Water preheat?

kmh5147

So this is one of those random ideas that pops into my head...... I'm sure someone has thought about this, but here it goes.  Would there be any gain on having a ultra high efficient tank-less heater preheat your water short minutes before the boiling cycle starts (basically delay cycle for preheat)?  First few negatives I can think of is the need for high gpm and the loss you would have anyway heating the cast of the boiler....... Anyway, random idea....... I'll throw it out for everyone to tear apart.



Thanks!

MTC

my understanding

is that those tankless heaters are actually far LESS efficient at heating water than a good condensing type tank. the efficiency comes in not having storage losses when nobody is using hot water, such as overnight or while at work. I think this system would be a net loss. besides that, hitting cold cast iron in a boiler with very hot water could shock fracture it, just like hitting very hot cast with cold water.



If you were to do something like this, I would think that a like 97% efficient condensing water tank would be the better bet... dump your condensate into it and add a boiler feed pump to it to return hot water to boiler.



I also think that the cost of such a setup, and its maintenance, would eliminate any benefits, unless maybe on a very large system.



I'm not really a pro, just a building manager/contractor that works with all these things, so take all this with a grain of salt. Just my initial impressions...

MTC

further similar thought...

I have always thought that heavy insulation on a condensation tank would be a smart way to return condensate hotter to the system, but I don't seem to see that. Why don't we have like R-16 foam insulated tanks? Or do they have them and I just don't seem to run into them? We insulate our return lines sometimes for this purpose, why not the tank?

Pumpguy

I assume

you're asking about pre-heating condensate for boiler feedwater?



Keep in mind that most of the heat the boiler's burner puts out is not used to raise water temperature, but to change water to steam.  It's called change of state and is referred to as latent heat. 



One (1) BTU will raise one pint (pound) of water 1 degree F.  It takes 960 BTU to change that same pound of water from 212 F. water to 212 F steam. 

ChrisJ

exhaust heat

Wouldn't it be more beneficial to superheat the steam using the exhaust heat?  This would at least pull some of that wasted heat back into the home.

Jean-David Beyer

Wouldn't it be more beneficial to superheat the steam using the exhaust heat?

A lot is known about superheaters and their use with steam locomotives. Some of that may well carry over to lower pressure systems, but I am not sure they are as important in home heating as they are in operating reciprocating (or turbine) engines, where condensation is a serious problem. It might be that you could get less condensation in the steam mains if the steam is superheated, but if you heated the steam enough, it would not even condense much in the radiators. It would still heat, but you might not get the benefit of the phase change between the steam and water. If planning to superheat, the entire design of the entire system would be desirable. Just adding it on to an existing system might be interesting, but it might never repay the cost in materials and trouble of doing it.



http://www.sdrm.org/faqs/boilers/page65.html

Hap_Hazzard

Exhaust Heat

Exhaust heat is already being used to make the steam. Even in an atmospheric boiler, the exhaust gases have transferred most of their heat to the steam before they reach the flue collector. They're in no condition to superheat steam at that point. What you'd need is a kind of countercurrent exchange system where the hottest gases are in contact with the hottest water/steam, so there is always a high thermal gradient favoring the transfer of heat. This would require some kind of downdraft burner, which is apparently too impractical to build or someone surely would have come up with one by now. The closest thing I've seen is in some of the big four-pass horizontal fire-tube industrial boilers, but they're not even superheating steam; in fact, most of them are just hot water boilers.

ChrisJ

exhaust

Hap,



I recall seeing stack temperatures for atmospheric boilers being in the 500-600F range. Sure sounds like they could superheat 212-215F steam to me.

SWEI

maximum efficiency

when the exhaust temp is closest to the steam temp.  You'd be starting to condense on a low pressure system, but it would be interesting to see what could be gotten.

Hap_Hazzard

Look at it this way

How much surface exchange area would it take to bring the two materials closer to the same temperature? If you have steam at 212°F and hot gases at ~550°F, and both are rising, and assume for the sake of simplicity that the steam gains one degree for every degree the exhaust gases give up, then for every degree you transfer, the gradient decreases by two. (Yes, I know you don't transfer degrees. Degrees are a measure of temperature, not heat, but I'm trying to avoid having to talk about transferring enough heat energy to increase/decrease the temperature by one degree.) Given enough time and area they could reach an equilibrium of 381°F, but even if you doubled the heat exchange area ina steam boiler you wouldn't nearly double the efficiency due to the ever-shrinking thermal gradient.

Hap_Hazzard

If you think about it for a minute,

there's a very good reason why you can't make a condensing steam boiler. ;-)

SWEI

flue temperatures

Should be quite a bit lower than that on a properly tuned three pass.

ChrisJ

three pass

Besides my neighbors 80+ year old three pass boiler the only residential one I know of now is a megasteam. What are the expected flue temps on a megasteam?

SWEI

Mega-Steam burns oil

at least officially.  Enreynolds reported 344F with NG on his.



http://www.na-stordy.com/seminarCD/pdfs/hbs39.pdf covers the non-condensing territory.



Given the almost nonexistent approach mod/cons demonstrate, it should be possible to create 240F steam with a 245F flue temp.

Hap_Hazzard

Trouble is,

Once water turns to steam it becomes very difficult to heat it further because it has about 35 times less thermal conductivity than water.

SWEI

heating it further

implies higher pressure.



So 1 PSI (215.6F) steam should be able to be produced with a flue temp somewhere around 220-225F.  Might not prove to be cost-effective, of course.

Hap_Hazzard

Now imagine the hot gases and water/steam are going in opposite directions.

The easiest way to do this would be with a downward flow of combustion gases from a power burner. The hottest gases would exchange her with already heated steam rising though the system, then as it continues downward, it meets increasingly cooler water. If the heat exchange is really efficient, the exhaust gases would condense. That's how countercurrent exchange works.



Countercurrent exchange mechanisms are very common in biological systems. It's surprising how rare they are in engineering.

Jamie Hall

Well, they really aren't that rare

in larger systems.  But they do take some thought.



I'm not sure about superheating; as Jean-David noted it's beneficial in steam turbine applications (which really don't like wet steam!) and also reciprocating engines -- most late steam locomotiive designs used it -- but the comment that it would actually be slightly counterproductive on space heating, where the benefit is from the phase change, has merit.



Where one could recover heat from the flue gas, though, with some benefit, would be by using a preheater heat exchanger on the condensate return (which is also done in some power steam applications).  But even there I'm not at all sure that the additional complications (it would certainly involve a heat exchanger -- which would need maintenance -- as it seems to me that it would also almost certainly involve at least one pump, with associated controls.  Depending on the head loss on the gas side of the heat exchanger, it might also require an induced draught fan) would be worth the effort -- since one of the beauties of steam heat is that it is so simple!

vaporvac

Insulate returns?

I've often wondered if there might be some advantage in insulating the returns. I think you just answered my question...Is there a downside? I would think insulating a condensate tank couldn't hurt either. They're both quite cheap things to do.



Regarding condensing steam boiler, as mentioned later in this post...I thought I read they did exist, but not in the US. Maybe this was just for high pressure systems and not for home use.

SWEI

uninsulated returns

are often what keeps the basement habitable.  I'd proceed carefully.

MTC

Insulating returns

does have a small benefit to the efficiency of the system. As others I think on this thread have pointed out, the vast majority of the energy use is in the state transfer from water to steam, not in heating condensate up to 212/215 whatever based on your pressure. But there would be some benefit, particularly in very cold weather when the system is running a lot (less standby losses in the returns).



The main reason to insulate the returns though is that warmer condensate returning to your boiler contains less damaging acids, and prolongs the life of the equipment. I don't understand all the hows and whys, I just trust those who do understand it and have told me so.



If you do have a "habitable" basement, then you will notice quite a difference when you insulate the returns. It will be much colder, but for most basement uses, I don't find this to be a problem. I insulate the basement ceiling in general and treat the basement as unconditioned space. But if you do want heat down there, the returns are the place to get it, not the steam mains. You'll pay a lot of money running your system hard to get heat that way. You're better off adding a hot water zone or a more complex system using the boiler pressure to lift condensate from rads up through a one way valve back to the returns if necessary. Leaving the mains uninsulated greatly increases fuel consumption as much of your steam condenses before it can get to the true heat sources (the radiators).

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jumper

good points

You can put a recuperator on the boiler stack and try to use high temperature fluid to dry the steam. Sort of superheat junior ? Hopefully more btu per pound of steam improves circulation. If you have enough head room you can also try a flash tank on return. The flashed steam goes to a terminal that condenses steam at a lower temperature than main radiators. It's not to lower temperature of return water but to lower pressure in return. Again the hope is to improve circulation. We consume fuel to circulate the steam so any improvement saves fuel. It's probably not worth the trouble for buildings under a few million btu per hour.

Larry_52

Superheated bad heat transfer

In strict heat transfer superheated steam is not a good method. Superheated steam is a dry gas and more of an insulator. Trying to get the sensible heat transfer from superheated steam is a slow process due to its insulating quality. Saturated steam rejects its latent heat energy in phase change and wallops you with a large quantity of heat energy. Superheated steam in heat transfer also creates thermal differential issues in coils and radiators. The reasons turbines and piston engines work better with it are because it is dry and does not erode blades and valves but most important the energy value versus saturated. It is all about the transfer of energy.