Head loss and canister filters.

[Pepere]

A frequently misunderstood topic.

I have seen several youtube presenters and posts on forums referencing head loss in canister filters that get it wrong.  
 

It has been opined that the lower you have your canister filter from your aquarium the more loss in flow you will get due to head loss.  This is not exactly true.

It is true in the case of a sump with free gravity return flow that the height of head from the pump reduces available head of pressure the pump has to move water.

In the case of a canister filter though, the height of the distance between the surface of the aquarium water and the filter does not impact flow in the same manner.  
 

The weight of the water the pump has to push up against does have head loss, but it is nearly completely offset by the weight of the water pushing down from the aquarium from the hose feeding water to the sealed pump.  Ie, one side is adding feet of head to the pump, the other side is substracting it.

2 points here though…

1.). The greater the height difference, the longer the hose.  Frictional losses of water flowing through hose impacts head loss.  And corrugated hose imposes greater turbulence and head loss per foot than smooth wall hose.  The smaller the diameter of hose the greater the head loss per foot.  Ie, 3/4 inch nominal pipe flows roughly twice as much as 1/2 pipe.  Small increases in diameter yields much bigger increases in surface area in a circle.

2.) canister manufacturers still list a total height difference that should not be exceeded.  It has nothing to do with head loss, but rather it has to do with static pressure. The greater the height difference, the greater the water pressure that is inside your canister filter.  The manufacturer engineered the seals to resist a set amount if pressure. Exceeding height difference puts you at risk for seal failure.

Most flow volumes given are rated at 0 head loss. That is a pump circulating water at the same water level with no restrictions.  Any restriction in the flow can be categorized as imposing head loss.  So, the media we put in to a canister filter imposes a restriction that can be reflected in terms of the head loss it imposes. The more restrictive the media in question, the more head loss imposed.  Fine sponges will impose more loss than coarse ones.  Polishing pads i pose more loss than coarse sponge.  We experience that head loss as flow reduction.

as the filter runs over time and the filter media gets progressively more filled, the head loss increases and the flow decreases…. When you clean or replace media, restriction goes down, head loss is decreased, and flow is increased..

[nabokovfan87]

On 6/16/2023 at 2:01 AM, Pepere said:

1.). The greater the height difference, the longer the hose.  Frictional losses of water flowing through hose impacts head loss.  And corrugated hose imposes greater turbulence and head loss per foot than smooth wall hose.  The smaller the diameter of hose the greater the head loss per foot.  Ie, 3/4 inch nominal pipe flows roughly twice as much as 1/2 pipe.  Small increases in diameter yields much bigger increases in surface area in a circle.

This is a wonderful point.  Bend radius also plays a role, generally speaking.
 

On 6/16/2023 at 2:01 AM, Pepere said:

2.) canister manufacturers still list a total height difference that should not be exceeded.  It has nothing to do with head loss, but rather it has to do with static pressure. The greater the height difference, the greater the water pressure that is inside your canister filter.  The manufacturer engineered the seals to resist a set amount if pressure. Exceeding height difference puts you at risk for seal failure.

Most flow volumes given are rated at 0 head loss. That is a pump circulating water at the same water level with no restrictions.  Any restriction in the flow can be categorized as imposing head loss.  So, the media we put in to a canister filter imposes a restriction that can be reflected in terms of the head loss it imposes. The more restrictive the media in question, the more head loss imposed.  Fine sponges will impose more loss than coarse ones.  Polishing pads i pose more loss than coarse sponge.  We experience that head loss as flow reduction.

Very interesting!  Thank you for explaining all of this. 🙂

[Pepere]

On 6/17/2023 at 3:01 AM, nabokovfan87 said:

Bend radius also plays a role, generally speaking.

Yes. A 90 degree right angle bend fitting imposes as much flow restriction as adding 3 feet of smooth wall pipe.