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Replacement of Pump Impellers at a Synthetic Yarn Plant

Energy audit was conducted in a large synthetic yarn plant. The plant was installed only 2 years back; however, the management was interested to know the possible improvements that can save energy.

The major utility loads in the plant were compressed air system, chilled water plant and cooling towers. All these utilities are very critical in plant operation; even a momentary stoppage of the fibre plant would mean production loss of a few crores.

We studied the cooling water system in detail to identify saving opportunity. The cooling water was supplied to chiller condensers, air compressor coolers etc.

Schematic diagram of cooling water system is given below.

The utility equipment comprising 3 nos. centrifugal chillers, 3 to 4 nos. air compressors, and 2 nos. refrigeration dryers were cooled by 3 nos. cooling tower pumps. The cooling water piping was very liberally designed and pipe friction losses were very low (schematic drawing shown in fig.).

The specifications of the pump is as follows.

Make: Greaves Pearless
Head: 30 m
Flow: 550 m3/h
Speed: 1450 rpm
Efficiency: 86%

Pump specifications shows a very good, efficient pump if it operates at the rated duty point. Does selection of an efficient pump mean good overall efficiency of the pumping system? The overall efficiency of a pumping system will be good if the pump operates efficiently with minor pressure losses in the entire system.

Let us understand the meaning of head specification for a pump. When we specify 30 metres head for a flow of 550 m3/h, it means that we are expecting a system resistance (total head) of 30 metres WC when a flow of 550 m3/h take place. If our system design is such that the resistance is more than the rated (smaller pipes, valve losses etc.) head, flow will be less than 550 m3/h. Similarly, visa versa.

Total Head is the sum of static and friction head. Static head is, crudely speaking, the height difference between water intake and discharge points. For a cooling water system, this can be taken as the height of cooling tower. In this case it is about 6 meters. That leaves us with a friction head of 24 metres. Friction head includes pressure loss in the piping, valves etc and the pressure drop across the heat exchanger. A typical chiller condenser will have pressure drop of 5 to 10 metres WC depending on the number of passes in the heat exchanger. Pressure drop in piping and valves generally does not exceed 5 to 6 meters in large systems.

What does all the above numbers mean? The total head requirement could be sum of the static head, friction head in heat exchanger and piping i.e. 16 to 22 meters. A pump of 25 meters would have been more than sufficient, as compared to the actual rated head of 30 metres.

Measurements on the pumps is given below.

Impeller diameter, mm 342
Head,m 35.00
Flow, m3/hr (per pump) 380
Motor input, kW (per pump) 52.7

During the energy audit, it was observed that the valves on the return line of each cooling tower cell were kept about 70% closed and that the pumps were operating at about 35 m head. Detailed pressure measurements indicated that about 70% of the pressure drop in the entire system was in throttled valves. This implied that there was a large mismatch between the pump and the piping system for the required operating flows. After a detailed study, it was decided to replace the existing 342 mm dia. impellers with 307 mm dia. impellers and only slightly throttled to balance the flow. The savings are estimated to be 376800 kWh/annum i.e. Rs.15 lakhs per annum. The pay back period on an investment of Rs. 75,000/- (for 3 nos. new impellers) was 18 days.

Energy saving by impeller replacement (3 nos. pumps in operation)

Impeller diameter, mm 342 307
Head,m 35.00 28.00
Flow, m3/hr (per pump) 380 380
Motor  input, kW (per pump) 52.7 37
Power savings for 3 pumps, kW   47.10
Energy savings, kWh   376800
Money saved, Rs.   1507200
Investment, Rs.   75000
Payback, days   18

In cases where the mismatch between the system and the pump is small, trimming (reduction of impeller diameter by machining) of impellers can shift the pump head/flow characteristics downward to match the system head. The flow and pressure will reduce, the efficiency may also reduce. However, energy is saved as throttling losses or higher pressure/flow, as may have been the previous case, is avoided. It is advisable to study the pump type performance characteristics and the test certificate before trimming. The reason for this caution is that the manufacturer, in order to meet the purchase specification, may have already supplied the pump with a trimmed impeller originally or may have supplied it with the smallest impeller that is designed to operate in the particular casing; further trimming may lead to drastic drop in efficiency and may fail to meet the operating pressure and flow requirement.