Managing Air and Steam

FlyingScotsman

Hello! I'm advising some residents in the building where I live to change air valves on their one-pipe radiators, and encouraging the board to replace vents on the main, as the ends of the system aren't getting steam (215-205-184F surface temperature drop off in the twenty feet of the main pipe approaching the main vent after boiler had been running for 30min).

Something that has been on my mind as I talked through my understanding of the need to vent air, airlocking etc, is that air and steam are both gaseous, and gases expand to fill available space.

I'm sure that cold air and hot steam would act as separate volumes, and that the boundary between the two would initially see little mixing, but they should equalize after a while and mixing would become easier.

How come after 30 min, half a radiator is (comparatively) cold and the other half is hot? Why doesn't the steam and air mix, and the mixture heat the whole radiator? I know air is an excellent insulator, but it is also a gas...

This instance I wasn't sure steam was even getting to this radiator (it's above the 205F portion of pipe in the basement)...so is this just the pipe heating up via conduction?

This is for a radiator with a drastic slope and a fully open hand valve, so I'm fairly certain condensate is getting out of there.

Jamie Hall

Good start. The problem is that while steam is a gas, it doesn't stay that way. Unlike air, steam at the pressure we usually run steam heat (less than 1 psi gauge) condenses -- near sea level -- at around 215 degrees F. The resulting condensate -- water -- has far less volume (around 1/1,700). So if you have a radiator -- for instance -- with no way for the air to get out, a small amount of steam well try to get in, but it won't mix. Instead, it will promptly condense -- right at the inlet. If some air can get out, there is more of a chance for steam to get in, so more of the radiator will get hot -- but there is still no mixing. Same sort of thing happens to a steam main or riser -- if it isn't vented, any steam making its way into one end will condense.

JUGHNE

Can you show us the main vents and the typical rad vents?

ethicalpaul

What @Jamie Hall said.

Plus, is this a baseboard? The steam will really condense when it hits the relatively cool pipe as it travels, and won't get past a given section until that pipe is hot (unlike a hot-water baseboard of course)

EBEBRATT-Ed

@FlyingScotsman @Jamie Hall has a better explanation than I could give. But why overanalyze it?

Whatever the cost of the vents and the installation is it's peanuts compared to the fuel your wasting

mikeg2015

Steam has a fairly low density so it takes fraction of ounces of differential to make it move at a fairly high velocity. Therefore if a radiator if not venting, it gets no steam. Additionally, as the leading front of steam travels and reaches cool air and pipes it condenses and creates a pressure reduction or in a vapor system even a vacuum momentarily that draws the steam behind it forward. So it travels almost like it’s water running downhill. OR imagine a paceline of cyclists rotating to take the lead. The leader blocks the “wind” for everyone else as it condenses and pulls it forward.

FlyingScotsman

Thanks everyone. Addressing everyone in turn...

@Jamie Hall Thanks. I suppose that process is a real inhibitor to mixing, until the air -a great insulator - is at the same temperature as the steam, which would permit mixing without condensation. To mix air and steam that way you'd need pipe at steam temperature (which it will likely never be, given as the surrounding environment is comparatively cold, and how close we are to condensation temp)...which is why it never happens! A-hah moment. Thank you. Was just trying to wrap my brain around the brownian motion/boyle's law side of it.

@JUGHNE We have single Gorton #1s on the main and while residents are responsible for their own vents I've posted the appropriate sizes per floor/location on the notice boards for a few years, and I'm getting good feedback on people using them.

@ethicalpaul Thanks. Yup. Jamie's explanation cracked it for me. Thanks.

@EBEBRATT-Ed Personal understanding. I found the problem and know how to fix it, but I just didn't know the answer to my own question.

@mikeg2015 That is a *great* analogy, thank you!

Following up generally I am putting multiple #1s in the location that was airlocked on the main, and expect some drastic improvement.

nicholas bonham-carter

It will be more cost effective to buy a few big mouth vents for the mains, instead of a lot more Gorton 1’s.The main venting is much more important than the radiator vents.—NBC

gfrbrookline

Totally agree with @nicholas bonham-carter on the main venting with Big Mouths. Figure the mains out then look at the radiators.

Since the heat is part of the building mechanical infrastructure and I assume heat is included in your HOA fee's I would address the vents as an HOA expense as we have done in my building. Doing this allows the HOA board to actually manage the heat distribution as opposed to individual unit owners deciding they like it hot causing another unit to go cold without having a choice. If you trust your unit owners not to play with adjustable vents I would by Vent-Rite No. 1's so you can dial in each radiator. If you do not I would buy a Hoffman 40 for each radiator and see how you make out. If the mains are vented correctly steam should reach each radiator at the same time so the Hoffman's should work well. If you have a cold room add a Vent-Rite 1 and turn it to above 4 until the room balances out.

gfrbrookline

@FlyingScotsman if what you sent to your other unit owners is the Gorton/MoM chart for vents please do not follow it. If your main venting is correct radiators should be vented based on size not proximity to the boiler.