Flow calculations

[ZaneTer]

Some of you may be aware that BRS recently did a YouTube video about flow. There was mention about Dana getting great results at 4” per second. This got me thinking quite a lot about what may be required for me to reach the same value in my own tank.
To achieve this as laminar flow:
A little maths:
I have a 1.8m x 0.6m x 0.6m tank.
It takes 18 seconds to change tank volume once @0.1m/s. This equates to 200x tank turnover per hour when using laminar flow length wise across the tank.

If we go front to back it becomes:
6 Seconds for 1x turnover which means that I have a turnover per hour of 600x

This leaves me with two thoughts. The first is that we generally don’t have enough flow in our tanks, I have just bought enough pumps to achieve 300x turnover but an average of 225x per hour. The second thought is people should be cautious how they measure flow when comparing turnover and laminar velocity. The problem with using laminar velocity is that when it gets calculated into turnover then your tank dimensions come into play. The safer bet will always be to work with turnover.

Thanks
Zane

 

[Brew12]

Some of you may be aware that BRS recently did a YouTube video about flow. There was mention about Dana getting great results at 4” per second. This got me thinking quite a lot about what may be required for me to reach the same value in my own tank.
To achieve this as laminar flow:
A little maths:
I have a 1.8m x 0.6m x 0.6m tank.
It takes 18 seconds to change tank volume once @0.1m/s. This equates to 200x tank turnover per hour when using laminar flow length wise across the tank.

If we go front to back it becomes:
6 Seconds for 1x turnover which means that I have a turnover per hour of 600x

This leaves me with two thoughts. The first is that we generally don’t have enough flow in our tanks, I have just bought enough pumps to achieve 300x turnover but an average of 225x per hour. The second thought is people should be cautious how they measure flow when comparing turnover and laminar velocity. The problem with using laminar velocity is that when it gets calculated into turnover then your tank dimensions come into play. The safer bet will always be to work with turnover.

Thanks
Zane

I'm not sure it is even that straight forward. There are so many other factors that come into play I don't believe any flow calculation that includes an entire tank has much value. If you have two islands with SPS on them, you need much less total flow than someone with a rock wall. An open structure with flow channels will need much less total flow than one with very short flow paths.
Doing a calculation like you propose would make a lot of sense for someone using a closed loop system. Probably less value for someone with flow pumps/power heads.

 

[ZaneTer]

You are absolutely correct brew. This was just posted so people don’t fall into the potential trap of saying they must have 10cm/s flow velocity.

In itself it’s a good goal but it doesn’t factor in that we have rock structures in our tanks and flow varies as it flow patterns meet. Personally I have a system of rotating power heads providing roughy 150kl/h of flow rotating every 5min (3 out of 4 pumps are always on) with an occasional burst of all four pumps on hitting 200,000l/h flow.