Does steam follow path of least pressure?

CoachBoilermaker

Let's say there are two radiators.

Gorton A & D.

D has bigger hole so does D get 100% of steam before A gets any steam?

Let's say steam is going to D and then D fills up and closes. Does the steam already half way to D reverse course and go to A? Or does it just get stuck there in the riser and condense?

patrykrebisz

depends.

i've made a video about how enlarging one main vent dramatically changes steam distribution in early stages of steam production. basically, all things equal, steam takes the path of least resistance. BUT as the boiler keeps on producing steam, the steam starts going anywhere there is an opening.

https://youtu.be/OUtYTFSRwbI

ethicalpaul

D has bigger hole so does D get 100% of steam before A gets any steam? 

No, but if everything else is equal the D radiator will get more steam while they are both open.

Let's say steam is going to D and then D fills up and closes. Does the steam already half way to D reverse course and go to A?

Or does it just get stuck there in the riser and condense?

"Stuck"? The steam is always moving. It's always condensing. The vent is a rather small part of the puzzle. Much more "movement" is due to condensing steam in the radiator than the small amount of air that can escape via the vent (this is opinion unproven by experiment but seems obvious beyond argument—but let's see 😅).

The steam isn't going to reverse.

PS: I don't know a Gorton "A" vent. Maybe you mean "4"?

Jamie Hall

It's really a lot more complicated, but basically vent size controls how FAST air will be able to leave at a given pressure difference from inside to outside, but not how MUCH air will leave. Then steam inside the pipes leading to the radiator will move at a velocity determined by the pressure difference between one end of the pipe and the other and the condition and other characteristics of the pipe. Then the volume is determined by the velocity and the size of the pipe.

So it won't follow the path of least resistance entirely. What will happen is that the steam will move faster with a greater pressure difference or, to look at it sort of in reverse, the pressure difference will be determined by how fast the steam is moving.

Now the pressure difference between the boiler and the outlet of every vent will be the same (neglecting pressure variations in the house, for simplicity) and so in a sense you could say that the steam will move in the path of least resistance — but as more steam moves in that path, the resistance will build and the steam will also move in any other paths available to it — until the mount of steam moving in each path is just the right amount so that all the pressure differences are equal.

So for your example, @CoachBoilermaker , the radiator with the bigger vent will get more steam, all else being equal, but they will both be getting steam at once.

Now there is a complicating factor here — nothing is ever simple! — which is that once the steam gets t o the radiator it starts to condense. This is not related to the vent size directly — but is related to whether the vent is open or closed. That reduces the pressure in the radiator — and hence increases the flow to that radiator. Once the vent closes, however, the pressure will rise slightly and the flow of steam is governed by how fast the radiator can condense the steam rather than how fast the air can escape.

This (and a few other factors) is why balancing the heat in a one pipe steam system is difficult. The output of the radiator, taken over time, is related to the radiator size (naturally) and the vent size (a given radiator with a larger vent will fill with steam faster than if the vent were slower) but also, and usually quite neglected in the thinking, by how long a given boiler steaming cycle is. As a result, a system which is beautifully balanced at one outside temperature and heat demand may be quite far off at another one.

Which is a nuisance…

Long Beach Ed

Jamie hits the nail on the head. Condensing steam and the vacuum (reduced pressure) it forms plays a significant role in the path steam will travel. And the condensing of steam in radiators is greatly affected by temperature of the iron and air around the iron.

This is why a riser ending in a tee arrangement with two identical radiators will choose one radiator or the other to enter seemingly at random.

jesmed1

Another interesting story about how steam "chooses" which path to take, at the end of this chart.

https://heatinghelp.com/assets/documents/Balancing-Steam-Systems-Using-a-Vent-Capacity-Chart-1.pdf

dabrakeman

And thus why we say vent the mains fast and the radiators slowly.