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Cathar · 02-04-2004, 05:42 AM

I've measured the amount of heat that various pumps dump into the loop. I typically achieve this in a bit of a kludgey way, but seems to be effective.

I use a foam esky, exactly 6 litres of water, the pump and some tubing. I estimate the thermal capacity of the system, and pour in water that's slightly colder than ambient. I then measure the time it takes for the pump to heat the water from 1C below ambient to 1C above ambient.

Using a 12V in-line pump I have here, and running it through the multi-meter to establish its exact power draw, I was able to match up what I measured the heat input into the water to be, to the pump's power input, and the two matched within 3% (which was incidentally 14.6W). The pump in question was a mag drive, but instead of the impeller sitting inside the electromagnet motor body, the magnetic impeller mechanism is shaped like a hole-saw bit and surrounds the electro-magnet mechanism, resulting in a pump that transfers very close to 100% of its power draw into the water as heat.

Different pumps transfer different amounts of heat, dependent upon their design.

ALL pumps (in-line or nor) will transfer at least as much heat into the water as the shaft power of the motor, which is typically around 50-60% of the actual motor power draw. Of that shaft power for centrifugal pumps, anything from 10-50% gets used as motive force for pushing the water, the rest is simply wasted as frictional energy (heat) due to the fairly large inefficiencies of the centrifugal pumping action. Closed impeller designs will get to the 50% value. Open impeller designs with sloppy clearances will be down around the 10% mark (or even lower).

On top of that, most pumps that give a single power rating will typically draw around 50-70% of their rated power with 100% of their rated power only being drawn at startup, unless an explicit power draw curve is given with the PQ curve, in which case that's the correct power consumption to trust.

It is highly unlikely that you will ever see less than about 30% of a in-line centrifugal pump's peak rated power being added as heat to the water.

A random test of how warm the water gets is useless. A test of how quickly the water temperature changes over a fairly small deviation from ambient in an insulated environment is a somewhat accurate method.

The Iwaki pumps do dump a lot of heat into the water, despite the motor being mostly separate/insulated from the pump body. The reason being that for my Iwaki MD30-RZ, the shaft power is about 45W, and immediately ALL of that is going to end up as heat in the water through one way or another. Then there's the actual pump motor heat, for which some of it will make it into the water. Even if it's just 10% of the pump motor's heat, that takes it up to the ~50W of heat output that I measured that the Iwaki MD30-RZ does dump into the system - and it's an in-line only pump.

02-04-2004, 05:42 AM	#10
Cathar Thermophile Join Date: Sep 2002 Location: Melbourne, Australia Posts: 2,538	I've measured the amount of heat that various pumps dump into the loop. I typically achieve this in a bit of a kludgey way, but seems to be effective. I use a foam esky, exactly 6 litres of water, the pump and some tubing. I estimate the thermal capacity of the system, and pour in water that's slightly colder than ambient. I then measure the time it takes for the pump to heat the water from 1C below ambient to 1C above ambient. Using a 12V in-line pump I have here, and running it through the multi-meter to establish its exact power draw, I was able to match up what I measured the heat input into the water to be, to the pump's power input, and the two matched within 3% (which was incidentally 14.6W). The pump in question was a mag drive, but instead of the impeller sitting inside the electromagnet motor body, the magnetic impeller mechanism is shaped like a hole-saw bit and surrounds the electro-magnet mechanism, resulting in a pump that transfers very close to 100% of its power draw into the water as heat. Different pumps transfer different amounts of heat, dependent upon their design. ALL pumps (in-line or nor) will transfer at least as much heat into the water as the shaft power of the motor, which is typically around 50-60% of the actual motor power draw. Of that shaft power for centrifugal pumps, anything from 10-50% gets used as motive force for pushing the water, the rest is simply wasted as frictional energy (heat) due to the fairly large inefficiencies of the centrifugal pumping action. Closed impeller designs will get to the 50% value. Open impeller designs with sloppy clearances will be down around the 10% mark (or even lower). On top of that, most pumps that give a single power rating will typically draw around 50-70% of their rated power with 100% of their rated power only being drawn at startup, unless an explicit power draw curve is given with the PQ curve, in which case that's the correct power consumption to trust. It is highly unlikely that you will ever see less than about 30% of a in-line centrifugal pump's peak rated power being added as heat to the water. A random test of how warm the water gets is useless. A test of how quickly the water temperature changes over a fairly small deviation from ambient in an insulated environment is a somewhat accurate method. The Iwaki pumps do dump a lot of heat into the water, despite the motor being mostly separate/insulated from the pump body. The reason being that for my Iwaki MD30-RZ, the shaft power is about 45W, and immediately ALL of that is going to end up as heat in the water through one way or another. Then there's the actual pump motor heat, for which some of it will make it into the water. Even if it's just 10% of the pump motor's heat, that takes it up to the ~50W of heat output that I measured that the Iwaki MD30-RZ does dump into the system - and it's an in-line only pump.