Induced draft fans pull flue gas through the boiler, air heater, and pollution control equipment, and their performance depends on the quality of the flow entering the impeller. A fan inlet box connects the rectangular ductwork of the flue gas system to the circular fan inlet while conditioning the flow so the impeller sees a uniform velocity profile. Poor inlet flow costs efficiency, increases noise, and shortens fan bearing life.
What an Inlet Box Does
Flue gas ductwork is almost always rectangular, while fan impellers are circular, and the transition between the two shapes must be made gradually to avoid flow separation. The inlet box performs this transition and also turns the flow when the duct approaches the fan at an angle rather than straight into the inlet. Inside the box, guide vanes or splitter plates distribute the gas evenly across the full inlet area.
Even flow distribution matters because an impeller fed unevenly develops vibration, and the blades closest to the high-velocity zone do more work than the rest. The result is reduced fan efficiency, higher power consumption, and accelerated wear on the blades and bearings. A well-designed inlet box prevents these problems by equalizing the velocity profile before the gas enters the impeller eye.
Construction and Materials
Inlet boxes are fabricated from carbon steel plate with stiffening ribs sized for the duct pressure and the weight of the box itself, including access platforms and insulation. The box must be rigid enough that it does not vibrate or drum under the fluctuating pressure field of the fan. Flanges at both ends match the duct and fan connections, and the internal guide vanes are welded in place with smooth transitions that avoid sharp edges.
Material selection depends on the gas temperature and corrosiveness. Standard carbon steel serves clean flue gas service, while stainless steel or coated construction is used where the gas carries corrosive constituents. The box interior may be lined with erosion-resistant material in the areas where ash-laden gas impinges on the walls. A manufacturer experienced in fan system equipment supplies the box with the necessary expansion joints and access doors for inspection.
Installation and Alignment
Installation requires careful alignment between the inlet box, the fan casing, and the connecting ductwork so that no stress is transferred into the fan bearings. The box is normally supported independently from the fan, with flexible connections at the fan flange to absorb thermal movement and residual misalignment. Field welding of the final duct connection must be completed without pulling the box out of alignment with the fan inlet.
The inlet box orientation must match the fan rotation direction and the duct routing, so the box is usually a right-hand or left-hand configuration depending on the site layout. This detail must be confirmed before fabrication, because a mirror-image box cannot be corrected in the field. Working with a supplier that coordinates the box design with the fan and duct drawings eliminates this common source of installation problems.
Performance Considerations and Upgrades
Inlet box pressure loss adds directly to the fan power requirement, so the box is designed for the lowest practical loss consistent with even flow distribution. Replacing an old, poorly performing inlet box with a modern design can recover measurable fan power in an existing plant. The improvement is often seen immediately in reduced motor current at the same fan duty.
When the fan duty changes, such as after adding new pollution control equipment, the inlet box may need modification to match the new flow conditions. Computational flow analysis during the design phase helps the manufacturer optimize the box geometry before fabrication. Buying the inlet box from the same supplier as other flue gas components keeps the interfaces consistent and simplifies the overall supply arrangement.
References
Air Movement and Control Association International. (2019). AMCA 210, Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating. AMCA.
American Society of Mechanical Engineers. (2022). ASME PTC 11, Fans. ASME.
American Society for Testing and Materials. (2022). ASTM A36, Standard Specification for Carbon Structural Steel. ASTM.
American Society of Mechanical Engineers. (2022). ASME B31.1, Power Piping. ASME.
