One-pipe control valves keeping air in the radiator

FlyingScotsman

(I only signed up yesterday and I'm asking a ton of questions - it's so great to have people to ask all this stuff after wondering for so long. I promise to stick around and give back where I can, maybe in the hydronics section...)

OK so, I'm reading about control for one-pipe steam systems. Dan mentioned the Danfoss air control valve in a seminar.

As I understand it, unlike a regular thermostatic air vent, instead of waiting for the radiator to fill with steam and then close on steam contact, the valve closes at an intended room air temperature, keeping air inside the radiator to prevent the steam filling it.

So now you have air and steam and condensate inside the radiator.

I thought that having steam condense in the presence of air led to that metal-on-metal sound, like a clanking, which is caused by the steam suddenly shrinking by 1700 times.I've heard that noise a lot in trapped two pipe systems with air issues.

Why doesn't this happen in the one-pipe radiator?


Oh wait...is it because there's pressurized steam waiting to rush in and replace it?
Am I wrong about the cause of that noise?

Jamie Hall

FlyingScotsman said:

I thought that having steam condense in the presence of air led to that metal-on-metal sound, like a clanking, which is caused by the steam suddenly shrinking by 1700 times.I've heard that noise a lot in trapped two pipe systems with air issues.

Why doesn't this happen in the one-pipe radiator?


Oh wait...is it because there's pressurized steam waiting to rush in and replace it?
Am I wrong about the cause of that noise?

Got it in one. The Danfoss valve works by closing and not allowing any more air out. If the air can't get out, the only steam which can get in comes to replace the steam which condenses. You still get a little heat, but nothing like as much. The condensate goes back out, as all good condensate should.

The cause of the noise -- correctly called water hammer -- is when a steam pipe (or sometimes, but much less often, a return) has water trapped in it -- usually by bad pitch, but sometimes by just being too small -- and the steam -- which is going along a pretty good clip -- picks up a slug of the water and slams it into the next handy obstacle, such as an elbow or a T. At which point it clangs. Water hammer is almost always completely avoidable, if the pipes are sized correctly and pitched correctly. (I'd say "always avoidable" without a qualifier, but I don't like to say things like that.)

JUGHNE

If you read up on the Danfoss 1-pipe TRV (air valve), they point out that the system needs to cycle....pressure to drop so air can reenter thru a vac bkr. If high pressure stays applied, the air will compress and overheat the rad. They need to "inhale".