Modern Building Services

FEATURE AIR HANDLING & VENTILATION 14 MODERN BUILDING SERVICES OCTOBER 2021 AIR HANDLING & VENTILATION D igital technology has opened up significant opportunities for the reduction of energy consumption in air conditioning systems. Variable Air Volume (VAV) systems in particular have derived enormous benefit from digital control. If your current control methodology is based along traditional lines however, there could be valuable additional energy savings waiting to be unlocked. This article outlines an alternative approach to the control of fan speed inVAVsystems which has delivered an additional 45%reduction in energy consumption (actual projects rather than theoretical simulations). By exploiting underutilised communication capabilities of digital control, this approach drives down energy consumption by employing damper blade positioning to optimise fan speedmore effectively. Current methodology In today’s VAV systems fan speed is typically controlled via duct pressure measurement, utilising pressure transducers to measure duct pressure and manage fan speed. If demand reduces, the damper closes to reduce the volume flow. This causes system pressure to rise, following the fan curve. In constant duct pressure systems, the controller addresses this by reducing the fan speed and reducing the system pressure to drop back to the set point (275 Pa). See Figure 1 . A disadvantage of this approach, however, is that when volume flow is lower (to meet the building’s reduced demand), the set point of 275 Pa, which is being maintained by the controller, is no longer appropriate in terms of the characteristic fan curve. A more appropriate set point, to follow the curve, would be 99Pa. A number of benefits are being missed as a result. Firstly, as constant duct pressure control forces a pressure drop to return system pressure to the set point of 275 Pa, the VAV units have to create the pressure that is not needed, involving a degree of energy wastage. Secondly, by creating a deviation from the characteristic curve (maintaining a set point of 275Pa, rather than the 99Pa that would better reflect the curve), constant duct pressure control forces the system to operate in its least effective working area (damper position below 40% open). This impacts on the acoustic performance of the system, elevating operational noise, and requiring higher energy consumption to meet the same load. See Figure 2 . More effective methodology In systems incorporating the latest generation VAV technology, there is the opportunity to rethink fan speed control using damper blade positioning to avoid the problems inherent in constant duct pressure systems. This involves exploiting the communications capabilities of the VAV technologymore effectively. VAV units are now readily available with BMS protocols including BACnet ms/tp, Modbus RTU and KNX. However, most projects today install VAV controllers with analogue 0-10v communication, and fit a local digital BMS controller to control the VAV via analogue signals. This limits the amount of data that can be communicated. By Ian Thomas , Product Technical Manager – Air Flow Controls, TROX UK Rethinking fan speed optimisation to drive greater VAV efficiency