Air Curtains – Dispel the myths
Andrew Saxon, Marketing Manager at Biddle Air Systems, tells us more about the science behind air curtains, and provides some advice on design, specification and installation.
When installed correctly, an air curtain will minimise the amount of cold air entering a building while keeping the warm air inside. However, there are many myths surrounding the technology, and a lack of knowledge and understanding has resulted in some poor installations in the UK.
Air curtains are ideal for any frequently used entrance, and can be successfully used to minimise the amount of cold air entering the building, while keeping the warm air inside. Typical applications including high street stores, shopping centres, factories, warehouses, hospitals, hotels, banks, cold stores, pubs and clubs, restaurants, airports, cinemas and reception areas.
Without the use of an air curtain, heat will naturally escape from a door when it is opened. Because of natural convection, warm air will spill out of the top, being replaced by cold air coming in at the bottom.
Air curtains actually work by disrupting and minimising this flow, providing a continuous stream of air circulated across a doorway serving a conditioned space. The incoming cold air is conditioned to reach a temperature of around 20-25oC, with the warm air being derived from various sources, such as direct electric heating and low, medium or high pressure hot water. Modern air curtains can operate with 60/40 condensing boilers and heat pumps.
The technology helps to provide a constant and comfortable environment around a doorway, to the benefit of employees and also visitors and customers – which is especially important in retail environments. It is been suggested that air curtains also improve productivity and reduce absenteeism.
However, the principal benefit is that air curtains reduce the amount of heating required – leading to lower energy bills and lower carbon emissions.
If designed, sized and installed correctly an air curtain will definitely save energy. It is estimated that an average double doorway could be losing as much as 32kW of energy every hour, and at 10p per kWh this can equate up to £7,000 worth of energy each year. By installing an air curtain, wastage can be cut by up to 70%. These savings are well documented, and can be easily illustrated with Computational Fluid Dynamics (CFD) models.
However, several myths and misunderstandings about how air curtains work, and how they should be installed, have unfortunately resulted in a significant number of poor installations in the UK.
This is a major concern, as energy savings will not be fully realised, and in some circumstances will be completely negated. In fact we would say that the effect that a badly designed, selected or installed air curtain has on energy consumption can actually be worse than not having an air curtain at all.
So, along with the other members of the HEVAC air curtain industry group, Biddle is committed to helping the building services sector to learn more.
One of the biggest problems we are trying to address is that air curtains are often treated as space heaters. While there are over-door heaters available on the market, it must be remembered that an air curtain isn’t a heating appliance.
While an air curtain’s air stream is warm (unless it’s an ambient/no heat model) and thus can contribute to the heating around an entrance, it will actually work alongside a building’s heating system, rather than replacing it. At this point it is worth mentioning that in air conditioned applications air curtains can be used to reduce the loss of cold air to outside.
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In terms of the actual installation, air curtains are usually fitted horizontally over a door, but are sometimes mounted vertically to the side. An air curtain must be installed inside the doorway, as close to the opening as possible. It must also be just wider than the doorway opening with an overlap at both sides (or at the top if it is a vertical installation). It’s important to remember that air curtains should be specified on the size of the door rather than the kW output.
The design of the air curtain must be suitable to discharge air across the whole height and width of the opening at a supply air temperature that is acceptable for the comfort of those passing through the doorway, and the heat output of the air curtain must be sufficient to temper the volume of air coming in at the entrance.
An engineering design procedure for calculating the supply air flow and thermal capacity of an air curtain is explained in the BSRIA Application Guide 2/97, Air Curtains – Commercial Applications. Manufacturers should be able to provide help and assistance, and illustrative CFD graphics. It is really important to spend time getting the jet stream right; if it has too little velocity or is too powerful it won’t be effective.
The characteristics of the building and the outdoor and indoor climate conditions must also be considered, as well as voltage and power supply.
In terms of products, building services engineers should find there are plenty of options available. Air curtains can be surface mounted or recessed and are generally available in 1m, 1.5m and 2m lengths – but can be seamlessly joined together if required. We find that brushed stainless steel is a popular finish, but they can be powder painted in a range of colours to more easily blend into an entrance or decorative theme.
It’s also worth considering the use of controls, which prevent unnecessary energy usage. A typical air curtain will have a manual control, set up at the time of installation. Although the end user can change the settings for the flow and temperature of the air stream, in our experience it’s unlikely this happens. Of course environmental conditions are constantly changing, so the settings will only be right some of the time, and wrong a lot of the time – wasting energy and money.
Automatic controls are now being widely introduced, which react to indoor and outdoor temperatures, so air curtains will be able to operate at an optimum setting all of the time. Controls are a major area of focus for the air curtain market and there will undoubtedly be more developments to come – especially for the retail sector.
Air curtains are a popular solution for retail applications, and some building services engineers might have heard some criticisms surrounding ‘open door’ trading policies. The HEVAC air curtain industry group has carried out extensive research into this area, and we have to say that the arguments against open door trading policies are actually quite flawed. In addition, there are some very positive benefits, including increased footfall. In fact it is thought that an open door trading policy is essential to the success of some retail stores.
However, air curtains can be used effectively with doors that constantly open and close – and we imagine that as controls become more advanced we will see this type of installation becoming more popular.
By minimising the amount of cold air that enters a building, air curtains significantly reduce energy consumption and carbon emissions. However, for energy savings to be delivered, air curtains must be designed, selected and installed correctly – so it’s worth learning more and spending time getting the basics right.