 Circulation pump
As a (radial) rotary pump, it belongs to the category of flow pumps. A typical feature of them is the shape of the outlet channel, which gradually tapers off at the outer edge. And then, with a slightly shallower
curvature, eventually turns away tangentially. The centrifugal forces of an impeller drive the fluid in here.
In a vented system, the appropriate fluid must then flow through the inlet channel that opens in the middle; otherwise, a vacuum would form. Such a pump must therefore always be completely filled; otherwise,
it will not pump. It cannot draw on its own without being filled.
A flow pump is essentially open, with no flaps or (backflow) valves. Ventilation is possible through it. High flow rates are possible, but not high pressures. However, there is a risk of material erosion due to
cavitation at the impellers, especially in the event of malfunctions and/or flow restrictions in the pump's flow inlet.
In the center are the carefully sealed bearings, lubricated with permanent grease (picture). On the outside, a wheel driven by a belt or gear; on the inside, a vane wheel. Apart from carefully shaping the
discharge channel and ensuring a proper seal, there are no issues with the design of a flow pump.
It is often said that seals, for example, those against shafts, always have a very slight leak. On the contrary, it is rather the case that a completely airtight seal is a sign that the oil seal is wearing in, and thus
points to a serious failure occurring soon. For this reason, some pumps had a special drain hole that prevented the grease seal from being endangered.
What's new is that it can also be easily soiled. Whereas the impeller used to be open on one side (top image), preventing dirt from accumulating, it is now often closed (bottom image). They probably do that
here for efficiency reasons as well. For modern engines, it would therefore be worth checking the inside of the pump as well if there are circulation issues.
In addition to its simple design, the flow pump offers another advantage: the lines remain open even after the motor is turned off. Have you ever heard of 'residual heat' or 'after-heating'? These effects are easy to
trigger: Drive as fast as possible on the highway, then exit and turn off the engine.
What do you think the engine has to say about this kind of behavior? It asks you how to shake off now the intensity of the last few lively bars. Well, in many vehicles, the fan continues to run, but the
mechanically driven coolant pump does not. And that is precisely where the advantage of the flow pump, which is practically always open, comes into play, as it allows for a certain amount of heat circulation.
One more question: Why is a pump typically installed at the coldest possible point, at the radiator outlet or the engine inlet? No, not because of the drive system. Pumps can suffer significantly from cavitation.
I've already mentioned her in this book. It is caused either by a localized build-up of heat or by very low pressure.
Very low pressure is generated, for example, when the impeller rotates at a higher speed and pushes the coolant through the system. A characteristic feature of a pumping motion is a pressure side and a
suction side that depends on it. The farther out on the wing, the greater its speed. In this process, the liquid molecules on the suction side can be subjected to such a strong vacuum that they change their
state of matter, and the distance between them corresponds to that of the corresponding vapor.
Since higher temperatures increase the distance between molecules anyway, heat promotes these processes, as mentioned above. But this isn't a stable state; it's more like a tightly coiled spring. Such a
very brief, partial switch to gas and back again can cause significant damage to the impeller, known as erosion.
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