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1 Closed impeller: The impeller has covers on both sides, with 4–6 blades between the covers, as shown in Figure (a). When the bending direction of the blade is opposite to the rotation direction of the impeller, it is called a shield-bent blade. Generally, the blades of impellers are backward-curving blades. This type of closed impeller has high efficiency and is widely used; it is suitable for transporting clean liquids that contain no solid particles or fibers. Closed impellers come in two types: single-suction and double-suction. A double-suction impeller can handle a larger volume of fluid than a single-suction impeller. A 2-opening impeller consists only of blades and reinforcing ribs (as shown in Figure b), without front or rear cover plates. Open impellers have fewer blades (2–5), resulting in low efficiency; they are used less frequently and are mainly employed for transporting liquids with high viscosity or slurry-like substances. 3. Semi-open impellers are generally divided into two types; one is the front semi-open type (as shown in Figure C), which consists of a rear cover plate and blades. Impellers with this structure have low efficiency, and an adjustable-gap sealing ring is required to improve efficiency. The other type is of the rear-opening kind, consisting of a front cover plate and blades; since the same sealing rings as those used in closed impellers can be applied, its efficiency is essentially the same as that of a closed impeller, and the blades not only convey liquid but also serve a sealing function. Semi-open impellers are suitable for transporting liquids containing suspended solids, fibers, and other particles. Semi-open impellers are easy to manufacture, have low costs, and exhibit good adaptability; as a result, their use in centrifugal pumps for chemical applications has been increasing in recent years.