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Industrial fans create a vacuum that causes air to flow through the system. This vacuum is called static pressure and is usually measured in inches of water column. Designers use a variety of mathematical models to estimate the static pressure required to achieve the required airflow. The required static pressure is influenced by the following system parameters:

• number and radius of elbows (bends) of the air duct;
• total length of system air ductwork;
• the diameter of the duct used and the air flow rate;APMT Insert
• the selected dust container;
• fine cleaning agents (inlet filter or inlet/outlet silencer).

Many of these parameters do not change over the entire life of the system. The exceptions are dust collector filters. As dust accumulates on the filters the pressure drop or resistance to air flow increases. The system will need to increase the static pressure to compensate for the effects of dust build-up on the filter surfaces.

Fans are typically selected to provide sufficient airflow throughout the life of the filters including additional static pressure to maintain airflow when the filters are approaching the end of their life. Filters at the end of their service life have a higher pressure drop than new filters. In order to extend the life of the filters in the dust bin a filter cleaning system is used during operation. Re-accumulation of dust on the filters followed by a self-cleaning cycle of the dust collector creates small fluctuations in the required static pressure in the system.

The most common fan control is the damper which puts additional load on the fan to maintain the required airflow. To maintain the required air flow rate in the duct the damper can be manually or electrically controlled. This method has a low accuracy which is not sufficient to constantly maintain the calculated air flow through the system. Manually operating the damper would require constant supervision by a qualified person. It would be expensive and difficult.

A more convenient way to control the fan and maintain a constant air flow in the system is to use a variable frequency drive. It changes the speed of the blower motor depending on the frequency signal. Three phase electrical networks in North America typically operate at 60 Hz. The use of a variable frequency drive allows the operator to select the frequency by slowing down or speeding up the fan. In an ideal system the maximum fan speed would only be achieved with adequate static pressure due to clogged filters. The rest of the time the fan would run at a reduced speed sufficient to generate the required static pressure. This method of management saves money. Compared to systems without flow control where the fan is constantly running at increased speed the VFD approach uses an intelligent system that maintains the required airflow while conserving energy.

There are mathematical models that validate the effectiveness of such a system using several assumptions and system parameters. Typically the installation of a variable frequency drive and airflow control system can pay off in less than two years and maintain the required airflow rate in the dust extraction system. This can reduce wear and tear on the system. When justifying the installation of a variable frequency drive and an airflow control system the following savings should be considered:

• labor costs;
• disposal costs;
• inventory creation costs;
• stable operation of the system and maintaining the required air flow in the system.

Having decided to use a variable frequency drive it is necessary to choose a method of continuous supply of a control signal. The purpose of the control system is to maintain the required airflow regardless of static pressure fluctuations in the system. An air flow meter installed in the duct system will allow the controller to adjust the fan speed accordingly. Flow measuring devices work best in clean air and are usually installed after filters. This could be the duct at the fan outlet. The duct length must be long enough to reliably measure the total airflow through the system.

In addition to measuring the air flow it can also be measured the static pressure at the point where the duct connects to the dust collector. The static pressure required to maintain the design airflow depends on a number of parameters that will not change until the system is changed. The simplest way to control a VFD in a dust extraction system is a static pressure controller. When the resistance of the filters to the flow increases the air flow generated by the fan decreases. This decrease in airflow causes the static pressure in the duct in front of the dust collector to decrease. So the controller will signal the VFD to increase power to maintain the static pressure. Conversely, after cleaning the filters with pulses of compressed air the flow resistance in the system drops. And the power of the VFD is reduced to keep the static pressure constant. BTA deep hole drilling inserts The result of this control is the maintenance of the design airflow and the attendant benefits and cost savings.

More manufacturers are realizing the importance of airflow control and the associated energy saving opportunities.

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