Without the fan, the hood is just a stationary cover, the air duct is merely an empty pipe, and the air in the filter cannot move at all. The fan is the only component in the entire LEV system that provides energy. Its task is simple: to create a pressure difference within the system, allowing air to flow from the high-pressure area to the low-pressure area, thereby establishing a continuous airflow from the hood opening to the exhaust outlet.
I. What is a Fan Doing?
A fan is essentially an energy conversion machine. Specifically, a fan does three things:
First, it creates a region of pressure lower than atmospheric pressure on the inlet side, commonly known as “negative pressure” or “suction”. This negative pressure is transmitted through the ductwork all the way to the hood opening, allowing the hood to draw in the workshop air along with pollutants.
Second, it generates a region of pressure higher than atmospheric pressure on the outlet side, known as “positive pressure” or “thrust”. This positive pressure pushes the filtered clean air through the final section of the ductwork and out to the outdoor atmosphere through the exhaust vent.
Third, it overcomes all the flow resistance in the entire system. Air encounters resistance when flowing through the hood inlet, the inner walls of the ductwork, elbows, reducers, dampers, and filters. The fan’s task is to provide sufficient pressure to overcome these resistances while maintaining the set air volume.
One can imagine a fan as a water pump, except that it pumps air instead of water. Air is much lighter and more compressible than water, which makes the selection and operation of fans more delicate than that of water pumps.
II. Two major players in the fan world: Centrifugal and Axial Flow Fans
The centrifugal fan has a shape resembling a snail shell. Air enters from the axial direction of the impeller, is thrown radially by the high-speed rotating impeller, and then exits through the volute. The centrifugal fan is characterized by high pressure and moderate air volume. It excels in dealing with “high-resistance” systems, such as LEV systems with long ducts, multiple bends, and high-efficiency filters (with high resistance). Another advantage of the centrifugal fan is that its noise is relatively controllable, and it is less prone to clogging when handling dusty air.
The axial flow fan resembles a large fan. Air passes directly along the axial direction of the impeller, with the inlet and outlet in a straight line. The axial flow fan is characterized by high air volume and low pressure. It is suitable for “low-resistance” systems, such as exhaust fans in workshops that directly vent to the outside. However, in LEV systems that need to overcome the resistance of filters and long ducts, axial flow fans often “fall short” in pressure, resulting in actual air volume far below the rated value on the nameplate.
A simple and straightforward rule of thumb: Centrifugal fans are the first choice for LEV systems. Especially for systems equipped with high-efficiency filters, HEPA, or activated carbon filters, the centrifugal fan is almost the only option. Axial flow fans are only suitable for situations where no filtration is required and the air is directly vented to the outside through short ducts.
III. The Core Concepts: Fan Curve and System Resistance Curve
Each fan has a performance curve that describes the pressure it can generate at different air volumes. Generally, the greater the air volume, the lower the pressure; the smaller the air volume, the higher the pressure.
Each LEV system also has a resistance curve that describes the pressure required to overcome resistance at different air volumes. Resistance is roughly proportional to the square of the air volume – if the air volume doubles, the resistance quadruples.
When a fan and a system work together, the actual operating point is the intersection of these two curves. At this point, the pressure generated by the fan exactly equals the pressure required by the system, and the air volume is the actual air volume of the system.
The correct selection process is: first calculate the total resistance of the LEV system at the target air volume (hood resistance, duct frictional resistance, elbow local resistance, filter resistance, exhaust outlet resistance), and then select a fan whose performance curve matches the resistance point at this air volume, leaving a margin of 10% to 15%.
IV. Motors, Drive Systems and Speed Control
There are three types of drive systems between the motor and the fan:
Direct drive: The motor shaft and the fan impeller are directly fixed on the same shaft. This is the simplest structure with the highest efficiency and no belt loss. However, the disadvantage is that the speed is fixed (unless a frequency converter is used), and the motor bearings have to bear the weight of the impeller.
Belt drive: The motor drives the fan impeller through a belt. The fan speed and air volume can be adjusted by changing the diameter of the pulley. The advantage of belt drive is its high flexibility, but the disadvantage is that the belt needs to be replaced regularly and there is a power loss of 3% to 5%.
Variable frequency drive: A frequency converter is installed in front of the motor, and the speed is continuously adjusted by changing the power supply frequency of the motor. This is currently the most advanced method, which can adjust the air volume in real time according to actual needs and has a significant energy-saving effect. Especially in multi-station LEV systems, when only some stations are in operation, it can automatically reduce the speed.
An additional benefit of variable frequency drive is that it can be set with a soft start function, allowing the fan to gradually accelerate to the set speed and avoiding the instantaneous impact force on the ducts and hoods each time the machine is started.
V. Position of the Fan in the LEV System
The most common configuration of the LEV system is to place the fan after the filter and before the exhaust outlet. In this case, the fan draws in clean air that has been filtered, and the impeller will not be worn by dust. The disadvantage is that the entire filter and the pipe section from the hood to the fan are under negative pressure. Any leakage will cause air to flow in, and pollutants will not escape.
The configuration of placing the fan after the hood and before the filter is rarely used in conventional dust treatment. This is because the dusty air directly passes through the impeller, causing the blades to be worn by dust. Wet dust may also adhere to the impeller, causing dynamic balance to be disrupted. This arrangement is only considered when dealing with gases that do not contain abrasive dust or when the filter itself cannot withstand negative pressure.
VI. Common Faults and Diagnosis Methods
When the fan malfunctions, the LEV system usually shows symptoms such as “reduced air volume” or “unable to draw air”:
Belt slippage or breakage: For fans driven by belts, the belt may stretch and loosen over time, causing slippage and a decrease in fan speed. The diagnosis is simple: listen for a high-pitched squeal when the belt slips; check the speed, and use a tachometer to measure whether the fan shaft speed matches the motor speed.
Dust accumulation on the impeller: When the fan operates on the dusty air side (not recommended), dust will accumulate on the impeller. When the accumulation reaches a certain level, it will disrupt the dynamic balance, causing severe vibration, and the “blade shape” of the impeller will be altered, significantly reducing efficiency.
Motor reverse rotation: If the wiring sequence of a three-phase motor is incorrect, the fan will rotate in reverse. A reverse-rotating centrifugal fan will have an air volume reduced to 10% to 20% of its normal capacity, and it will produce abnormal noise. Diagnosis method: Open the fan inspection door and observe whether the impeller rotation direction is consistent with the arrow on the nameplate.
Abnormally increased system resistance: Often, it is not the fan that is faulty, but the system resistance has increased. The most common reasons are clogged filters, dust accumulation in the ductwork, or mistakenly closed dampers. Diagnosis method: Measure the static pressure at the test points before and after the fan. If the pressure at the fan outlet is normal but the negative pressure at the inlet is much higher than the design value, it indicates that the ductwork or filter is clogged.
The fan is the heart of the LEV system. Without a fan, the LEV system is like a stagnant pool. However, a fan that does not match the system – either with insufficient air volume, excessive noise, or frequent malfunctions – is even more troublesome than having no fan at all. Only by choosing the right fan, placing it in the right position, and regularly checking the belt and dust accumulation can this system function like a healthy heart, beating steadily and continuously.