steam velocity question

mel rowe

Would appreciate comments on my last post- specifically the thought of underfiring my boiler to match the attached load, with the intent of reducing steam velocity and saving gas.

mel rowe

steam velocity question

I keep learning more every day, hopefully, with the patience and advice from fellow Wallies. I always try to feed back results and a big thanks, but in case I am remiss, I have made great progress with my one pipe sysstem and I'm very grateful to all who have contributed to my education. On steam velocity, I'm puzzled as to why the formulas use net EDR, condensate load, etc., rather than gross. Seems like that gross output represents the steam out of the boiler and is not affected at that point by the load, just the boiler output. What am I missing here?

ttekushan_3

Nailing down an easy point of reference

I can only respond based on my observation of a wide variety of steam heating systems. When using EDR or lbs of condensate, you are looking at normal operation, i.e. steam @1psig @215ºF room temp of 70º at sea level. You'll notice that the I=B=R pick up factor is about 1.33 for the average application. Unless its unusual. And unusual seems to be more usual than not.

My point is, that the time of the onset of steaming is a very good time to use the gross steam output numbers (lbs of steam per hour) to determine the steam exit velocity and the velocity in the near boiler piping. But this number is modified by the load of the system and its environs; and continuously so throughout the warm-up approaching saturation. So... You know the velocity near the boiler on start up and the velocity everywhere under saturated conditions. But every point in between is subject to virtual chaos theory based on boiler size vs load, cold start vs warm start, the temp at cold start, the venting efficacy, rate of steam pressure rise, piping layout, piping length, piping size, single pipe vs two pipe, parallel flow vs counterflow, etc.

So all your're missing is the "art" part of steam heating- the part that draws some of us to this wonderful heating medium, and sends others* running and screaming in the opposite direction!

*"And they shall be the purveyers of forced air heating" (attributed to God, I think)

-Terry

mel rowe

Terry, thanks for your response. Very enjoyable reading. I think you confirmed my thought that in the near boiler piping, which is where we work so hard to reduce the velocity, velocity is actually determined by the gross output of the boiler, rather than the net rating. This made sense to me but it only adds to the problem of trying to reduce steam velocity in today's modern boilers. With only 2 2in. tappings on my Utica boiler of 299,999 btu, and even with a 3 in. header I installed, 32 in. above the water line, I still don't have a chance to get the velocity to where I can get really dry steam. Oh well, at least it's working a lot better since I added the second tapping and the 3 in drop header. If I use the gross output rating and only one tapping, as originally installed, I calculate the steam velocity as about 72 ft./sec. It is a wonder that the house didn't lift off its foundation. LOL Thanks again.

clammy

velocity

Mel on all my steam install i calcaulate my exiting velocity and try to keep it under 14 fps i have pipe some with exiting velocity to about 11 and found them to work excellent with rads getting much hotter then before and loads of other promblems like noise in supply mains from wet steam disappearsed .This does require that you repipe your near boiler piping a good start is the manafactures lit they usually will give you the real poop but in my expereince over sizing the header and installing a drop header is the best insurance for dry steam production also having your near boiler piping and supply mains properly insulated really helps the system perform as it should peace and good luck clammy

Steamhead (in transit)

Terry, I think

that quote came from Dave Lennox or Willis Carrier.

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mel rowe

Thanks to all. Clammy, I became aware of the velocity issue after reading Dan's book. Since then I have followed much good advice here and opened the second 2 in. tapping, and run both into a 3 in. drop header, that was as far above the water line as I could get it to try and retain water droplets in the boiler. All this is a big help, but it seems I am totally limited by having only two 2 in. tappings on a 299,999 btu boiler. Don't understand why they would design a boiler this way. Since I have now learned more about EDR, by my calculations my rad EDR adds up to 612 sq. ft., and the boiler is rated at 760 sq. ft., so I am considering underfiring my boiler down to the attached load of 612. Seems there should be two benefits to this, both lower velocity steam and less gas usage. Any thoughts on this?

Steamhead (in transit)

Mel, if I remember correctly

that's a Utica PEG-300, correct?

On that boiler, and many others, the velocity slow-down and steam/water separation take place in the 2-inch risers and the header. The risers must extend at least 24-inches above the highest possible waterline and the header must be 3-inches in diameter and preferably a drop header. It might not look like enough, but it will do the job.

If the boiler is short-cycling, you might wish to HAVE A PRO down-fire it slightly. Ohterwise I'd leave it where it is.

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mel rowe

Thanks Steamhead, you have a good memory, or else I've been taking up too much time on the Wall. LOL. The before and after pics looked like this. As you can see I went to a 3 in drop header and the risers are 36 in from water line to header. I am getting much drier steam and the system is quieter and runs better, but due to being oversized, it only runs about 35 % of the time (30 min.on, an hour off) in the coldest weather. This doesn't seem very efficient, so that's why I keep thinking it would be better to underfire the boiler, maybe by about 20-25%, to match the attached load. Seems this would give longer cycles, and more efficient gas usage. What would a pro do to downfire the boiler like this?

mel rowe

Thanks Steamhead. Either you have a very good memory, or I post too much, or both. LOL. With the help from many here, as shown in the att. pics, I repiped my Utica boiler and added a second tapping and a 3 in. drop header, and risers that are 36 in. above the water line, so this is a good as it gets with only 2 2in. tappings available. Overall the result is very good. It just keeps rattling around in my head that since the boiler only runs 30 min. and then is off for an hour even in the coldest weather, I should be able to further reduce velocity as well as improve the efficiency by underfiring between 20-25% from current level. How would a pro go about downfiring a boiler like that? Is it just a matter of reducing the pressure?

Steamhead (in transit)

Possibly

but any changes MUST be verified with a digital combustion analyzer. That's why a pro is needed. It's for your own safety.

I don't remember seeing the "after" pic of your repipe previously. Looks like you did a good job!

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