Keg Transfer Math Question

[agentbud]

Things that make me go hmmmm. For the purposes of this question, let's assume a "semi-closed" transfer from a fermenter to a keg where neither of the vessels are pressurized and both are vented so that only gravity is being used. A single hose goes from the drain valve on the fermenter to the beer out post on the keg. The fermenter is set at a height where the drain valve is just above the top of the keg. Naturally gravity would do its thing and the beer would start transferring. But, what happens as the volume of liquid in the fermenter decreases and that in the keg increases. Since the keg is being filled from the bottom up, does it ever get to the point where the liquid pressure in the keg becomes greater than that of the gravity pushing the beer out of the long tube at the bottom of the keg such that it stops flowing? If yes, would the flow start again of the fermenter was raised higher, possibly increasing the forces of the beer coming down over that of the pressure of the liquid in the keg?

 

[bailey mountain brewer]

As far as I know the keg should fill... I'm sure someone will have a detailed answer as to why but I'm not that someone. I know if I set a big mouth bubbler next to my sink and drain it into another vessel with a hose going to the bottom of that vessel it will drain down til the levels meet even if that means there is 5.5 gallon in the recieving vessel and only .5 gallon in the big mouth bubbler.

 

[day_trippr]

Pretty basic stuff. The vertical distance of the drop from fermenter to keg matters - a lot. Too short a distance and you'll never empty the fermenter.
If you set the fermenter and keg on the same level the flow will likely stop somewhere in the middle of the transfer, but as long as the tubing remains full so its siphon ability isn't lost, raising the fermenter would cause it to empty further...

Cheers!

 

[agentbud]

If you set the fermenter and keg on the same level the flow will likely stop somewhere in the middle of the transfer, but as long as the tubing remains full so its siphon ability isn't lost, raising the fermenter would cause it to empty further...

Cheers!

I get that part. But if the fermenter is already above the keg, as the keg fills, does the downward pressure of the liquid in the keg (ie putting pressure at the entry point of the beer - the end of the liquid dip tube) affect at all the ability of the remaining liquid from flowing in or will that pressure eventually stop the flow? Think of it in the extreme - let's say you have a coke bottle with a hose connected to a valve on the bottom and that hose runs down to the bottom of a swimming pool. When you open that valve, will the coke run into the pool or will the pressure at the bottom of the pool prevent it from coming out of the hose?

 

[DuncB]

And the flow is proportional to the 4th power of the tube radius.
so double your tube diameter will increase flow by sixteen times.
Length, viscosity of the fluids is proportional.

 

[DuncB]

@agentbud Think of your analogy another way

If you put a hosepipe to the bottom of a swimming pool does the water flow out of the pipe?

 

[CascadesBrewer]

I don't know the math/physics, but my understanding is that the liquids in the two vessels will settle out to the same level. As long as the level in the fermenter is above the level in the keg, the liquid will flow.

In any case, I do what you describe and do not have an issue filling a 5 gallon keg from my fermenter with the fermenter about 1 foot above the keg.

 

[McKnuckle]

I do this all the time. The key is to equalize the pressure between the two containers and any connective tubing.

I ferment in an SS BrewTech bucket that sits in a small fridge. I stick an airlock in the bucket, and run tubing from that down to a keg standing on the floor. The CO2 purges the keg during fermentation. Another tube from the keg's liquid port runs to a blow-off jar where it bubbles just like a regular airlock.

I leave this system enclosed, so it is entirely filled with CO2 - but at atmospheric pressure. When it's time to fill the keg with beer, I remove the blow-off tube from the keg. The system is still closed. Then I connect the bucket's spigot to the keg's liquid port. I open the spigot, and beer flows into the keg. The gas port and airlock are still connected with tubing, so CO2 from the keg flows back into the bucket (no pressure). The bucket will empty down to the bottom.

If you aren't getting this fancy, it will still work - but you have to open the system to atmospheric pressure. Vent the keg via its gas port to a blow-off jar or just into the air. And open the fermenter at the top.

 

[agentbud]

I stick an airlock in the bucket, and run tubing from that down to a keg standing on the floor. The CO2 purges the keg during fermentation. Another tube from the keg's liquid port runs to a blow-off jar where it bubbles just like a regular airlock.

So, during fermentation when you are purging the keg, would you want the hose coming from the fermenter to connect to the gas-in post so that the CO2 pushes the oxygen down the keg and then up and out the liquid tube. Or would you want to connect it to the liquid-out post so that the CO2 comes in at the bottom of the keg and pushes the oxygen up and out the gas tube? The latter seems like it would purge better but that's just me.

For the record, the way I do it is to connect a CO2 tank to a cask breather and that to the top of the fermenter. The out at the bottom of the fermenter runs to the liquid out on the keg and the keg is vented to the air. As the fermenter empties, the cask breather allows CO2 to fill the void at atmospheric pressure, thus avoiding oxygenation. My only concern with this process is whether I am losing aromatics out the top of the vented keg.

 

[McKnuckle]

The short answer is that it doesn’t matter which post is used for the CO2 ingress and which one vents it. Either way, the CO2 has to push out gas on the opposite end of the keg, and it all gets purged (and then some) by the end.

I’ve used both. But the reason I now use the gas post is so I can leave that tubing intact for the return trip (i.e. CO2 back to fermenter). It’s fewer pieces to change out.

 

[Bobby_M]

The answer is no. As long as the top level of the liquid in the donor vessel always remains higher than the top level of the liquid in the receiving vessel, it will continue to flow. Another way to say it is that liquid will find level.

Quick thought experiment.... Which pressure gauge reads higher; 800 gallon tank or 250 gallon tank.

Here's another one that trips people up constantly. Assuming pumps A and B are identical, which tank will fill to the top first?

 

[DuncB]

@Bobby_M

You aren't getting many takers.

Pressure in tank with 250 gallons in will be higher. The deeper you dive the higher the pressure gets, doesn't matter if it's in the atlantic ocean or a submarine escape practice tank the pressure at 50 m deep is the same in both ( provided the atmospheric pressure same and they are both at sea level and the water is the same).

Regarding tanks X and Y I notice that tank X has more water in to start with.

 

[Brewdog80]

Ive drained for decades from my fermenter down into my keg. As long as the fermenter is higher than the keg it will drain. The lower the level the slower the flow, but it will empty.
OH, as far as keg purging with co2, if you really want to purge most of the o2, connect the inflow to the liquid, co2 is heavier than o2 and you need to fill bottom and it will push o2 out of the gas valve at the top. If you fill from the gas port, you will be mixing the o2 and co2 way more. Just sayin.science...

 

[Bobby_M]

@Bobby_M

You aren't getting many takers.

Pressure in tank with 250 gallons in will be higher. The deeper you dive the higher the pressure gets, doesn't matter if it's in the atlantic ocean or a submarine escape practice tank the pressure at 50 m deep is the same in both ( provided the atmospheric pressure same and they are both at sea level and the water is the same).

Regarding tanks X and Y I notice that tank X has more water in to start with.

It was more of a rhetorical exercise so I didn't mind but thanks for playing along. Yes, I actually meant for tank X to be slightly ahead at the start as a bit of bait.

The explanation is that pump A has a head pressure that starts and ends higher than pump B will ever see. In the picture below, the red line represents the starting head pressure for each pump. The green line is the final head pressure when the tanks are full. The reason I like this one is that I hear someone say that they don't want to pump-fill a vessel from the drain valve or bottom drain because it's slower than pumping to a port on the upper sidewall and they don't want to put excess strain on the pump. No exaggeration, I hear it about once a month for the past 10 years.

 

[DuncB]

@Bobby_M
Now that I know that X wasn't an error, but you've answered anyway they could still fill the same rate or defy the pump factors as they are 2D tanks and one could have a much bigger third dimension we can't see. But agree the head of pressure that pump A is up against is much more than pump B.

 

[Bobby_M]

@Bobby_M
Now that I know that X wasn't an error, but you've answered anyway they could still fill the same rate or defy the pump factors as they are 2D tanks and one could have a much bigger third dimension we can't see. But agree the head of pressure that pump A is up against is much more than pump B.

Ah, good catch. I should have clarified that both tanks have the same capacity.

 

[natmartin]

This is fascinating, and shows why I'm an Electrical Engineer and not a Mechanical Engineer. I've just started using a conical, and so far I had avoided bottom-filling because I was worried that the grainfather pump couldn't keep up... instead I was pumping to a port on the lid. (The grainfather is on the ground, the conical is on a counter, so there's pretty decent height difference.)

Now I know that bottom filling will actually be easier on the pump.

 

[agentbud]

The deeper you dive the higher the pressure gets, doesn't matter if it's in the atlantic ocean or a submarine escape practice tank...

LOL. I love this comparison. I, for one, have no clue what a submarine escape practice tank looks like. Maybe compare the atlantic ocean to a 20 BBL fermentation tank. That I can visualize!

 

[Deadalus]

It was more of a rhetorical exercise so I didn't mind but thanks for playing along. Yes, I actually meant for tank X to be slightly ahead at the start as a bit of bait.

The explanation is that pump A has a head pressure that starts and ends higher than pump B will ever see. In the picture below, the red line represents the starting head pressure for each pump. The green line is the final head pressure when the tanks are full. The reason I like this one is that I hear someone say that they don't want to pump-fill a vessel from the drain valve or bottom drain because it's slower than pumping to a port on the upper sidewall and they don't want to put excess strain on the pump. No exaggeration, I hear it about once a month for the past 10 years.

View attachment 760676

There's more friction loss in X's pipes too. They have the same number and type of bends.

 

[Deadalus]

I am skeptical that with typical tubing that the flow would stop but if your tubing was sufficiently rough and perhaps narrow that the flow could theoretically stop due to friction.

 

[DuncB]

@agentbud
This will give you the gist of a submarine training tower and it's purpose.
https://en.wikipedia.org/wiki/Submarine_escape_training_facility
I used to look out at the portsmouth one shown in the picture on the wiki page. Only 30 m tank in fact but the tower looks at least 50metres.
There was the odd " accident " in it, I never visited it.

 

[DuncB]

@Deadalus
Diameter of pipe is more important than Length ( horizontal ), if you double the pipe diameter flow increases 16x, that would easily counteract a rough tube but you wouldn't want a rough tube in your brew system.

 

[Deadalus]

@Deadalus
Diameter of pipe is more important than Length ( horizontal ), if you double the pipe diameter flow increases 16x, that would easily counteract a rough tube but you wouldn't want a rough tube in your brew system.

I think you are being imprecise and I did say theoretically. Theoretically I could make the pipe 100000 ft long and very narrow in diameter and very far away horizontally. Narrow enough that doubling it wouldn't do anything. I think what you are meaning to say though is that if you could change the length or the diameter by the same factor, which is more important? I didn't say anything about the length vs the diameter anyway though.

 

[scrap iron]

Riddle me this, sorry watched an old Batman movie last night.
Yesterday I was doing my normal closed transfer using a SS bucket from spout to out post on the keg. I run the gas line back into the top of the bucket. The only difference was I used some of the Evabarrier 4mm tubing I just bought. Before I had always used regular 3/16" Bevflex tubing for the beer side. I thought I'd try it out with the Duotight fittings I bought too. Everything was working fine until the last part of the fill. The beer stopped draining and I weighed the keg and found it was light. I peeked in the bucket and seen there was still about 1+ gal still there. I then changed out the 4mm [0.15748"] for the 3/16" Bevflex [0.1875"] lifted the bucket for a few seconds and the flow started again. The bucket was on the counter and keg on the floor.
Why did the flow stop, was it the different inside diameter of the lines? the shorter length of the Eva then the Bev? I don't know but thought maybe a chunk of yeast in the pickup tube but didn't notice any when it started draining again.

 

[DuncB]

I think you are being imprecise and I did say theoretically. Theoretically I could make the pipe 100000 ft long and very narrow in diameter and very far away horizontally. Narrow enough that doubling it wouldn't do anything. I think what you are meaning to say though is that if you could change the length or the diameter by the same factor, which is more important? I didn't say anything about the length vs the diameter anyway though.

I'm thinking that you are a little over sensitive. I'm just using information from poiseuilles law which is well recognised scientific fact, not your theory. You can make your pipe as long as you like but doubling the diameter will still increase the flow by 16x. I didn't infer " that if you could change the length or the diameter by the same factor " but just stated that doubling the diameter would increase the flow by 16 times which would more than compensate for the roughness of the tube you had.
PS your 100000 ft long narrow ( not a dimension incidentally ) tube wouldn't have to be very far away if you made it into a coil.

 

[Deadalus]

I'm thinking that you are a little over sensitive. I'm just using information from poiseuilles law which is well recognised scientific fact, not your theory. You can make your pipe as long as you like but doubling the diameter will still increase the flow by 16x. I didn't infer " that if you could change the length or the diameter by the same factor " but just stated that doubling the diameter would increase the flow by 16 times which would more than compensate for the roughness of the tube you had.
PS your 100000 ft long narrow ( not a dimension incidentally ) tube wouldn't have to be very far away if you made it into a coil.

No I just don't like that you have created a straw man argument. I never said anything about the length initially when you directed your post to me. Also you have to know the tubing material before you can calculate friction loss. You can't assume doubling the diameter is an automatic fix, because besides the material used the length can also be important. And this is what you exactly said

@Deadalus
Diameter of pipe is more important than Length ( horizontal ), if you double the pipe diameter flow increases 16x, that would easily counteract a rough tube but you wouldn't want a rough tube in your brew system.

I'm not disagreeing with what you are saying about doubling diameter by itself but what you have said about length isn't necessarily true. I provided a counter example. Plus I wasn't suggesting how to fix any friction issues, I was providing a theoretical example that might be a possibility but for a different reason than the OP suggested.