Maximising the efficiency of fixed-speed pumps

When a fixed-speed pump is preferred over a variable-speed unit, there is no need to compromise energy efficiency, as ROY HENDERSON explains.There are several ways to maximise the energy efficiency of pumps. The best known include specifying a variable-speed pump rather than fixed speed, using high-efficiency motors and balancing the system properly during commissioning. Other opportunities include designing the system for parallel pumping and effective control logic.
Key ways of maximising energy efficiency of pumps
• Trimming impellers
• Variable speed drives
• High efficiency motors
• System balancing during commissioning
• Parallel pumping
• Control logic

One important aspect of energy-efficiency best practice is, however, often overlooked — trimming the impeller. Although variable-speed pumps are viewed as an easy solution, there are instances where fixed-speed pumps are better and preferred by the specifier. In applications where demand is constant, for example, variable-speed pumps may involve higher upfront cost and greater system complexity; they are not necessary, particularly for applications greater than 11 kW. In such fixed-speed applications, specifiers can readily make significant energy-efficiency improvements by considering impeller size. It is a well-established best practice that to deliver optimum efficiency the size of the impeller should be accurately matched to the desired operating point of the pump. In some international markets all pump manufacturers trim impellers as a matter of course to meet the specific requirements of flow and head designated by the customer to maximise energy savings. In Europe, however, this practice is not as widespread. Trimming impellers There are several manufacturing challenges involved in trimming impellers. Pumps cannot easily be standardised with this approach as each impeller has to be a one-off. The material of the impeller also has to suitable for machining. Whilst traditional bronze is ideal, spun cast iron, pressed stainless steel or plastic (resin) impellers are more brittle and problematic to machine. To overcome these challenges, a practice has grown up whereby each customer requirement will be met by the next-nearest impeller size from a standardised range. This practice can have a significant impact on the energy efficiency performance of the pump throughout its lifetime. Specifiers, however, are often unaware that the energy efficiency of their system has been compromised. It is therefore extremely important to look carefully at the small print of the technical submittal from the pump manufacturer before purchasing. Problems arise because the pump manufacturer is making a near rather than exact match. To the customer, this seems perfectly reasonable — on the surface. This seemingly innocuous decision however results in long-term wastage of energy. To meet the load requirements for the pump, the nearest impeller size will always be the one which over-performs rather than under-performs — or the pump would not be able to do its job. The outcome is a built-in oversizing of the pump from design stage, which can cost dearly in the long run. Instead of being built around the customer’s desired operating point, the specification is now focused on an operating point dictated by the pump manufacturer, delivering a higher flow and head than required by the customer for the task. Commissioning The commissioning engineer very quickly discovers that the pump flow is higher than specification. To bring it in line with requirements, the pump will be throttled to achieve the desired design flow. This, of course, raises up the head. As the diagram shows, the resultant operating point at design flow can be far above the customer’s desired operating point. Wasted head has been introduced from the moment of commissioning and will continue throughout the lifetime of the pump. For the site user, the result is increased energy costs. The pump motor is being forced to work harder, absorbing more power than planned, to create a head (and maintain an operating point) that nobody actually wanted in the first place! In these circumstances, the figure stated for notional efficiency of the pump becomes a nonsense. A pump capable of 80% efficiency, operating exactly at the customer’s desired operating point, is a very different proposition to the same pump being forced to deliver wasted head and absorbing wasted energy in order to do so. The efficiency of a system that has been near-matched for impeller size rather than matched exactly with a trimmed impeller may be around ten percentage points lower than the efficiency for that pump stated by the manufacturer. As the diagram shows, trimming a pump impeller offers a significant opportunity for reducing energy costs. The potential cost saving on the example shown (operating duty of 250 m3/h at 19.15 m head) demonstrates annual energy savings of 14%. It should also be noted that these cost-savings are achieved with a pump costing less than £4000. It is important to see this in the correct context. For smaller pumps, where the range of available motor sizes is at its widest, the trimming of impellers is unlikely to have anywhere near the same impact. Trimming an impeller will also, of course, have a slight effect on efficiency; on a properly selected product, the efficiency could be reduced by one or two percentage points. Caveats These caveats do not, however, negate the fact that running a large pump at a needlessly elevated operating point throughout its life wastes valuable energy. It is also contrary to the guidelines regarding over-sizing of plant as outlined in Part L of the new Building Regulations. Pumps already being manufactured with trimmed impellers (which does not tend to involve a premium price) can deliver valuable long-term energy-efficiency benefits and lower lifetime costs. Best practice dictates that the customer should ask a few shrewd questions before selecting the pump. 1. What material is the impeller made from? Bronze lends itself to machining, whilst more brittle materials such as plastic and pressed stainless steel do not. To our knowledge, bronze impellers are available as an option from all pump manufacturers. 2. Will the impeller be trimmed to match the desired operating point or will the impeller size be dictated by the standard range of sizes from the manufacturer? 3. If energy savings can be realised at some point in the future, either after commissioning or at a later date due to change in building use, is the material of the impeller suitable for trimming to achieve these savings? This capability is already a guideline under the ASHRAE code in North America. 4. What will the rated operating point for this pump be in terms of flow and head? If this is not the same as your desired operating point, what head and flow is the pump manufacturer saying you will achieve with the pump? How would this increase the head once the design flow is adjusted during commissioning? 5. What is the actual power consumption of this pump at the desired operating point? 6. If the manufacturer suggests a higher-than-requested operating point, what is the ‘adjusted efficiency’ and how much more energy will the pump use compared with the specified operating point? Whilst it is not usual for customers to judge the effectiveness of a fixed-speed pump based on its power consumption, this is in fact a more valuable indicator of true energy efficiency. Comparing the power consumption of alternative pumps at your desired operating point when specifying the system will give a far more accurate and reliable guide to the cost of incorporating that component than you would get from a notional estimation of efficiency stated by the manufacturer. Small print As mentioned, a number of manufacturers serving the European market, such as ourselves, trim impellers as a matter of course and can test the pump at your desired operating point prior to delivery. A few timely questions to the manufacturer and a look at the small print of the technical submittal will soon establish whether this important aspect of energy efficiency has been taken care of by the pump manufacturer on your behalf. Roy Henderson is sales director with Armstrong Pumps, Wenlock Way, Manchester M12 5JL.
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