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Article no. 1 -
(Variable speed pumps explained - taken from H&V News)

Variable speed pumps can save energy, cut maintenance costs, reduce harmful emissions, boost reliability and optimise system performance, but only if applied correctly.

WHAT ARE VARIABLE SPEED PUMPS?

Simply put, a variable speed pump offers variation in speed and the potential to improve efficiency levels beyond those achieved by a fixed speed pump. Looking deeper, pumps are based on affinity laws. These are used in hydraulics and HVAC to express the relationship between several variables involved in pump performance (such as head, volumetric flow rate, shaft speed and power)
The affinity laws state that: (click on the image to enlarge)

THE MARKET

Three companies control approximately 80 per cent of the European market for circulator pumps - Grudfos, Wilo and Smedegaard. There are around 20 times more pumps in service in existing installation than are supplied new every year. According to British Pump Manufacturers (BPMA), it is common for fixed speed pumps to be oversized, with most system designers allowing a contingency on the system head required. ''Retrofitting variable speed drives could match pumps to actual system requirements more accurately and save considerable amounts of energy'', says the BPMA, adding: ''Many existing systems use control valves and bypasses, all of which absorb energy not required to satisfy system demands. It is therefore apparent that a major opportunity exists for modifying installed systems to make them more energy efficient.'' Europump, an international reference body and source of harmonised regulations for efficient and environment-friendly fluid pumping systems, agress. It says: ''Traditional flow-control methods - using throttling valves or bypass lines - waste energy and frequently increase operating and maintenance costs. However when properly applied, variable speed drives (VSDs) have the potential to significantly lower energy costs, improve overall system reliability and minimise pumping system life cycle cost''. Variable speed technology has indeed been around for many years and has been heavily promoted, but Grundfos Pumps' marketing manager Linda Dingley says: ''We [in the UK] still lag behind many other European markets; despite the fact our perception is that we are ahead of the game.'' One reason for this is that variable speed control is not as straightforward as it may at first appear. Chris Parsloe of technical consultant Parsloe Consulting explains: ''If pumpm speed is controlled so as to maintain constatnt pump pressure then, at low flow rates, pump efficiency can become poor, thereby cancelling out some of the energy saving.''

ADVANTAGES AND DISADVANTAGES OF VARIABLE SPEED PUMPS

Benefits:
- Energy savings between 30 and 50 per cent have been achieved in many installations by installing variable speed drives (VSDs). It has been claimed that using variable-speed pumps in secondary chilled-water systems can reduce pumping energy by more than two-thirds.
- Tighter control is possible beacuse, by matching pump output flow or pressure directly to the process requirements, small variations can be corrected more rapidly by a VSD than by other control forms, which improves process performance.
- Lower speeds results in less wear and tear - particularly in bearings and seals - and sytem relaibilty therefore is improved.
- Vibration and noise are reduced and seal life is increased providing the duty point remains within allowable operating range.

Drawbacks:
- Cost; variable speed pumps can, in some cases, cost around twice as much as their fixed speed cousins.
- Structural resonance in variable speed equipment can cause vibration, which is potentially harmful to equipment.
- Small increase in the speed of a pump significantly increase power absorbed, shaft stress and bearings loads. It shoul be remembered that the pump and motor must be sized for the maximum speed at which the pump set will operate.

FURTHER RESEARCH

- Variable Speed Pumping: A Guide to Successful Applications, from Europump and US - based Hydraulic Institute. See www.europump.org
- CIBSE Knowledge Series guide KS7 Variable Flow Pipework Systems explains how to design re-circulating heating or cooling water systems incorporating variable speed pumps.
- The Variable Speed Driven Pumps Best Practice Guide, from members of the BPMA, the variable speed drive group of Gambica. The national organisation represents the interest of companies in the instrumentation, control, automation and laboratory technology industry in the UK, and experts from electric motor industry. See www.gambica.org.uk/pdfs/VSD_Pumps.pdf

Other Articles and Case Studies:

Case study no.1 (System upgrade to energy class 1)
Article no. 1 (Variable speed pumps explained) 
Article no. 2 (Energy saving tips) 
Article no.3 (Variable Pumping Systems p.1)
Article no.4 (Variable Pumping Systems p.2)

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