Two or more closed loops - counterproductive?

[dkeller_nc]

Some good options given above regarding DT water movement. There is a difference between the flow's given off by a close loop system and that of MP powerheads. With a well designed close loop system you can have several areas of Turbulent flow with a MP system using at least 2 you can have 1 area. That area would be only where their flow paths crossed/meet around the middle part of the DT.
MP's are a nice product and have their place just a close loop system does, but they are different in what they can do.

Um... That's not actually correct, at least from an fluid mechanics standpoint. The output from a nozzle from a CL system and the output of a propeller or traditional powerhead is fundamentally laminar (i.e., not turbulent). It would take incredible (and pretty much unachievable) flow rates through the loop in a CL system to convert the flow inside the pipe from laminar to turbulent. On a propeller pump or traditional powerhead, of course, it's laminar by its very nature.

So, that leaves one with creating a zone of turbulence by interaction of the flow field with the tank walls, rocks, or other flow fields. In this respect, flow from a nozzle of a CL system is completely equivalent to the flow from a powerhead/propeller pump. What one can do, of course, is drill more holes and put more output nozzles on a closed loop operated by a single pump, while in the case of powerheads/propeller pumps one must add an additional unit.

However, in all cases (no matter how many output nozzles are installed), a CL system is drastically less efficient than a propeller pump - by a factor of at least 5, and depending on how much knowledge of fluid mechanics the aquarist has when the loop is designed and installed, it may be less efficient by a factor of 10 or 15.

Closed Loop systems are also far less flexible than propeller pumps with regard to changing the aquascaping inside the tank and changing the flow pattern to accommodate the new lay-out. With a propeller pump, one simply moves it to a different part of the glass, or a different part of the rockwork.

Finally, the flow from a propeller pump can be changed far faster than a closed loop. Rapidly ramping up/down a centrifugal pump in a closed loop arrangement in a second or two as a propeller pump is capable of will dramatically shorten the life of the pump. Even if one could operate a centrifugal pump in a rapid on/off fashion, the flow coming out of the nozzles won't change nearly as rapidly, since the outlet piping constitutes a pressurized system where the longer the pipe length/larger the diameter of the pipe, the slower the flow response.

All this said, there are a few advantages to a closed-loop arrangement:

The first, as mentioned by the OP, is aesthetic. I do agree that powerheads/propeller pumps visibility in the aquascape should be minimized as much as possible. A closed loop drilled through the bottom of the tank makes concealment easy.

The second is the amount of equipment to maintain/repair. A closed loop has one pump, and possibly one drum-type flow switch. And practically speaking, one could have as much as 3 to 4 outlet nozzles on such a loop (any more than that and one starts getting into low back-pressure issues with a centrifugal pump, which will destroy the pump). To get the equivalent, one would need to purchase/maintain 3 or 4 powerheads, though the total flow will be a good deal higher with this option.

The third is systems that need a very large amount of water movement that only an industrial-sized centrifugal pump can provide. This is one reason that public aquariums almost exclusively use closed-loop systems for circulation, although Venotec is trying to change that with huge propeller/penductor pumps.

However,

 

[acesq]

"one could have as much as 3 to 4 outlet nozzles on such a loop (any more than that and one starts getting into low back-pressure issues with a centrifugal pump, which will destroy the pump)."

dKeller_NC: tell me more about this statement. Thanks.

 

[IANick]

Dkeller:

Thank you for the insightful comments. I will take a look at the FlowWolf digital diverter you mentioned.

Two things:

1) You mention the locline will reduce the volumetric flow rate. I think I can see why this might be the case. But I had mentioned a possible venturi mod to the loc line that would - hopefully - gain this volume back and possibly more (without adding bulky/difficult to conceal penductors). The link to the mod is here http://reefcentral.com/forums/showthread.php?t=1632025. Do you have any thoughts on this plan?

2) I second Acesq's query. Can you expand upon your comment that more than three or four outlet nozzles may lead to low back pressure issues?

Thanks.

 

[Aquatron]

Your funny turbulence isn't created in the pipe it can only happen when water from two different direction cross paths. How many times can the flow from two MPs cross
paths? Now how many times can the flow from lets say 4 close loop outlet's cross paths?
Like I said they both have their place but they are different in what they can do

 

[dkeller_nc]

Your funny turbulence isn't created in the pipe it can only happen when water from two different direction cross paths. How many times can the flow from two MPs cross
paths? Now how many times can the flow from lets say 4 close loop outlet's cross paths?
Like I said they both have their place but they are different in what they can do

No, turbulent flow in pipes is very much possible - it depends on something called the Reynolds number. The Reynolds number is a dimensionless constant that depends on the velocity of the flow, the diameter of the pipe, and the density and viscosity of the solution. For the same type of solution and temperature, the higher the flowrate, the closer one is to the transition point between laminar and turbulent flow. Another word for turbulent flow in a pipe is "plug flow".

But as I said in the post, one simply has to add the same number of propeller pumps/powerheads as the number of output nozzles on a closed loop to get an equivalent system. Since, however, there are substantially greater inefficiencies in an enclosed centrifugal pump head and large head losses created by the piping, the amount of power needed to generate a given number of flow fields from a closed loop versus the same number of flow fields generated by propeller pumps is considerably higher.

Other than the inefficiency associated with closed-loop designs and the much faster ramp up/down that a propeller pump is capable of, the flow fields generated from a CL and the same number of propeller pumps is equivalent.

 

[dkeller_nc]

Dkeller:

Thank you for the insightful comments. I will take a look at the FlowWolf digital diverter you mentioned.

Two things:

1) You mention the locline will reduce the volumetric flow rate. I think I can see why this might be the case. But I had mentioned a possible venturi mod to the loc line that would - hopefully - gain this volume back and possibly more (without adding bulky/difficult to conceal penductors). The link to the mod is here http://reefcentral.com/forums/showthread.php?t=1632025. Do you have any thoughts on this plan?

2) I second Acesq's query. Can you expand upon your comment that more than three or four outlet nozzles may lead to low back pressure issues?

Thanks.

One thing to realize about eductors/penductors is that they do not increase the output of a flow nozzle in the total energy sense - that would be a violation of the first law of thermodynamics (conservation of mass and energy).

What a educator/penductor does is exchange velocity for mass flow rate. The relevant equation in rectilinear coordinates is "kinetic energy = 1/2 m*v^2", where m=mass and v=velocity. The total kinetic energy of the fluid flow out of the opening with a normal nozzle and with a penductor remains the same (that's a bit of an approximation - the penductor may result in a higher head pressure/frictional losses, so the kinetic energy may actually be less).

What you can see from this equation is that if you hold the kinetic energy constant, you can drop the velocity of the fluid and gain a higher mass flow rate.

Note that this explanation is a bit simplified - the appropriate equations include the integral of the velocity field over the entire diameter of the flow field, and "m" is actually the mass flow rate integrated over the area of the flow field.

But regardless of the simplification, the conclusion is the same - a venture/penductor/educator exchanges velocity for mass flow rate. That can be a good thing in a reef tank, where one wants flow of water over as wide an area as possible at low velocities. It's a good way to convert the inherent high velocity, small cross-sectional area jet of water from a centrifugal pump into flow that more closely resembles the output of a propeller pump.

 

[dkeller_nc]

Dkeller:

2) I second Acesq's query. Can you expand upon your comment that more than three or four outlet nozzles may lead to low back pressure issues?

Thanks.

What I'm referring to is operating certain types of centrifugal pumps at an extremely low discharge pressure. There are several reasons why this may damage a pump, but one of the big reasons is cavitation, which is vaporization of the water on the low pressure side of a pump impeller surface. Whether or not this happens depends on the speed of the impeller, and the back pressure against which the pump is working. Higher back pressures ensure that the absolute pressure on the back side of the impeller blades is not dropping below the vapor pressure of the water going through the pump, which would result in cavitation and premature failure.

Note that the exact minimum back pressure below which the pump shouldn't be operated is sensitively dependent on the design of the impeller and centrifugal pump housing, the speed of the pump, and the temperature of the water going through the pump.

 

[acesq]

Thanks for that explanation. I think I may try to incorporate penductors in my C/L design to increase the mass flow rate at the expense of velocity particularly since they will be at the bottom of the tank nearer the sand bed and LPS.

Can you now expound on the low back pressure issue on centrifugal pumps caused by more than three or four output nozzles?

 

[dkeller_nc]

What I meant by that comment is that 3 or 4 nozzles would be relatively safe from the point of view of not running into a cavitation problem with your centrifugal pump. But I shouldn't have implied that there's an easy-to-reference hard number where less than that is OK and more than that is not.

There's a lot of uncertainty with this aspect of designing a piping system - whether or not you will run into issues will greatly depend on the piping diameter relative to the pump size, the number of nozzles/openings you have on the loop, and exactly what type of centrifugal pump you intend to use.

One thing you might consider is installing a ball valve immediately after the discharge of the pump. You can then restrict the output of the pump if you run into too low of a back pressure. If your setup will include a ball valve at each nozzle, then that would accomplish the same effect - you could simply adjust the total back pressure on the system by adjusting each one of the nozzles.

By the way - there's yet another aspect of this that you may wish to consider in your design. Some systems that incorporate automatic "slam shut" kinds of flow control (like closing/opening ball valves with actuators and some kinds of drum valves) can introduce something called "water hammer". When the valve closes, a pressure wave travels back through the piping system that cause vibration, and in extreme cases, component rupture.

Here's a wiki link to the subject:

http://en.wikipedia.org/wiki/Water_hammer

 

[IANick]

Darn you Mr. (Miss/Mrs.?) DKeller... Just when I start to think I'm getting a good handle on things, you give me new things to think and learn about (certainly hadn't heard of the "hammer" before). BUT, in spite of these possible new concerns, I'm thinking my general plan is becoming somewhat reinforced. That was a great description of how velocity is exchanged for volume via the Venturi effect on penductors (or, in my case, modified locline). But I see this as a good thing.

Regarding low pressure back flow, that is somewhat concerning I suppose... especially since I have no knowledge of how to calculate such a thing. For sure I wanted more than four outlets on each loop. Probably 6 to10 was what I had planned (certainly on the horizontally oriented incoming and outgoing tide loops). The barrel roll loop I may be able to get away with less - maybe 6 to 8. The idea was to get lots of outlets powered by powerful pumps (probably more powerful than needed) that would allow me leeway to "ramp up" if my flow seemed to be lacking. As mentioned I'd been thinking of the Waveline DC 20,000.

I took a look at the derkoon digital flow diverter and think this could be an ideal product to allow me to switch the incoming and outgoing tidal loops so that I can avoid having to power these with separate pumps and, more importantly, avoid having to feed them with separate intakes. The only problem on this front is that the specified maximum flow rate on the digital diverter is 12,000 liters per hour. This won't handle the flow rate of the DC 20,000. I wish I was more confident in calculating head loss through my piping system and then add back in the extra flow gained from the Venturi modded loc line to see if a dc 12,000 would be sufficient. While I guess I could (with effort) figure out the head loss in the piping, the gains in the loc line mod seem much more subjective and will leave me guessing at my final flow rate.

 

[acesq]

I'm having a hard time understanding how a lack of back pressure can cause cavitation problems. Can you explain this?

 

[dkeller_nc]

Darn you Mr. (Miss/Mrs.?) DKeller... Just when I start to think I'm getting a good handle on things, you give me new things to think and learn about (certainly hadn't heard of the "hammer" before). BUT, in spite of these possible new concerns, I'm thinking my general plan is becoming somewhat reinforced. That was a great description of how velocity is exchanged for volume via the Venturi effect on penductors (or, in my case, modified locline). But I see this as a good thing.

Regarding low pressure back flow, that is somewhat concerning I suppose... especially since I have no knowledge of how to calculate such a thing. For sure I wanted more than four outlets on each loop. Probably 6 to10 was what I had planned (certainly on the horizontally oriented incoming and outgoing tide loops). The barrel roll loop I may be able to get away with less - maybe 6 to 8. The idea was to get lots of outlets powered by powerful pumps (probably more powerful than needed) that would allow me leeway to "ramp up" if my flow seemed to be lacking. As mentioned I'd been thinking of the Waveline DC 20,000.

I took a look at the derkoon digital flow diverter and think this could be an ideal product to allow me to switch the incoming and outgoing tidal loops so that I can avoid having to power these with separate pumps and, more importantly, avoid having to feed them with separate intakes. The only problem on this front is that the specified maximum flow rate on the digital diverter is 12,000 liters per hour. This won't handle the flow rate of the DC 20,000. I wish I was more confident in calculating head loss through my piping system and then add back in the extra flow gained from the Venturi modded loc line to see if a dc 12,000 would be sufficient. While I guess I could (with effort) figure out the head loss in the piping, the gains in the loc line mod seem much more subjective and will leave me guessing at my final flow rate.

Ha! - It's actually "Dr. Keller (male)" but I don't go by the title - that's too much puffery for me. ;-)

I'm pressed for time right now, but I will write you back later today with more specific information. Here's a couple of quick thoughts - it's actually the number of wide-open nozzles on the discharge side of the pump that might cause low back pressure issues. You can certainly install all of them that you want, so long as you either install a ball valve immediately after the discharge on the pump, or a ball valve on each nozzle so that you can throttle them (and therefore increase the back pressure) if the need arises.

You won't actually get more flow through the loop by installing the penductors - the flow through the loop will be identical with and without them. The extra mass flow comes from the tank water being pulled through the fins on the penductor.

 

[dkeller_nc]

Regarding the suitability of the Der Kroon FlowWolf and the pump you've chosen - I really don't think this will be a problem. While the pump is rated at 20,000 liters per hour, manufacturers most often state their pump's capacity at zero head pressure.

Most centrifugal pumps commonly used by hobbyists lose about a third to half their flow once installed in a system, though because your installation will have a significant inlet head pressure, you might do better than that. As you noted, you would need to calculate the head pressure and compare that against the pump curve to see exactly what you'll get out of the pump, and so far as I'm aware, the pump curve for the DC 20000 hasn't been published yet.

One last plumbing design consideration - you will want to install vibration isolation both before and after the centrifugal pump. All pumps vibrate to some extent, but that doesn't matter with a submersible pump in a sump. On a closed loop, however, you want to be sure that vibration transmitted through the pump to the PVC plumbing doesn't cause your bulkhead gaskets to prematurely fail, since replacing these will mean draining the tank. Suitable vibration isolation can be as simple as installing a short piece (say, 6" or so) of reinforced vinyl flex tubing on both the intake and discharge of the pump.

 

[dkeller_nc]

I'm having a hard time understanding how a lack of back pressure can cause cavitation problems. Can you explain this?

Cavitation in a pump occurs (generally speaking) on the back side of the impeller blades. The absolute pressure in this location is a function of the speed of the impeller and the absolute pressure in the centrifugal impeller housing.

Once that pressure drops below the vapor pressure of the fluid being pumped, the liquid will vaporize and cause bubbles to form. These bubbles rapidly form, collapse and coalesce in a complicated fashion, but the process erodes the material that the impeller is made of, causing premature failure.

Because it's the absolute pressure that causes cavitation, and absolute pressure is the sum of the pressure in the pump housing and the pressure drop caused by the impeller moving in the fluid, once can often stop cavitation from occurring simply by increasing the head pressure in the pump by valving down the output.

 

[acesq]

Cavitation in a pump occurs (generally speaking) on the back side of the impeller blades. The absolute pressure in this location is a function of the speed of the impeller and the absolute pressure in the centrifugal impeller housing.

Once that pressure drops below the vapor pressure of the fluid being pumped, the liquid will vaporize and cause bubbles to form. These bubbles rapidly form, collapse and coalesce in a complicated fashion, but the process erodes the material that the impeller is made of, causing premature failure.

Because it's the absolute pressure that causes cavitation, and absolute pressure is the sum of the pressure in the pump housing and the pressure drop caused by the impeller moving in the fluid, once can often stop cavitation from occurring simply by increasing the head pressure in the pump by valving down the output.

Perfect! Thank you. Am I correct in assuming that if I do have cavitation, I'll see it by the discharge of bubbles into the tank and can turn a ball valve to increase the head pressure to eliminate the cavitation?

 

[IANick]

Gotta say, I'm quite pleased with how this thread has developed. Thank you Dr. DKeller for your insights. It's been informative. But I'd still encourage any others with thoughts on the topic to please contribute... both on my specific proposed setup, and perhaps other generalized closed loop observations or recommendations.

 

[IANick]

Just so I'm clear... ~6" of flexible pipe on both intake/outlet side of pump. You mean attach these lengths to the pump and then attach the rigid PVC to the flexible piping? And not attach the rigid PVC directly to the pump? I hadn't heard this recommendation before, although I can see why it might make sense.

 

[Gandolfe]

don't forget to use unions at the inlet and outlet sides of the pump so you can disconnect the pump in the future if it fails or for cleaning!

 

[dkeller_nc]

Perfect! Thank you. Am I correct in assuming that if I do have cavitation, I'll see it by the discharge of bubbles into the tank and can turn a ball valve to increase the head pressure to eliminate the cavitation?

No, not necessarily. Cavitation is a transient effect caused by the passage of fluid over an obstacle in the flow field. In this case, the passage of the impeller through the water in the pump housing. The bubbles immediately collapse when the pressure rises above the vapor pressure of the fluid - in fact, it's the rapid collapse of these bubbles that actually causes the damage to the impeller or other pump components.

Instead, you can hear cavitation - it's a bit difficult to describe, but you will instantly recognize it once you hear it. It's kind of a high-pitched "tinking" sound. You may be able to find an audio example on You Tube.

Yes, you're correct that a ball valve placed immediately on the discharge side of the pump will allow you to place a bit of back pressure on the pump and prevent cavitation. Alternatively, you can slow the pump speed if your centrifugal pump is a DC model with a speed controller, or an AC model with a VFD (variable frequency drive).

 

[dkeller_nc]

Just so I'm clear... ~6" of flexible pipe on both intake/outlet side of pump. You mean attach these lengths to the pump and then attach the rigid PVC to the flexible piping? And not attach the rigid PVC directly to the pump? I hadn't heard this recommendation before, although I can see why it might make sense.

Yes, that's exactly how to do it. Not only will the flexible tubing dampen normal vibrations of the pump, it will also work to reduce (but perhaps not entirely eliminate) water hammer from the switching of the flow valve. By the way, another way to eliminate water hammer is to plumb a small opening in the loop immediately before the switching valve. That outlet will provide a vent for the pressure wave that's created when the valve temporarily closes off before it switches to another fluid path.