Pumps used in hydronic systems are often made out of cast iron as water within the loop will have either been treated with chemicals or de-oxygenated to prevent corrosion. However in hot water systems or any system with a steady stream of oxygenated liquid the pump will have to be made of a more expensive material such as bronze.
Most circulators use a rotating water wheel called an “impeller”. As the impeller rotates it creates a centrifugal force increasing water pressure in an outward radial direction giving the water an appreciable velocity. Curved vanes along the impeller then direct the water back out of the pump. The circulating pumps outlet will often be smaller than that of inflow producing an effect similar to that of holding your thumb over a garden hose. Circulating water within a closed circuit this way means that the only friction the circulator is required to overcome is that of the piping systems itself rather than having to lift fluid from a lower point of potential energy
The energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher the velocity of the liquid at the vane tip
This kinetic energy coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.
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