AIR CONDITIONING, COOLING & VENTILATION
Keep the indoor air in and the outdoor air out
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. 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?
Energy effectiveness of an air curtain unit (HVACR application) The energy effectiveness of an air curtain unit installation over an open doorway can be described by the following equation:
where, E
is the energy effectiveness
Qa is the energy exchange through an open doorway WITH an air curtain unit fitted plus the power consumed by the air curtain units
Qb is the energy exchange through an open doorway WITHOUT an air curtain unit fitted
The closer the energy effectiveness is to 1 (unity) the better the energy effectiveness with,
1 = Ultimate Barrier (equivalent to a closed door, if Qa = 0) 0 = Bad (equivalent to an open doorway with no air curtain units, if Qa = 1)
The energy effectiveness could also become a negative figure, if Qa >1 e.g. -0.25, in the case of a badly designed, badly selected or badly installed air curtain unit, which could make the energy situation even worse than just having an open doorway.
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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 building entrance and suitable controls need to be fitted to adjust the heat output and air stream characteristics if necessary.
Energy exchange loss
A good air curtain unit installation could be up to 70% effective at controlling the original energy exchange loss through a doorway without an air curtain unit. To a significant extent it does this by disrupting the natural convection (buoyant flow) effect of warm air spilling out of the top of the open doorway and being replaced by cold air in at the bottom. The new “lower” energy exchange (30% of the original) is added to the power consumed by the air curtain unit and the energy effectiveness then calculated.
Controls
Occupant comfort and energy efficiency do not have to be mutually exclusive. Controls with the air curtain unit are essential to prevent unnecessary energy usage. They can, if necessary, also be used to adjust the air stream characteristics and to optimise penetration across the doorway. For convenience, user operated controls are mounted remotely and configured to satisfy the needs of the building occupants. At the simplest control level only manual operation of the fan(s) and fan speed may be required, however more advanced control options may be included such as:
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Thermostatic control Timer On/Off control
Step or modulating control of electric or water heating BMS control interface Energy management
Automatic door activation
Optimisation controls for on/off and temperature adjustment
Installation and maintenance
Specifying the correct air curtain unit size and performance is part of the building design. What follows is installing the air curtain unit correctly and commissioning it to optimise performance. Thereafter, as the units will be working 10 to 24 hours a day, every day, they do need checking periodically for safety, cleanliness and operational efficiency. This vital stage of regular service and maintenance is often ignored, yet it does impact the credentials of organisations that make ESG key measures of performance within their company’s stated objectives. The Air Curtain Industry Group is a member of the Federation of Environmental Trade Associations (FETA). Further information can be obtained from manufacturers who will be able to provide the most suitable product for individual applications.
Benefits -
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Allows ‘open door trading’ invitation to customers
Increases comfort for customers and staff
Increases available useable space
Increased productivity
Reduces employee absenteeism
Reduces product spoilage
Applications
Air curtain units have been used in a range of buildings, including:
Airports Care homes Distribution centres
Educational establishments Factories Galleries Hospitality Hospitals Hotels Leisure complexes Museums Retail
Shopping centres Showrooms Supermarkets Warehouses
BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2023 23
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