In heavy industrial manufacturing, people are often more attracted to the smooth operation of digital control systems or the high-speed rotation of precision-balanced impellers on dynamic balancing machines. However, on the production floor of industrial ventilation equipment, there is a critical process that is often regarded as a "traditional technique" but directly determines the service life of the equipment - the welding of the local ventilation fan casing.
Many people believe that the casing is merely a "cover"; as long as the steel plate is thick enough and the welds are connected, it should be fine. But for local ventilation fans (such as contra-rotating fans and axial-flow fans) that operate year-round in harsh environments such as mine excavation sites, tunnel construction areas, and highly corrosive chemical workshops, the casing is far more than just an air-guiding passage. It is a core structural barrier that withstands continuous high pressure, resists structural resonance, and prevents gas explosions in explosion-proof fans. A defective weld is like a "ticking time bomb" during fan operation.
I. Rejecting "Assembly-Line Automation": Why Do High-Quality Fan Casings Still Rely on Highly Skilled Manual Gas-Shielded Welding?
In today's era of advanced automation, why are experienced welding technicians still irreplaceable when manufacturing the air collectors and cylindrical casings of high-power axial-flow fans and local contra-rotating ventilation fans?
As shown in the workshop environment, even though the factory is equipped with large CNC laser cutting machines, the welding of fan casings - a typical cylindrical structure - still requires skilled welders to perform manual operations involving overhead welding and vertical welding.
Industrial fan casings are usually made by combining high-quality carbon steel or stainless steel plates of different thicknesses (such as Q235B and 304 stainless steel). During the rolling and assembly process, internal stresses in the plates may be released, and small gaps caused by dimensional tolerances may appear. This makes it difficult for fully automated welding robots to perfectly adapt to complex and variable curved surfaces and gaps in real time.
A skilled welder, however, can instantly judge the arc color and molten pool behavior, and dynamically adjust the welding angle, oscillation width, and travel speed to ensure complete penetration at the root of the weld.
Especially in the welding of casing flanges, support stiffeners (V-shaped or channel-steel reinforcing rings), and the main cylindrical body, the welds are often fillet welds. Through precise control of CO₂ gas-shielded welding, experienced welders can effectively reduce spatter and avoid grain coarsening in the heat-affected zone caused by overheating. This minimizes stress concentration and provides the fundamental protection against fatigue cracking of the casing under long-term high-speed airflow excitation.
II. In-Depth Analysis: How Does Welding Quality Directly Affect Two Core Performance Factors of a Fan?
1. Eliminating "Hidden Resonance": The Causal Relationship Between Welding and Structural Rigidity
When a local ventilation fan operates at high speed, aerodynamic noise and mechanical vibration generated by the impeller are continuously transmitted to the casing. If the welding process is poor and defects such as incomplete fusion, porosity, or slag inclusions exist inside the weld, the overall structural rigidity of the casing will be locally weakened.
This weakening can cause structural resonance at certain operating speeds. Over time, it not only increases noise levels but also accelerates bearing damage and may even cause tearing failure of the casing along the weld seams.
2. Preventing "Airflow Loss": Geometric Accuracy and Air Tightness
The control of thermal deformation during casing welding directly determines the roundness tolerance of the fan duct.
During manufacturing, if the welding sequence is improper (for example, without using symmetrical segmented reverse welding), the significant thermal stress generated can distort the fan cylinder.
This may cause uneven radial clearance between the impeller and the inner wall of the casing. In severe cases, it can lead to rubbing between components and sparks - a critical hazard in explosion-proof environments.
At the same time, deformation can cause severe turbulence and backflow inside the casing, wasting motor power and reducing the total pressure efficiency of the fan.
III. Strict Quality Inspection at the Source Factory: What Does a Qualified Fan Casing Weld Need to Go Through?
In a professional manufacturing system, a perfect local ventilation fan casing is never considered finished immediately after welding.
To meet demanding industrial export standards (for example, the strict requirements of CIS countries for industrial equipment regarding durability and low-temperature impact resistance), manufacturers must establish a complete quality-control cycle:
Precision Groove Preparation
Before welding thick steel plates, mechanical beveling must be carried out to completely remove oxide scale and oil contamination, ensuring that the deposited metal fully fills the weld root.
Proper Welding Sequence
Symmetrical welding and staggered segmented welding processes are adopted to balance thermal stress as much as possible and control overall deformation within millimeter-level accuracy.
Non-Destructive Testing (NDT)
For critical load-bearing welds of high-pressure and explosion-proof local ventilation fans, 100% ultrasonic testing or dye penetrant testing is performed to ensure zero internal pores and zero cracks.
Stress Relief Treatment
After welding, vibration stress relief or thermal stress relief processes are applied to release residual internal stresses, ensuring that the casing will not experience dimensional creep during ten years of high-intensity operation.
IV. Purchasing Guide: How to Judge the Welding Quality of a Fan Casing from External Appearance?
For industrial buyers or international procurement companies, when internal inspection data is unavailable, the processing quality and expected service life of a fan casing can often be judged through the following three key indicators:
1. Check the Uniformity of the "Fish Scale" Weld Pattern
A high-quality manual weld should have evenly spaced fish-scale patterns, a smooth transition, and no obvious undercut or excessive buildup. This reflects the welder's stable control of molten pool temperature.
2. Check the Continuity of Reinforcing Ribs
To reduce labor and material costs, poor-quality fans often use intermittent spot welding for casing stiffeners. Under long-term high-pressure vibration, these areas are highly prone to weld failure.
High-quality industrial fans require continuous full welding at critical load-bearing points and around the reinforcing rings.
3. Check Surface Treatment and Corrosion Protection
Reliable manufacturers perform strict slag removal, grinding, and overall shot blasting after welding, followed by immediate application of anti-corrosion zinc-rich primer.
If welding surfaces retain excessive slag or are not properly finished, corrosion will first occur at the weld areas in humid industrial environments, potentially cutting the service life of the entire fan by half.
