Modern Building Services

22 MODERN BUILDING SERVICES JUNE 2021 FEATURE COOLING & VENTILATION VENTILATION A chieving a consistently high-quality indoor environment in classrooms is critical to children’s ability to learn. There is evidence to show that Carbon Dioxide (CO2) concentrations of above 1,000 parts per million (ppm) in any space strongly impact one’s cognitive ability. This effect (The Bohr Effect) is amplified when we consider that classrooms are typically 60 m2 with an occupancy of 32. This is felt in the form of tiredness and a lack of concentration that a stuffy room can cause. Proper ventilation has therefore become a key talking point when considering optimal indoor environments that are both comfortable and energy efficient. BB101: Guidelines on ventilation, thermal comfort, and indoor air quality in schools stresses the importance of proper ventilation in classrooms. The guidelines highlight a spectrum of ventilation strategies ranging from natural ventilation, in the form of opening windows, to centralized mechanical ventilation, each having its own advantages and disadvantages. While natural ventilation has been the historic solution in the UK, energy efficiency, air pollution and viral transmission make the use of natural ventilation less appealing. With COVID-19, air pollution, and controlling CO2 factoring strongly into design decisions, manufacturers of mechanical ventilation systems are finding themselves in the spotlight. By using mechanical ventilation with heat recovery, filtered fresh air can enter classrooms at a controlled flow rate, without the risk of draughts typically associated with naturally ventilated spaces. Within the mechanical ventilation space, there are dramatic differences between equipment types. Decentralized mechanical ventilation is vastly different from centralized mechanical ventilation, both in the mechanics of the equipment and in the control strategies. This article therefore aims to put a spotlight on Smart Mechanical Ventilation (SMV). Under the categorisation of BB101 (Ventilation, thermal comfort and indoor air quality 2018), SMVs are categorised as duct-free ventilation with automatic bypass. The workings of Smart Mechanical Ventilation In essence, SMVs are decentralized mechanical ventilation systems with heat recovery. The term SMV can be equally applied to any system that monitors the indoor environment and reacts to changes to the environment, such as adjusting the flow parameters of the equipment. The goal of an SMV, in the case of classrooms, is to supply enough fresh air to the rooms to control indoor air quality, whilst reducing the risk of discomfort due to draughts. SMVs consist of a pair of ductwork connections, a matching pair of fans, filters, dampers, and a heat exchanger. To explain the unit’s operation more thoroughly, on the intake pathway fresh air is pulled from outside, where it is filtered before being passed through both dampers and the heat exchanger. In the heat exchanger, it picks up heat from the extracted air, before being supplied into the room for distribution. On the extract pathway, air is pulled from the room where it is filtered before passing through the heat exchanger, giving off up to 92% of its heat. It is then exhausted to outside by a fan at the rear. SMVs like SAV System’s AirMaster make use of heat recovery and the Coanda effect to distribute air within classrooms evenly whilst avoiding draughts. The air enters the room at a velocity of 3 to 4 metres per second at a Delta T of approximately 2°C below room temperature. This enables the sub-cooled air to stick to the ceiling, to carry it across the classroom. As it makes its way across the ceiling surface, the fresh air will mix with the air already in the room, increasing in temperature and reducing in velocity. Therefore, by the time the fresh air makes its way into the occupied zone, its temperature should be close to the room temperature, with a velocity of approximately 0.15 metres per second. This enables an ample supply of air to classrooms with no risk of draught. If the Coanda effect is the lungs of and SMV, the heat exchanger is the heart. Whilst air is entering the room, the air already in the room containing a great deal of CO2 and heat energy, is extracted. This warm, soiled air is passed through both a filter and the heat exchanger before being exhausted too outside. The exhaust air is ejected as a high-pressure jet, preventing it from being recirculated into the supply duct, which has a much lower pressure and lower entry velocity. Every SMV has several different temperature control processes which are used to maintain the ∆T of 2°C throughout the year, all of which work in parallel with one another. On a daily basis, one of three processes will take place, which we call balanced ventilation, fan differential, and automatic bypass. In the balanced ventilation mode, the unit will be achieving the supply air temperature of 2°C below room temperature as designed. Both the main damper and heat exchanger bypass damper adjust to mix up or down the supply air temperature as required within a 1°C tolerance band. If the supply air temperature falls below the tolerance band, the SMV may enter the fan differential mode, in which the intake fan reduces its throughput by a small percentage, whilst exhaust fan speed remains the same. The same amount of heat is extracted from the room, but the supply air moves through the heat exchanger at a lower velocity. As such, more heat can be picked up by the supply air, increasing air temperature. Spotlight on Smart Mechanical Ventilation (SMV) - insights shared by Jonathon Hunter Hill , Product Manager, AirMaster SVUs Clean air for all