3, 4 Characteristic curves selection chart and Figs. Their H/Q curve is steep compared to that of other centrifugal pumps the cut-off point marks the operating behaviour of this pump type. Propeller pumps are suitable for large flow rates and low heads (up to approximately 15 m in a stage with an axial flow propeller and 20 m in a stage with a mixed flow impeller). The higher the specific speed, the smaller the number of impeller blades, the less pronounced the profile convexity and the smaller the hub-to-tip ratio. Of all centrifugal pumps, propeller pumps have the highest specific speeds (ns > 110 rpm) but are used most commonly for ns > 160 rpm. For this reason a pump with adjustable vanes in a mixed flow impeller is also referred to as a propeller pump. Propellers can be designed with fixed-pitch blades (fixed-pitch propeller), blades whose pitch can only be adjusted with the propeller dismantled (adjustable-pitch propeller) or blades whose pitch can be adjusted during operation (variable-pitch propeller). A common feature of a ship's propeller, a propeller-type mixer and a propeller pump is the axialflow impeller (see Axial flow pump and Propeller).
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