Author Topic: Small boiler - feedwater heating.  (Read 1402 times)

Offline Gas_mantle

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Small boiler - feedwater heating.
« on: November 21, 2017, 04:05:33 PM »

I'm in the process of setting up my 5" vertical boiler on it's own base with the feedwater tank, pump etc permanently coupled. I'm intending to route the engine exhaust up the chimney but have considered using the exhaust to heat the feed water first.

I'm planning on a feedwater tank of perhaps about 1ltr but the boiler can be thirsty so I'm wondering if trying to preheat the water while needing to top up is going to be a waste of time. Realistically will a coil preheater raise the feedwater temp enough to make any performance difference before running the tank dry ?

If I were to go ahead would doing this condense the steam to the point of getting too much water entering the chimney ?

Thanks :-)

Offline MJM460

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Re: Small boiler - feedwater heating.
« Reply #1 on: November 22, 2017, 12:31:59 AM »
Hi Gas Mantle, great progress and a lot of fun with that boiler, running it on coal already.

Feedwater heating is a great topic for Talking Thermodynamics.  You will find some relevant information in the posts on 27/7/17.  As you are ready to build something, don't worry about the order of you have questions, what you want to do know now is more important than general knowledge for future use.  I can get back to that later.  Or I can make some suggestions here or if you prefer it.

You will need a separator to not produce hot, oily rain in an annoying amount, again more information in Talking Thermodynamics posts on 8 July 17 for some ideas.  Have a look at those, and you might be able to answer your own questions and just proceed to describe what you build and how it operates, that is the only real test of theory.  I find a separator is important even without the extra condensation in the feedwater heater.  If my outline has been too brief, don't be shy about asking questions, others will be in the same position and will almost certainly appreciate your asking.  I can post my sketches if the pictures do not give enough information.  No fancy Fusion drawings, but pencil sketches are quite in keeping with the steam era.

Second, you can heat the Feedwater tank, easiest path as it does not involve a pressure vessel for the water.  So long as you do not get the water so hot that it vapourises in the feed pump suction.  If your burner is a bit borderline, it will help, and you can quantify this with the steam tables.  Again, just ask if my outline was too brief when you get down to using it.  But your boiler got to pressure/ temperature pretty quickly, so probably not important to preheat the water if you are starting with an appropriate level in the boiler with enough separation space above the water level.  So the pump may be the first priority.

Hope that helps.  Looking forward to your progress.

The more I learn, the more I find that I still have to learn!