Low carbon buildings


www.heatingandventilating.net


Keeping the right air indoors


Chris Lister, Chair of the FETA Air Curtain Group and owner and managing director of Envirotec, explains why the deployment of an air curtain unit is an effective way to help businesses meet their environmental, social and governance (ESG) criteria


A


s a customer, the moment of entry into a commercial building is your first impression of the business and is likely to have an


influence on whether you spend money there or want to return at a later date. On some premises, as you cross the threshold you might feel a blast of air. That blast of air has more than likely come from an air curtain unit - a device that provides a continuous broad stream of air circulated across a doorway serving a conditioned space. The primary purpose of air curtain units is to


help keep the indoor air inside and the outdoor air outside. A secondary purpose is to help condition any outside air that manages to break into the indoor space. The conditioning can include: a temperature rise or fall; filtration of particles and insects; and introduce a welcome breeze - warming or cooling. Another benefit of deploying air curtain units is that the air barrier they produce helps to reduce the air capacity load on central air conditioning equipment.


12 February 2023


Air curtain units are commonly mounted horizontally over the door or they can also be mounted vertically at the side of the door or under the floor directed upwards. However, air curtain units should not necessarily be seen as a space heater for warming up internal areas, especially if it is electrically heated. An air curtain unit is most effective as an air barrier when utilising ambient air, as ambient air will be less buoyant than heated air, therefore the air stream will travel further before it breaks up. Building performance and energy usage is a


key part of the process in building design and decisions based around heating, cooling and ventilation can be vital in ensuring good indoor air quality.


How do air curtain units work?


For winter operation, air curtain units disrupt the natural convection (buoyant flow) effect of warm


air spilling out of the top of an open doorway and being replaced by cold air coming in at the bottom. This convective flow is the primary heat loss infiltration mechanism and by minimising this flow they can save energy compared with a doorway without an air curtain unit. Wind and building stack effects (when warm indoor air rising and escaping causes cooler outside air to be drawn into a building) can cause air to flow into the doorway and whilst air curtain units do not necessarily act as a barrier to prevent the entry of this outside air, when used in air conditioned or industrial climate control areas (HVACR applications), they will condition the incoming air at the entrance and minimise cold draughts.


The heat output of an air curtain unit must be sufficient to temper the volume of air coming into the entrance. An air curtain unit will become less effective if the velocity of the incoming air is excessive. This can occur as a result of under-pressure within the building from extract systems, stack effect with leaky or tall buildings, or wind effects on an exposed site. If conditions are not extreme, an air curtain unit with a non- heated air stream (ambient air curtain units) can also be effective in reducing energy losses from an air conditioned entrance by disrupting the natural convective heat transfer at the doorway. The width of an air curtain unit’s discharge should be wider than the doorway opening; an air curtain unit’s air stream that is narrower than the doorway is not an effective barrier. Opening and closing of doors can disturb the air stream, which may take some time to re-establish. The heating capacity of an air curtain unit can have an effect on the space temperature within the


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