The motor fan is the heart of a dust collection system, and its selection is as important or more important than that of the filter itself.
Design and Initial data
The following data must be established and known for a fan to be properly designed:
- Effective gas flow at the fan inlet, taking into account the temperature, depression at that point and height above sea level.
- Temperature.
- Gas properties, specific weight.
- Height above sea level.
- Required pressure.
Required pressure in fan design
It must be the sufficient to overcome the pressure drop of the circuit, and consists of several components added together:
- Initial pressure drop at the suction point, hood, enclosure, kiln, mill, etc.
- Head loss in pipes, to be calculated by the designer.
- Pressure loss in components upstream of the filter
- Pressure loss in the bag filter itself
Pressure loss in a bag filter
In a simple circuit, without pre-separators, suction from atmospheric pressure, etc., the pressure drop in the filter may be the most important.
Selecting a fan: Characteristic curves
GORCO has its own range of fans for medium-sized airflows, between 3,000 and 80,000 m3/h approx., and has first brand suppliers that can cater for cases outside GORCO’s availability or for special needs.
To select the appropriate fan for a problem, design curves are used, grouped into flow/pressure families.
The design point of a fan will be found on the curve for it, and preferably in an area where it is performing strongly, and where it intersects with the system resistance curve.
Obviously this is a theoretical point, and the system will accommodate the reality as it is suited. For example, if the pressure drop is less than expected – a very typical case when starting a clean bag filter – the absorbed flow will be higher, and it may be necessary to regulate it in some way, otherwise the installed motor may be not enough.
Absorbed power: Motor selection
The consumption of a fan at its design point must always be provided by its manufacturer, as it depends on both the starting data and the machine performance.
The fan motor must be calculated with an excess over the absorbed power of at least 15-20%; and provided that due precautions are taken into account so that the absorbed power does not overshoot the theoretical calculation. Thus, the draft will need to be regulated whenever sudden flow changes are anticipated. And in a bag filter system, the starting conditions with clean bags may lead to a flow surge that blows the protection fuses.
