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Pumping system improvement

This case study illustrates the importance of integrated systems approach, with multidisciplinary inputs helping to achieve large savings. The plant manufactures Pesticides products. Chilling is a major end use of energy, roughly about 15% the plant energy consumption. There are two ammonia based vapour compression system to produce chilled water at 8 C. The specifications of the plant are as follows.

Compressor make: Kirloskar
Model: KC6
Capacity (at 0ºC SST) = 120 TR
Rated specific power consumption at above SST = 0.72 kW/TR
Rated primary flow = 61 m3/h
Rated condenser flow = 105 m3/h

The chilled water system has primary and secondary pumping arrangements. The primary pump specifications and measurements are given below.

Make: Kirloskar
Model: DB 65/13
Head: 30 mWC
Flow: 130 m3/h
Motor: 20 HP

Measured values:
Discharge Head: 37.5 mWC
Suction head: 4 mWC
Flow: 112.2 m3/h
Power input: 14.0 kW
Operating efficiency: 80%

The pressure at pump discharge was 37.5 mWC and the pressure at chiller inlet was 28 mWC. This indicated that the pressure drop in the 4” supply piping from pump to the chiller was 9.5 mWC. This is very high. Not more than 2 mWC is considered to be good. Similar pressure drop was observed in return line also.

The pipe sizing of 4” in generally adequate for a rated primary flow of 61 m3/h. However, due to improper selection of pump, the pump is giving about 112.2 m3/h; almost double that of the required flow rate. This flow through a 4” pipe is expected to produce a pressure drop of about 10 mWC.

The solution suggested was to reduce the primary flow, by reducing the impeller diameter. The plant personnel wanted to know if reducing primary flow would effect the chilling capacity. A trial was taken by reducing the flow by controlling valves to evaluate chiller performance. Flow reduced from 112 m3/h to 75 m3/h. Total reduction in pressure in the supply and return was 1.5 kg/cm2.

The following graph shows the variations in chiller inlet/outlet temperature and capacity before and after reducing the flow.

Figure 7 4: Chiller performance

Note that, with reduction in primary flow, inlet and outlet temperatures found to be reduced. Most importantly, the capacity of the machine remained unaffected.

After gaining confidence from the above exercise, the impeller diameter of primary pump was reduced from existing 174 mm to 145 mm. Since the existing impeller diameter and the new impeller diameter were very different, a new 145 mm impeller was purchased, instead of trimming.

After installing the new impeller, the performance is as follows.

Head: 23 mWC
Flow: 75 m3/h
Power input: 8.2 kW

Energy saving for 9 months,10 hours/day operation is found to be 15,660 kWh/annum. i.e. Rs 70,000/- per annum. Investment for a new impeller was Rs 4000/- with a payback period of one month.

Lessons learnt:

Excessive pressure drop in pipelines is perceived to be a problem with inadequate pipe sizing. In this case study, we have tried to elaborate on another reason, such as excessive flow resulting due to incorrect pump sizing. The above case study is on a small system. Many larger systems, especially with cooling water, there has been many cases of excessive flows in heat exchangers, without any commensurate benefit in heat transfer.

If you note the cooling water/chilled water inlet/outlet temperatures and compare with the values given by the process designer at design loads, this fact would be very clear. For same heat load, if you actual ‘delta T’ is 2 deg.C compared to the rated value of 4 deg.C, you can begin to analyse for excess flow and to achieve savings. Use a smaller pump, smaller impeller or switch off pumps in multiple pumping systems are typically suitable for achieving savings by flow reduction