Are There any problems with "cold" condensate returning to a boiler?

JUGHNE

I worked on a 1918 steam system in a house that has 10 radiators. 2 pipe steam, trap guts long gone (removed). The redo involved supply valve orifices sized at 80% EDR for 18 oz. The system actually runs at 12 oz so less EDR is delivered. 7 CI rads have no trap, 1 CI rad has new trap, 1 cabinet convector and 1 short baseboard each have traps.
House attic insulation has been updated and owner is more than comfortable even when ODT is well below design temp. The 8 cast iron rads heat across on average about 70%. The condensate lines for these are cool (room temp) to the touch. The cabinet & BB cond lines are hot/warm only up to the dry return.
Consequently the condensate returning to the boiler is cool, basement temp. Just a different situation...used of almost burning my hand on condensate lines at the boiler.
There are 2 steam mains that come around and are dripped into the wet return which add some temp to the actual inlet of the boiler return. Could this lower than usual return temp cause any issue that anyone can think of??

Fred

I can't see that that would pose any problem but I have to ask, are there any vents at the end of those mains near the drips or anywhere on the returns? If there are, I would think steam getting at least to that point would keep the return condensate warmer than the basement temp. In any case, by the time that return water mixes with the near boiler wet return water, it's plenty warm.

JUGHNE

Thank You Fred, There are two Gorton 2" on each steam main end drip right at the boiler. ( the last rads are really close to the boiler). They get hot and close, really hard to hear them venting with the boiler running.

The dry returns have 2 Gorton 2's on them, they never get hot and I assume never close. There is a test port valve at that location and when opening I get only air on start up, never steam.
This is my long distance job (140 miles one way) so I didn't get to be there when it was really cold and have not been able to monitor operation as I would like. Most of my work is within 1/2 mile of my house.

Fred

I think you are fine.

Jamie Hall

I hope you're fine! The wet returns on my system are always somewhere between cool and downright cold!

JUGHNE

So if steam can achieve cool return water, (what modcon boilers thrive on for efficiency). Where are condensing steam boilers? The first heat exchanger could do the heavy lifting of boiling water. Then the secondary heat exchanger has the condensate water pass thru it at even let's say 100*, this would pull the latent heat out of the flue gases, condensing the vapors and we would get to use PVC venting which would solve a lot of bad chimney issues. Maybe 2 boilers in one unit, CI for the steamer passing its flue gases into a SS heat exchanger for the water return reheat. The less going out the chimney is higher AFUE...right??

But the rad emitters would have to milk out all the heat possible. Another modcon wish. This would really kick some HW/FHA butt.

So what am I missing here? There must be some fly in the ointment somewhere. Seriously why isn't this looked into?
(Just in case, I mentioned it first.....

Larry_52

It won't work because there is no constant flow. You would a need a pump recirculating condensate through this economizer idea and even still you would end up with not enough heat sink and high exhaust temps.

Running the system pressure at vacuum is the key to get a squeeze on exhaust temp.

JUGHNE

How about adding the constant flow of low temp bb heaters to the cond return. Would need a lot of water and heat sink you suppose?

So with pressure in vacuum is less fire needed in boiler or is it the same fire would have lower stack temp because of more btu's absorbed?

Jamie Hall

Economizers -- basically a way to preheat the feed water -- are used on power boilers, steam (railway) engines, marine boilers, and some big heating boilers. The actual installation varies with the application. Trouble is, they are an added layer of complexity and capital cost never mind maintenance, as they all require a continuing (not necessarily constant) flow of feedwater -- or a way to take them out of the stack gas circuit or waste steam circuit (depends on the application) when they aren't wanted.

Big power boilers and marine boilers are using the steam for power, not heat (although the heat of the low pressure steam is used in the economizers) and condense at very high vacuums (well over 20 inches), using cold water. That latent heat may or may not be usable, depending on the temperature of the feed water.

It's all a matter of balancing cost and efficiency of operation against capital and maintenance costs.

JUGHNE

Thanks Larry and Jamie for responses to my now vaporized pipe dreams/crazy ideas. We would not want to lose the simplicity of residential steam systems as they are now.

However the question of vacuum operation reducing stack temp? Are we talking 2 stage or modulating burner firing?

Larry_52

Yes a means of down firing ( staging or modulation)would be necessary to develop a vacuum condition on a normal one pipe steam system, on top of vacuum vents and possible vacuum pump to purge all air 1st cycle. But at say 27" vacuum your saturation steam temp is 100 degs, so you get to squeeze the exhaust energy further.

Jamie what do you mean the heat of low psi steam is used in economizers? I think what you mean is feedwater heaters, the economizer is always associated with flue gas. Economizers are more prevalent the higher the psi steam boiler regardless of use. The saturation temp of your primary steam psi zone dictates the flue gas pinch point. Basically at say 200 psi you cannot theoretically (perfection) lower the flue exhaust any more than around 400 degs. Hence the economizer to grab the rest of the heat energy. Boilers when being designed are always thought more as gas coolers than actual steam generators. Economizers do not always need continuous feedwater if it is pressure regulated after the economizer with a feed pump keeping pressure above saturation temp. Although here the issue will be what is called economizer approach temp, stagnant water raising above saturation steam temp will just flash after the feed reg when it leaves the economizer. Or incorporate economizer re circulation from boiler water which checks economizer inlet when feed demand is low and allows actual boiler water to circulate through the economizer to prevent boiling.

you could possible design a flue gas tubed or regenerative air preheater for the burner and gain the efficiency of condensing units, but that would prove maintenance and cost inhibititive as mentioned with economizers.

Dave in QCA

There is no problem with cold condensate coming back to the boiler. In my building we have a Dunham vapor system and the condensate comes back at room temperature. However, as you describe in your system, we have end of main drips next to the boiler that run hot. Also, something that you do not describe is the equalizer pipe dropping down from the header. This pipe naturally stays very HOT, and the condensate flow into the equalizer through the Hartford loop mixes the cold returning condensate with the hot condensate from the steam main drips and the equalizer. Because of this, the boiler will never see that cold condensate.

JUGHNE

Thanks for responses and all good info, food for thought.
Yes we have two steam main drips plus the equalizer connection so boiler water is hot. Just overthinking again.

Probably next winter I may get the opportunity to have the time to stay on site long enough to observe constant operation.

If I can get pictures to work I'll post more about the system.

ChrisJ

I agree with others.
My wet return is often pretty cool, especially this time of year.

Though, when its near 0F out it gets too hot to touch. I have my dry returns insulated now which helps.

EBEBRATT-Ed

Believe it or not boiler mfg determine boiler ratings on the temperature of the condensate returning to the boiler. We had a job with 180 condensate and the water line wouldn't stay steady. The boiler mfg said we were overfiring the boiler even though we were not according to the name plate. The condensate returning to the boiler was above there stated temperature so we reduced the firing rate we were already adding those btus from the condensate.

ChrisJ

EBEBRATT-Ed said:

Believe it or not boiler mfg determine boiler ratings on the temperature of the condensate returning to the boiler. We had a job with 180 condensate and the water line wouldn't stay steady. The boiler mfg said we were overfiring the boiler even though we were not according to the name plate. The condensate returning to the boiler was above there stated temperature so we reduced the firing rate we were already adding those btus from the condensate.

They should supply what the expected return temperature is. I don't think I've ever seen this in a manual, at least not for steam boilers.

icesailor

@JUGHNE:

"" Just overthinking again ""
It al

JUGHNE said:

Thanks for responses and all good info, food for thought.
Yes we have two steam main drips plus the equalizer connection so boiler water is hot. Just overthinking again.

Probably next winter I may get the opportunity to have the time to stay on site long enough to observe constant operation.

If I can get pictures to work I'll post more about the system.

There's no such thing as "Overthinking".

That's how we learn.

When HH.com runs slow, it doesn't always keep up and drops material. What is below is an example.

ways learn to learning. If you want to learn something, over think it. Take what you need, and leave the rest.

RobG

icesailor said:

@JUGHNE:

"" Just overthinking again ""
It al

JUGHNE said:

Thanks for responses and all good info, food for thought.
Yes we have two steam main drips plus the equalizer connection so boiler water is hot. Just overthinking again.

Probably next winter I may get the opportunity to have the time to stay on site long enough to observe constant operation.

If I can get pictures to work I'll post more about the system.

There's no such thing as "Overthinking".

That's how we learn.

When HH.com runs slow, it doesn't always keep up and drops material. What is below is an example.

ways learn to learning. If you want to learn something, over think it. Take what you need, and leave the rest.

Ice, I think the problem might be on your end as I have no problems.

Fred

I suspect it may be the version of his browser. I had a lot of similar problems 2 or 3 months ago and since installing some patches released by MS (if it's Explorer), I haven't had those problems.

ChrisJ

Fred said:

I suspect it may be the version of his browser. I had a lot of similar problems 2 or 3 months ago and since installing some patches released by MS (if it's Explorer), I haven't had those problems.

Or a slow 10 year old computer running Windows XP. I come across this way more than I should.

vaporvac

I sometimes have issues even on a 1yr old HP.

EBEBRATT-Ed

This is from Skidmore condensate pumps see page 6 RH side

Jamie Hall

Firefox. But the kinds of computer/website problems you are looking at can happen on pretty much any browser or computer, if the internet connection is at all flaky. Which a lot of them are.

JUGHNE

Thanks ED, yes, we were talking about cool returns. Whenever I get back to the job on a good heating day, probably months from now, I will check the actual temp going into the boiler. Though as mentioned there are 2 steam main drips plus the equalizer into the wet return section so I believe it is probably alright. Thanks for looking, it is logical that steam boilers would suffer with too cool returns as hot water boilers would.