New Stainless Economizer

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fredrosse
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New Stainless Economizer

Post by fredrosse » Mon May 20, 2019 5:14 pm

I am in the process of increasing steam pressure for better speed, and that will require more heat energy to produce extra steam generation. I have installed a new copper exhaust steam heat exchanger, which will bring raw feedwater from about 70F up to maybe 150F, giving about 6% more steam flow with the original heat input, propane fire.

At the surplus metals operation (Glick's, Reading PA) I have found (and purchased) a heavy wall stainless steel coil heat exchanger, 5 inch coil diameter, 1/2 inch tube diameter, 5 square feet surface area, which will make a good economizer in the stack. This can bring feedwater up to about 270F, giving yet another 10% steam generation.

My entire feedwater system is all copper and brass piping, fittings, tubing, pumps, etc. Does anyone have experience with the new stainless steel inserted between the copper feedwater system and the steel boiler? Corrosion issues?
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Re: New Stainless Economizer

Post by RGSP » Mon May 20, 2019 8:47 pm

I suspect that almost everyone with a steamboat is aware of the chloride enhanced stress corrosion cracking in Austenitic stainless steels. 304 is fairly prone to it, wherease 316 is often called a "marine" grade, whereas in fact it is only a factor of around ten better than 304.
To be honest, I think that many people (including me probably) worry too much about this: it is STRESS corrosion cracking, and if stainless items are only lightly stressed, the problem can be slow enough to happen to effectively mean never. A lot of people use cheap stainless ball valves, which can be of 316 alloy, but are often 304, and I've never heard of anyone having problems with corrosion cracking in these: other problems yes, but in non-critical parts of the steam circuit they work well, and are cheap enough to replace every few years.
What certainly seems to be true is an absence of corrosion problems associated with connecting stainless components to copper or bronze. Let's face it, few steamboats get used for very many hours per year, and we aren't in the same league for either pressure or operating hours as power stations or large steamships.
I'd still avoid using 304 in any really hot and critical parts of the system.
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Re: New Stainless Economizer

Post by DetroiTug » Tue May 21, 2019 3:56 am

Electrolysis issues with our usage periods and intervals is probably a matter of very low concern.

Might want to sit down for this one, but a buddy of mine builds economizers out of 1/2" EMT thin wall conduit and they work just fine at 400-500 psi. The only issue he's had is they tend to rust through rather quickly where they are brazed together. I think it's a bad design as it is a grid that lays in the top of the boiler, like a tube assembly of a Derr boiler on it's side. Although it works, I think it would be better if it was a single route instead of multiple paths.

Fred, on my steam system for the car, I started out with just an economizer in the top of the boiler, I added an exhaust feedwater heater and there was a marked improvement. A new car I'm assembling has only a feedwater heater and so far in testing it worked out well.

What I've noticed is the "Little and often" really holds true with the feedwater heaters, run them too long and they chill down and no longer have any affect. So short cycling of feedwater works best. Pump velocity through the heaters can help too.

-Ron
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Re: New Stainless Economizer

Post by Lopez Mike » Tue May 21, 2019 12:34 pm

What do you mean by "run them too long"? Physically too long or too much running time?
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Re: New Stainless Economizer

Post by DetroiTug » Tue May 21, 2019 4:33 pm

Mike,

Little and often. Too long is the period of time without bypassing. Eventually, unless the economizer is very long in feet (surface area), they will start to chill down. It's better to open and close the bypass at closer intervals, of course this all leads in to why automatic boiler controls are so much better, because that is what they do. And I don't mean the steampump/middle trycock level control that i have, it works but it's wasteful.

Some of the Stanley guys use 2-300 feet of economizer, they call it a 'water wall", but it's essentially an economizer. They wrap it around the circumference of the boiler shell under the insulation, I think it's just robbing Peter to pay Paul, but that is how they do it.

-Ron
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Re: New Stainless Economizer

Post by Lopez Mike » Wed May 22, 2019 1:31 am

Ah. I understand.

I use a hot well and a bypass float so I shouldn't get into much trouble. Both an exhaust feed water heater and an economizer are in my future and I didn't want to walk into some unforeseen issue.

In a few days I will be lifting the boiler and engine out of my old hull and looking for an unsuspecting fool to get the old hull and trailer out of my yard.

I had a woman down the road who wanted the hull to plant asparagus in but her husband got wind of it. Sigh. Time for some chainsaw and acetylene action I suppose.
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Re: New Stainless Economizer

Post by fredrosse » Wed May 22, 2019 8:43 pm

".....the steampump/middle trycock level control that i have, it works but it's wasteful. "

Sidetracking the original thread for a moment, I have also used the middle trycock level control, and it works very well. The trycock orifice is adjusted to let down full feed pump mass flow (liquid saturated boiler water), so any steam use results in a net loss of boiler water mass. When the water level in the boiler is high, the liquid mass flow through this fixed orifice is also high, bringing down the water level in the boiler. When the level is low, the steam mass flow thru the same fixed orifice is saturated steam, which is only about 10% of the high level flow, so the feed pump can easily keep up and restore boiler water level.

This by itself is wasteful of energy, dumping this orifice flow back to the feed tank, or to atmosphere, where it does little to none in the way of thermal economy. A way around this is to put a small counterflow heat exchanger with cool feedwater flowing toward the boiler, and the boiler orifice flow going back to the feed tank (or overboard), heating the feedwater. This recovers most of the energy in the orifice flow by heating the incoming feedwater. While not perfect efficiency, it provides a reliable boiler water level control with no moving parts or controls whatsoever, and recovers the great majority of orifice flow heat energy, also with no moving parts in the fluid system whatsoever.
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Re: New Stainless Economizer

Post by Lopez Mike » Wed May 22, 2019 8:52 pm

Does this provide a higher efficiency than a hot well float? Assuming that the the feed water pump isn't way oversized, that is.

Speaking of energy loss, I'm contemplating small bypass line with a needle valve from just before the whistle valve back to the hot well to keep the whistle feed line from accumulating condensation. I don't expect it to totally eliminate the hot water shower but one hopes.
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Re: New Stainless Economizer

Post by fredrosse » Thu May 23, 2019 3:31 pm

"Does this provide a higher efficiency than a hot well float? "

The hotwell float method of boiler water level control assumes the principle that the entire steam/water system has constant mass, and the two variable storage vessels (the boiler and the hotwell) each will have some defined water mass, according to the hotwell float valve setting, which maintains constant mass in the hotwell. Hence constant mass in the boiler liquid, hopefully at the right boiler water level. This system works well, is very energy efficient, and maintains proper level in both vessels as long as there is no external loss of steam or water. Of course, steam leakages, packings, blower use, as well as whistle use causes mass loss from the system, and the operator must keep up with replenishing these losses or else the boiler water level will go down.

The "Try cock orifice" type level control actually controls boiler water level, vs the hotwell float system, which does not actually involve any sensing of boiler water level. The hotwell float system only works with full condensing plants, and occasional operator intervention. Puffers without a condenser dump all their exhaust steam, and there is no opportunity to use the hotwell float method. The "Try cock orifice" method works with both puffers and condensers.
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Re: New Stainless Economizer

Post by TahoeSteam » Thu May 23, 2019 3:58 pm

Something to keep in mind that we have encountered is overheating the economizer when not pumping water.

We've mainly encountered it when we've had a hot fire going while waiting to leave and the engine isn't running. The economizer gets so hot that when we did start running the engine and pumping water it acted like a monotube boiler; producing a lot more steam, but not helping raise the water level at all. The solution ended up being to plumb in a circuit for additional pumps (hand, aux, etc) to pump into the economizer to help with additional cooling. When encountered, shutting off/dampening the fire and pumping tons of water into the system helped to cool it down. Minimizing time with heavy firing and no water flow helps mitigate encountering the situation in the first place.

Being on top of your game and recognizing potential conditions for this to occur, or recognizing it when it does occur can save you a lot of heartache. Nothing more embarrassing than having to drop anchor with a bunch of passengers aboard because you have low water, can't get any into the boiler, and can't have a good fire to have enough steam to run the engine to pump water 😂
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