restricting powerhead flow?

[kcress]

I realize that it goes against the common belief but it is true. I also thought the same thing until it was explained to me by a fellow engineer. I then did tests that completely validated it.

Let me describe something that may help.

You've all used a vacuum I presume?

Now what happens when you are sucking along and pick up something that blocks the nozzle? The vacuum whines up hideously causing you to quickly snatch at the offending blockage, your curtains.

Now think about it.. The vacuum's motor whines up! It goes from say 20,000rpm to maybe 30,000rpm. The motor has a fixed horsepower ability. At that horsepower,(whatever it is), it can only move so much air through the vacuum resulting in a speed of only 20,000rpm. How can it speed up?!?!? It speeds up because it suddenly has NO work to do! It has been unloaded by virtue of the blockage. If you put a watt meter on the vacuum you would find the power drops dramatically.

The 'work',(and I mean 'work' in the strict scientific definition), a vacuum does is called "mass flow" and that equates directly to power. The mass is the air. Flow is how much per unit time. If you reduce the mass flow the power requirement drops - it has to.

The very same thing happens with centrifugal pumps. This is such an important concept that in some cases, actually in many cases, if you run pumps with NO output restriction their motor's can be destroyed because the mass flow is excessive - demanding more than the motor is designed to put out continuously. This is especially common with blowers. Most home furnace motors will fail if not enough back pressure is presented to reduce the motor load.

So again. Feel free to restrict the outlet of any centrifugal pump. Not completely, as the water will recirculate in the pump heating up eventually. (Not likely though in an aquarium submersible.) And never block a pump's suction as that can cause an excessive pressure drop. That leads to the water boiling due to low pressure not temperature. The boiling results in cavitation that can erode the pump's impeller.

Restricting the outlet will save you money both in pump life and electricity. Diverting is certainly fine too.

Please note that I was wondering where this misconception was coming from so often. Several months ago I tracked it to Melve's Reef where he made this same wrong statement for exactly this same sump/diverting case. I asked him to correct the statement and I believe he did.

 

[Sploke]

Just a point to note - restricting the output is fine, but don't restrict the intake. This can cause the impeller to cavitate and can shorten the life of the motor.

 

[clown-lover]

In response to that one part where you state if you don't restrict the flow you will shorten the life of a pump because of letting it run full throttle. Isn't there a normal restriction to the pump in head pressure? I can understand what you are trying to say, its not that I'm ready to put my chips in the pot as I'll have to validate this with some engineers that I know at Ulteig. But as others have said what you are posting is contrary to everything I have read and been advised on boards about pumps.

 

[Amphiprion]

Maybe there are other factors involved, because of the fact that most aquarium pumps are already "throttled" back so that overloading the driver/system doesn't occur (though that still doesn't explain my results, though maybe there is an issue with the meter...). I understand where you are coming from, kcress, however there are other factors that take precedence when dealing with aquariums. While it seems as though energy consumption by the motor itself is lessened, the energy that isn't being used for mechanical work is converted into heat (because the pump maintains driver speed and there is added friction due to lack of lubrication--not just water recirculating), which, as many will know, can be significant. The amount of energy theoretically saved by throttling is nominal to begin with. Now, add into that, if the temperature were to rise above the intended point, the energy requirement for cooling devices. When it all comes down to it, at least in the overall picture, I would rather a single pump use a bit more electricity than to have to make up for added heat and the need for more cooling devices. The trade off is slim and negligible, IMO, making sort of a moot point. One can easily confirm the difference with a smaller container, a pump, something to throttle it with, and a thermometer.

Edit: I should add that this makes the efficiency rating drop substantially, since more energy is converted into heat versus work. The added heat also raises the corrosive properties of the water, shortening impeller life.

 

[kcress]

In response to that one part where you state if you don't restrict the flow you will shorten the life of a pump because of letting it run full throttle. Isn't there a normal restriction to the pump in head pressure? I can understand what you are trying to say, its not that I'm ready to put my chips in the pot as I'll have to validate this with some engineers that I know at Ulteig. But as others have said what you are posting is contrary to everything I have read and been advised on boards about pumps.

Yes clown-lover the head presented by our aquarium pumps is a 'restriction'. Let me point out that while our pumps will last longer with some throttling, with no throttling, they will still get their 'design lifetimes'. They might just run longer if they were throttled. But then hopefully we didn't buy too large pumps that need a bunch of throttling.

Again I am sorry if it is contrary to your established beliefs. I know that can be unsettling. It is still true. Try to look at it from the point of "conservation of energy". A pump is designed to move a fluid against a pressure. That is the work it does. If you do something to reduce the flow, it is doing less work. It requires less power.

Maybe there are other factors involved, because of the fact that most aquarium pumps are already "throttled" back so that overloading the driver/system doesn't occur (though that still doesn't explain my results, though maybe there is an issue with the meter...).

Most aquarium pumps are small inefficient devices to start with. Things like power heads are actually "impedance controlled" devices. They don't much care one way or another if they are pumping or even have an impeller installed. They won't burn up and they won't change their energy draw. They could get warm if not immersed. Likely a meter won't show any detectable change. Heck the power draw is likely about the resolution of most wattmeters.

Now with regard to a lift pump or mag drive pump we should see a difference.

I understand where you are coming from, kcress, however there are other factors that take precedence when dealing with aquariums.

Whoa whoa! Aquariums are no different than any other water pumping application. The rules are not different.

While it seems as though energy consumption by the motor itself is lessened, the energy that isn't being used for mechanical work is converted into heat (because the pump maintains driver speed and there is added friction due to lack of lubrication--not just water recirculating), which, as many will know, can be significant.

There is no "unused energy". An induction motor, as found in a circulating pump, (not a power head), is an interesting device. It's mission in life is one thing and one thing only. "Spin at the design speed". If it has no load, just a shaft, then it uses only the energy required to overcome frictional and parasitic loses. That is the bearing drag, the windage of the rotating rotor, and the losses caused by the electrical resistance of its windings. If you then hang a pump on this motor it will pump all the water it can fit thru the impeller and the pump volute while maintaining its design speed. If anything happens to increase the load the motor will start to slow down. In a effort to get back to its design speed it will draw more current and hence do more work. Blocking its output reduces its load greatly as I've explained. However as you point out the 'pump' would still get warm. This is because some small amount of water recirculates around the impeller and this small amount of work turns into heat. If you have completely blocked the outlet none of this heat can leave except thru the pump case to the surrounding air which is much less effective at cooling than water cooling.

There is no added heat. No added energy. The small amount that is always there is just not going out in the normal water flow. You will note that I said one should not block the output for this very reason.

The amount of energy theoretically saved by throttling is nominal to begin with.

Yes with aquariums that is true. But with anything that runs long hours, and most aquarium pumps run 24/365, any energy saved adds up eventually. Rather like a dripping faucet. Two days, nothing! A year? A lot!

Of course better would be the exactly correct sized pump as that is the most efficient solution of all.

Now, add into that, if the temperature were to rise above the intended point, the energy requirement for cooling devices. When it all comes down to it, at least in the overall picture, I would rather a single pump use a bit more electricity than to have to make up for added heat and the need for more cooling devices.

Again there is no added heat. Throttling will reduce the energy that arrives because the motor does less work.

Another point I always find interesting:
Do you realize that every erg of energy that you put into your system in the form of pumped/moved water ultimately results in heated water? Every bit of of moving water in your aquarium results in heated water. Even the water that falls to your sump ends up heating that same water. So having the water flow freely thru your pump/system results in heating the water of your system more than a throttled pump ever could.

If you have a 100W pump circulating water in your tank then you have essentially a 100W heater running in your tank 24/7. Yes the agitated surfaces that result and the radiator effects of the piping all help to carry away this heat so it may not directly equate with a 100W heater.

The trade off is slim and negligible, IMO, making sort of a moot point. One can easily confirm the difference with a smaller container, a pump, something to throttle it with, and a thermometer.

Absolutely I agree. Aquarium pumps are so small and often inefficient that the trade offs are very likely moot. Furthermore running a test may be hard due to the difficulties of measuring such small energy gradients.

My entire point was and is that you guys not labor under a misconception! All I'm saying is, "NO do not go to the hassle of bypassing if it is a hassle or causes other problems". You can throttle a pump with NO negative ramifications. That is the only point I wanted to make.

I would always appreciate someone telling me that some belief I held was a misconception. That allows me to proceed with all the facts.

Just so you know, I'm an electrical engineer. I do this pump stuff for a living.

 

[clown-lover]

I would always appreciate someone telling me that some belief I held was a misconception. That allows me to proceed with all the facts.

Just so you know, I'm an electrical engineer. I do this pump stuff for a living.

Please don't read my message wrong kcress. I do appreciate the information but like I've said in other forum posts I'm not the type of person to take one statement at face value. While I appreciate that your an engineer and do this for a living and it is your field of expertise I always back up information with another source. Take it as a corroborating evidence type of theory. I understand posting information that is contrary to beliefs as I have done it myself on this board and appreciate the debates that I do see about information in this hobby. That is the one way we progress and increase our knowledge base.

 

[kcress]

clown-lover; No offense taken. I commend your further research. Please do check with your buddies.

 

[Amphiprion]

Yes clown-lover the head presented by our aquarium pumps is a 'restriction'. Let me point out that while our pumps will last longer with some throttling, with no throttling, they will still get their 'design lifetimes'. They might just run longer if they were throttled. But then hopefully we didn't buy too large pumps that need a bunch of throttling.

Again I am sorry if it is contrary to your established beliefs. I know that can be unsettling. It is still true. Try to look at it from the point of "conservation of energy". A pump is designed to move a fluid against a pressure. That is the work it does. If you do something to reduce the flow, it is doing less work. It requires less power.




Most aquarium pumps are small inefficient devices to start with. Things like power heads are actually "impedance controlled" devices. They don't much care one way or another if they are pumping or even have an impeller installed. They won't burn up and they won't change their energy draw. They could get warm if not immersed. Likely a meter won't show any detectable change. Heck the power draw is likely about the resolution of most wattmeters.

Now with regard to a lift pump or mag drive pump we should see a difference.





Whoa whoa! Aquariums are no different than any other water pumping application. The rules are not different.




There is no "unused energy". An induction motor, as found in a circulating pump, (not a power head), is an interesting device. It's mission in life is one thing and one thing only. "Spin at the design speed". If it has no load, just a shaft, then it uses only the energy required to overcome frictional and parasitic loses. That is the bearing drag, the windage of the rotating rotor, and the losses caused by the electrical resistance of its windings. If you then hang a pump on this motor it will pump all the water it can fit thru the impeller and the pump volute while maintaining its design speed. If anything happens to increase the load the motor will start to slow down. In a effort to get back to its design speed it will draw more current and hence do more work. Blocking its output reduces its load greatly as I've explained. However as you point out the 'pump' would still get warm. This is because some small amount of water recirculates around the impeller and this small amount of work turns into heat. If you have completely blocked the outlet none of this heat can leave except thru the pump case to the surrounding air which is much less effective at cooling than water cooling.

There is no added heat. No added energy. The small amount that is always there is just not going out in the normal water flow. You will note that I said one should not block the output for this very reason.




Yes with aquariums that is true. But with anything that runs long hours, and most aquarium pumps run 24/365, any energy saved adds up eventually. Rather like a dripping faucet. Two days, nothing! A year? A lot!

Of course better would be the exactly correct sized pump as that is the most efficient solution of all.





Again there is no added heat. Throttling will reduce the energy that arrives because the motor does less work.

Another point I always find interesting:
Do you realize that every erg of energy that you put into your system in the form of pumped/moved water ultimately results in heated water? Every bit of of moving water in your aquarium results in heated water. Even the water that falls to your sump ends up heating that same water. So having the water flow freely thru your pump/system results in heating the water of your system more than a throttled pump ever could.

If you have a 100W pump circulating water in your tank then you have essentially a 100W heater running in your tank 24/7. Yes the agitated surfaces that result and the radiator effects of the piping all help to carry away this heat so it may not directly equate with a 100W heater.



Absolutely I agree. Aquarium pumps are so small and often inefficient that the trade offs are very likely moot. Furthermore running a test may be hard due to the difficulties of measuring such small energy gradients.

My entire point was and is that you guys not labor under a misconception! All I'm saying is, "NO do not go to the hassle of bypassing if it is a hassle or causes other problems". You can throttle a pump with NO negative ramifications. That is the only point I wanted to make.

I would always appreciate someone telling me that some belief I held was a misconception. That allows me to proceed with all the facts.

Just so you know, I'm an electrical engineer. I do this pump stuff for a living.

I can appreciate that you are an electrical engineer. I am a chemist and biologist . I was trying to compare a mechanical pump to a biological one, hence my inaccuracy (biological pumps, such as the heart, work to the contrary. Any impedence results in more energy input and a shortened usable life). The rules are different in aquaria because of the various parameters concerned, hence my focus on heat. I was not referring as to how they work. I do acknowledge that, after working through some equations, the overall energy drawn is less. But the other tradeoffs are too severe, IMO/E.

Here, you seem to be missing the overall picture. You are only focusing on a (very) narrow gain in energy savings. Again, throttling does indeed increase the friction of the parts, making greater heat above and beyond what energy was put into it. Again, most aquariums don't need the added heat. Again, it shortens the life of the impeller by making the properties of water more corrosive around it, therefore shooting yourself in the foot in the long term.

I do agree, however, that an appropriately sized pump is what most people should consider. Given all of those other factors, I still think the trade off is too slim to make the compromise. I wouldn't want extra cooling devices or having to replace the impeller more often (or cause more abiotic precipitation than what already occurs. I'd like to keep calcium in solution).

 

[schigara]

So where is the tipping point? I am not an engineer so humor me.

For example: a MAg 12 pump @ 0' head pressure will run at a higher wattage than the same pump at 10' head pressure and consume less energy? It takes more energy for the pump to "freewheel" at 0' head than to pump at 10' head pressure?

Say I put a Mag12 pump in a bucket of water and measure the watts pulled and then take the same pump and put a valve on the output, restricted by 99%, which configuration is going to draw more wattage?

So where does the balance shift?

Groan... Sorry schigara that's not actually true.

You can restrict any centrifugal pump as long as you don't block it totally. The pump wattage will ALWAYS go down, not up. The pump will last longer and run cooler. The pump will in NO WAY work harder or be negatively effected in any way by restricting the output.

You can also divert either is fine. Diverting just uses more power.

 

[schigara]

In this scenario, the inlet of the pump is restricted. This reduces the pressure inside the impeller housing and ofcourse, increases rpm and reduces drag. On the other hand, block the outlet of the vacuum and you will see an increase in pressure inside the impeller housing and increased drag on the motor.

The analogy you are trying to make is totally wrong and inverted.

The vacuum speeds up because the compressor is forced to pull and push from the outlet side of the housing which creates a neutral pressure inside the compressor housing.

I realize that it goes against the common belief but it is true. I also thought the same thing until it was explained to me by a fellow engineer. I then did tests that completely validated it.

Let me describe something that may help.

You've all used a vacuum I presume?

Now what happens when you are sucking along and pick up something that blocks the nozzle? The vacuum whines up hideously causing you to quickly snatch at the offending blockage, your curtains.

Now think about it.. The vacuum's motor whines up! It goes from say 20,000rpm to maybe 30,000rpm. The motor has a fixed horsepower ability. At that horsepower,(whatever it is), it can only move so much air through the vacuum resulting in a speed of only 20,000rpm. How can it speed up?!?!? It speeds up because it suddenly has NO work to do! It has been unloaded by virtue of the blockage. If you put a watt meter on the vacuum you would find the power drops dramatically.

The 'work',(and I mean 'work' in the strict scientific definition), a vacuum does is called "mass flow" and that equates directly to power. The mass is the air. Flow is how much per unit time. If you reduce the mass flow the power requirement drops - it has to.

The very same thing happens with centrifugal pumps. This is such an important concept that in some cases, actually in many cases, if you run pumps with NO output restriction their motor's can be destroyed because the mass flow is excessive - demanding more than the motor is designed to put out continuously. This is especially common with blowers. Most home furnace motors will fail if not enough back pressure is presented to reduce the motor load.

So again. Feel free to restrict the outlet of any centrifugal pump. Not completely, as the water will recirculate in the pump heating up eventually. (Not likely though in an aquarium submersible.) And never block a pump's suction as that can cause an excessive pressure drop. That leads to the water boiling due to low pressure not temperature. The boiling results in cavitation that can erode the pump's impeller.

Restricting the outlet will save you money both in pump life and electricity. Diverting is certainly fine too.

Please note that I was wondering where this misconception was coming from so often. Several months ago I tracked it to Melve's Reef where he made this same wrong statement for exactly this same sump/diverting case. I asked him to correct the statement and I believe he did.