Boiler stack performance vs. relative humidity & dew point

[Mfoxchicago]

Evening guys. I am curious, today we went steaming late in the afternoon when the cloud cover and the humidity was on the rise and found our boiler firebox not so willing to allow natural draw up the stack to occur. We had a full load of wood fired up, plenty of water, everything seems nominal, but just could not get the stack to draw the heat up through the pipes. They were brushed out only two weeks ago and should not need brushing this soon. We really struggled to get pressure up with such low draft.

My Fireman, who also is a volunteer fire fighter, made the observation that today's atmospheric conditions were similar to what fire fighters loved for battling wild fires = relative humidity 60%, dew point 62 degrees, winds less than 10mph, overcast and air temp 72 degrees. He said fire fighters have learned that high RH and low temps tend to make the smoke of grass fires to actually sit down on the flames and help put them out.....????

Has anyone experienced this sort of anomaly or condition in their steaming.........? We have a stack blower, but our battery levels were down and that was not able to help (sucks to be us). I am wondering how the old classic river steamers of the mid-19th century dealt with the same sort of conditions on hot steamy mid-summer days along the great rivers when the RH was high, winds dead calm, high dew point and no real way to force draft other than having boiler crews literally fanning the flames manually.

Your thoughts.

[fredrosse]

I have been involved with combustion in boilers for over 40 years, and have never seen anything like you mention as a cause for lazy fires or poor stack draft.

Dry combustion air is basically 78% Nitrogen, 21% Oxygen, 1% other gasses. Humid air typically adds in 1% to 2% H2O (water vapor, steam) not enough to materially change things as far as combustion and stack draft goes.

So I think the observed conditions are due to other factors, perhaps moist fuel, which robs much of the heat value available, lowering combustion temperature. That can happen when the fuel gets a healthy dose of morning dew, mist, or even rain.

[dhic001]

Lack of wind over the funnel will definitely have an effect on draft, but I can't see it being so very different from one day to the next.

The vessels of old all had decent funnel height, to encourage the natural draft to their fires. Your funnel at best resembles that of an early internal combustion engined vessel. How much of a difference it will make I don't know, not knowing how your boiler is configured, but I would be seriously considering putting a proper steamer funnel on her. Not only will a decent tall funnel improve your draft, but the vessel will look more like a steamship, and that will help attract passengers! You may also want to consider fitting a steam blower into the funnel to allow you to force your fires more when you don't have sufficent battery power to do so.

Daniel

[Mike Rometer]

Anyone who ever had a coal fire in the home knows that there are days when they just will not draw. I too have had days when steaming locomotives when it has been difficult to get the fire going enough to get steam (no problem then, just turn on the blower) Damp heavy air doesn't like to move.

[johngriffiths]

Tubes can block very quickly if cold and there is a heavy resin/tar constituent in the smoke. This I know from both direct and indirect experience. A tall stack is a great help and stack extensions are frequently seen when traction engines are being fired up. Others use computer cooling fans arranged to blow into the sealed ash pan and these will work off a car battery, which can be carried on board if no other power is available. Many use a steam blower, fitted at the base of the funnel where it leaves the smoke box, when there are a few lbs on the gauge.
When firing up get a thin hot wood fire going that covers the grate and only start putting large material on after this is burning well, some say it should almost be at the glowing embers stage for coal.

JohnG

[fredrosse]

I agree with Daniel.

And to some extent can make sense of what Mike says.

However I ran a Dmestic Heat-Power Module for4 years, 24-7, and never experienced the difficulties mentioned by Mike,

"Damp heavy air doesn't like to move." Actually the more water vapor in the air (high humidity), the less "heavy" it becomes. Because the steam (water vapor) has a significantly lower molecular weight than dry air, the high humidity damp air is actually lower density, "lighter", even though the water vapor is only a couple of percent of the humid air mixture. The only time humid air becomes more dense than dry air is when actual liquid water (in the form of a fine mist) is mixed in as very small droplets entrained in the air. This condition of course corresponds exactly to 100% relative humidity, and looks like a cloud.

This might be the condition discussed here, but with actual relative humidity anything less than 100%, the draft of the boiler and flue system will behave virtually the same as normally expected. Add moisture in the form of "mist", or introduce extra moisture from wet fuel, then combustion, ans stack performance can suffer.

A taller stack,and/or stack blower overcomes the problem, as mentioned by several others in these replies.

[Mike Rometer]

I agree with Daniel.

And to some extent can make sense of what Mike says.

However I ran a Domestic Heat-Power Module for4 years, 24-7, and never experienced the difficulties mentioned by Mike,

.

Yes but with 24/7 your flue never got cold. Lighting up on a cold damp morning could be extremely difficult with an open fire. My mother sometimes held a sheet of newspaper across the opening to force the miniscule draft to pull through the fire instead of over it, but then open fires are notoriously inefficient for many reasons.

Consider you must have been lucky.

[ron parola]

I can't really tell from the pic of the Princess on the side but I looked up my copy of Live Steam, Feb '85 and on the cover photo of the boat it SEEMS the stack is quite a bit taller. Maybe just the angle of the photo or has the stack been shortened? rp

[Mike Rometer]

I can't really tell from the pic of the Princess on the side but I looked up my copy of Live Steam, Feb '85 and on the cover photo of the boat it SEEMS the stack is quite a bit taller. Maybe just the angle of the photo or has the stack been shortened? rp

Certainly wouldn't help the draw. Full size traction engine men put an extension on to get the fire started. You see them strapped to the roof on Showman types.

[TahoeSteam]

I can't really tell from the pic of the Princess on the side but I looked up my copy of Live Steam, Feb '85 and on the cover photo of the boat it SEEMS the stack is quite a bit taller. Maybe just the angle of the photo or has the stack been shortened? rp

Ron, if you take a look at the video I posted of the 1995 B&W steamboat meet at about 25:50 you'll see your observations are correct

Mack, your fireman/volunteer firefighter's explanation isn't quite right... I'll leave that for another day.

I will echo the suggestions for a stack extension, or adding back the removed height of the stack, as well as a steam stack blower which are actually quite effective even with only a couple pounds on the clock.