Delivering energy-efficient pumps

Designing systems with duty and standby pumps that can both meet maximum demand is must not efficient. Wayne Rose explores other approaches. Simply installing pieces of equipment that are, individually, energy efficient does not deliver the lifetime performance necessary today. Whilst the requirements of Part L of the Building Regulations may be met, such systems are unlikely to satisfy today’s end-customers, who are increasingly aware of lifetime cost and carbon footprint. The emphasis now is on achieving maximum overall system efficiency — in other words, optimising wire-to-water efficiency. Parallel pumping and multi-pump, multi-zone control have a major role to play in this process, particularly in larger scale, more complex applications where the potential lifetime cost-savings is huge. The opportunities presented by parallel pumping and multi-zone control are, however, frequently under-exploited. This article aims to explain why and suggest how to maximise the benefits. Parallel pumping The benefits of parallel pumping remain largely unexploited because the designers of HVAC systems have traditionally favoured a full duty/standby approach — missing out on significant cost and energy savings. Over the last year, however, system designers have begun to realise that habit is no excuse for knowingly over-specifying the installed pump power. Doubling the kilowatts installed ‘just in case’ is beginning to be recognised as outdated thinking with today’s technology. The reason this practice has continued for so long might be due to a common assumption that if one pump of a pair operating in parallel fails, the remaining pump will cover only half the duty — insufficient for demand. This is not so; half the installed power does not equal half the capability. To meet a system design duty of 200 l/s at 200 kPa, the traditional approach is to install two pumps, each with a 75 kW motor, to provide full duty and standby — a total of 150kW installed. A parallel pumping alternative for the same application would split the duty between two smaller pumps, each with a 37 kW motors — a total of 75 kW installed compared to 150 kW. If one pump should fail, the remaining pump could deliver far more than half of the full design duty. One pump could, in fact, deliver 167 l/s, which is 83% of the full design duty, while the other pump undergoes maintenance. Since most systems require full design duty very rarely, it is likely that the impact on the system will be minimal during maintenance. A parallel pumping approach therefore effectively halves the carbon footprint relating to the manufacture of the equipment itself and halves the installed kilowatts. The two smaller and lighter pumps are easier to install and maintain and, as they have a lower upfront cost than two much larger pumps, there are cost benefits for the contractor as well as for the client. Variable-speed drives are also smaller, as is the electrical supply cabling. Clinging to full duty/standby is also outmoded in the light of advances in technology. Split coupled pumps with external seals, for example, enable seal changes to be carried out by just one operator in minutes, rather than hours, without the need to disturb other pump components or the motor connection. Multi-pump, multi-zone control When considering multi-zone pumping, the reasons for under-exploitation of the energy efficiency advantages are very different and stem from pump configuration and system design. If we look as some common control strategies, the reasons become clear. One common strategy is to monitor differential pressure across the loop at the pump, with all loads and distribution lines downstream of the sensor. Mounted in the system at this point, differential pressure is measured across the supply and return piping to the system in the mechanical room. A maximum plant head pressure limit is established, and then maintained using a variable-frequency drive. The controller can now reduce pump speed to maintain the sensor set point. For example, if pump speed is reduced from an original 1450 rev/min to 1365 rev/min at design flow, the power consumption will be reduced by 22.4% compared with a constant-volume system at 75% design flow. Such a saving is 15% greater than with a constant speed pump in a variable-volume system at 75% flow. However, a shortcoming with this control strategy is that the pressure setting we have established actually limits further reductions in speed and, therefore, the potential energy savings. Alternative strategy An alternative strategy is to measure the differential pressure across the loop at the remote load (or index point) across the supply piping and return piping encompassing the valve and coil set. A sensor setting is established at the remote load zone (for example, 40% of the original maximum head), this being the differential across the load coil and the control valve. A variable-frequency drive modulates the pump speed/plant energy input. At reduced flows the differential pressure drop across the valve is itself significantly reduced. As we no longer build the distribution piping losses into the sensor setting, the pump head can reduce at lower flows and the speed reduction also keeps the efficiency closer to the original design. Pump speed can be reduced to 1250 rev/min at 75% flow compared with the original 1450 rev/min, whilst still maintaining the sensor setting, representing a power saving over a constant-volume system of 38.7%. However, further reductions in speed and energy consumption are limited by the sensor setting at the remote load zone. There is also a risk that sensing the pressure across a remote load could theoretically result in loads close to the pump being under-pumped. Even better control A more effective control strategy is to measure differential pressure across the control valves. Sensors local to the pump, across the system, normally build design flow piping and load friction losses into lower pump operating speed. Sensors across control valves, however, remove these losses from the control loop so that only ‘real’ proportional piping will be pumped, and then only to satisfy the volume needed to match demand. The result is that all needs are met with minimum power penalty. Pump speed can be further reduced to 1194 rev/min whilst still maintaining the system requirements. The pump head would be reduced to 180 kPa and the efficiency held close to full -low efficiency. This results in a huge 47.5% reduction in power compared to a constant-speed constant-volume system. It also demands 30% less power than measuring differential pressure at the pump head and represents a further 12% improvement on a ‘remote load’ control strategy. The Armstrong IPS range of controllers for these types of multi-zone, multi-pump applications process field signals to determine the sequencing of any optimal duty point of any pump. Pumps are controlled for maximum efficiency and utilise wire-to-water efficiency control. There are three controllers in the range to suit the application and all integrate with building-management systems. A particular benefit of these solutions is that detailed knowledge of pump characteristics is used in the design of the controllers, providing sophisticated protection against pumps running at end of curve. As these controllers are pre-engineered rather than bespoke, they are a reliable and proven solution at an economic price. A growing trend is for the IPS controller to be supplied as part as a fully-integrated pre-assembled package. In conclusion, the future lies in optimising wire-to-water efficiency — not just the specification of system components that are individually energy efficient. The key to delivering cost savings and reductions in carbon footprint lies, particularly for larger scale and more complex systems, in harnessing the true potential of multi-pump control. Wayne Rose is marketing manager with Armstrong.

Related links:

• www.armstrongpumps.com