Maximising the energy-saving potential of variable-flow systems

Stephen Hart
Maximising the benefits of variable-speed pumps — Stephen Hart.

Variable-speed pumps used 15% less energy than fixed-speed pumps. Good design of variable-slow systems can increase that saving to 75%. Stephen Hart explains what you need to do.

CIBSE Commissioning Code W for water-distribution systems was last revised in 2003 at a time when the vast majority of medium to large secondary circuits were designed for constant volume and featured 3- or 4-port temperature control valves. The intervening years have seen a dramatic increase in the installation of variable-volume circuits where a combination of pressure, flow and 2-port temperature control valves are combined with variable-speed pumps and pressure sensors to deliver some of the energy savings required by Part L2 of the Building Regulations. Many system designers, installers, commissioning specialists and manufacturers have modified their behaviour or their products over recent years to better suit the requirements of variable-flow systems, and this is likely to soon lead to a revision of Commissioning Code W. Over the years we have, in the UK, developed a professional approach to the commissioning of water-distribution systems. A number of codes and guidance documents are currently available from CIBSE and BSRIA upon which the commissioning specialist and manager can rely. The value of fixed-orifice devices to help proportionally balance or verify flow rates is widely recognised, and the trusty ‘water box’ or U-tube manometer is still regarded as the commissioning specialist’s ‘best friend’, despite recent improvements to the portability and reliability of digital manometers and ultrasonic flow meters. Variable-flow systems should be balanced at the full-load condition with the 2- port control valves fully open and at the minimum-load condition with the 2-port control valves fully closed. Unfortunately, accurate water-side balancing at full load only will only ensure that a variable-flow system is in balance for a small percentage of operating hours per year. Further measures, such as the use of reverse-return networks or the installation of differential-pressure control valves, flow-limiting valves or pressure-independent control valves are required to maintain an acceptable balance at all part-load conditions. If a variable-flow system becomes unbalanced at part load, excessive pressure variations around the system result will result in overflows and underflows through individual heat exchangers, hunting control valves, pumps running too high, increased heat losses from return pipes and chillers and boilers operating when they should not. Good system design and commissioning, however, can easily reduce average system flow rates by 20% or more, resulting in a 50% reduction in energy absorption by variable-speed pumps — not to mention the reduced energy consumption by chillers cooling or boilers heating water that is not required to keep the building at a comfortable temperature. A recent research study demonstrated that the installation of a variable-speed pump alone can reduce pump energy consumption by 15%, whereas the additional installation of pressure-independent control valves to maintain system balance at part load can reduce the energy consumption of the pump by up to 75%. Irrespective of the devices used, it is crucially important that the water-distribution system is designed, installed and commissioned as a whole if the true energy saving potential of variable-flow pumping is to be realised. Irrespective of the variable-flow system design and the temperature, pressure and flow devices and methods used, certain requirements are common. • Identify the index circuit and tune the pump to deliver just enough pressure to maintain design flow around that circuit. • Position the pump pressure sensor as far into the circuit as possible to maximise its ability to reduce speed and save energy at part load whilst generating enough pressure to operate all of the installed pressure independent balancing and control valves at all load conditions. • Ensure that the pump always circulates enough flow to distribute the inhibitors, ensure chilled or heated water is available on demand and to protect the pump, chiller or boiler from potentially harmful low flows. • Install enough flow-measurement devices in branches and sub-branches to enable a good commissioning report to be generated, remembering that regulation valves are not required upstream of pressure-independent balancing or control valves. It is important to verify the system performance at part load, with the control valves throttling. We should be less concerned over the amount of flow data produced in a water-side commissioning report, but rather the quality— especially regarding system performance at part load. An effective commissioning procedure, implemented by a knowledgeable specialist, can make a greater difference to the efficient and effective control of a variable-flow water-distribution system than ever resulted from the accurate proportional balancing of a constant flow system. A good variable-flow system can be greater than the sum of its components if it is designed, installed and commissioned correctly. Stephen Hart is managing director of Frese Ltd.

Diagram
To maximise the energy-saving potential of variable-speed pumps at part load, the pump pressure sensor should be as far into the circuit as possible.
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