Ducting

A duct is a tube through which air may be moved from one location to another. Air is a fluid (like water but much less dense) but the difference is that air (a gas) can be compressed bur water (a liquid) cannot.

Ducting can take many forms and can be made from various materials. The most common for larger section duct is sheet metal, which can be made into square, rectangular, circular and oval section. Square and rectangular ductwork is usually custom built to a certain size and is relatively simple to manufacture, install and join. Most circular ducting is 'spirally wound' (i.e. a narrow strip or metal is formed into an airtight spiral).

Plastics can also be used for ducting, but because of the high cost of either moulding or extruding the section, is limited to smaller size up to 150mm diameter and small, square and rectangular sections.

Flexible ducting, using a flexible sheath around a wire helix is very popular in the range 100 to 400mm diameter.

When it comes to the lowest resistance of moving air, a circular dust is the best, as it has the greatest cross sectional area and a minimum 'skin surface'. An oval is acceptable to a maximum width to height ratio of 2 to 1 (the aspect ratio). Square and rectangular section (the same aspect ratio applies) allows air turbulence in the four corners and has a greater 'skin surface'.

Friction between the internal surface of the duct and the adjacent moving air, has the effect of slowing the air. The rougher the internal surface of the dust the greater the friction. For example, an unlined brick built chimney is a duct for the smoke and fumes from an open fire, but has a far greater friction factor (50 times greater than spirally wound metal duct). Flexible ducting also has a high friction factor, about double that of spirally wound. Air does not like being squeezed or allowed to expand suddenly, of being forced round corners and through grilles. All these have the effect of resistance to the airflow.

Air Velocity

Air can only be heard when it is moving. Still air is quiet. The faster the air moves the louder the noise. Writers describe the wind 'moaning gently', 'whistling round a building' or as a 'howling gale'. When air changes direction the noise level is increased. What we are actually hearing is caused by fluctuation in the pressure of the air.

A fan is a means of moving air and will create noise, which will be aggravated by an obstruction in the airflow. The fan itself may also cause mechanical and electrical noise and create vibration, but these are all subjects for a later section - The important factor is to keep the speed of the air (its velocity) as low as possible. This is usually achieved by having a large size fan moving the same volume of air through a larger aperture or duct, keeping the velocity low.

There will always be a conflict of interests here. It may be that there is not room for a larger fan or duct, there may be financial reasons for using a smaller (cheaper) fan. The actual installation will also govern fan selection. A quiet library will require lower noise levels (hence air velocities) than an engineering workshop. Another consideration for air velocity is the proximity of moving air to people, not only from noise aspect, but also because of 'draughts'. A draught is created either the air velocity is too high, or the temperature of the air is too low.

The advantage of mechanical ventilation over natural ventilation is that it is controllable, thus the velocity can be governed and the temperature varied if required. A natural air velocity between 0.15 and 0.5m/s is usually acceptable. Mechanical air movement up to 3m/s can be acceptable provided people are not directly in the airflow.

When air is moved down a duct, all the duct characteristics will have an effect on the air velocity. These include changes of direction, section and terminations.

Resistance

Continuing the air/water analogy, in a domestic water system the pipes supplying the bath taps are of a larger diameter than those supplying the hand basin. This is so that water under the same pressure can pass a greater volume and fill the bath more quickly.

An air duct can also be likened to a motorway. If duct size is reduced and the air pressure remains the same, the air flow is reduced. On a motorway, if the width is reduced (three lane coned down into two) the traffic slows down as the lanes merge. Theoretically, if the traffic was to increase its speed, the same volume could flow in two lanes instead of three. To increase the velocity of air in a duct requires a considerable increase in power required, and also increases the noise.

There are three things we all are familiar with, that follow similar laws of physics. Electricity in a wire, water in a pipe and air in a duct. Most people appreciate that the wire supplying power to a kettle has to be "fatter" than the wire to a table lamp. Water pipes are mentioned above, but for some reason, people think you can shove as much air as you like down a small duct.

System Resistance

The resistance of a ventilating system is caused by:

•	The loss of energy at the point of entry of the air due to a sudden increase in air velocity from practically zero to the velocity along the duct. Keep air velocity at entry around 2.5m/s or less.
•	The friction between the air and the inside surface of the duct. Keep duct velocities low - about 2.5m/s for general use.
•	Changes of cross-sectional area of duct, where there are expansions and contractions, or changes of shape (say from square to oblong section). Expansions, contractions, and changes of size or shape should be made by gradual taper sections, not abruptly.
•	Changes of direction, such as bends and Tee-junctions are large wasters of energy. Changes of direction should be by easy bends and well-rounded corners, not by sharp elbows, unless fitted with guide vanes (expensive).
•	Auxiliary items, such as grilles, louvers, filters, heaters. These items should be large enough to keep air velocities through them down to a reasonable level, consistent with the velocity in the main duct.

NOTE: the resistance of any system of ducts, grilles, filters, etc, is proportional to the square of the air velocity through it - keep velocities down by using recommended duct, grille and filter size.