Two blow offs, only one bubbling - science check

[Komodo]

I've got a fermenting vessel with a 1/2' silicone blowoff into a bucket of water. I ended with more volume than anticipated (lots of variables), and the headspace is less than usual. It's going off like a rocket, and since I had to go back into work, I got paranoid and added a second 1/2" silicone blowoff from the other triclamp port. But I had to connect two short lengths of hose to make it long enough to reach the bucket. I'd say the first is around 28", and the second longer one is around 40".

As soon as I hooked up the longer hose, it began bubbling hard, but the first, shorter tube did nothing. I came back after, and still nothing from that tube. I'm sure there is some sciencey pressurey explanation for what is happening and hope someone can fill in that blank.

 

[Agent]

If they aren't hanging the exact same amount into the water bucket, then there will be two different pressure requirements to overcome the liquid in the bucket before bubbling out.

And flow always goes for the path of least resistance first.

 

[fluketamer]

yeah what he said least resistance

 

[Komodo]

Wouldn't the longer hose have more resistance? They are both hanging similarly.

 

[Agent]

Resistance due to tubing length is relative to flow rate, which in this case is practically negligible.

When you say "hanging similarily" if you have one 6" in liquid and the other 5.75" in liquid, the 5.75" one will still release all the bubbles - until the flow rate through the additional tubing creates more than 0.25" water column of drag.

 

[mac_1103]

In addition to what he said, it doesn't matter. All the gas that needs to escape is escaping. If you had two perfectly balanced blow off tubes, each one would be bubbling half as much as the one is now. Think of the second blow off as a backup.

 

[Kickass]

What benefit does two blowoff tubes add?

 

[day_trippr]

I believe the intent was to double the potential bandwidth of gas escaping the fermentor...

Cheers!

 

[hotbeer]

CO2 production isn't what it was when the blowout happened. So of course between multiple exits, the two blow off tubes, the one with the path of least resistance is where everything goes.

Until the CO2 production is near the limits of the volume capacity that one tube can handle, then that's the one that's going to blow the bubble.

The depth of the tubes in the liquid of the blow of tube might play a part too for what that capacity is that two tubes are needed. So putting both a tiny amount below the surface might get you some bubbles out of both. But for typical fermentations, I'd think you are well beyond that time when that might happen.

 

[Komodo]

The intent was:
- I was going to work and couldn't monitor the tightly sealed stainless vessel
- the fermentor was filled more than usual and was at peak ferment shooting a constant stream of 1/2 bubbles
- If the tube clogged, I wanted a backup vent. If one clogged, the other probably would too, but I wanted to do what I could.

@mac_1103 You seem to be misunderstanding my post. I was curious as to why the longer one was bubbling and the other wasn't. I had expected them both to bubble, or just the shorter one.

 

[mac_1103]

You seem to be misunderstanding my post. I was curious as to why the longer one was bubbling and the other wasn't. I had expected them both to bubble, or just the shorter one.

Or maybe you're misunderstanding my reply? I think my statement that the two tubes would have to be identical in order for both to bubble is on point and I also think that Agent covered the most likely reason why the longer one might still be the path of least resistance.

 

[PCABrewing]

The intent was:
- I was going to work and couldn't monitor the tightly sealed stainless vessel
- the fermentor was filled more than usual and was at peak ferment shooting a constant stream of 1/2 bubbles
- If the tube clogged, I wanted a backup vent. If one clogged, the other probably would too, but I wanted to do what I could.

@mac_1103 You seem to be misunderstanding my post. I was curious as to why the longer one was bubbling and the other wasn't. I had expected them both to bubble, or just the shorter one.

Take a piece of tubing and a bucket filled with water.
Put one end of the tube in your mouth and suck vigorously, just kidding.

1. Put one end in your mouth and blow air through it.
2. Now take the free end and hold it in the water to a depth of 2 inches and blow through it, it will require more effort.
3. Finally take the free end and immerse it to near the bottom of the bucket and blow through it, it will take a lot more effort.

That is due to "Hydrostatic Pressure" which varies increases as an object, the end of the tube, gets deeper in a liquid.
If your tube ends are at different depths in the water you will have at least some effect from it.

 

[Deadalus]

Soemthing not mentioned is that the tubing with the coupling will have added resistance from it. If the coupling is on the shorter tube length, the coupling resistance may exceed the additional line resistance on the longer tubing.

 

[mac_1103]

the tubing with the coupling will have added resistance from it

And I suppose the coupling could also be leaking. But if I'm following correctly, it's the longer hose that has the coupling and is the only one bubbling.

 

[Deadalus]

And I suppose the coupling could also be leaking. But if I'm following correctly, it's the longer hose that has the coupling and is the only one bubbling.

I reread but I couldn't quite tell which one had the coupling. I was leaning towards the shorter one because the OP said the one with coupling needed the coupling because the one half couldn't reach on its own when unless a length was added. It could be leaking as well. Couplings don't incur a lot of resistance either as far as fittings go, 45s, 90's and tees are usually higher. I'm not sure its much of an issue with gases either. I just don't think the two different blowoffs are equal resistance.

 

[Rish]

One of the things I love about HBT is how there can be this much detailed discussion about why one tube is bubbling and one isn't!

 

[mashdar]

Large air handling systems operate in the range of inches of water column. That stuff's heavier than you'd think. A fraction of an inch will swamp any pressure drop across the tube, unless you've got some serious flow.

 

[Komodo]

One of the things I love about HBT is how there can be this much detailed discussion about why one tube is bubbling and one isn't!

This is why I asked it.
I’ve been around here for a long time.

BTW - absolutely no leaks at coupling. Its a SS pickup tube and I could just barely fit the 1/2” hose over the ends. I can’t say for a fact that both tube ends were exactly the same depth, but I think so as they were side by side so easy to tell.

 

[Deadalus]

This is why I asked it.
I’ve been around here for a long time.

BTW - absolutely no leaks at coupling. Its a SS pickup tube and I could just barely fit the 1/2” hose over the ends. I can’t say for a fact that both tube ends were exactly the same depth, but I think so as they were side by side so easy to tell.

Which length of hose is the coupling on, the 40" or 28"?

 

[mashdar]

Which length of hose is the coupling on, the 40" or 28"?

Yes!

edit: coupling couples two pipes : )

 

[sbro]

Have you tried raising the shorter one in the water until they are both bubbling?
The depth difference of each hose will show you how much "resistance" or pressure differential exists between the 2 hoses.

 

[Bobby_M]

I think the answer has been provided but I'll say it concisely. The length of the hose doesn't matter in this case, It's the depth of the insertion into the bucket and even a small variation will make a difference.