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This post was last edited by 3983596_FPPZ on 2018-11-14 at 15:37. 1.1.6 Efficiency Efficiency is the percentage ratio of useful power to shaft power, represented by the symbol η. Generally, the efficiency of pumps ranges from 40% to 70%; mixed-flow and axial-flow pumps can achieve efficiencies of around 90%, while pumps with low flow rates but high head capacities typically have efficiencies in the range of 10% to 30%. Let’s take a look at what efficiency is and what impact it has on pumps. The efficiency of a pump can be broken down into three components: volumetric efficiency, hydraulic efficiency, and mechanical efficiency. 1. Volumetric efficiency: The ratio of the actual flow rate of the pump to its theoretical flow rate is known as volumetric efficiency. In simple terms, the pump is supposed to deliver a certain flow rate theoretically, but in reality there are losses due to backflow of liquid inside the pump. This topic is covered in the section on flow rate; examples include leaks from the sealing ring balance holes, self-flushing mechanisms in mechanical seals, the return pipes of the balance drum and balance disc, and the cooling circulation system inside magnetic pumps. These mechanisms cause some of the flow rate to flow back from the high-pressure area to the low-pressure inlet area. This backflow represents a kind of internal short circuit in the pump’s flow rate, and the actual flow rate that the pump delivers outwardly is the amount remaining after this backflow has been accounted for. This loss is referred to as volumetric loss, and it is denoted by the symbol ηv. 2. Hydraulic efficiency: In the pump’s suction inlet, impeller flow channels, and discharge channels, the flow velocity and direction of the liquid vary at different points within the same area; this results in the liquid at different locations colliding with each other and interacting ； The sudden changes in the position of the liquid relative to the pump, such as at the inlet of the blades, the outlet of the impeller, and the baffle in the discharge channel, cause a certain degree of local loss of energy in these areas. The combination of these effects results in a loss of liquid energy, which leads to a decrease in head pressure; this is known as hydraulic loss. The percentage ratio of the actual head pressure to the theoretical head pressure is called hydraulic efficiency, denoted by the symbol ηh. 3. Mechanical efficiency: Liquids have a certain viscosity, and when the rotor system rotates within the liquid, resistance is generated. This resistance is particularly significant for impellers with large diameters and large surface areas, as losses due to this resistance are greater. We refer to such losses as disk friction losses ； Furthermore, as a machine, a pump inevitably experiences losses related to mechanical transmission, such as friction from mechanical seals, losses in rolling bearings, and losses in couplings. The loss in these two parts, when added together, is referred to as mechanical loss. The actual energy transmitted to the impeller is not equal to the energy transmitted by the coupling. The ratio of the power input to the impeller to the shaft power is known as mechanical efficiency, denoted by the symbol ηm. The overall efficiency of a pump is the product of these three efficiencies, that is, η = ηv*ηh*ηm. For some special types of pumps, such as magnetic drive pumps and shielded pumps, there is also magnetic eddy current loss; should this loss be included in the mechanical efficiency or considered separately? Actually, this isn’t that important; what matters is how to reduce losses and improve efficiency. What does it matter where these values are calculated? The efficiency of pumps with different structures and designs varies greatly, and this difference arises from the varying weights that each of the three types of efficiencies mentioned above holds in the overall efficiency of the pump. After-class questions: (Multiple choice) Centrifugal pumps with low flow rates, high head, and large-diameter impellers generally have low efficiency. The reasons are: A. Low volumetric efficiency B. Low hydraulic efficiency C. Low mechanical efficiency D. All of the above ABCD