Article: Avoiding draught with industrial air curtains - Biddle - #1

Air curtains—Q Backgrounds to the design of industrial air curtains

Avoiding draught with

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Roberto Traversari, MBA (TNO) III \Jl \A O LI I K I I V* \A I L. 1113

In industrial buildings balanced ventilation is infrequently used. Increased pressure difference over the building facade causes substantial cold outside air to flow in and much discomfort in the entrance area. A well-designed and properly installed air curtain will be able, to heat the large amount of cold outside air entering the building, provide optimal energy savings and high comfort.

Causes of draught

The indoor environment in buildings is influenced by venti­lation (a controlled, often mechanical, fresh air supply) and infiltration (an uncontrolled fresh air supply). When a door is open, most of the fresh air from outside enters through the doorway, causing draughts and loss of heat. Industrial buildings, like warehouses and assembly plants, often have substantial amounts of cold air flowing into the building. This has various causes (figure 1).

First of all, there are the specific properties of an industrial building. Unlike buildings such as shops, offices and the like, industrial buildings are very large and are often situated in wide open spaces. And there are often several very large doorways. These doorways may be positioned on one side of the building, or there may be several doors on opposite sides of the building. Older factory buildings are not particularly leak-tight and may cause large infiltration. All these factors can lead to large volumes of outside air coming in through open doors and leaving the building somewhere else.

The second cause is mechanical ventilation. Mechanical venti­lation will give the building a sufficient amount of fresh air in a controlled manner. Without the controlled intake of the same volume of air a pressure difference between inside and

outside will occur: When a door is left open, most of the fresh air will enter through the door because the resistance in the doorway is low compared with open windows or cracks. Another source of ventilation and infiltration is the local suction of industrial processes, such as welding or polishing. Without compensation by the controlled inflow of outside air; local suction will cause draught when a door is left open. A third cause of draught problems is wind. On a windy day, there is substantial draught if doors on either side of the building are open. In this situation wind can freely flow at high speed through the building. At home we make use of this effect in summer by leaving the front door and the back door open at the same time. With only one door open, the wind will not flow through the door entirely because the pressure will build up in the building and give resistance to inflowing air However wind will cause overpressure on the windward side and underpressure on the lee side of the building. This will cause airflow through the building from the windward side to the lee side. The force of such airflow will depend on the wind velocity, the wind direction and the leak-tightness of the building. In addition, the turbulent nature of wind may also exert a pulsating force on the open door so that diffe­rential pressures may vary with time and as the position of the doorway changes.

Finally there is the so-called chimney effect or stack effect. Factory workers on the shop floor should feel comfortable and the lower areas of a room are heated accordingly. Hot air will rise and escape from the building through gaps or open skylights. At floor level, the rising air is replaced by cold outside air entering the factory through the door.

Temperature difference or pressure difference?

The cause of the airflow through an open door can be deducted from the flow profile through the doorway The flow profile is the sum of one-sided flow (through pressure differences) and two-sided flow (through temperature diffe­rences). By using the so-called neutral height we are able to distinguish four categories (see table 1). The neutral height is the height in the doorway where the horizontal air speed is

Building properties

- large doors

- many doors

- doors on opposite sides

- not leak-tight

Mechanical ventilation

local extraction of industrial gases

Infiltration by wind effects

- isolated buildings

- wind velocity

- wind direction

Infiltration by stack effect

- internal heat sources

- open skylights

February

2008