18:03
• The material question wasn't there when I answered. I'll edit. I'm more familiar with mining and manufacturing chemicals, and can confirm the corrosion resistance of those stainless - I didn't know they were foodsafe, other than 316L is preferred for welding applications, and is primarily used for surgical equipment.
– Mark
Mar 28 '16 at 18:09

In addition to the excellent answers on pressure vessels already posed a steam boiler has the additional complication that, as you are dealing with a change in phase under pressure there is a particular hazard with steam that you can get very rapid pressure spikes under certain conditions so the safety regulations for steam boilers are rather more stringent than for pressure vessels in general. For example interruption of the flow of water may cause a rapid pressure rise which may not be immediately apparent see steam explosion

You will also find that your jurisdiction requires specific welding codes for welding of steam pressure vessels and/or individual testing and inspection.