DIY Tesla Impulse Turbine

[Toymaker]

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.

 

[HMEL]

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.

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

 

[Toymaker]

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

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.