Why are backward-curving blades used in centrifugal pumps

An impeller with backward-curving blades increases the fluid’s potential energy more than its kinetic energy; since kinetic energy is converted into potential energy in the volute with minimal loss, the pump has high efficiency. Its drawback is that it requires a larger pump volume to generate the same theoretical head compared to impellers with forward-curving blades.

To answer this question, it is generally considered from the perspectives of hydraulic efficiency and the ease of selecting a motor. The analysis is as follows: 1. With forward-curving blades, as the flow rate increases, the head rises, causing a sharp increase in shaft power, which poses difficulties in designing the motor. If the motor power is selected too low, a high flow rate can easily burn out the motor ； If the motor power is set too high, the motor will be larger in size, more expensive, and have lower efficiency. 2. Another major drawback of forward-curving blades is that when the liquid is ejected from the outer edge of the impeller, it causes significant impact losses against the flowing liquid in the volute channel, resulting in a marked decrease in hydraulic efficiency.

It seems it’s mentioned in the principles of chemical engineering; I’m not sure if I remember it wrong.

When bent forward, the internal friction and impacts of the liquid result in a significant loss of mechanical energy

Although the theoretical head of forward-curving blades is high, although the kinetic energy of the liquid can be partially converted into potential energy in the volute, this conversion process results in significant energy loss. Therefore, due to the failure to achieve a high energy efficiency, centrifugal pumps always use backward-curving blades.

I. The blade shape is determined by the inclination angle at the outlet end of the blade: blades with an angle of 90 degrees are radial blades, those with an angle less than 90 degrees are backward-curving blades, and those with an angle greater than 90 degrees are forward-curving blades. II. The leaf shape impression theory involves a theoretical head, which includes potential energy and kinetic energy. At the same flow rate, the kinetic energy of the forward-curved blades is greater than that of the backward-curved ones, which are radially centered. III. The main purpose of a centrifugal pump is to transport liquids by overcoming potential energy differences; although liquid kinetic energy can be converted into potential energy, significant energy loss occurs during this conversion process. Therefore, to achieve a higher energy utilization rate, backward-curving blades are mostly used!

The theoretical head generated by a forward-curving impeller is the highest. However, centrifugal pumps actually use mostly backward-curving blades. Reason: The theoretical head of a centrifugal pump includes static head and dynamic head; for transporting liquids, it is the static head that is desired, not the dynamic head. Although some of the dynamic head in the volute and guide vane can be converted into static head, the high flow velocity causes intense vortices to form within the pump, resulting in increased energy loss. To improve the economic performance of centrifugal pumps, backward-curving blades are advisable.

Modern centrifugal pumps all use backward-curved blade profiles, rather than forward-curved or radial blade profiles, mainly for the following reasons: (1) For impellers of the same diameter and speed, the absolute velocity of forward-curved and radial blade profiles is greater than that of backward-curved profiles, although the theoretical head also increases as a result. However, the proportion of dynamic head is relatively large. In terms of the operation of water pumps, it is the static head that needs to be increased, rather than the dynamic head. Before the water flows out of the pump, most of its kinetic energy must be converted into pressure energy; this conversion takes place within the pump casing and diffuser surrounding the impeller. Excessively high flow speeds lead to increased hydraulic losses, thereby reducing the efficiency of this energy conversion. (2) The curvature of the forward-curved and radial blade grooves is greater than that of the backward-curved ones, which increases the hydraulic loss due to relative motion and also raises the difficulty of casting. The leaf channels are tortuous; otherwise, vortices will form at the inlet, resulting in greater hydraulic losses. (3) The power in the forward-bending and radial modes increases rapidly with flow rate, making it easy to overload the power machine.

Backward curvature helps to increase static pressure energy, and this is the function of a centrifugal pump

Although forward bending can provide a larger pressure head, the energy loss is too high