Air system components
A typical air system will contain one or more of the following components:
• System inlet
• Distribution system
• Fan and prime mover
• Control apparatus
• Conditioning apparatus
• System outlet
These are shown in Figure 5.1 taken from AMCA 200-95.
An air system will usually include a device such as a louvre, filter, mesh screen or guard, grille etc., where the air or gas enters the system. These elements are necessary for personnel safety as well as to preclude the entry of rain, dust and other unwanted materials which we do not wish to collect.
Some of these items may be an architectural feature such that their appearance may be of more importance than their functional efficiency as they may be visible from the exterior of a building.
This will be made up of the straight ducting, bends, junctions, diffusers and reducers. It will be purpose-designed to convey the air or other gas from the system inlet(s) to the system outlets). In certain cases, enclosed spaces in the structure such as plenum chambers or other enclosures above ceilings may be used to confine the flow. Holes in walls may also direct the air.
A fan is necessary to produce a pressure difference between the inlet and outlet of the system such that the required flow of air or gas is passed. The fan must be correctly designed and selected to produce the requisite flowrate against the specified pressure differential for satisfactory system operation.
Different fan designs produce different flowrates against different system pressures. The absorbed power will be a function of these two properties and the fan efficiency. Their variation with time may also affect prime mover selection. For consideration of the factors involved see Chapter 1, which not only gives typical characteristic curves but also the history of how these differences arose.
In most air systems it is desirable to regulate or control the flowrate according to some external requirement. This might be the variation in ambient conditions through the year, the reduction of a boiler output, the change of drying capacity according to stock moisture content etc, etc.
Control and regulation of the flowrate through the system is usually in response to some monitoring signal such as air velocity, pressure, temperature or humidity. It may also be desirable to regulate the flowrate in the individual branches of the ducting according to whether they are in use or not. Examples of this would be the individual rooms of a hotel air conditioning system, the extract points of a wood refuse extract system or the outlet connections of a multi-boiler induced draught plant, etc.
Control devices such as dampers function by increasing or decreasing their pressure loss and thus reducing or increasing the flowrate. Variable inlet vanes act on the air or gas entering the fan to give a controlled amount of pre-swirl. This reduces the amount of work carried out and thus the pressure developed by the fan.
In recent years the control of the fan, by varying the rotational speed of the prime mover, has become ever more popular especially with the introduction of inverters with induction motors. Chapter 6 gives a resume of the methods used including other types of variable geometry designs of fan.
Most ventilation systems are designed to take the air or other gas from the inlet and change its condition before discharging it at the outlet. These changes could be:
• Altering its temperature by passing through a heater or cooler
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SYSTEM OUTLET |
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SYSTEM OUTLET |
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SYSTEM OUTLET |
SYSTEM
INLET
Figure 5.1 Typical fan system
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M |
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Equ 5.2 |
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Equ 5.3 |
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Equ 5.4 |
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Pl |
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Where: F L V M |
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= m |
• Altering its humidity by passing through a dryer or washer
• Altering its solids content by passing through a filter or dust collector
Many conditioning devices require an outside energy source such as hot water, or electrical resistance for a heater, or chilled water for a cooling coil. Other apparatus such as filters or cyclonic dust collectors are passive and have no external energy connection. All such apparatus however has a pressure loss, increasing the fan pressure requirement and therefore having an important effect on the fan selection and the absorbed power.
A ventilation system usually terminates with a special component at the end of each of the outlets. This component may be a simple wire mesh screen, a ceiling diffuser or a special grille. In many cases these may incorporate control devices such as dampers and/or mixing boxes. In air conditioning, the distribution requiring careful outlet positioning and diffusers to achieve the desired air motion and temperature conditions.
Posted in Fans Ventilation A Practical Guide