Modern Building Services

FEATURE AIR HANDLING & VENTILATION A more effective methodology is to use a digital VAV controller, making all data available digitally over the BMS network. As digital control technology provides the opportunity for increased levels of data management in comparison to analogue technology, the position of the damper blade itself can be used to indicate the appropriate fan speed to meet demand. This eliminates the need for the fan speed to be controlled by a pressure transducer, meaning that duct pressure does not have to be measured and maintained at an unnecessarily high set point. By contrast, as demand falls, both fan speed and pressure can fall together, following the characteristic curves of the devices, as shown in Figure 3. Energy savings, improved acoustics and easier commissioning There are a number of advantages of this approach. Firstly, enabling the fan speed and pressure to reduce uniformly along the device’s characteristic curve harnesses the potential of the fan laws (power is proportional to rotary speed cubed: P α N3), reducing energy costs throughout the equipment’s lifecycle. Reducing the flow rate by 40% as shown in the examples here will result in a reduction in energy consumption of 78%, but the real-world energy saving then is dependent on demand generated by occupancy patterns. Installations employing this approach to date indicate that fan energy consumption can be reduced by around 45%. Secondly, the damper blade position will be between 40% and 100% open at this lower pressure point, facilitating better control of the VAV units and improving acoustic performance. Lastly, controlling fan speed in this way removes complexity during commissioning, avoiding the difficulties associated with the positioning of pressure transducers in the duct. This approach also provides a more stable control circuit on which to design and operate the system. To deliver these benefits, the systemmust, of course, have controls capable of monitoring and acting upon the damper blade positioning data. A central BMS might fulfil this function, but could involve complex (and therefore expensive) bespoke programming. So, look instead for VAV systems/components which already incorporate, as standard, the capability to achieve these additional energy savings. A number of standard TROX products and solutions, for example, have resident, integrated control technology capable of fan speed optimisation without the need for bespoke BMS programming. It can take a while to understand the full potential of technologies as they emerge. But now that digital control of VAV is in daily use, it is clear that there are valuable additional energy savings waiting to be unlocked by rethinking aspects of control methodology. The approach to fan speed optimisation described here is one such example, in which exploiting underutilised features can open up new functionality. By moving to this methodology for fan speed optimisation, utilising damper blade positioning, it is possible to deliver significantly improved environmental and acoustic performance, whilst simplifying commissioning and creating a more stable control circuit. More information can be found at www.troxuk.co.uk MODERN BUILDING SERVICES OCTOBER 2021 15