Inlet vortices and uneven airflow in dust-extraction and purification industrial fans can often be eliminated using inlet straightening vanes or guide vanes. If the fan inlet is too close to a wall or obstacle, or if throttling occurs in the inlet chamber, fan performance will be reduced.
The gap effect in the air chamber or inlet chamber is also considered an integral part of the entire system. When determining system characteristic curves, the pressure loss through the air chamber or intake chamber should be considered as an additional resistance to the system.
First, some dust extraction and purification industrial fans primarily use inlet ducts. The contraction of the airflow leads to a reduction in flow rate, and the subsequent rapid expansion results in losses, which can be regarded as additional resistance to the system.
If a circular collector is installed at the inlet of the duct or the dust extraction and purification industrial fan, these losses will be significantly reduced.
If such a smooth collector cannot be installed, installing a conical connector will significantly reduce energy loss; even installing a simple flat flange at the end of the duct will reduce losses by half compared to using a flangeless collector.
According to standards, the cross-sectional area of the inlet duct must not exceed 112% or be less than 92% of the inlet area of the dust-extraction and purification industrial fan. The angle of the connecting piece is defined as 150° for convergence and 70° for expansion.
Secondly, uneven airflow from the inlet elbow to the intake duct is one of the common causes of poor fan performance. Elbows at the inlet of dust-extraction and purification industrial fans, particularly those at the bag inlet, do not allow air to enter uniformly. Consequently, the fan impeller generates vortices, resulting in uneven airflow distribution.
If air has mass, the flowing air will move. The additional resistance curve for a circular elbow system with a given diameter and diameter ratio.
The intersection of the average fan inlet velocity and the system additional resistance curve represents the system additional resistance coefficient for that specific elbow.
The additional resistance coefficient should be added to the friction and dynamic losses determined for that specific elbow; this applies only when the elbow is located at the fan inlet. The additional resistance curve for a 90° bend at the system inlet or other inlets that generate non-uniform inlet airflow is as shown.
It should be noted that when duct vanes are used between the inlet of a dust-extraction industrial fan and the elbow, or when 3–8 appropriately sized ducts of sufficient length are employed, the system’s additional resistance coefficient varies little with changes in rotational speed. These modifications help maintain the uniformity of the airflow at the fan inlet, bringing it closer to the airflow conditions of laboratory test equipment.
For special inlet boxes with specific airflow and inlet conditions, most manufacturers of industrial dust extraction fans can provide information regarding their design and additional system resistance.
It is not possible to tabulate the flow rate and pressure drop under these fan inlet conditions. Since the width and height of the duct vary significantly, which can affect and reduce fan performance to varying degrees, the fan inlet should be avoided.
Results indicate that flow losses can reach as high as 45. By installing guide vanes in these units or replacing the guide vanes with square guide vanes or 45° elbows, these units will see significant improvements.
