Warming buildings — not the planet
Mike Price discusses how to keep heated air in a building — even with doorways
that are used often or deliberately kept open.
Air curtains have an important role to play in saving carbon and energy losses at building entrances and in providing a comfortable environment for occupants.
Even when doorways to commercial premises are not left open intentionally, if the pedestrian traffic is high enough the entrance effectively becomes open, even with double door lobbies. Problems can then arise with cold draughts entering the entrance from wind and building stack effect. Even with no wind or draughts entering the doorway, natural convection causes warm air inside the building to spill out of the top of an open doorway and cold air to enter at the bottom to maintain the pressure regime, resulting in a substantial loss of energy and associated carbon emissions (Fig. 1, top).
Air curtains project a continuous broad stream of air across the whole doorway to disrupt this natural convective effect and, therefore, minimise energy loss. This disruption of the convective effect can be seen in the computational fluid dynamics image (Fig. 1, bottom).
In practice, an air curtain is not an impermeable barrier, and wind and stack effects may still result in some air exchange across the doorway.
A correctly selected and installed air curtain will significantly reduce the amount of air movement across a doorway, but there may well be some draughts remaining. To help overcome this environmental problem, which can lead to customer and staff complaints, it may be necessary to heat the air stream from the air curtain to produce a comfortable climate, especially if staff work near the entrance.
Various types of heating can be used such as electric resistance, low-pressure hot water, gas, steam and heat pump.
Some air curtains can also cool, providing a chilled air barrier in the summer or acting as another heat pump indoor unit where a closed-door policy is in operation.
As an industry group, the HEVAC Air Curtain Industry Group believes that doorways should be closed whenever possible but acknowledges they will become effectively open during times of high pedestrian use — whatever the kind of door system. Air curtains then serve an essential purpose by saving energy and providing comfort to occupants compared to an open doorway with no air curtain.
The width of an air-curtain discharge grille should be just wider than the doorway opening for horizontal units or taller than the height of the opening if a vertical air curtain is used. An air curtain smaller than the door opening is ineffective.
The air curtain must discharge air across the whole height or width of the opening at a supply air temperature which is acceptable for the comfort of people passing through the doorway. The heat output of the air curtain must be sufficient to temper the volume of any air coming in at the entrance and be positioned as close as possible to the opening to avoid cold air bypassing the air stream.
It is important to understand where air-curtain performance could be compromised, such as the following.
• When external wind factors are too severe.
• Leaky buildings with high air infiltration rates.
• Poorly located entrances (e.g. building corners).
• When indoor/outdoor temperature difference is extreme.
• High entrances and tall buildings creating high stack effect.
• Where manufacturers’ O&M requirements are not followed.
• Where there is poor control strategy.
Controls for an air curtain are essential to prevent unnecessary energy usage, to provide the correct-temperature air stream and to prevent overheating within the building entrance. They can, if necessary, also be used to adjust the air-stream characteristics and optimise penetration across the doorway. For convenience, user-operated controls are normally remotely mounted and configured to satisfy the needs of the building occupants. For simplest control only manual operation of the fans, the fan speed and the heat output may be required. However, more advanced control options may be included such as the following.
• 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.
Air curtains should not necessarily be seen as substitutes for the space-heating system, especially if they are electrically heated. Heat output is primarily to allow the unit to operate as an air curtain.
Overdoor heaters are small electric or water-heated fanned units with a low air volume flow rate. They are intended to be installed at doorways having low pedestrian footfall where the door is mainly closed (Fig. 2). They are useful for providing warmth to a cold area near a doorway which could become uncomfortable, similar to a radiator positioned under a window. They should not be seen as an alternative to an air curtain.
More information on air curtains and overdoor heaters can be obtained from the FETA web site (address below), where there is information on air curtains, including an ‘Air curtain guide’ and a ‘Cold store air curtain guide’ written by the HEVAC Air Curtain Industry Group.
Mike Price is chairman of the HEVAC Air Curtain Industry Group.
