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post #22 of (permalink) Old 02-13-2013, 04:03 AM
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Originally Posted by AndreyT View Post
That's incorrect.
Ummm No its not incorrect, Physics is still physics. Canisters work on a gravity fed siphon. A canister filter relies on gravity to feed and prime the filter to fill it with water. With out it you cant get it filled with water. The pump does not create a vacuum - if it did, the canister would be capable of self priming. The filter doesnt use internal equalization to create flow. All it use the self equalization for is getting the water to the medium and to the pump.

Try this experiment.

Stand on a ladder or chair above your tank. Using a hose try to suck on the hose to create a syphon and pull water out of the tank. You can suck on the end of the hose and you might get the water to go up the hose while your sucking so hard your about fall over. BUT stop the suction and what happens. The water will run right back down the tube and back into the tank. Why.....?? No gravity to pull it down the tube to start the syphon effect. Do that over the side of the tank and you can pull the water right out of the tank. pull it back over the level of the tank and its no longer syphoning. So yes it takes gravity to feed a syphon. Thus a canister needs that to even fill the filter up to create equalization inside and gravity must be constat to keep a syphon happning.

Originally Posted by AndreyT View Post
In any canister filter, the weight of the column of water in the intake tube is always precisely counterbalanced by the the weight of the column of water in the return tube. For this reason, the net effect of water weight in the tubes is always precisely and exactly zero. I.e. gravity plays no role in moving the water through the filter whatsoever.
If this was the case then a pump wouldnt even be needed all one would need to do is create a syphon and it would circulate. It takes the weight of the water falling (this is caused by gravity as weight is a form of gravity and its pull on things) through the tube on the intake to create the syphon thus filling the canister What happens if you get an air bubble in the intake tube but the canisters full? You get a canister filter that doesnt flow. How does the syphon keep flowing? Falling water through the tube continously filling the filter so it can reach the pump. Raise the canister above the tank and the syphon slows to a halt. Why.. Gravity has less effect. Resistance is higher and cretaes a flow restriction on the intake. Volume in is not equal to volume out. As mentioned before Canister are unable to pump/create vacuum inside a canister to keep a syphon or start a syphon. They are PUSH pumps only. There is a reason why manufactures of canister tell you they need to be placed lower below the tanks for a reason.

Originally Posted by AndreyT View Post
Canister filters are not "fed by gravity", as it is often incorrectly believed. Wet-dry filters are fed by gravity, canister filters are not. This is the defining difference between open (wet-dry filters) and sealed (canister) filters. Canister filters, by design, do not require "feeding" at all. They are based on the idea of circulating continuous volume of "weightless" water. From the principle of operation point of view, canister filters do not have "intake side" or "return side": they are perfectly symmetrical in that regard. Speaking informally, both sides of canister filter always provide exactly the same "intake pressures", which meet inside the canister and cancel each other.
BOTH canisters and wet drys are fed by gravity and syphon effect. The only difference is the forces of atmospheric pressures between the two as the wet dry is exposed to such pressures and canisters are not. Canister filters are basicly a controled leak. Your creating a leak by means of gravity to flow down an intake tube fill a fliter and then get it to a pump that pushes it up a outlet. All the while following the least pasth of resistance. The leak being the path of least resistance.. Aka Intake leaking flow into the canister via gravity and output being push via pump as long as you dont excedee the head pressure.
Originally Posted by AndreyT View Post
This is actually the fundamental principle that justifies the very existence of canister filters. Since gravity and weight of the water plays no role at all (the water in this system is effectively "weightless"), the pumps in canister filters does not have to "lift" the water. It only has to push the water through the media. Canister filters were invented specifically to take advantage of this principle.
Lets look at the MAJORITY of designs of canister.. Where is the pump located? Before or after the filter media? MOST ALL if not ALL pumps on canisters are AFTER the media. The filter brings in the water from the intake side via siphon flow caused by gravity. THEN it flows into the filter and goes to the bottom and fills the filter from the bottom up. Going through the Media trays from bottom to top. Its this way by design for 2 reasons. Using this method allows us to push out the air in the canister easier thus easier filling with water. It also allows for a Smaller pump to be used to get the job done if the pump is in the top of the filter at the outlet fitting as it then only has to force the water up the outlet hose. So a pump NEVER pushes the water through the media. It only pushes the water out the outlet hose. How does the water manage to get to the canister and to the bottom of the filter and through the media to reach the pump to allow the pump to do its job pumping it through a smaller hose back into the tank? Gravity and the path of least resistance. Whne using a sealed canister its forced in that direction of least resistance.
Originally Posted by AndreyT View Post
As long as the system is filled with water and sealed, it can be placed below the tank, above the tank, next to the tank - anywhere. The filter itself and will not be able to feel any difference. The pump load will remain constant. This is just school physics.
Really? then why do canister manufactures tell you specifically placement of a canister needs to be below. Not next to, not above, not even with. Pump loads will NOT remain constant. If you change head pressure on any given pump a pump with lower head pressure will pump more. BUT its also proven that a syphon can not work if it is above or to equal with its souce. This is school physics. Its also proven the pump does not create a vacuum. If it did it would prime itself.
Originally Posted by AndreyT View Post
P.S. The reason canister filters have height-difference limitation is not the pump load, but rather seal strength. The lower you place the canister - the greater the water pressure inside the canister becomes. If the canister is placed too low below the tank, the seals will begin leaking.
Sorry this description is just silly and makes no sense. A canister system is NOT a 100% sealed system. It has a inlet and an outlet. Pressure should NEVER build high enough to create a leak at a seal. Why? again.. Due to having a outlet in the filter. The reason for height limitations for the canister is the head pressure it applies on the pump trying to push the water up the hose. If it gets to high it will have to much head pressure and reduce the flow greatly or to nothing. Has nothing to do with causing a seal in the filter to leak. Remember Physics about water.... It follows the path of least resistance. The canister is it self a sealed unit except for the inlet and outlet. If you increase the resistance of the water with a higher head pressure you dont cause a leak. you just create zero flow due to the amount of head pressure fighting the waters abillity to return to the tank. At this point you dont have a syphon anymore you have zero flow out. Its just sitting stationary whith a pump running doing really nothing. Has ZERO to do with creating a seal to leak.

I know several Fluid Engineers and real world people that would disagree with your post on how it works. Gravity is involved. Does a syphon exist in zero gravity? I dont believe so..

Canister filters and wet drys all work off of syphon effects, as you state yourself. (were not talking on back mounted filters) Syphons are the results of gravity.

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