Rethinking the control of VAV systems

How do you halve the energy used by the fan in a VAV air-conditioning system? ANDY BARLETT has a suggestion.Buildings are estimated to account for 40% of the energy consumed in Europe, and Part L of the Building Regulations aims to highlight inefficient buildings and promote a ‘greener’ agenda. Reducing energy demand will help reduce greenhouse-gas emissions and provides a quick win for the EU, which has estimated a 22% saving in energy by 2010 just by focusing attention on this area. The regulations also apply to the refurbishment of existing buildings, both domestic and commercial, and the new Part L means that buildings over 1000 m2 are covered by the regulations if they undergo any extension or material alteration, change of use, or alteration or revision of a controlled element or service — such as air conditioning. The fans, pumps, chillers and boilers that comprise the main plant of some air-conditioning systems are big consumers of energy and are thus a natural focus for any energy-saving efforts. Fan optimisation is one way of reducing these energy costs. In VAV (variable-air-volume) air-conditioning systems with conventional pressure control, the supply pressure is selected to provide enough air to the most unfavourably placed VAV unit during full-load operation. As a result the remaining units are oversupplied, and the excess pressure has to be eliminated by closing the dampers to reduce the airflow to the space. If a damper is closed by more than 40% a unit is operating in the most unfavourable range for control characteristics (noise, pressure losses and energy consumption). To complement the use of dampers to regulate airflow in a VAV, Belimo’s Fan Optimiser makes it possible to control the fan according to overall volume of air required, based on feedback from damper actuators. As the speed of the fan is reduced in response to the lower demand for air, dampers are opened to the fullest possible extent to reduce their pressure drop. Supply-pressure regulation can be dispensed with and fan energy consumption cut by as much as 50%. Energy costs will be reduced and the convenience for users increased. The two diagrams show how the same airflow can be supplied by opening dampers and reducing the pressure developed by the fan. With fan optimisation the required volume, actual volume and damper position are all recorded via the MP-Bus that links the actuators and controller, analysed by the optimiser and specified as a set-point for the frequency converter. This enables the system to be operated in the optimum range for control characteristics, energy consumption and noise. The damper which is open the widest serves as a reference, as shown in the diagrams. The system is operated by the fan optimiser with optimum damper positions based on current demand signals. The objective is to keep the pressure loss across the VAV units as low as possible and thus permanently reduce operating costs by decreasing the fan output. The damper position of each VAV unit is recorded and transferred to the fan optimiser via the MP-Bus. These values are used there as a control variable for regulating the fan controlled by the frequency converter. As a result of this technology, an energy saving of up to 50% can be achieved compared to conventional systems in which fans are controlled by air duct pressure. Controlling damper position and fan speed in this way brings a host of other benefits. • Lower costs, because supply and exhaust-air pressure controls are eliminated. • Quicker installation — standard cable for the 3-wire MP-Bus. • Easier commissioning, due to the elimination of pressure controls. • Quieter operation; flow noise through the units and ductwork is reduced by lowering the supply pressure. • Operational reliability is increased, with pressure losses due to filter contamination automatically being compensated. Complaints such as the system not supplying enough air are a thing of the past. • Optimum cost-benefit ratio; the investment pays even with small and medium-sized buildings. • Can be used for new systems, retrofitting for system optimisation and renovation of existing systems. All VAV compacts (LMV-D2M/NMV-D2M) from 2001 onwards, support the optimiser function. • Simple engineering and efficient commissioning — thanks to the pre-configuration, LCD display and self-adaptive control function. Andy Bartlett is managing director of Belimo Automation www.belimo.co.uk
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