Fluid Dynamics

[advena]

i posted this under "reef discussions" but didnt get any responses, so i figured some ADVANCED folks might know the answer...



gotta fluid dynamics question for yall that i cant find an answer to.

is it the height of the water column or the height of the drop that is the deciding factor on gravity returns??

more info:

if you (or in this case I) have a 'fuge mounted above the display (say the bottom of the fuge is level with the top of the display & is a 30gal that is 19" high) & is drilled about 2" below the top of the tank (plumbed with 1" PVC), with bulkhead installed & line leading to the display...

and you want to have the gravity return going to the BOTTOM of the display (150 gal tank--24" high)...

is this possible??

my sister got me worried because, to make a long story short, im getting a new display tank (150gal -- 60x24x24) & would have to drill the hole for the gravity return about 6.5-7" below the water surface (on the new display), and shes a'thinkin' that with just 1-2 inches of water column above the outlet for the 'fuge (even though it is almost 1.5 feet above the display) wont flow into the display (that is almost 1.5 feet below the fuge) because the display has 6.5-7" of water above the proposed 'fuge INLET...


i really hope this makes sense... im a weeeee bit punchy right now :spin2:

hope someone can help me where no post could...

-scott

 

[isuchopper]

punchy translation

punchy translation

Scott-

If I understood your question right, you have a fuge whose water level is HIGHER than the water level in the tank. If this is the case, you can run your return line to any point in the tank and you'll have water flow. Being a complete noob to salt water, I'm not sure if there's some special plumbing that changes the fuge water level in certain spots or some other voodoo like that, but given what I understand, it will work fine. A picture perhaps?

Mike

 

[Putawaywet]

advena, interesting question.

And as Isuchopper explained, you'll be good to go.

But now you have me wondering... so taking it one step further, does the fact that there is a greater (however slight) pressure differential at the bottom of your display affect the speed at which water drains from the fuge?

In other words, will the fuge drain at the same speed with it's output 1" below the surface of the display as it would with it at the bottom of the 24" tank?

inquiring minds want to know.

Brett

 

[advena]

sillyness & suchlike...

sillyness & suchlike...

thanks guys...

Mike-
you understood my question exactly!
i could take a pic but im not sure it would be necessary because with my current setup i have the gravity return going directly into the top of the display (no plumbing in the display) so it wouldnt apply to my question...
there is no plumbing voodoo taking place though...
Plumbing is all 1"... bulkhead on the fuge-->ell-->4' of pipe at slope to display bulkhead level-->ell-->display bulkhead.

here are a couple quick sketches to clear anything up (especially if more ppl have ideas!!):

OK- actually as i was sketching this stuff out i realized that awhile back i had the 'fuge plumbed totally differently & the return was going to the BOTTOM of an unused overflow!!!

so yes it does work... & yes my sister was really wrong, and YES we are all very lucky that i dont operate heavy machinery for a living!!!

this has gotten me interested though...
i have a question if there is anyone out there that knows about this stuff:

is there a limit for this type of situation? is there ever a situation that a body of water WONT drain into a lower body of water??
like (theoretically) if you had a 30gal tank & you wanted it to drain 50' below the water surface (like into the ocean or something and you plumbed a 50' pipe from that tank down) and the tank was ABOVE the water surface... would it drain???

curiosity hasnt killed me yet...
in fact thats why im obsessed with this hobby in the first place!!!

i guess this is a longwinded way of asking the same thing as Brett!!!

now TWO inquiring minds want to know...

-scott

 

[BeanAnimal]

Very simple answer, the total "drop" is the distance between the surfaces of the two liquids. This distance between the surfaces of the bodies can be defined as the "head", and it is what will dictate the flow rate (along with pipe size and friction). Of course we are talking about a "free flow" system. In a siphon or other type of system (where air is present or introduced into the plumbing for example) then the "head" calculations become a bit more involved.



As long as both bodies of water are open to the same air pressure, then the body of water with the higher surface level will drain to the lower body.



Enjoy

 

[advena]

cool... thanks for the answer.

so the larger the distance between the surfaces of the bodies of water, the faster the flow no matter how deep the plumbing goes from the upper body into the lower body of water??

is that right?

thanks again for the answer.

-scott

 

[BeanAnimal]

Yup

 

[Mclendonmdw1]

Without getting two deep in hydraulic theory BeanAnimal is absolutely correct. If you check the pump curve for your return pump it will give the available flow at any of a number of head values. These numbers are ideal values in a perfect system with little or no losses. This can do two things for you.
1. you can calculate about what your flow rate is through your system by measuring the head differences between the two surfaces of your tanks. Very useful to us reefers.
2. This is not something we are typically concerned with but you can also measure the actual loss of head in the system. Do this by measuring the output (time required to fill a five gallon bucket in minutes and convert to hours giving GPH) from the return line in this case it’s the gravity return to the main tank. This is tricky because you need to maintain a constant surface elevation of the water in the lower tank, so the pump is sucking from a constant head lower tank to a constant head top tank. This is known as a constant head test. You then take your Gallons per hour (see 5gal bucket) and your head difference between the surfaces and plot them on the graph that shows the pump curve and BAM head loss of the system…..BORING and unless you are designing a pump station I just made you dumber…lol.
Knowledge is power

 

[advena]

thanks for the added info...
came just in time 'cause i just got the commish for the new NYC main pump station

anyways i gotta agree that more you know the better... the stuff you "dont need" just gets filed in the "use later file".

i also gotta agree with your sig, LOL...
im from oahu, and just moved here to denver & i must say that i miss surfing the most...
snowboarding here is GREAT but it AIN'T surfing & no matter how much it dumps it will never equal a good session even with li'l head-high barrels

lates,
-scott

 

[goby1]

How will you stop air getting in to the line, causing bubbles to rise in the display?

G1

 

[advena]

there are two ways:

1) set your feed pump (from the sump) at such a level as to make the fuge level HIGHER than the outlet leaving the sump.
2) put a valve on the outlet line (from the 'fuge) to control the level of water exiting the sump ...

i use the second way... this way you dont need to keep the water level above the outlet because the water "piles up" behind the valve & so just water goes into the display.

-scott