Author Topic: Boiler feedwater preheating.  (Read 1400 times)

Offline Gas_mantle

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Boiler feedwater preheating.
« on: February 02, 2018, 10:06:10 PM »
I'm in the process of mounting my 5" vertical boiler to a baseboard and adding a water tank / pump etc and have considered using the engine exhaust to pre heat the boiler feedwater via some coiled copper pipe within the water tank. I want the exhaust to also vent up the boiler chimney to increase draught when running on coal but wonder if running the exhaust via the feed water tank will mean I'm am left trying to exhaust water up the chimney ?

The steam is superheated from the boiler but after leaving the water tank it will need to run upward to the chimney so I'm concerned I'll end up with too much condensate in the rising pipework.

Given the amount of water my boiler drinks I'm not sure preheating will have a great improvement in efficiency but if I can extract a bit of extra heating without a negative effect elsewhere I'd like to do it.

Anyone got any thoughts ?

Cheers
Peter  :)

Offline Ramon

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Re: Boiler feedwater preheating.
« Reply #1 on: February 02, 2018, 11:34:46 PM »
Hello Peter

I think perhaps you are answering your own question as by running the exhaust through the water tank you are, in effect, creating a condenser.

It would be better to run the feed water from the pump through an area which can heat it and preferably just before it enters the boiler - not too hot of course as you don't want to boil it before hand.

On my Wide a Wake set up I have a coil just in the end of the flu tube that is shielded from direct heat by a stainless plate, 'tis but a short route from there to the boiler clacks.

By doing it this way you would place no restriction on your exhaust in the form of condensate collecting in the pipe work.

Just my thoughts - hope they help in some way

Regards - Tug
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Offline Steamer5

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Re: Boiler feedwater preheating.
« Reply #2 on: February 03, 2018, 04:14:24 AM »
Hi Peter,
 Like Ramon says think you have answered your own question! I remember reading the construction series of the Clayton steam wagon were Mr Dryer added a feedwater heater, first outing he had a nice warm, but oily, shower! He then redesign the heater  by removing quite a large amount of the feedwater tubing.

As a thought can you add a coil of tubing, or part there of, to the top of the boiler above the tubes but inside the flue area, route the feedwater up thru the coil & out & down to the feed clack?

Cheers Kerrin
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Offline Jasonb

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Re: Boiler feedwater preheating.
« Reply #3 on: February 03, 2018, 07:42:41 AM »
I've also seen a "sleeve" around the steam pipe that the water enters one end and exits the other which would add some heat.

One thing with using the heat of the exhaust steam to heat the water is you will be removing heat from this steam so it will condense and you will therefor have less volume coming out of your blast pipe in the chimney so will reduce the draft.

Offline Zephyrin

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Re: Boiler feedwater preheating.
« Reply #4 on: February 03, 2018, 08:02:59 AM »
The best way is to use the "wasted heat" in the chimney to heat a loop of tube feeding the water to the boiler, hence this heat will no more be wasted...
as said above, don't use the steam for that except if you install a true condensing.
if the water tank is very close to the boiler (as is the case with tank or saddle locos), water becomes hot...and may hinders the use of an injector, which requires cold water !
« Last Edit: February 04, 2018, 09:13:27 AM by Zephyrin »

Offline Ramon

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Re: Boiler feedwater preheating.
« Reply #5 on: February 03, 2018, 08:25:39 AM »
I've also seen a "sleeve" around the steam pipe that the water enters one end and exits the other which would add some heat.

If you do mean the 'steam' pipe as opposed to the 'exhaust' pipe Jason, then again you wouldn't you be condensing the steam but this time before it gets to the engine  :ThumbsDown:

As Zephyrin recommends Peter - use the 'wasted' heat at the top of the boiler to do some more work. A simple coil in the chimney is all that's probably needed to do what you want.

Ramon
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Offline ChuckKey

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Re: Boiler feedwater preheating.
« Reply #6 on: February 03, 2018, 12:04:54 PM »
Exhaust steam feedwater heaters have been used as far back as Richard Trevithick's first use of 'strong steam'. His circa 1806 engine in the Science Museum, London, has one.

http://collection.sciencemuseum.org.uk/objects/co51033/trevithicks-high-pressure-steam-engine-and-boiler-c-1806-engines-steam-engines-high-pressure-engines

Yes, it does not want too much surface area, and I suspect you will be surprised how fast it heats the water, unless you are already familiar with the speed with which a Windermere kettle makes the tea.

If you want to use an injector, it won't like hot water.   

Offline Gas_mantle

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Re: Boiler feedwater preheating.
« Reply #7 on: February 03, 2018, 04:10:47 PM »
Many thanks for the comments guys :-)

I guess like as others have said, I had a feeling my idea could be problematic and you have all confirmed that so I'll bin that idea :-(

Although I haven't yet measured how much water my boiler uses, it appears to well over a gallon per hour so realistically I don't suppose a small heating coil would achieve a lot anyway, the best way forward seems to be to just route the exhaust up the chimney as I had originally planned.

The idea of having the water feed pipe circulate through the smoke box area above the fire tubes is something I have considered but there isn't a lot of room so isn't really practical  :(