Does a return pump warm the water?

[Gregcoyote]

It's pretty simple physics. If more work is done it requires more energy and that energy creates waste heat.


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[jeffaquarius]

That is exactly what I said. Less water flowing less power.

 

[PB_Smith]

That is exactly what I said. Less water flowing less power.

no, not exactly.
If you are pumping the water through a restriction such as a fine filter media or a going from one diameter pipe/hose to a smaller diameter, than you have less water flow BUT the pump is working harder thereby producing more heat.

Like I said, it really depends on the application and if the pump is pushing or pulling the water.
So in a typical sump set-up the pump is pushing the water and the only restrictions will be head pressure and any flow restrictors/nozzles.

The difference is much more apparent in canister set-ups and is why they always have the pump after the canister pulling water, that way the pump is doing little work and the majority is done by gravity because the water always seeks it's own level.

 

[Gregcoyote]

"What we got here is a failure to communicate."


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[PB_Smith]

I recently changed some things on my DIY canister set-up that resulted in less flow restriction and much higher gph turnover through the pump and it runs much, much cooler with the higher flow rate/less restriction.

 

[jeffaquarius]

I would say it depends on what type of motor driving the pump. The canister is designed to have the intake water level to be almost at the same level as the discharge. While the pump for the sump will require power related to the flow. Bear in mind the motor does not really maintain a constant speed but slows down when restricted. The best way to prove it is you take an infra red thermometer and measure the pump radiated heat.

 

[Gregcoyote]

Jeff, I did that. As the motor is placed under higher loads (head pressure or restriction), it heats up accordingly. The coolest operating temperatures is when there is no head pressure. Lifting water equals work and work equals energy usage which in turn creates heat. Maybe we are saying the same thing.


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[Peixe]

I think I've solved the issue at least for now. I turned off the heaters (2) and still had warm water. So after 1 night with the skimmer off I noticed some cooling water, back on the 79 mark, where I like it to bw actually. So I opened the pump of the Octopus skimmer and cleaned it with some pure vinegar and re assembled it. Worked like a jewel... but to be honest, I haven't seen anything blocking or clogging the pump. Now, anyway, problem is solved.
Thank you guys for all the info and discussion over the issue, I think I've learned a lot!

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[PB_Smith]

I would say it depends on what type of motor driving the pump.

true, if you are talking about the difference between magnetic drive submersible pumps vs direct drive external pumps.
The direct drive pumps transmit virtually no heat and can pump a helluva lot more water at much greater head pressures, so those would be the best choice hands down. too bad they cost almost 4x as much $$$ and require more plumbing.


but as I was rolling this topic around my cranium I kept thinking of little "what if's" and "yeah but's" and came to the conclusion that my ideas sound logical, but I am far from an expert in fluid dynamics, so who knows....

at least it offers a little exercise for the ole' gray matter.

 

[jeffaquarius]

I am glad that the OP has figured out what causing his tank temperature problem. However, I would like to clarify my stand which may help others with similar issue. The heat generated by pumps is simply caused by the electric current in its motor coil or winding. The more current, the more heat will be generated. The water flowing helps cool it down but it does not mean that it generates less heat. When less water is flowing, it just retain those heat within. The pump has to run at its optimum operation to reduce heat. We know that the more water flowing the more power is consumed and thereby more current is passing through the motor coil. Here is an excerpt from the link below if you really wanna know what is causing the abnormal heat on the pumps.

"Control the flow of the pump by slightly close the discharge valve till the pump operate at its best efficiency point."

Cause of pump overloading | Enggcyclopedia

.

 

[mr_X]

Cycle Stop Valves, Inc.

 

[Gregcoyote]

Very good article. That settles it for me...my assumption was wrong as the article points out. My bad.


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[jeffaquarius]

That is funny. Doug has not said a word but settled the whole thing lol

 

[Ingy]

Greg, your assumptions were not wrong. I read that article by "cycleStopValves" and boy is it full of holes.
First off, engineers do not believe the only way to reduce power is to reduce RPM. In fact the 'Affinity Laws' for a specific centrifugal pump head/flow rating can be affected by; 1. throttling, 2. bypassing, 3. impeller diameter changes, and 4. impeller design. RPM is not even in the affinity laws, as it is a dynamic factor affected by the other 4 factors.
The best way to change a pumps power/head rating is in fact to change the speed of the pump through a series of gears/belts or clutches or variable speed drives, but each of those are expensive and in a category of their own (with their own set of affinity laws for similar pumps).
So, assuming speed changes are out of the question (although speed variable DC pumps are getting quite cheap now), then throttling is in fact the best way to reduce flow. BUT, it reduces flow much more than it reduces power, so the net result is a overall loss in efficiency. If you completely close off the discharge valve, the pump will still run, producing ZERO head for about 50% power consumption. Your tank may not see an inflow of warmed water, but the sump will start heating up.

Don't take my word for it, here's a link to an engineering site that is not trying to sell discharge valves;
Discharge Regulation of Centrifugal Pumps
look about 1/3 the way down the page for the graph depicting power consumption vs Flow (q)

Other things we don't/rarely use in centrifugal pumping systems, pressure, because its all about head and flow.
There is no "magical" counter intuitive property to these pumps. there is no "free by-product of horsepower"

But most importantly, his statement "With centrifugal impellers, restricting the flow rate with a valve reduces the power required proportionally" is FALSE. It is close to linear, but the [q] intercept is around 50% power. So a 50% reduction in flow results in about a 25% reduction in power consumption. Therefor the resulting heat gain would INCREASE, not decrease.

 

[jeffaquarius]

So you really wanted to be technical to get to the bottom of this. When you restrict the flow in a centrifugal pump the blades will somewhat reduce its speed a bit and the Counter Electromotive Force will increase reflecting a high inductive reactance (ac resistance) thereby reducing the flow of current to the motor. You can not consume more power when your waterflow has been reduced. When you restrict the flow with the control valve you have reduced the flow of water so there is no way that you will consume more power. I am an electrical engineer and I have been performing acceptance tests of both power generators and fire pumps in high rise buildings. However, it seems to be a big challenge arguing about return pumps in an aquarium.

 

[Ingy]

The concern raised was about temperature. Throttling a pump will not decrease temperature increase in the water. The article from the stop valve selling site incorrectly said a decrease in flow via a throttle valve would result in a PROPORTIONAL decrease in power. That is just not true.
If your high rise fire pumps worked like the fire pump system in the power plant I used to work in, then they will pump water into an elevated resevoir until the head level is reached. At that point no more water will be lifted. But, although flow has been reduced to 0 the pump is still turning drawing power and trying to pump. A decrease in flow does result in a decrease in power consumed, but it is only about 1/2 the rate of flow reduction.
Even the stop valve guy does say throttling is not recommended in temperature sensitive systems which was the concern.


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[jeffaquarius]

Your example is in the extreme state. Of course the motor will keep on running when the valve is totally closed. My point is that the pump motor has an optimum operation of which it is designed for. Once you let it work over its limit like bigger discharge pipe diameter and lower head it will overheat. Partially closing the valve will make it work less which has a similar effect when using a smaller size discharge pipe. Although you increase the head pressure at the lower end of the valve when partially closed you are decreasing the upper side pressure which is reducing the flow. We can not ignore the heat loss of the motor but making it work more by overloading will either trip the breaker or its overload protection.
Edit: If you have a clamp on meter take the current readings when the valve is fully opened and then compare it when valve is gradually closed. I can guarantee you that the current will start going down.

 

[Gregcoyote]

From a basic thermodynamics point of view, the more work any pump or motor does (head pressure) the more heat it creates as waste heat. If the motor uses less current under higher head pressures, it doesn't compute to me.

My field is aerodynamics and structural engineering so I am fascinated by the comments.


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[PB_Smith]

Ya know, after giving it much contemplation, I think maybe there really needs to be a differentiation made between direct-drive pumps and magnetic impeller drive pumps, as I think we are beginning to mix apples with oranges a little.

As far as I know, the vast majority of magnetic impeller drive pumps we employ in aquariums have a more or less free floating impeller and are essentially frictionless using the water as it's coolant & lubricant and the electro-magnet in the casing is switching at 60 cycles per second regardless of what the flow rate is.

It simply does not make any sense to me that the speed of the impeller as effected by flow rate via valves is going to have any impact on the amount of current being drawn.
I can not see any direct link between the two, the magnet is switching at a constant rate regardless of the flow rate.

Now the heat generated is being generated by the current in the electro-magnet, and that is a rate generally set by the cycles of the AC current. (remember we are talking small aquaria pumps, not pumps to power a fire sprinkler in a high rise, large variable speed pumps as mentioned in the article linked or similar type systems.)
So with that in consideration, the heat generated should also be relatively consistent.
Flow rate will influence the subjective heat felt by virtue of the design of the pump being water cooled/lubricated.
The math really is simple, (I think) lower flow rate=longer contact time with a given volume of water as it moves through the pump and around the pump housing if submerged=greater the subjective heat transfer to the water.

I say subjective because the cumulative heat transfer/build up in the system would/should be relatively the same because more heat overall isn't being transferred, it is just raising the temp in one section higher, IE; the water in the sump with restricted flow may be hotter than the display than in unrestricted setup, but the overall heat exchange is generally the same given all other factors are in common.

simple way to test it is go restrict the flow through your cars radiator by half and see if it gets hotter or cooler.

I hope that all makes some sense.

now a direct drive pump with a fixed impeller if blocked or restricted too much will transfer that need for more energy to the pump and cause more heat build up.

Simple test, go jam the impeller of a Little Giant pump and see how hot that bugger gets. I've seen them almost catch fire because of a jammed impeller, but then again they are air cooled.....

 

[Gregcoyote]

Does a return pump warms the water?

I tend to agree, but work done needs energy to do it. If a pump is lifting water to a certain height, energy is expended doing that no matter the physical setup to transfer the mechanical energy. The more resistance the harder the pump works, even magnetic drive pumps. Yes the 60hz switching occurs even if the impeller is stuck, but it would seem to me the resistance to overcome the magnets, rather than turn the impeller, is again a function of current and as a result, heat. There is no doubt in my mind this effect is far more noticeable in direct drive pumps, but the original question was heating the tank water.
The lower the flow the greater resistance also limits the volume of cooling water circulating in the pump.

Okay, this is easy to test with a bucket of water. Which gets hotter, a power head with no flow restriction vs one with a stuck impeller?


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