CASE STUDY: ENERGY CONSERVATION IN FLUID MACHINE

A pumping system is designed to supply water at a rate of 3.0 m3/min through a 15 cm diameter, 24 m length pipeline. The total static head is 8 m and the system incorporates a globe valve for flow controlling. The friction factor for the flow through the pipe could be considered as a constant at 0.006 and the sum of the coefficient of fluid resistances (K factors) of pipe fittings is found to be 34.0 (without the globe valve). You may use the following equations for energy losses with usual notations:

(a) A particular pump selected for the above flow system has the performance characteristics as given in Figure 1. The rated speed of the pump is 1500 rpm.(i) Identify the best operating point of the pump, the corresponding maximum

efficiency and the pump category.(ii) Determine the operating point of the pump and the corresponding efficiency, when

it is installed in the system with the globe valve at its fully open position.(iii) The required flow rate is obtained by controlling the globe valve position.

Determine the position of the globe valve to be set and the corresponding electric power requirement for pumping.

(iv) If the average annual operating hours of the system is 3200, estimate the annual cost of electricity. Take cost of unit of electricity as 10 Rs/kWh.

Figure 1: Performance characteristics of the pump

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(b) It is noticed that a set of smaller, identical pumps is available in the stores, which could be used in place of the pump in part (a) above. The performance characteristics of the smaller pumps are given in Figure 2.(i) Identify the connecting configuration and the minimum number of pumps required

for the application. You may assume that the total load characteristic of the system does not change due to the replacement.

(ii) Estimate the electric power requirement for the pumping in the new figuration.(iii) Estimate the annual cost of electricity. Give your opinion on the replacement.

Figure 2: Performance characteristics of the second set of pumps

(c) Another alternative flow control method is the use of variable speed drive (VSD) with the original pump.(i) Estimate the speed of the original pump that should be set through VSD for the

application. You may use the pump similarity laws

(ii) Calculate the electric power requirement for the pumping with this set-up with the globe valve at its fully open position. Compare the percentage energy saving potential of VSD with relation to the previous two set-ups.

(iii) If the additional investment cost involved with the VSD is Rs. 225,000, estimate the payback period. Neglect maintenance costs. You may use suitable parameters needed for the financial analysis. Give your recommendation on the use of VCD.

(iv) Is there any other alternative method that could be more cost effective?

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