Mystery two pipe job, I'm stumped

gerry gill

put an aquastat on a certain radiator section to take out the vacuum pump once the steam has distributed itself..after all, once its in the radiator, is the pump necessary at that point?..thats how our experiments have been working with a vacuum pump on a mouat water seal system..

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GW

Mystery two pipe job, I'm stumped!

OK, I had some of you guys comment on a two pipe vapor system last week, but after going back to the job today I have confirmed that there are no orifices or any other trap fitting (elbow). I took the convection unit right off the piping at one convector and even blew down the return pipe, and looked down the pipes with a light; there are orifices at all!

The piping was installed in a typical two-pipe water-like fashion, with mains and sub-mains. The pipe sizes to the convectors are only 3/4" and 1", the returns are 3/4" and 1/2", depending on how big/small the unit is. The house was built in 1890 or so. The boiler is a Smith/Mills coal unit, a huge beast.

It's got copper-tube convectors with no air vents (there is one cast iron rad with an air vent). One convector has had a trap cut in on the return in the basement, who knows when that happened. The convestors are flat like normal; no pitch towards the return.

There is a swing check on the return down by the floor. The pressuretrol is set at 2psi cut in, with a 2psi differential. The A dimension is only 16" on the lowest steam carrying return (again, there is a swing check, there are 5 returns coming back, all up high. There are NO boiler traps at the boiler, nothing/nada, it's like looking at a one-pipe system as far as returns are concerned.

It's coming in at 823 sq ft of EDR. I found an old 1.5 gph nozzle kicking around. Will I get spanked if I pipe a new boiler in this? I'm needing some hand-holding here; any grandfather-figures out there that could help me out? Thanks, Gary

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Steamhead (in transit)

In some convectors

notably Trane units, the orifice was cast into the convector element itself. Don't let the lack of disc orifices throw you, this is an Orifice Vapor system. It is definitely newer than the house.

To avoid getting spanked:

Pipe the new boiler in with an oversized drop header- 3-inch minimum, more if you can get away with it.

Control the boiler pressure with a Vaporstat. No more than 8 ounces pressure, and less if you can get away with it.

Vent the heck out of the steam mains. Use the Gerry Gill/Steve Pajek charts. Also put some decent vent capacity on the dry returns.

If any convectors or heat transmitters have been replaced, they will need orifices. Without them, steam will get into the dry return and shut the vents mounted thereon. This will lock up the rest of the system with air and you won't get much heat. You can use disc orifices for this, but you might have to experiment with their sizing.

And take plenty of pics!

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Christian Egli_2

All holding hands together before we jump

I'm guessing the system works fine as is. What would seem real important is to make sure you send dry steam down the tubes the same way the old cavernous boiler did. Modern and small chested boilers don't separate the steam so much as the header does. A carefully upgraded header like Steamhead mentions is key.

Where does the system air currently escape? Are there any main vents?

I suspect this system could benefit greatly with the installation of a vacuum pump that sucks on the returns. Something to keep in mind if the pipes appear to be undersized.

Good luck

Steamhead (in transit)

That won't work on Orifice Vapor

sorry Christian, if you have a vac pump you need traps. The increased delta-P from the vacuum would overcome the orifices and pull the steam right into the returns.

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Christian Egli_2

Still holding hands, or just pushing me off the cliff?

The beauty of the things that suck is precisely more beautiful with the orifice.

We know orifice traps work fine... in steady applications, and they are slow, sloooow to vent the air out of the radiators. That relative slowness is the major draw back to using them in home heating applications where the job of heating is all up and down and quick gyrations. The Navy adores orifice traps, many industrial systems do too.

There are also the orifice on the radiator inlet.

Looking at the whole system with the boiler, of which every square foot of heating surface goes on to supply about fourteen square feet of radiator. When everything is in balance and properly matched, everything hums along. In ordinary piping, the steam is free to go wherever it wants, and without traps it would go where we don't want it.

Orifices throughout this whole system will cause bottle necks and traffic jam everywhere they are. They are good bottle necks because they allow only the amount of flow that can be condensed in the respective radiator. The trick of such systems is that everything has to be in balance. Remove one orifice limiter, and the steam has found itself the hottest shortcut; all of the orifices work as one unified squad of the steam traffic police.

The predetermined holes on the orifices are designed for the load and a set pressure differential. If you change any loads (or install a mismatched boiler) it is obvious you run into imbalance problems, suddenly one of your cop on the squad is no longer equipped to handle its load. That's a problem.

But, when you change the whole system set pressure, all of the orifices are equally affected. The squad still does the same repartition job it did before, all proportions are kept.

Bizarrely, the steam load remains the same, no matter what the differential pressure - that's because the boiler has not changed, its fire still produces the same (or so, for all practical purposes) amount of steam in pounds. Each pound taking away about 11xx BTU of fire. Orifice sizing to load remains valid enough.

However, there is a big difference in the pressure to volume relationship. Half pressured steam is twice as voluminous but it still carries (about) the same amount of heat, measured in BTU/lb. Does it make a difference, orifice wise, when we try to push through the small volume with a low pressure differential, or when we try to cram the large volume with the higher pressure push? Not for steam and its energy content. Steam only exists as long as it contains energy; once that same energy content is gone, the steam is gone.

The main beauty of the vacuum pump, is that is moves the steam artificially if the boiler can't on its own, and because of that, you get an advanced opportunity to get rid of the always annoying air. This pre-start feature, compensates for the slowness of the orifice venting. Neat. After that, everything is about the same.

There, I made this sound like a lecture, fun maybe, but we all know how things go in practice: adjustments are necessary, so, I'm in no way discounting Steamhead's advice. Plus, there are definite limits to the amount of vacuum you want.

A common occurrence of this situation pops up in combined furnace and AC systems. The grilles and dampers are all adjusted to dump the proper balance of air in each room and this works... summer and winter, even though the summer fan blows more air than the winter speed setting.

There, all head scratching fun.

Steamhead (in transit)

But the problem with scorched-air/AC systems

is that the largest heating load is on the lower floor, whereas the largest cooling load is at the top. So the system is out of balance at least half the year.

And in Orifice Vapor, the orifices are sized for a certain delta-P. Adding a vac pump increases the delta-P the same as raising the steam pressure. The effect is the same: steam in the dry return, water hammer, frustration.

Balance is the key.

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Steamhead (in transit)

Interesting concept

let us know how it works out!

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GW

swing check?

OK, so I can certainly get a jumbo header installed; I'm cool with the supply side. What about the return? That swing check is making me nervous, take it out? My A will increase big time, the old boiler's water line is about 5 feet off the floor.

Another thing I fear is the possible need for a boiler feed pump, and IF i do put one in, will that change anything that we've discussed so far? Any coments on this? Perhaps I don't need one..

Any yes, there are main vents at the end of the 5 returns. There are NOT any vents on the supply mains. Again, it's just like a hot water two pipe, but it's steam and there are no vents on the supply piping.

Thanks again! I;m trying to stay in touch, I'm away at Comfortech, using the Convention Center's computer at the moment.

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clammy

5 ft water line?

If the old boiler water line is that high you should also consider installing a false water line to keep all your returns under water so you don't lose the water seal between your rads returns and your ends of the main drips if your supply drips tie into the wet returns ,i also believe in what steamhead has stated use a vapor stat and over size you header to bring your exiting velocity below 14 fps 11 fps is heaven very dry very quite also changeout the mains and install the properly sized main vents and you should be good to go . Also insulate all your near boiler piping at a mimuin it does make a difference Best of luck Peace and good luck clammy

Christian Egli_2

Well, let's say I meant a low ceiling one floor home. Fineprint

You're right, the vacuum pump does the same trick as anything that increases the pressure differential. The major problem that comes along with too big a pressure differential in ordinary systems is that you get high stacking A dimensions that cause water to back up and pile up into the returns, and bammo, you get water hammer.

The check valve below the 5ft column prevents no such backflow out of the boiler as we may think (it only does so strictly in an emergency rupture of the wet returns). Technically, the check valve stops (if it doesn't leak) the water inside the boiler from backing up. True. However, in a steam system, condensate never stops pouring back towards the boiler, it stacks itself in front of the check valve with the hope of being admitted to the party. Those check valves are some airheaded bouncers.

The height of the back stacking column of water waiting to get back in is purely a function of boiler to return pipe pressure differential; the check valve serves no purpose in limiting that height. Only the Hoffman differential loop does, or alternatively, a condensate pump or a pumpless pump (the alternating return trap).

In your case Gary, a Hartford loop connection is much better protection than the check vavle.

Also, it seems you only have dry returns (since there are air vents on them) in this case only, lowering the boiler water line shouldn't mean anything since the returns were already dry at 5 ft, they'll be that much drier at a lower water line. This will also give you more wiggle room in the A pressure differential dimension.

Also

Choose a boiler with a large water surface to fluctuate and compensate for slow or delayed condensate return, rather than considering a feed tank.

Lastly

I run the vacuum pump exactly the way Gerry described, in the middle of the heating cycle, they serve not much purpose (lifts excepted) and why risk the dangers of cavitation?

Air is bad to breathe and vacuum pumps get rid of if for you.