Realising the energy-saving benefits of variable-speed pumps

Integrating variable-speed pumps into the system they serve is the way to maximise energy savings. Wayne Rose explains how.The energy efficiency and cost benefits of variable-speed pumps are widely acknowledged, but are rarely optimised in installed systems. Analysis of the overall wire-to-water efficiency of installed systems frequently reveals that, whilst individual components have energy-efficient features, the potential for maximum energy saving from the overall system remains untapped. This article reviews the theory behind energy efficiency of variable-speed pumping and suggests changes in control philosophy and product specification capable of optimising performance. Power absorbed by the pump is a function of flow multiplied by the square of the head. So, if pump flow and head can be reduced together, following a square-law curve similar to a system curve and with all valves fully open, pump efficiency will remain nearly constant and maximum savings can be achieved. Why this objective fails to happen in so many system designs could be due to the control philosophy or the positioning of differential sensors. Typically, a sensor controls pumps with variable-frequency drives to maintain a constant pressure across the pump. While this provides a simple solution with advantages over fixed-speed alternatives, the pressure drop in distribution piping at full load is transferred to the control valves at low loads. This can waste energy and affect the ability to control the valves effectively. An alternative strategy (Fig. 1) is to measure the differential pressure across the loop at the remotest load (or index point) across the supply and return piping encompassing the valve and coil set. This reduces significantly the differential pressure drop across the valve at low flows. 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 is reduced from 1450 rpm at full flow to 1250 rpm at 75% flow, 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, and there is a risk of loads close to the pump being under supplied. There is another, and more effective strategy that reduces energy consumption even more. That is to measure differential pressure across the control valves (Fig. 2). Sensors local to the pump, across the system, normally build design-flow piping and load friction losses into a 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. Using this second approach, we see that pump speed can be reduced from 1450 to 1194rpm whilst still maintaining the system requirements. The pump head would be reduced to 180 kPa and the efficiency held close to full flow efficiency. This results in a huge 47.5% savings 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. A barrier to achieving these energy-efficiency improvements has traditionally been how to achieve the necessary levels of control over variable-speed pumps within the wider system. A solution available from Armstrong is the IPS range of controllers, which interpret load requirements and adjust the pump speeds to the minimum whilst ensuring all load requirements are met. Control valves are therefore allowed to operate as close to fully open as possible without sacrificing user comfort. Pumps are controlled on a ‘best-efficiency’ basis and utilise wire-to-water efficiency control. Built-in protection prevents pumps from running at end of curve. As they are pre-engineered rather than bespoke, IPS controllers are a reliable, proven solution at an economic price level. The IPS controller can be supplied as part as a fully-integrated pre-assembled package (Fig. 3). Another recently launched solution is Armstrong Pumpbox (Fig. 3). Developed in response to demand for a fast-to-install, fully-integrated pump system package, it has built-in optimisation of energy efficiency. Pumpbox is a self-contained HVAC pumping solution, manufactured off-site and housed in a compact thermally and acoustically insulated weatherproof enclosure. Available with a range of 4300 Series pumps from 80 to 200 mm bore, with 2-pole or 4-pole motors up to 55 kW and 60 Hz frequencies, Pumpbox suits a wide range of applications and provides a space-saving alternative to the traditional plant room. Incorporating maximum performance components such as vertical inline pumps, Armstrong IVS variable-speed drives with sensorless in-built inverters, suction guides, triple duty valves and controls, the Pumpbox has inherent energy-efficiency advantages over other systems. It is a fully-integrated solution, so all sub-systems and controls are matched and designed to work in harmony with water to wire efficiency optimised to ensure low operating costs. As Pumpbox is manu­factured offsite and tested prior to delivery at site, where it requires only final connections, installation and commissioning are simple and fast. It also has significant ease-of-main­tenance advantages and a space-saving design to reduce footprint. In conclusion, it has been suggested that the inherent energy-efficiency benefits of variable-speed drives are not, in them­selves, enough to achieve the sorts of carbon reductions demanded by today’s specifiers and site occupants with increas­ingly demanding Building Regulations. That does not mean to say, however, that these energy-efficiency benefits are any less valuable or achievable within the overall system. Careful consideration of control philosophy and/or the employment of highly effective ready-integrated solutions such as IPS or Pumpbox can bring variable-speed pumping to its full potential. Optimum wire-to-water efficiency is easily achievable. Wayne Rose is marketing manager of Armstrong. Further information on this subject is available from Armstrong asf a CIBSE-accredited CPD seminar available free of charge to groups of consultants, contractors and engineers. Fig. 1: Using the differential-pressure measurement across the remotest load (or index point) to control the speed of a variable-speed pump limits the energy savings that can be achieved and may result in loads close to the pump being under supplied. The table shows the savings at 75% flow and represents an additional 21% over maintaining plant differential pressure. Fig. 2: Measuring differential pressure across the control valves can save more energy than measuring it at the index point. The table shows the power saving at 75% flow, an additional improvement of 12% over Fig. 1. Fig. 3: To make it easier to exploit the benefits of variable-speed pumps, Armstrong offers two solutions that integrate controls and pumps. On the skid is a preassembled packages with IPS controllers (right). The other solution is the recently launched fully integrated Pumpbox (below).
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