Does a return pump warm the water?

[jeffaquarius]

I would say that we are having a good discussion on the topic since everyone is learning including myself as we go along. With the throttle valve the pump discharge pressure will be higher than the valve outlet (upper side) and not equal. Water is liquid and compressible just like oil used in hydraulics. Have you wondered why there is pressure gauge on the pumps? Don't forget that water has weight and the higher you are lifting it, the higher the pressure you need at the bottom to raise it up to the top. The pressure at the bottom is higher than at the top (they are not equal). If your read the link below and really understand it, hopefully it will clarify all the issues we are dealing with. Take note of the MINIMUM HEAD. It stops to a point where the pump will be overloaded once it is below that curve, Try to run your return pump with the discharge at the water level of your sump or even lower and you will see how the heat will rise noticeably.

What is head?

With regards to the HEAT of the pump motor, there is always heat being radiated regardless of how much work is being done. However, we know that the more power being consumed the more heat loss is present. At 90% efficient you already have 10% heating up the pump. One example when a throttle valve is necessary is when the pump has been installed in a system that is more than it can handle. Meaning it is undersized or overloaded. So what do you do? Your option is either to replace the discharge pipe to a smaller size to reduce the flow or you can install a throttle valve. I would choose the later since it is variable while the former is not.

 

[Ingy]

Water is liquid and compressible just like oil used in hydraulics. Have you wondered why there is pressure gauge on the pumps? Don't forget that water has weight and the higher you are lifting it, the higher the pressure you need at the bottom to raise it up to the top. The pressure at the bottom is higher than at the top (they are not equal). If your read the link below and really understand it, hopefully it will clarify all the issues we are dealing with. Take note of the MINIMUM HEAD. It stops to a point where the pump will be overloaded once it is below that curve, Try to run your return pump with the discharge at the water level of your sump or even lower and you will see how the heat will rise noticeably.

What is head?

With regards to the HEAT of the pump motor, there is always heat being radiated regardless of how much work is being done. However, we know that the more power being consumed the more heat loss is present. At 90% efficient you already have 10% heating up the pump. One example when a throttle valve is necessary is when the pump has been installed in a system that is more than it can handle. Meaning it is undersized or overloaded. So what do you do? Your option is either to replace the discharge pipe to a smaller size to reduce the flow or you can install a throttle valve. I would choose the later since it is variable while the former is not.

Water is NOT compressible under normal (and with our wee little pumps).

Water Compressibility, USGS Water Science School

The gauge on a hydraulic system is to measure the power of the pump, not the amount of compression going on as it does in an air compressor. Water acts the same as a solid under compression. A pressure gauge attached to the end of a steel rod will measure who much force, or pressure is applied to the other end. There is no build up of the pressure as the rod is compressed between the pushing force and the gauge. Similarly there is no build up of pressure as the liquid (oil or water) is compressed. The gauge will quickly, and at the same rate, increase as the pump comes up to speed.

As for pressure loss, in a standing head pipe, the ONLY pressure difference is that applied by the head pressure. There is no pressure loss caused by the throttle valve. If you had a pipe longer than the max head the pump is capable of, and you installed a gate valve in the bottom, the max height the water would achieve would have nothing to do with how open the gate valve is (as long as its open a bit). If you took pressure readings throughout the pipe at different heights, they would be the same if the valve was wide open or almost closed (not the same at different heights, but the same regardless of valve position). If you were to attach an outlet pipe below the max head height creating a flow in the head pipe, there would be no change in the pressure at different heights no matter what the amount of flow were.

AS for motor efficiency, 90% is wayyy too high. 60-65% is much more accurate. The rest (35-40%) becomes heat. Adding a throttle valve to a optimally designed system renders it less efficient at a close to linear, but not proportional rate. But they will still consume less power as the valve is closed (on that we agree), although they will create more heat at the same time.

 

[Ingy]

BTW someone tried to hack my account on Friday Evening. They failed, but please be cautious if you see any defamatory remarks from my account in the future. I will always try to be a polite as possible, even in a heated debate.


On a final note about tank heating, DC pumps have provided a much more efficient way of decreasing flow rate without heating the water. And they are no more expensive (or not much more anyhow) than AC pumps. Just avoid the knockoff pumps out of China so you don't get burned (financially, not figuratively) like I did.

 

[mr_X]

Just avoid the knockoff pumps out of China so you don't get burned (financially, not figuratively) like I did.

From what country of origin would you say we should purchase our pumps?

 

[jeffaquarius]

Because the impedance increased, the current also increases proportional to impedance, drawing MORE power (I=V/Z).

The formula says the impedance is inversely proportional to the current. Therefore the higher the impedance the lower the current is.

 

[jeffaquarius]

We agreed that more flow the more power required thus more heat. We also agreed that less flow will require less power. Should it not follow that there should be less heat? The only reason why there is more heat in the pump is because you reduce the flow of water to dissipate the heat. With maximum flow the pump will have less heat. It is because the heat is absorbed by the water flowing and it goes to the tank.

 

[jeffaquarius]

If you took pressure readings throughout the pipe at different heights, they would be the same if the valve was wide open or almost closed (not the same at different heights, but the same regardless of valve position). If you were to attach an outlet pipe below the max head height creating a flow in the head pipe, there would be no change in the pressure at different heights no matter what the amount of flow were.

This is already out of the topic but may I ask where did you get that kind of information? Do you know why a booster pump is sometimes installed on the upper floors of a multi-storey building?

 

[Ingy]

First when I worked in the steam plant on a recovery boiler. Then as an electrician. Booster pumps are to overcome headloss. If they put enough pressure in a pipe on the ground floor to overcome headloss and still provide say 40 PSI at the top floor, the pipes could/would burst from the very high pressure at the bottom. Of course that's why they generally have water tanks at higher floors, to reduce cycling water hammers.

As for your other thought about pump heating, you would be correct if the pump had the same efficiency at varying speeds. But as you throttle the flow back the pump becomes drastically less efficient. To the point where if you completely shut the flow off, the pump still draws about 1/2 the power, but now it all goes into heating. Pumps are most efficient when they are run flat out.


Sent from my iPhone using Aquarium Advice

 

[Sniperhank]

Well, though this thread was put to rest several days ago, it has once again come back to life. All sides have been presented at this case and we are just beating a dead horse at this point and the OP's question was answered pages ago. Thread closed.