【2026 Conveying Machinery】To truly determine whether a fan is suitable, it is necessary to consider air volume, air pressure, power, and speed together

I. Air volume: Air volume refers to the amount of air that a fan transports per unit of time, with the common unit being m³/h. For example: If a fan is specified to have a capacity of 10,000 m³/h, this only indicates that it can achieve such an air volume at the corresponding wind pressure. Discussing air volume without considering wind pressure is meaningless. It is a common occurrence on site: the fan’s nameplate and specifications are correct, but the actual air volume is insufficient. The cause is often not the fan, but rather the pipeline resistance. A narrowing of the ducts, too many elbows, improper connections at tees, valves that aren’t opened fully, clogged filters, and collapsed flexible connections can all reduce the air flow. A fan is not a water pump, nor is it a device that operates on its own. After it is connected to the ductwork, the amount of air flow it can handle depends on the resistance imposed by the entire ventilation system. So when looking at air volume, it is essential to consider air pressure as well.

II. Wind pressure: Wind pressure can be understood as the ability of a fan to push air against resistance, with the unit being Pa. The wind exits the fan and passes through pipes, elbows, reducers, valves, filters, silencers, and outlets; pressure is lost at each component it passes through. The function of wind pressure is to overcome these resistances. The wind pressure has dropped, and the air flow isn’t increasing ； The wind pressure is high, and it’s not easy to work on site either. Many projects prefer to choose a higher wind pressure value as a form of insurance. After operation, the system resistance is not as high, so the fan operates in a range with higher air flow rates. As a result, the wind speed at the outlets becomes too high, humming is evident, the motor current increases, and it is difficult to achieve a balanced air flow at the outlets. In the end, the only option is to reduce the air pressure by closing the air valve. While this seemingly solves the air volume issue, it actually results in extra energy consumption being used for air valves and noise control. In many workshops, basements, and equipment rooms, the noise problem is not caused by poor quality of fans, but rather by excessively high wind pressure and airflow rates. An optimal wind pressure condition is simple: the air volume meets the requirements, the current is not overloaded, the noise level is acceptable, the air valves do not need to be kept closed tightly for long periods, and the system is easy to adjust. Only when such a fan is installed on-site can it truly be useful.

III. Power: Many people, upon seeing that a fan is equipped with a 7.5 kW motor, assume that the operating power of this fan is 7.5 kW. This understanding is incorrect. Motor power is the rated capacity that the motor can sustain over a long period of time, and it is not the power consumed by the fan at all times. To determine the actual power consumption of a fan, it is necessary to consider the wind volume and pressure under which it operates. For the same fan, the greater the air volume and wind pressure, the heavier the load, and the higher the current will be. With low pipeline resistance and the air valve fully open, the fan operates in a high-volume mode, causing the motor current to increase. When the filter gets clogged, the air flow decreases, and the current and pressure values change to another state. Judge whether the power is appropriate on-site; don’t rely solely on the nameplate. It is necessary to check the operating current and also refer to the fan performance curve. If the current remains close to the rated value for an extended period, it indicates that there is little remaining headroom ； If the current exceeds the rated value and operation continues, the motor heats up, the protection system trips, and the load on the bearings increases, leading to a failure eventually. During fan commissioning, current is one of the most direct signals. It is necessary to measure whether the air volume is sufficient, whether the air pressure is correct, and whether the current is stable.

IV. Rotational speed: Rotational speed has a significant impact on fans. When the rotation speed changes, the air volume, air pressure, and power will also change accordingly. For the same fan, as the speed increases, the air volume increases, the wind pressure increases, and the power also increases. What is most easily overlooked here is power.

Take a common example from the field: a fan that originally rotated at 1000 revolutions per minute now rotates at 1200 revolutions per minute. The rotational speed increased by only 20%, the air volume increased by 20%, the air pressure increased by 44%, and the power increase was nearly 73%. This is why the pulley cannot be replaced casually at the site, nor can the frequency conversion frequency be increased arbitrarily. For some projects where the air volume is insufficient, the first reaction is to increase the speed. After the speed was increased, the air volume indeed rose, but the motor current exceeded the limit, the belt heated up, the bearing temperature increased, and noise and vibration also increased. In the end, instead of solving the problem, it was shifted to the motor, bearings, and impeller. Extra caution is needed with old fans. The strength of the impeller, the main shaft, bearings, belts, and the margin of the motor all need to be checked. A fan cannot simply be sped up just because it can rotate; exceeding the safe range has immediate consequences.

V. How to solve the problem of insufficient air volume? The air volume at the site is insufficient, so many people request a model one size larger. This processing is too simple. First, check a few key points: First, check the rotational speed. Is the frequency of frequency conversion correct, is the belt slipping, and is the pulley ratio appropriate? Second, check the rotation direction. When a centrifugal fan runs in reverse, it still expels air, but the airflow volume and pressure decrease significantly. It’s meaningless to just look at whether there is wind; the direction of rotation must also be determined. Third, check the filters and air valves. Clogged filters and valves that aren’t fully opened are the most common basic problems. Many times, after spending half a day on the site, it turns out that just the valve is in the wrong position. Fourth, check the pipes. Discrepancies between construction and the drawings, smaller pipe diameters, an excessive number of elbows, and hasty fabrication of tees can all increase resistance. Fifth, check the import conditions. The presence of elbows in front of the fan inlet, too rapid a change in diameter, and airflow directed inward can all affect the air intake to the impeller. When the incoming airflow is chaotic, the performance of the fan decreases significantly. Sixth, check the impeller. Dust accumulation, wear, and deformation of the impeller can all affect air volume and cause vibration.

After dealing with these issues, then consider the possibility that the fan is too small. The worst thing that can happen on site is to change equipment without having measured the data first. Replacing it with a larger fan is costly, and it also introduces problems related to noise, current, and regulation.

VI. Why does variable-frequency speed control save electricity? There are two common ways to adjust the air volume of a fan: one is by using an air volume control valve, and the other is by adjusting the speed. The air damper is used to artificially increase resistance in order to reduce the air flow rate. The fan is still operating at a high speed, with much energy being wasted on throttling. Adjusting the speed means causing the fan to do less work. When the load decreases, the speed is reduced, the air volume drops as well, and the power drop becomes more significant.

Take a simple example: if the fan’s speed is reduced to 80% of its original level, the air volume will drop to about 80%, the air pressure will fall to around 64%, and the power consumption will be reduced to roughly half. Therefore, in situations with large load variations, variable frequency operation is more energy-efficient than keeping the valve closed for long periods. Variable frequency control is suitable for air supply and exhaust in air conditioners, dust removal, cooling ventilation, as well as clean rooms. But frequency conversion is not a panacea. If the fan is too small, frequency conversion alone cannot save the situation ； The pipeline resistance is too high; frequency conversion cannot replace the necessary repairs. Select the right fans and pipelines first; only then can energy-saving controls yield solid results.

VII. It is essential to examine the fan performance curve; this curve shows the variations among air volume, air pressure, power, and efficiency. When selecting a fan, pay attention to the wind pressure, power, and efficiency corresponding to that air volume. The operating point is too close to the low airflow end, causing the fan to be unstable and resulting in increased noise and vibration. The operating point is too close to the high-air-volume end, increasing the motor load and causing the current to rise. Only when it falls near the efficient range can the fan operate well, consume less power, and remain stable.