Pump rated wattage vs actual power draw vs pumping power

[murray13]

deleted...

Damn it takes me WAY to long to answer a post. Someone else almost always beats me to it.

And does a better job...

Back to just reading all of this.

[redleader]

Rated power is sort of "cover your ass number" as i understand it. Sort of like the 8 amp rating on 250w power supplies

[Ewan]

myv65 has answered this realy thoroughly, but the I'll just add a little in case some people aren't buying it. The job of the pump is to add kinetic energy the water. This is represented by mass times velocity squared. The higher the velocity of the water, the harder the pump works.

The fan effect myv65 spoke of has been experienced by all of us when vacuum cleaning. When something gets clogged in the vacuum cleaner, the motor pitch increases. That is because the motor is no longer sucking a flow. While the vacuum is blocked the flow there is no flow so the motor doesn't realy do any work, and runs free up to it's max rpm.

Also with regards to water heating from pumps. All the energy imparted by the pump to the water gets converted to heat. How efficient a pump is, has got very little to do with it, it will simply spend less overall energy in giving the water it's energy.

Example; a system loop is sized so that it will have a flow of 3L/min. You have the choice of either using a low head/high flow pump rated at 80W, or a high head /low flow pump rated at 20W. You'd naturally pick the later, thinking that the first will impart much more heat to the water. However the first pump will be working so far back on it's pump curve that it will be drawing nowhere near 80W. It could well be drawing 20W, just like the second pump.
Since the flow of the water is the same for both pumps, it is safe to assume that frictional losses between the water and the piping will be exactly that same. Therefore the heating of the water should be exctly the same.
I.E. it's the flow thorugh the tubing that's causing the heat, and not the motor itself. A certain flow through a system will always result in much the same heating, regardless of what pump you use.
The pump will of course have some effect, but that effect will be limited to how much thrashing and friction there is within the pump. A pump which is far from it's optimum working range will impart more heat since it will be thrashing and casuing more friction with the water, than a pump that's designed to work at the flowrate in question.

Be sure to correct me if I'm wrong about this.

[brucoman]

Originally posted by myv65

I haven't done what you suggest, but it's pretty routine to keep pumps running 24/7 in the systems we used in the paper industry. The pump energy is enough to keep the oil warm during machine outages. A fun one was our hot oil systems. At room temperature the oil is pretty thick and our flow control valves would be wide open yet system flow well below setpoint. Pump energy would get you up to ~140°F and almost bring the valve off its wide-open position. By the time the burners took it to 550°F and the oil's viscosity dropped below water the flow control valve would darn near close to keep from overshooting the desired loop flow. [/b]

<Off topic>
Who's black liquor recovery boiler do you have??