What is an air curtain?

Steve Brown, Development Director with Envirotec, provides some facts on air curtains

What is an air curtain?

Essentially, an air curtain creates a barrier of air across a doorway to prevent an exchange of air between the inside and outside of the building, while enabling the doorway to remain open. Typical applications include the exterior doorways of shops, shopping centres, factories and warehouses.

Why are air curtains used?

For much of the year in the UK the temperature inside buildings is higher than the outside temperature. When doors are left open, the heated air inside the building will spill out of the top of the open doorway and be replaced by cold air coming in at the bottom of the doorway (warm air is more buoyant than cold air). This situation may be reversed in the summer, where air conditioned air leaves the building to be replaced by warm air from outside.

Both situations waste significant amounts of the energy used to heat or cool the inside air, and cold air passing through a doorway at low level also creates uncomfortable draughts.

While most buildings prevent this by closing doors, this is not practical in shops where there is a steady flow of customers entering and leaving, as the doors would be open for much of the time.

Consequently, air curtains are used to maintain comfortable conditions inside the building and save energy in doing so. They also enable ‘open door trading’, making the shop more inviting to customers.

How does it work?

The air barrier may be created by blowing air vertically down from above the doorway to the floor, or from the side of the doorway horizontally across the opening. This flow of air disrupts the inward and outward flows of air described above to prevent the waste of heated or air conditioned air.

Heated air curtains are most commonly used in the UK, and it is common to ‘temper’ the air by heating it slightly, so that people passing through the air barrier do not experience uncomfortable draughts.

How do air curtains save energy?

Tests with air curtains have shown that a properly designed air curtain will reduce heat loss around a door by up to 80%, so that less energy is used by the heating system. This more than compensates for the small amount of heat used to temper the air passing through the air curtain. In fact, air curtains with thermostatic control can be configured to contribute to the heating of the interior of the shop, so the energy consumption of the main heating system can be reduced in proportion to the energy consumed by the air curtain.

In addition, in buildings with high ceilings heat tends to collect at the top of the space (stratification) where it does not contribute to keeping people warm at floor level. Air curtains can be designed to draw this warm air down from the ceiling and re-introduce it to the room at floor level, so less heat is wasted.

Can air curtains be used with low carbon heat sources?

The heat source for air curtains is typically hot water or electric elements – or a combination of both. As the heating requirement is only to temper the air, air curtains are ideal for use with low temperature hot water as might be generated by a heat pump. Similarly, they can also be configured to work with water temperatures that maximise the energy performance of condensing boilers.

Heat pumps are becoming increasingly popular, because of their low energy consumption and carbon emissions. However, the heat generated by some heat pump systems can sometimes be of “low grade”, so more innovative air curtains may also incorporate some electric heating to ‘top up’ the heat pumps output in particularly cold weather.

How is air curtain efficiency maximised?

Each air curtain needs to be designed in relation to the height and width of the particular doorway it is protecting. The air curtain should be wide enough to ensure the air barrier overlaps the doorway at the sides. The velocity of the air also needs to be configured to suit the height of the doorway, so that the air reaches the floor but does not come into contact with floor at high velocity as this will disrupt the laminar flow of the air.

For these reasons, specialist expertise is required to ensure air curtains deliver maximum efficiency.

Are air curtains the best option for all shop doorways?

It really depends on the likely footfall of customers. In a shop that only receives occasional customers a closed door will usually be the most cost-effective option. In busier shops and shopping centres with frequently opened doorways, an air curtain in conjunction with automatic doors will be the best option for reducing energy consumption and maintaining customer comfort.