Average heat input

[Wearyman]

I am continuing to research steam engines and boilers as I work out my plans for my own steam launch, and one of the things I am wondering is just what is the average expected operating temperature of a boiler that powers a 2-4 HP steam engine?

I know that the steam itself can run into very high temperatures, and depending on the construction and fuel used in a boiler can reach temperatures as high as 1500 degrees F. (maybe even more?) But that is at the extreme high range. I am currently planning on an all-copper tube watertube boiler with no superheater. Since copper's melting point is just over 1000 F, I know that I cannot reach those extreme temperatures. Indeed, temperatures pushing 900 F would be ill advised as the copper will soften and lose strength at those temperatures. Of course, water boils at 212 F (at sea level) so I shouldn't ever need to even get close to the failing point of my boiler.

So what I am hoping to find out is what temperatures should I be trying to reach during normal operations of my boiler? 400F? 500F? Higher? What are you all running at?

[stevey_frac]

I am continuing to research steam engines and boilers as I work out my plans for my own steam launch, and one of the things I am wondering is just what is the average expected operating temperature of a boiler that powers a 2-4 HP steam engine?

I know that the steam itself can run into very high temperatures, and depending on the construction and fuel used in a boiler can reach temperatures as high as 1500 degrees F. (maybe even more?) But that is at the extreme high range. I am currently planning on an all-copper tube watertube boiler with no superheater. Since copper's melting point is just over 1000 F, I know that I cannot reach those extreme temperatures. Indeed, temperatures pushing 900 F would be ill advised as the copper will soften and lose strength at those temperatures. Of course, water boils at 212 F (at sea level) so I shouldn't ever need to even get close to the failing point of my boiler.

So what I am hoping to find out is what temperatures should I be trying to reach during normal operations of my boiler? 400F? 500F? Higher? What are you all running at?

I don't own a steamboat, so take what I say with a grain of salt.

Copper pressure ratings drop quickly with temperature, so be careful. at 100F a 1" schedule K copper pipe is rated for a working pressure of 655 psi. At 400F it's half that at 327 PSI. If your boiler get hotter then you expect for any reason, you don't have a lot of reserve. Keep it full of water, and you should be ok, I think.

[Edward]

Dear wearyman ,

For the conditions you describe , non superheated saturated steam , there is a precise and unbreakable relationship between pressure and temperature . So the temperature in your boiler and of the steam it produces will depend on the pressure . ( if you don't maintain the boiler properly a build up of scale or oil can inhibit the heat transfer from the boiler/tube wall to the water leading to local overheating , but this is outside the scope of this posting. )

By Googling " Steam Tables " you should be able to find out all the information you need , as a generality you are probably not going to excede 420 degrees F 200 degrees C , This equates roughly to 200 psi .

It is probably worth buying a set of steam tables , they're not expensive and most will cover much more than the temperature/pressure range in which you're interested .

Regards Edward .

[Narwhale]

Wearyman,
edited out
Rich S.

[johngriffiths]

Hi Wearyman,

The SBA did a very good paper on the use of copper for making boilers. Now only available in cd format it can be obtained from Geoff Newton, cost £5. One member of my model engineering club de-rated the design for his proposed water tube boiler from 105psi to 77psi after reading it and doing more calcs. The problem is that poorly conducting scale on the water side results in a substantial temperature rise.

All the best with your project

John

[fredrosse]

The American Society of Mechanical Engineers (ASME) produces the boiler code rules used in many countries. Their boiler code prohibits use of copper or copper alloys at any temperature exceeding 406F. This temperature corresponds to saturated steam at 250 PSIG, about twice the pressures used in many steam launches.

[Wearyman]

If I am understanding you guys correctly, I can use copper as long as I am not exceeding pressures of around 200psi at temperatures around 400F. The question is, is that 400F STEAM temp or 400F heat source temp? (I would assume steam, but you all know what happens when you assume.) If heat source rather than steam, then the heat input requirements are much lower than I originally suspected. (My backyard propane grill makes 500F in under 5 minutes!) if steam, then I would have to calculate the heat input needed to see if it is safe.

Am I also correct that I would need to braze rather than solder the tubes for extra strength and heat resistance?

[artemis]

If I am understanding you guys correctly, I can use copper as long as I am not exceeding pressures of around 200psi at temperatures around 400F. The question is, is that 400F STEAM temp or 400F heat source temp? (I would assume steam, but you all know what happens when you assume.) If heat source rather than steam, then the heat input requirements are much lower than I originally suspected. (My backyard propane grill makes 500F in under 5 minutes!) if steam, then I would have to calculate the heat input needed to see if it is safe.

Am I also correct that I would need to braze rather than solder the tubes for extra strength and heat resistance?

You should keep the temperature under 406 deg. F and the pressure under 250psi gauge The temperature is that of the water-turning-into-steam and therefore as long as the steam under pressure IN the boiler does not exceed 250psi gauge, the temperature INSIDE the boiler will not exceed 406F. Steam present IN the boiler will not exceed the temperature of the water IN the boiler (that's why it's called saturated). Heat transfer of copper is such that there is approximately 6 deg. F temperature drop between the outside of the boiler - fire space - and the inside of the boiler - water space. Although you may have a much hotter fire - as much as 2500F - the surface temperature of the fire side of the boiler will be no more than 6 deg. F hotter than the steam/water inside the boiler. Period. Accept it. Don't make life difficult.

You can braze or silver solder.

[preaton]

The difficulty occurs in the steam space. Although by definition ,saturated steam at 200psi will be about 200 deg C. In some boiler designs there is the potential to superheat the steam. Once the steam is dry the temperature of the steam is related to how much heat is applied and independent of the pressure. This is where copper becomes a problem. Most boiler designs that use copper tubes get around this problem by making sure that the tubes are "drowned" ie below the water level of the boiler. Then the above situation cannot arise.

[Maltelec]

Ideally your boiler should be designed so any steam parts are out of the way of the direct heat. E.g. if you have a horizontal drum at the top, the lowest water point should be 1/2 way down the drum, thus the hot gasses which are not directed on to the top 1/2 of the drum can't overheat it.

In reality you find that you can quite safely run the water a bit lower than that, mainly because copper is such a good conductor of heat.

As for the super heater, I would personally never use anything except steel. The super heater has the potential to become very very hot, especially if you have a large fire while not running the engine. steel, while being a much poorer conductor of heat, has as much better heat tolerance.

So basically in my opinion, if there is water next to the copper it shouldn't be a problem. Where there is steam next to the copper it should be out of the way of direct hot gas flow, and anything which is deliberately in the hot gas flow with no water should be steel.