Hot water boiler preheat steam boiler

Bestway_Dave

I would like to replace a 3million btu steam boiler with a 2.5 million btu condensing hot water boiler to heat the 80 deg F condensate return water, then use a 500k steam boiler to bump the hot water to steam. My thought is that I could gain the high efficiency and technology of the condensing boiler and then achieve the required steam to finish things off. Will this work? Has anyone done this? Or am I just missing something?

ttekushan_3

what you are missing

is the latent heat of evaporation.   It takes about 970 times the heat energy to turn a unit of water into a unit of steam.  The same unit of steam now takes roughly 1700 times the volume of space of the unit of water.



Your 500K boiler will be the new size of your heat source.  You will get less than 500K into the space.  Your 2.5 million hot water boiler will run about 1 minute an hour.



This will not work.



What will work is to keep the 3 million btu boiler and utilize a stack economizer that will preheat the condensate with waste heat.  They come in non condensing and condensing models depending on you situation.



Here's an article-   http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/steam3_recovery.pdf



Companies like Cain industries  or  Heat Sponge in addition to many reputable boiler manufacturers offer these.

nicholas bonham-carter

Steam economies

Is this boiler used for heating? In addition to the economiser, you could also try to make the burner operates as a hi-lo configuration. These are usually controlled by pressure:

1.controls call for heat, pressure is at zero. Burner fires high.

2.pressure rises to 12 ounces, burner switches to low fire to maintain that pressure.

3.temperature outside drops (ice age), pressure drops, burner switches to high fire to maintain the 12 ounces.

This burner may already have the hi-lo option, and only needs to be turned on!--NBC

Bestway_Dave

Not getting it

I guess I'm not understanding. If I've got liquid returning back to the boiler at a low temp, why couldn't I use a hot water condensing boiler to bump up the water temp to the point where a steam boiler could bump it up even more? If I've got liquid, and I pour 2.5 million BTUs into it, it seems that I would be accomplishing most of the battle with the hot water boiler. It seems that 2.5 million BTUs water would be the same as 2.5 million BTUs steam.

Note: the 3 million btu steam boiler needs to be replaced no matter what. And the boiler is being used to space heat through radiators.

nicholas bonham-carter

Making steam

Raising the temperature of water from 210 degrees to 212 (steam) requires much more energy than raising the temperature of water from 90 derees to 200 degrees.

When you replace this boiler, make sure you do a radiation survey to size the replacement properly, instead of just replacing like for like. The old boiler may have been over-sized.

Don't forget the modulating burner on the new one, as well as the economiser.--nbc

Jamie Hall

Heat of evaporation vs. temperature change

Terry stated it correctly, but perhaps a slight rephrase might help...



You are starting with returning condensate at, let's say, 80 degrees Fahrenheit.  You are adding heat to that water which, at atmospheric pressure, will steadily raise the temperature until you get to the boiling point -- 212 degrees F.  To do this will take 1 BTU per pound of water per degree, or about 132 BTU per pound.  When the water reaches 212, however, instead of the temperature rising further, some of the water will evaporate -- boil.  That takes 970 BTU per pound of water evaporated; this is called the latent heat of evaporation.



So to convert a pound of water at 80 F into steam at 212 F takes about 1100 BTU: 130 to raise the temperature to boiler, and 970 to boil it.



2.5 million BTUs in water is most assuredly not the same as 2.5 million BTUs in steam.  To absord 2.5 million BTUs in water, going from 80 to 212, would take 19,000 pounds of water.  If you allow the water to boil, creating steam at 212, it would take only about 2,000 pounds of water.



Now you could use a hot water boiler to do the temperature raising part -- but it wouldn't condense, since the output temperature of the water is too high.



As Terry suggested, if you want to improve the efficiency on an installation of this size, there are stack economizers (feed water heaters) which will do this.

ttekushan_3

It takes more than a bump to create steam

Assume the 3Mill boiler is about 71 HP.   It will produce about 2474 pounds of steam per hour, requiring about 2,400,000 BTU/hour to do so.  You will get about 2474 pounds of water back (about 297 gallons) every hour.



2474 lbs/hr x temp rise required up to 212F but not steaming (212F-80F=132degF)=326,568 BTU/hr.



326,568 BTU/hr is all that is required to bring your condensate water up to near boiling.



2,073,432 BTU/hr is required to "bump it up" to steam.



The discrepancy is obvious.



Preheating water for a steam boiler is, well, just heating water.  Again, it take 970 times the amount of energy to turn water to steam than it does to raise a quantity of water 1 degree F. without any temperature differential at all.



Steam is not water.   With water heating, you are releasing heat into the building in proportion to the temperature differential across the heat exchanger.  Steam is totally different.    We are not heating with a temperature differential at all.  We are releasing the latent heat that was pumped into the water at 212 to get it to turn to steam in the first place.  The steam and condensate have the same temperature at the moment of condensation in the heat exchanger.



They may both be called boilers, but heating water and creating steam are two different animals.

Mark Eatherton

Really, 80 degree F return temperatures??

Other than seeing a system in the process of shutting down, or staring up, can't say as I have ever see 80 degree condensate. It usually comes back at around 180 + depending, of course.



Remember that in THIS situation, the boiler will only be 86% efficient, not 95%.



And then there are the heavy oxides to deal with in the HXers passage ways.



I can see what's going on in your minds eye, but if you REALLY want to reduce energy consumption of a steam based system, consider application of a Tekmar Steam boiler control, along with non electric TRV's at the points of use. I also like the idea of a modulating burner, and the ability to do a finite reset using co-relating temperatures, instead of unreliable pressure sensors.



One of these days, I am going to take possession of an old modcon boiler, and I am going to modify its operation to make it a steam generator. That, or steal the burner out of it and drop it into another boiler, like a multiple pass (MPO?) boiler and see what happens.



Demand is not a constant. Why should fire be a constant?



But I like your thinking outside of the box....



ME

ttekushan_3

oops didn't mean to step on your toes.

sometimes a long time passes by the time I hit the "post" button!



Anyway a clarification of a clarification won't hurt.  I don't see any other way to get the point across without gesturing with my arms.   I can't do that  here! 



Our math tells the story, but sometimes I am accused of using too much detail.   I once read an amusing definition of the word "clarification" that was used to describe a paper I had written:



Clarification:  To fill the background with so much information that the foreground goes underground.



I've always liked that one:-)

The Steam Whisperer (Formerly Boilerpro)

latent heat....

It takes 132 btu to raise 1 lb of water from 80F to 212F.

Once the water is at 212F, It takes another 972 btus to take that 1 lb of water and boil it into 1 lb of steam.

Out of that 3 million btu boiler,only about 360,000 btu is used to raise the water from 80F to 212F.  The other 2,640,000 btu are used to convert the 212F water to 212F steam.

Welcome to the wonderful power of steam.  In this simple formula lies the reason why steam is inherently so efficient and powerful as a heat source.  It takes only about 3 lbs of water converted to steam to heat an average room in a home for an hour on the coldest day of the year (3,000 btu/hr). With a typical hot water system in the U. S., it would take 150 gallons of water, plus all the power to pump it for an hour, to heat that same room on the coldest day of the year.