High Efficiency Steam Boilers

Brad White

There is an inherent conflict between condensing boilers (92% plus in residential level equipment) and a final heating medium to your radiators that is above the dewpoint of the flue gasses as Terry pointed out.

85-87% is about as good as I can imagine getting.

Charlie Wolfe

High Efficiency Steam Boilers

I cannot find a gas fired steam boiler with an AFUE above 92%. Are there any? If not is there some technical obstacle that precludes this?

ALH_4

steam

And if I may expand on this...

Is it somewhat an issue of heat exchanger size relative to burner size? I do not know about temperatures over 200°F, but condensing boilers can be 97% efficient at low modulation with a supply temperature of 167°F. Could a down-firing burner create steam at the top and condense at the bottom of the heat exchanger? Water level would be critical.

I have a feeling the biggest "technical" obstacle is the cost.

ttekushan_3

Self contained vs discrete

A high efficiency hot water boiler allows low temperature operation, and therefore its easy to have return temps that are low enough to allow condensation of flue gasses and the heat energy therein.

Steam is always above boiling and therefore precludes low temp operation, requiring a different strategy for boosting economy. The industrial steam sector does this all the time, even with boiler pressures and temps far beyond what a heating system will ever see. These measures include separate stack economizers which recover flue gas heat, of which there is a class of condensing types. These are separate items which are never reflected in boiler efficiency ratings alone. Its difficult to condense flue gasses when you're running 750 psig at 750 degrees F, but less so when dealing with saturated steam at 215.

A condensing stack economizer first heating a boiler return water preheat resevoir and then preheating combustion air would do the trick. But still it'd be a discrete item that wouldn't be reflected in boiler efficiency alone.

The cost of such things usually gives no return on investment. But realistically, a similar arguement can be made on condensing boilers in general, at least for the time being. Greater economies of scale and increasing fuel prices could tip the balance.

In steam, the Burnham MegaSteam is a three pass design of very nice efficiency. Total heating costs with such a unit are competitive with mod/com HW heating systems.

Now imagine the megasteam with an economizer...

Terry

Steamhead (in transit)

It's being done already

Hoval (in the U.K.) and Gasmaster (in Canada) make and sell 90+% steamers in larger sizes. I don't think they use "add-on" devices- they sell the whole thing as an engineered package. Why American boiler makers don't do this is beyond me- the economies of scale would keep the extra cost down. Maybe they're just lazy.

The Burnham Mega-Steam is by far the best residential steam boiler out there now. Burnham deserves credit for bringing it to market, but it's only the first of several needed steps.

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McKern

No ROI on condensing boilers?

"The cost of such things usually gives no return on investment. But realistically, a similar arguement can be made on condensing boilers in general, at least for the time being."

Is this a proven fact or your personal opinion?

J.C.A._3

Hey Frank....

Are vaccuum systems worth taking a "new look" at?

I worked on quite a few in my younger days, and to see water boiling at 180° mystified me...for a while, but then I saw the light.

Are there any new materials that will help make vaccuum valves hold negative pressures longer than the old stuff? Better yet, are there any techs. that could believe and/or conceive of such a system?It wouldn't be "condensing" ...but could go a LONG way to making steam a better option for the long run.

Maybe I'm thinking way too far outside the box...but I really did see it work before! Chris

ttekushan_3

Beyond arguement?

you're point's well taken. Compared to atmospheric boilers, the ROI on paper isn't there especially based on a unit's life expectancy, but in practice it usually is. Eff numbers on atmospheric installations usually don't meet their ratings. Sometimes not even close. So installation technique and installer knowledge will shift the numbers one way or the other. Condensing/sealed combustion is the opposite, since eff ratings are met or surpassed in most installations.

Combustion control and special attention to thermal transfer efficiency is the first priority. Like the Burnham MegaSteam. Its that final heat extraction in its latent form that sometimes costs today more than it will pay back in residential installations "for the time being." Increasing advanced boiler production, rising fuel costs, etc should change the balance. And future government mandates could make ROI discussions moot.

Perhaps its time for stricter mandates, so some of this great boiler technology will become mainstream. As Steamhead points out, the technology is out there in current production but for some reason our choices are limited.

So "the arguement can be made" is just that. Specific situations call for specific solutions.

Oh. Almost forgot. Snarky response to your "just your opinion" remark: I didn't realize that there are sacred heating cows on this site that are not open to rational discussion unless duly footnoted with annotated bibliography.

Uni R_3

No topic/idea should be sacred...

You said it quite factually and I wondered what was the basis of your claim. I was just curious, no sacred cows here either.

Steamhead (in transit)

Two of our Wallies are experimenting with this

Boilerpro and Long Beach Ed, using exhausters that work in a way similar to those used on the Paul and Moline systems. We know that if you can fill a steam system with steam at ounces rather than pounds, the steam is not compressed and the boiler doesn't have to generate as much steam. This reduces fuel consumption. Pulling a vacuum on the system takes this further.

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Perry_3

And why stop at 180F

Why not build a modulating variable vacuum system such that the steam temperature ranges between say 100 F and 180 F depending on outdoor temperature. That would allow the boiler to usually - if not always - operate in condensing mode.

There really is not much difficulty in building pipe systems that are leak tight. The radiators would not have air vents (that would be an air leak); and the makup water would probably have to be deairiated prior to adding to the system.

The theory is there; I know the equipment is there for larger systems.

Perry

Steamhead (in transit)

That's been done too

it was called the Dunham Vari-Vac system. Many installations of this system have been completely knuckleheaded, but when it worked it was great. Can't remember how deep a vacuum it could pull though- my Dunham stuff is at the shop.

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ttekushan_3

A thousand pardons UniR

I didn't have my second cup of coffee at the time I posted. Weekdays are different, what with half a pot dispatched before walking out the door.

My apologies.

BTW, I recall a thread perhaps within the last year here that suggested that at Major Manufacturer's training seminars the point was made that ROI on the difference in cost between more traditional boilers and condensing boilers (both of their making) is quite long, perhaps as I recall, 10 to 15 years. Thats assuming the "traditional" isn't an oversize slapped in place! And ohhh how often that occurs.

That leaves out a lot of the competition, though. Others, for example, Triangle Tube, might have an attractive ROI. Viessmann too, due to its build and longevity. And this very idea of competing mfrs should push the prevalence of "traditional" designs out of the mainsteam. Frankly, I think the cost differences will diminish to the point that ROI is virtually irrelevant and we can get down to the business of conservation.

Now its steam's turn. Really, techniques for elimination of fuel waste in steam world is over 100 years old. It was forgotten soon after intermittent firing of gas and oil boilers came along. After WWII's building boom and cheap energy prices energy efficiency was lost and forgotten for steam, HW, air, everything.

The advent of the more recent HW boiler tech has upped the ante for steam, and so it should. We've been there, and as Frank said, why not now? Ironically, electronics and sensors (even simple ones) make variable load matching, etc so much easier than it was then.

-Terry

ttekushan_3

Deairation

I never thought about that for low pressure and vacuum steam. Deairation's critical in high pressure applications, but it may be great for low pressure also.

Assuming we're actively modulating the atmospheric pressure in a subatmospheric steam system, how about this:

Why not just "can" the exhausted air? Like shuttling the air through the pump between a compressed air tank and the heating system. A small amount of water treatment (with oxygen scavenger) in the boiler water and the limiting of fresh atmospheric air admitted would give us a pretty tight system. Blowdown frequency is reduced as well.

Thoughts?

-Terry