Pre-heating boiler water

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: Harold_V, WJH, cbrew

James Powell
Posts: 396
Joined: Fri Oct 28, 2011 10:42 pm

Re: Pre-heating boiler water

Post by James Powell » Sat Apr 10, 2021 8:16 pm

1/8" copper tube offers a lot of resistance to flow even in a 3/4" model...you can do the calculations for velocity of the water and the resultant losses, but they will be substantial for a 1" or 1.5" model.

I would go with something like 1/4" copper tube (about 4x the area, or 1/4th the velocity & pumping losses).

We have fitted steam:feedwater feed heaters before- on a Stuart #9 with the 504 boiler. It was very marginal with the spirit burner without the feed heater, and a lot more comfortable with it. I would guess that as posted upthread, on a marginal boiler the added energy that you can recover with a feed heater is probably worth it. The same thing is true on full size, where a gain of a couple of .1% of n spread across all the locos could exceed the fuel costs difference. Here at work in a heating plant, we have preheaters running exhaust: water.

On the ship, running with extraction on the turbine (typically at the cross under pipe, so HP extraction not LP), the difference was about 4% _plant_ n vs using boiler steam for preheating (285F/33-38 PSI). At 10 000 hp, that was a substantial amount of fuel, and far more if we were at 21 000 shp and getting it off the LP turbine (about 5% n, from ~28-> 33% n overall)

Remember the above is on turbines running extracting steam into a pressurized hotwell, which is NOT what we would have on a railway engine.

James

Berkman
Posts: 304
Joined: Wed Aug 16, 2017 7:55 pm

Re: Pre-heating boiler water

Post by Berkman » Sat Apr 10, 2021 8:50 pm

Every oil fired 7.5 gauge loco I've seen has the exhaust steam plumbed up the exhaust nozzle just like the real ones. Nothing wrong with the change in draft, just design the front end arrangement with oil firing in mind.

1/8 is way too small.

Emfinger
Posts: 177
Joined: Tue Oct 01, 2019 3:30 pm
Location: Yucca Valley, CA

Re: Pre-heating boiler water

Post by Emfinger » Sun Apr 11, 2021 11:18 am

The axel pump that I have has two check valves with an 1/8" diameter seats. That is why I picked 1/8" (ID) tubing.

Between the firing valve, my blower control valve and the atomizer flow control (pressure regulated) I can go from just burning to a roaring blaze and anywhere between. I don't need or want the exhaust to be part of the burner control. If I were burning rocks I would want more draft when the throttle is opened.

User avatar
Bill Shields
Posts: 6478
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Pre-heating boiler water

Post by Bill Shields » Sun Apr 11, 2021 4:28 pm

1/8 orifice is very different than 1/8 pipe.

Theoretically it will work but will create a lot of back pressure and the water rushing through will not get as warm as if it were passing through a larger pipe at lower velocity.
Too many things going on to bother listing them.

Emfinger
Posts: 177
Joined: Tue Oct 01, 2019 3:30 pm
Location: Yucca Valley, CA

Re: Pre-heating boiler water

Post by Emfinger » Sun Apr 11, 2021 7:01 pm

Bill Shields wrote:
Sun Apr 11, 2021 4:28 pm
1/8 orifice is very different than 1/8 pipe.

Theoretically it will work but will create a lot of back pressure and the water rushing through will not get as warm as if it were passing through a larger pipe at lower velocity.
This tubing is 1/8" ID just like the check valve

User avatar
Bill Shields
Posts: 6478
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Pre-heating boiler water

Post by Bill Shields » Sun Apr 11, 2021 7:11 pm

You will do better with larger tubing...but if that is all you have..
Too many things going on to bother listing them.

Berkman
Posts: 304
Joined: Wed Aug 16, 2017 7:55 pm

Re: Pre-heating boiler water

Post by Berkman » Mon Apr 12, 2021 9:57 pm

No doubt cost is a major problem, but wouldn't something like monel be a decent solution.

AnthonyDuarte
Posts: 361
Joined: Tue Jan 19, 2010 10:46 am
Location: Orange, CA
Contact:

Re: Pre-heating boiler water

Post by AnthonyDuarte » Mon Apr 12, 2021 11:30 pm

Having one or two points with 1/8” ID is ok, but you shouldn’t size your tubing based of the internal bores of your check valve. Restriction increases with the length of the tube in addition to the diameter. Think about how easy it is to suck air through a straw. Make that straw 3 feet long and see how easy it is... (spoiler, it’s not).

Probably want 1/4” as a minimum.

User avatar
Bill Shields
Posts: 6478
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Pre-heating boiler water

Post by Bill Shields » Tue Apr 13, 2021 4:45 am

Monell is not the best for thermal conductivity. It is slightly better than stainless.....but nowhere near as good as copper.

If you ate going to do it for the purpose of heating water then use some good basic thermodynamic principles..just make it easy to remove and repair and stay out OJ f the way for routine maintenance
Too many things going on to bother listing them.

Emfinger
Posts: 177
Joined: Tue Oct 01, 2019 3:30 pm
Location: Yucca Valley, CA

Re: Pre-heating boiler water

Post by Emfinger » Thu Apr 22, 2021 10:09 am

Got it guys, going with 1/4" copper.
Thanks to all.
Tom

Post Reply