physics of radiator vents

stanleysteamer

Maybe I am missing something obvious, in which case I hope that the community here can set my mind straight.



The density of steam at 212 F and 0 psig is about 0.037 lb/cf, and density of air at similar conditions is about 0.059 lb/cf. So left to their own devices, the air should tend to move to the bottom as the steam fills the radiator. But the radiator vent is installed near the top of the radiator, which would seem to create an airlock.



Maybe it has something to do with a non steady-state condition, with steam entering the radiator pushing a 'plug' of cold air toward the top and the vent?

Jamie Hall

If you were working with still gasses

your analysis would be just fine -- but the conditions inside a radiator are anythiing but still.  The turbulence in there is terrific.  Further, as soon as some steam comes in, it hits the cold radiator (relatively speaking) and it isn't steam any more -- it's water.  The vent won't see steam until enough steam has gotten in there to have heated the major part of the radiator all the way up.

ChrisJ

Weird

Out of 10 of my radiators I've got one that heats across the bottom and shuts the vent pretty fast now.  It started doing this when I switched from Hoffman 1As to Gortons.   I'm tempted to mount a vent up higher, or vent the radiator slower and drill and tap the runout and vent that separately.   Its a tiny little 18sqft radiator with 10 feet of runout going to it.



The other 9 for the most part heat across the top and then fill in and finally near the end steam hits the vent and closes it.  All radiators are tube type.



My point being, steam doesn't seem to always behave the way you think it should yet once you realize what its doing, it always seems to make sense. 

MTC

ChrisJ,

does your radiator inlet go to the top or the bottom of the radiator? I'd assume it goes to the bottom if you're getting that effect. The path of least resistance with a high flow rad vent in that setup will be straight across the bottom ports and to the vent, leaving the top airlocked.



I think you're on the right track with the idea of venting it slower, and if you have problems getting enough heat, install a varivent or something in the runout. They're nice and small (so you can often install it on the riser right before the valve) and vent very fast, but are adjustable.



A high and low vent on the radiator might also help, like you sometimes see on a large radiator, but I think it would be less dependable than the other system... you could potentially get the same effect you're getting now.



stanleysteamer, the vents should be about halfway down. Near the top is usually more problematic for the reasons you're thinking of, but it will still heat... Jamie describes it well, the steam is turbulent in the fins, and seems to work its way across the top in general, but condenses along the way and the condensate running down the sections continues to heat them further. You end up with sort of a triangular heating pattern... the valve side has heat/steam most of the way up and down the columns/tubes, the vent side mostly heats at the top, and you have some air left at the bottom of the vent side. In one pipe heat that is, the steam traps on 2-pipe change that dynamic a bit.

JeffM

IR images

The attached photos show what differences in venting rate do to a radiator's heat up pattern. The one with just one section and the top hot has a slow vent, and the larger radiator has a faster vent and you can see that the steam shoots further into that radiator so that multiple sections heat simultaneously. You can pretty clearly see the pattern of heating mentioned above. Also I couldn't help taking an IR image of the dog while she was watching me and wondering what was going on...

ChrisJ

Fast

The 18sqft rad has a Gorton 5 on it, mainly to get steam to it at the same speed as ones with much shorter runouts.  In fact it still ends up lagging behind a little and I would like more heat in that room.  I'm also thinking about drilling and tapping the runouts to my bedroom rads and installing Gorton 6s on them.  The rads have TRVs and work best with Gorton 4s, but on mild days I'm betting I won't get steam up to them (2nd floor).



I've got an 1/8" pipe tap and I'm dieing to use it. 





Jeff, the two pics you posted are of two different rads.  You would need to compare several radiators all with 2 or 3 sets of vents (slow, med, fast) to compare how they all heat.  I'm betting a small section rad like mine would heat more evenly with a slow vent than with a fast.