With intermediate values, a mixed-flow type impeller, sharing elements of both radial and axial designs, is more suitable. This gives a high flow rate but low head. With this design, fluid enters and is discharged parallel to the rotating shaft. At the other end of the scale, axial flow impellers are preferred if the specific speed is greater than 175 (US units: 9000). Efficiencies are also low because of high friction and mechanical losses. Compared with other impeller designs, however, flow rates are low. These produce high head through centrifugal action as the fluid is expelled perpendicular to the pump shaft. If the specific speed is less than 40 (US units: 2000), radial impellers are suitable for the application. When selecting a pump for a particular application, and knowing the shaft speed, flow and differential head, a calculation of specific speed will identify the impeller shape most suited to the task (Figure 1). Ns (SI: l/s) = 0.614 S (US: gpm) What is Specific Speed used for?.To convert values between US units (gpm) and SI units (m 3/h, m 3/s, l/s) the following factors can be used: When doing calculations, it is important to ensure that compatible units are used. Is the differential head at the BEP (US units: ft SI units: m) Is the flow rate, per impeller eye, at the BEP (US units: gpm SI units: m 3/hr) flow is taken for the largest impeller diameter available for a pump and should be halved for double inlet pumps) Is the rotation or shaft speed of the impeller (rpm) Is the specific speed (often shown as ‘S’ when used with US units)
Specific Speed is an important factor in the design and selection of pumps and especially pump impellers. Although it is not actually a speed, it depends on shaft speed and is also a function of the flow rate and differential head at the pump’s Best Efficiency Point (BEP – see What is the Best Efficiency Point?). Specific Speed (sometimes called Pump Specific Speed) is a dimensionless rating for centrifugal pump performance. Useful information on specific speed and suction specific speed What is Specific Speed?
0 kommentar(er)
