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External auqarium airpump for hotend cooling?

Posted by sungod3k 
Re: External auqarium airpump for hotend cooling?
April 05, 2019 10:11AM
Just remember that pressure is not equal to flow.

A fan might not be able to deal with a lot of back pressure, but in free air, it can generate a lot of air flow, which is what we need for cooling. A pump can generate a lot of pressure, but not always a lot of flow.
Yes and no. You are quite right about the amount of air moved, but with compressed air you can use a much smaller area for heat exchange to get rid of the same amount of energy. With fans you need large cooling fins, with pumps they can be tiny as long as you can direct the airflow directly through them. With my Merlin hotend the cooling fins is the M10 thread. The close encapsulation of the thread makes the compressed air very effective in cooling the hotend. If you were to combine compressed air with standard coolers like the E3D uses them they would be very inefficient.

Re: External auqarium airpump for hotend cooling?
April 17, 2019 10:39AM
a lot of air flow, which is what we need for cooling

I don't think so.

If you want to quickly freeze the filament that has just been deposited, for example to improve details or do bridging, you have to remove its thermal energy as quickly as possible, and since the exposed surface for this heat exchange is very small, you can only rely on one thing: the speed of the air passing on this minuscule surface.

Therefore, if the objective is to control the behaviour of the extrudate, it is always better to supply cold air at high pressure, in a fine jet, just around the nozzle.

Stiring air masses at low speed has another objective, which is to homogenize the temperatures in the printed part. And in this case, warm air, just below the glass transition temperature of the material, should be stirred instead, to minimize shrinkage constraints.

The question of whether the cooling jet should be laminar or turbulent is paradoxical:

A laminar jet is by definition oriented, and is therefore only optimal for certain printing directions.

On the other hand, in a turbulent area, there is a chaotic mixture of vortices. However, in vortices, the air velocity is much higher than in the average flow. That is why I tend to think that turbulent conditions are more efficient, at the very small scales that interest us.

Two symmetrically opposed fine jets create a chaotic, changing air velocity field around the nozzle, evenly distributed in all directions, with very high peak air velocities. On the other hand, the average flow is slower, since the two jets are opposite, but it is not this average flow that interests us.

Edited 1 time(s). Last edit at 04/17/2019 10:42AM by M_Xeno.
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