DIY Tesla Impulse Turbine

You can increase the temperature limit of your copper boiler by increasing the wall thickness of the copper. Recall Barlow's formula gives maximum pressure inside a tube as:

P = 2*T*S/D

Where S is the allowable stress of the material (copper), which we know becomes smaller as the copper is heated, which results in lowering the maximum allowable pressure, P. However, we can increase P by increasing T, the thickness of the copper.

From the chart below we can see that copper looses about 1/2 it's strength at about 900 F. But if we can compensate for the resulting lower "P" by doubling the wall thickness "T"


So, if you want to use super heated steam at temperatures well above 230 C (447 F) you need only increase the thickness of the copper you're using to make your boiler.

Assuming you're not using a monotube design for your boiler, another solution to increase allowable pressure would be to use a different shape for the boiler. The formula to find max allowable pressure for a sphere is:

P= 4*T*S/D

Using the same material (copper) and the same wall thickness, a spherical pressure vessel will hold twice the pressure of a cylindrical vessel.

Toymaker said:

NOTE: The 231 C number I sighted as a temperature limit for copper is the number I'm using for my specific monotube boiler and is based on what I personally feel is safe for my copper monotube boiler. Other design factors such as wall thickness, boiler diameter, brazing metal used will all play a part in determining a safe working temperature. I arrived at the 231 C (448 F) number in part from calculations I've done and in part from the experience of other monotube builders. Steam car builders whom have posted their experiences on the SACA (Steam Automobile Club of America) forum web pages have demonstrated through multiple builds that copper tube boilers are safe to operate at 440 F at 400 psi.

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Do not ignore the fact that wall temperatures subjected to heat transfer can be anywhere from 50 to 70 degrees higher then steam temperature.

HMEL said:

Do not ignore the fact that wall temperatures subjected to heat transfer can be anywhere from 50 to 70 degrees higher then steam temperature.

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As shown in the "Strength of Metals" graph shown in post #361, a 70 F increase will result in less than a 10% decrease in the strength of copper.

Can you post the source of your information as I would find it useful to know what materials and wall thickness are applicable.

Look up "The Engineering toolbox" on the web. The credit is on the graph. (I have used it - and numerical interpretations also).
Simply: a piece of pipe/tube at Room temperature, rated for 6 bar (88psi), is only rated at about 5 bar for steam, owing to the temperature degradation of the tensile stress of copper.
BUT if the SUPERHEATER passes through a fire box where it can get to 1000F before the water boils and steam flows through the superheater to extract the heat and cool the copper, then it is only good for about 30psi (2 bar) pressure.
That's what I base my design upon. And because the superheater is silver soldered (at a cooler point) to the boiler body (on my small boilers), it becomes an integral part of the system being tested and certified. I.E. the reason "WHY" my boiler inspector will not increase the pressure allowance to certify my boilers. If I use the bolted connection (on other larger boilers) the superheater becomes a system component added after the boiler, therefore not a part considered for the boiler certificate.
It is as much "Regulation politics" as it is real Engineering. Most recommendations for boilers - that use steam at pressures higher than 2 bar - are to use stainless steel superheaters.
Notwithstanding, using ASME regulations for the design for boilers ensures all components are "managed" to sensible wall thicknesses, joints are correct to Engineering standards, and it is very straightforward when presenting calculations to a boiler inspector for certification. Some people "bitch" about these Regs being "inappropriate" for model boilers, but in my world, Steam is steam is steam whether for a model or full size. (likewise stress, material properties, etc.).
I hope this clarifies my reasoning?
K2