You haw the following data set from a pump that is operating

You haw the following data set from a pump that is operating at 1725 rpm with a 16 cm diameter impeller: To match your current system, you plan on changing both the impeller and the operating speed. What impeller diameter and pump speed should you specify so that it will operate at its best efficiency point with a pump head and a flow rate as close as possible to 15 m and 7.5 Lpm respectively? Assume the best efficiency point for the pump at the conditions in the table occurs at a volumetric flow rate of 20 Lpm.

Solution

D1 = 16cm = 0.16m = 6.29inch

The impeller diameter and pump speed can be found using Affinity Laws

Find the diameter using Law 2 of Affinity Law:

Flow is proportional to Impeller Diameter

Q1 / Q2 = D1 / D2

20 / 7.5 = 6.29 / D2

D2=2.36 inch

Using Law 2b

Pressure or Head is proportional to square of impeller diameter

H1 / H2 = (D1/D2)^2

H1 = 15 (6.29/2.36)^2 = 19.17 m (1inc =0.0254m)

Using Law 1b

Pressure or Head is proportional to square to shaft speed

H1 / H2 =(N1/N2)^2

19.1 / 15 = (1725 / N2 )^2

1.2786 = (1725 / N2 )^2

1.1307 = 1725 / N2

N2 = 1525rpm

Hence the impeller diamter is 2.36 inch and impeller speed is 1525 rpm

Note: I have used the conversions wherever required