A number of VSD case studies are presented in this Chapter to illustrate the potential savings possible, and also the less quantifiable improvements in pump and system reliability. For example, in one case the greatest benefit of using a VSD was not the cost saving, but that the process (irrigating a golf course) was carried out in a more effective and controlled way.
The industries in which the applications were based are described along with the ways in which the benefits were achieved. This should allow similar opportunities to be found, whatever the process or type of industry.
The amount of the saving will be dependent on the cost of energy for the user. Where the study recorded energy cost per kilowatt hours (kWh), it is stated.
The value of the Euro and US dollar are considered sufficiently close, for the purposes of the case studies, to make conversion unnecessary, the monetary values are expressed as €/$.
Where gallons are used in case studies, they are US gallons.
Actual company references for equipment suppliers or user industries are intentionally excluded.
Case study 1: Variable speed drives on water pumps supplying a ring main at a domestic appliance factory
(Based on Energy Efficiency Best Practice Programme, case study 88)
Case study summary
Energy savings were achieved by using variable speed drives (VSDs) to control two water pumps supplying the ring main in a domestic electric appliance factory. Previously control was by pressure-operated bypass valve, with pumps oversized for normal operation.
Other potential applications
Any applications which require varying flow rates, where present control method is by valves.
Water distribution system
The process water distribution system at a domestic electrical equipment manufacturer consisted of a 150 mm (6 inch) cast iron distribution ring main, which was fed from two sources:
1. Mains water from the local water supply company (termed raw water in this study) was pumped into the ring main from a 910 000 litre (240 000 gallon) storage tank using one of two pumps, each driven by a 15 kW (20 hp) induction motor. The pumps were used on a duty/stand-by basis and both were capable of providing much higher pressures and flow rates than required.
2. Water from the company’s own borehole was pumped directly into the distribution main by a submersible borehole pump with 30 kW (40 hp) motor.
Original control method
With the raw water pumps, control of both the ring main pressure and the pump flow rates was originally achieved by mechanical means. Pressure in the ring main was controlled by a pressure-operated bypass valve, which diverted excess water back into the storage tank. The system caused the raw water pump to be fully loaded at all times.
When using the borehole pump, the allowable flow rate was governed by the company’s water abstraction contract. To maintain flow rates within this limit a partially closed valve between the pump and ring main was used to throttle the flow, with considerable energy wastage.