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Flow Rates

RichardJW

Member
Joined
29 Oct 2013
Messages
101
Location
Shaw, Oldham
There seems to be much discussion about good flow of water throughout the tank - what I'm interested in is Flow rates . I'm running at 1000L / hr in a 180L setup ( I.e x 5.5 times tank vol per hour ) and am happy that the coverage throughout the tank is good - BUT I do feel it is a bit on the heavy side . What is considered a bare minimum turnover / hr ??




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I think a lot also depends on the way it's delivered. Obviously the less outlet points the water has, the more aggressive it will be.

5.5x turnover is by no means over the top as you probably know, but the way you describe it makes me think you're delivering it in the wrong way perhaps.

I have a 260l and turning over 2500 lph which is near enough 10x and the flow is very gentle. I have a full length spraybar delivering 2000 of that and just one outlet taking care of the other 500.

What sort of set up are you running?
 
Flow rate and flow velocity are two different things, however they are related. My 28L nano tank has a 250l/h hob filter with an outlet with a large hydrulic diameter and the current within the tank is very gentle. My 125l tank has a 1200L/hr canister filter with a spray bar which has an hydrulic diameter much smaller in relation to the amount of flow. The water velocity in this tank is much more of a torrent. I could reduce this torrent by increasing the numer or size of holes in spray bar.

Since Q=VA where Q is the volumetric flow rate which is constant, I increase A which is the cross-sectional area of the flow, this in turn reduces V which is the flow velocity.

There's other factors at play with energy losses but this is the basics.
 
There seems to be much discussion about good flow of water throughout the tank - what I'm interested in is Flow rates . I'm running at 1000L / hr in a 180L setup ( I.e x 5.5 times tank vol per hour ) and am happy that the coverage throughout the tank is good - BUT I do feel it is a bit on the heavy side . What is considered a bare minimum turnover / hr ??




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10x
 
I think a lot also depends on the way it's delivered. Obviously the less outlet points the water has, the more aggressive it will be.

5.5x turnover is by no means over the top as you probably know, but the way you describe it makes me think you're delivering it in the wrong way perhaps.

I have a 260l and turning over 2500 lph which is near enough 10x and the flow is very gentle. I have a full length spraybar delivering 2000 of that and just one outlet taking care of the other 500.

What sort of set up are you running?

All of the flow is via a Lily pipe - see my thread under Toy Room Project . Now I've got some fish in and have watched how they move , react and flourish I'm convinced that I was getting a touch paranoid . I'd visited Stockports Pets@home store and seen the Big Tank setup there - I didn't like the fact that some fish were swimming really hard against the current ( as opposed to flow ) .
Spray bar is certainly an option , if as it matures I'm not happy with how things are flowing . ATM there is none of the usual algae startup hitches so I'm guessing that something is right - never one to rest on my laurels I intend to be ever vigilant though.


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Flow rate and flow velocity are two different things, however they are related. My 28L nano tank has a 250l/h hob filter with an outlet with a large hydrulic diameter and the current within the tank is very gentle. My 125l tank has a 1200L/hr canister filter with a spray bar which has an hydrulic diameter much smaller in relation to the amount of flow. The water velocity in this tank is much more of a torrent. I could reduce this torrent by increasing the numer or size of holes in spray bar.

Since Q=VA where Q is the volumetric flow rate which is constant, I increase A which is the cross-sectional area of the flow, this in turn reduces V which is the flow velocity.

There's other factors at play with energy losses but this is the basics.

Bit of applied physics there - explains well how 10x turnover can be managed without stressing the fish .


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I currently have over 25x turnover in my 80l using a Fluval 105, a Koralia 1200 and an Eheim nano skimmer, going by the manufacturers ratings anyway (in practice it will be much less). I've had up to 45x in the past too.

I like dense plant growth and the "jungle" look so I find this flow necessary for adequate CO2 distribution. Could even do with more tbh.

Never had a problem with the fish, even Siamese fighters have been fine in the tank.

IMO a lot of fish seem to like the extra flow, I observe many species periodically choosing to swim directly against the filter/wave-maker outlets.


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There seems to be much discussion about good flow of water throughout the tank - what I'm interested in is Flow rates . I'm running at 1000L / hr in a 180L setup ( I.e x 5.5 times tank vol per hour ) and am happy that the coverage throughout the tank is good - BUT I do feel it is a bit on the heavy side . What is considered a bare minimum turnover / hr ??
Hello,
The 10X rule is a rule of thumb which helps to guarantee that there is sufficient energy to move the fluid to most, or all regions of the tank. Flow rate should always be considered within the context of the distribution of that energy throughout the various zones of the tank (high, low, left, right, center and so forth). So having some arbitrary flow rate is meaningless if the distribution of that kinetic energy is uneven, because if the distribution and flow profile are incoherent then there will be areas that have too much energy while other areas suffer insufficient flow. In that case, both areas are susceptible to the risk of nutrient and CO2 deficiency.

By far, the most critical function of flow/distribution is the delivery of CO2 and Oxygen to the leaf. Terrestrial plants have the advantage in this regard because CO2 diffuses and transports easily in air. On the contrary, CO2 or Oxygen dissolved in water diffuses and transports at a rate of approximately 10,000 slower than in air. This is a major disadvantage for aquatic plants. Gaseous transport in plants is, by a huge margin, the most important mechanism to their survival and prosperity, so in water, the first trick is to increase the CO2 partial pressure, which is accomplished by pressure injection. Higher pressure helps to transport the gas but the diffusion and transport rates are still appallingly low.

There is a much less well known obstacle to the movement of the gas in fluids. Fluids moving across a solid object have a frictional component known as viscosity. We all know the term from motor oil commercials but it applies to any fluid. This friction slows the movement of the fluid to nearly zero at a short distance from the surface of the object. In our case, the leaves will have a layer of stagnant water surrounding it, referred to as the boundary layer. Boundary layers having near zero velocity also therefore have a high static pressure. This high pressure pushes against the rest of the water above it and is an impediment to the movement of nutrients and CO2 from the larger water area. So, CO2 and nutrients inside the boundary layer will slowly move across the plants exterior membrane but replenishment of the gasses and nutrients from the free stream into the Layer are impeded due to the high static pressure of the Layer. As fluids are put in motion their static pressure decreases and the Boundary Layer thickness becomes thinner up to a certain speed. Above that speed, the Boundary Layer again thickens up. So there is a small range of fluid velocities near the leaf surface where increased flow reduces the thickness of the Boundary Layer and facilitates transport of nutrients/CO2 to the leaf. At velocities below this range the thickness and pressures are high, and above the range the thickness and pressures also increase.

Clever distribution schemes therefore allocate enough energy to all locations so that the fluid velocities are within the acceptable range in all the zones.

We normally advise that if all the leaves can be seen to be gently swaying, then this is an indication of good flow distribution. If some are moving like flags during a storm while others are static then this is an indication of poor distribution.

You might want to have a look at the video in the post http://ukaps.org/forum/threads/inline-devices.2965/#post-32127 to get a better illustration of flow theory.

Cheers,
 
Last edited:
Hello,
The 10X rule is a rule of thumb which which helps to guarantee that there is sufficient energy to move the fluid to most, or all regions of the tank. Flow rate should always be considered within the context of the distribution of that energy throughout the various zones of the tank (high, low, left, right, center and so forth). So having some arbitrary flow rate is meaningless if the distribution of that kinetic energy is uneven, because if the distribution and flow profile are incoherent then there will be areas that have too much energy while other areas suffer insufficient flow. In that case, both areas are susceptible to the risk of nutrient and CO2 deficiency.

By far, the most critical function of flow/distribution is the delivery of CO2 and Oxygen to the leaf. Terrestrial plants have the advantage in this regard because CO2 diffuses and transports easily in air. On the contrary, CO2 or Oxygen dissolved in water diffuses and transports at a rate of approximately 10,000 slower than in air. This is a major disadvantage for aquatic plants. Gaseous transport in plants is, by a huge margin, the most important mechanism to their survival and prosperity, so in water, the first trick is to increase the CO2 partial pressure, which is accomplished by pressure injection. Higher pressure helps to transport the gas but the diffusion and transport rates are still appealingly low.

There is a much less well known obstacle to the movement of the gas in fluids. Fluids moving across a solid object have a frictional component known as viscosity. We all know the term from motor oil commercials but it applies to any fluid. This friction slows the movement of the fluid to nearly zero at a short distance from the surface of the object. In our case, the leaves will have a layer of stagnant water surrounding it, referred to as the boundary layer. Boundary layers having near zero velocity also therefore have a high static pressure. This high pressure pushes against the rest of the water above it and is an impediment to the movement of nutrients and CO2 from the larger water area. So, CO2 and nutrients inside the boundary layer will slowly move across the plants exterior membrane but replenishment of the gasses and nutrients from the free stream into the Layer are impeded due to the high static pressure of the Layer. As fluids are put in motion their static pressure decreases and the Boundary Layer thickness becomes thinner up to a certain speed. Above that speed, the Boundary Layer again thickens up. So there is a small range fluid velocities near the leaf surface where increased flow reduces the thickness of the Boundary Layer and facilitates transport of nutrients/CO2 to the leaf. At velocities below this range the thickness and pressures are high, and above the range the thickness and pressures also increase.

Clever distribution schemes therefore allocate enough energy to all locations so that the fluid velocities are within the acceptable range in all the zones.

We normally advise that if all the leaves can be seen to be gently swaying, then this is an indication of good flow distribution. If some are moving like flags during a storm while others are static then this is an indication of poor distribution.

You might want to have a look at the video in the post http://ukaps.org/forum/threads/inline-devices.2965/#post-32127 to get a better illustration of flow theory.

Cheers,

Wow , thanks ! I'm glad I've got a Science degree !!!
In a nutshell , if the all plants are moving GOOD only some BAD .



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