Balancing for variable-speed pumps

Maximising the energy-saving potential of variable-speed pumping systems in air-conditioning systems using fan-coil units demands action up close — says LARS FABRICUS.It has long been recognised that variable-speed pumps offer significant potential for energy saving by varying the speed of the pump to match system demand. Because of the cube-law relationship between pump speed and energy consumption, small reductions in pump speed lead to dramatic reductions in energy consumption. Pump energy savings of 60 to 70% have been predicted for heating and cooling applications in typical buildings. Challenge However, designers of air-conditioning systems based on fan-coil units face the challenge of utilising the undoubted energy-saving potential of variable-speed pumps while achieving thermal balance in the system. Pressure has to be controlled so as to ensure that some parts of the system are not starved, while others receive excessive flow. The solution lies with using differential-pressure control valves (DPCVs) to maintain a constant differential pressure between two points in a pipework system that are either side of a variable resistance. To work effectively, they require a minimum differential pressure across them — typically 15 to 20 kPa. Large DPCVs have traditionally been located on main branches on each floor, or, in a very big building, in each floor subdivision. To maximise the energy-saving potential of variable-speed pumps, the closer the pump sensors are located to the terminals the better. In this way, efficient pump control can be maintained over not just the pressure drop in the riser but in the main branch line too. Sensor and DPCV should be close together, which becomes impossible if there is only one large DPCV per floor, and the sensors are to be as near as possible to the terminal units. Moreover, large DPCVs are expensive and can lead to poor valve authority at the terminal. It is possible to use DPCVs that are small enough to be fitted at the fan coil — but space is the problem, and the last thing needed is another valve to be crammed into a ceiling or floor void already filled with rigid pipes, joints and strainers. Solution A space-saving, modular solution to this problem is now being applied in buildings across the UK from Aberdeen University to the Swiss Re tower in London. Commissioning modules used with flexible multi-layer pipework make it possible to sub-divide the system into smaller, self-contained sub-systems, each served by a single distribution module with a cost-efficient DPCV already built in. The distance from DPCV to control valve is minimised and accurate modulating control of the pumping system is maximised.* Capital cost is also reduced, and the problem of good valve authority is overcome — turning the energy-saving potential of variable-speed pumping into reality. Reduced demand off peak For most heating or chilled-water systems in the UK, the requirement for heating or cooling will probably be at its peak for less than 10% of the time. For the remaining time, the demand will be significantly reduced. Pumping less water during these low-demand periods can make big cuts in energy consumption.   Pump energy can be saved because there is a straight correlation between pump speed, flow rate and energy consumed. If only half the flow is required, then the pump only needs to operate at half speed. At half speed, the energy consumed will be reduced to 87.5%.   To properly quantify the energy saving, the system resistance must be calculated at full and minimum load conditions and plotted relative to the pump curve.   The stirring effect of pumping results in heat being added to the water; in a chilled-water system, this extra heat has to be removed by the chillers. Reducing pumping power input in chilled-water systems at times of light load reduces the power input to the chillers. There is also less heat loss from return piping for heating systems and less heat gain in chilled-water systems. Where condensing boilers are used in heating systems, the lower return temperatures will result in greater operating efficiencies. A commissioning module enables one compact DPCV to be shared by up to six terminal units with a maximum loading of 11.6 kW heating and 6.3 kW cooling per terminal — a very cost-efficient ratio. If the pressure available from the pump increases, the DPCV will close to take out the excess pressure. If the pressure available should reduce, the DPCV will open so that more pressure becomes available. Similarly, if 2-port valves in the variable load begin to close, the DPCV will also begin to close to maintain the same overall pressure drop between flow and return pipes. Moreover, because the DPCVs are located close to the terminals, the pressures they control are relatively low (typically 0.2 to 0.5 bar), so crucial good valve authority can be achieved every time. Each DPCV is sized to give good valve authority over all the terminals linked to the module, which can be fan coils, chilled beams etc. These compact units are competitively priced so the problem of capital cost is reduced. Pre-insulated The commissioning module has all components, including multi-port manifold, automatic air vent, isolation valves and DPCV where required, housed in a pre-insulated galvanised steel casing. This box is no bigger than a standard fan-coil unit and can be easily and quickly located in the ceiling or floor void. Connecting up the flexible multi-layer pipes is a matter of minutes, and the commissioning module is ready for action. Other key system components, including large, fine-mesh strainer, pressure gauges and specially designed commissioning valves, can all be accommodated inside the box. Commissioning is greatly simplified. Balancing of the system flow rates is achieved by opening or closing the specially designed double-regulating valve, which has vertical indicator to give an easy-to-read setting. The metal-to-metal thread enables the valve setting to be rigidly locked, so it can be closed and re-opened to its exact setting with great accuracy. Lars Fabricus is Managing Director of SAV Valves Systems, Scandia House, 131 Armfield Close, West Molesey, Surrey KT8 2JR.

*The technique is fully described in a new publication ‘Variable-volume pumping systems — a practical design guide’.