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Old 11-12-2014, 01:46 AM   #41
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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.

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I totally agree with that statement Greg. The question is do you really believe that reducing the flow of liquid will require more work? In a pump design there is what we call the Best Efficiency Point. It is true that closing the valve will create the maximum pressure with no flow that can overheat the motor. The same thing will also happen if you let the pump run freely with zero pressure (no water). We avoid and never advice anyone to let the pump run that way. The pump has to run in between for maximum performance.

One time a pump takes too long to fill up the tank so it was decided to replace the discharge pipe to a bigger size diameter to make it easier for the pump. Guess what happened? It burned the motor. It was then replaced with a bigger sized pump. It is true that you are making it easier for the pump but you are also increasing the power demand.
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Old 11-12-2014, 02:01 AM   #42
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Does a return pump warms the water?

That doesn't make sense to me that reducing the demand on a pump increases its energy usage. But in a induction motor, since the coils alternate at the same 60hz speed all the time, it wouldn't react to a load from a pump the same way a direct drive motor would. Maybe that's the difference.


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Old 11-12-2014, 02:05 AM   #43
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So your statement is conflicting because you are saying that if we reduce the flow by closing the valve (not totally closed) it will require more work.
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Old 11-12-2014, 02:07 AM   #44
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Does a return pump warms the water?

No, reducing flow is equivalent to head pressure and that places more load on the pump. That's why I never restrict flow but use a Y valve to dump excess water for adjusting flow in things like reactors. Now the difference there is I'm not lifting water very far, I'm just recirculating it.

Good topic. I need to look into it a bit deeper.


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Old 11-12-2014, 02:12 AM   #45
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Its true that you increase the pressure by slightly closing the valve but in turn you have reduced the water flowing therefore you are spending less energy.
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Old 11-12-2014, 03:04 AM   #46
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Greg, it is counter intuitive but a throttle valve does reduce flow AND power consumption. But, and this is VERY important in this situation, the reduction is NOT proportional.

If you reduce the flow via a throttle valve buy 50% you will not see a reduction in the consumed power of 50%. It will be closer to a 25% power saving. So if you use 75% of the energy to move 50% of the mass the same distance, what does the extra 25% of the energy do? It creates heat (and a wee bit of extra noise). (And these numbers are in addition to the normal power factor rating of the motors). That is why throttle valves are not used in temperature sensitive situations such as the OP was asking about. They cause temperature increases.

As for my credentials, I am an Electronics Technologist and a practicing Electrician.


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Old 11-12-2014, 10:34 AM   #47
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If you reduce the flow via a throttle valve buy 50% you will not see a reduction in the consumed power of 50%. It will be closer to a 25% power saving. So if you use 75% of the energy to move 50% of the mass the same distance, what does the extra 25% of the energy do? It creates heat (and a wee bit of extra noise). (And these numbers are in addition to the normal power factor rating of the motors).

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Assuming those numbers are reliable, you will also have similar effect when the pump is operated more than its manufacturer's suggested specifications. The depth or height of water where your return pump is and the height of your display tank (total head) plus the size of the discharge pipe will have influence on your pump. As Greg has been saying the more work (more flow) the more power you consume and so thus the radiated heat. So it still comes down to the right adjustment to get the Best Efficiency Point.
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Old 11-12-2014, 10:56 AM   #48
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My protein skimmer heat the water (2C° over ambiant temperature)
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Old 11-12-2014, 11:06 PM   #49
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My protein skimmer heat the water (2C° over ambiant temperature)
How did you find out that its not the other pumps in your tank?
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Old 11-13-2014, 02:36 AM   #50
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Recently I pulled out my skimmer (SRO 2000 Internal) and reactor. The reactor only had a small pump. I saw a 4C drop in tank temp before I plugged in the heaters again.

[ BTW my ATS feeds directly from my overflow drains so no pump is needed for that; yeah. ]


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