Can the fan box be adjusted according to the air volume? First distinguish between set speed and closed-loop control
The fan box can be adjusted according to the air volume, but two completely different methods must be distinguished first. The first is open-loop speed regulation: give the driver a speed command, the fan runs at this speed, and the actual air volume changes with the system resistance. The second type is closed-loop control: the actual value is measured with an air volume or differential pressure sensor, and the controller continuously corrects the rotational speed to keep the air volume within the target range. The former is simple and the latter is accurate, but the latter requires reliable measuring points, appropriate control parameters and on-site calibration. Many projects say they need to 'adjust according to the air volume', but in fact they only do open-loop speed adjustment. After running for a period of time, they find that the air volume is off, so they go back and replace the sensor.
Open-loop speed regulation: Give instructions, but do not guarantee the air volume
Open loop speed regulation is the most common approach. The controller outputs an analog, PWM or communication signal, and the driver runs at the speed corresponding to the signal. When the control cabinet displays 50%, the fan will turn to the corresponding speed of 50%. As for the actual air volume, it depends on the system resistance.
System resistance will change. After the filter accumulates dust, the resistance increases and the air volume decreases; after the air valve is closed, the resistance increases and the air volume also decreases; air duct leakage or branch adjustment changes will also affect the actual air volume. Open-loop speed regulation does not sense these changes. It only executes the speed command and does not measure the air volume results.
Therefore, open-loop speed regulation is suitable for systems with relatively stable resistance or low requirements for air volume accuracy. If the system resistance changes significantly and the air volume is required to be stable, closed-loop control is required.
Closed-loop control: measure, compare, correct
The core of closed-loop control is feedback. The sensor measures the actual air volume or pressure difference, and the controller compares it with the target value and adjusts the speed command according to the deviation. If the air volume is low, increase the speed; if the air volume is high, reduce the speed. The entire process is completed automatically without manual intervention.
It sounds simple, but there are several key points in its actual implementation.
First, what to measure. Directly measuring air volume requires an air volume sensor, which is more expensive and requires installation location. What is more common in engineering is to measure the pressure difference, such as the pressure difference before and after the filter, room pressure difference or air duct static pressure. Using the pressure difference to indirectly reflect the air volume is low cost, but it is necessary to confirm the corresponding relationship between the pressure difference and the air volume.
Second, where are the measuring points installed? The sensor is installed at the outlet of the fan box, which can only reflect the outlet air volume and cannot guarantee that each branch receives enough air. The multi-branch system should first complete the balancing debugging, and if necessary, install the sensor at the critical end or the most unfavorable branch. Elbows, diameter changes and eddy currents near the measuring point will affect the readings, and the installation location should avoid these disturbance areas.
Third, how to set the control parameters. If the proportional, integral, and differential parameters are set incorrectly, the system may oscillate. The air volume command fluctuates frequently, and the driver constantly adjusts the speed, which in turn shortens the life of the equipment. During debugging, it is necessary to observe the response under several typical working conditions and confirm that it is stable before putting it into automatic operation.
A common pitfall of closed-loop control
Some projects were equipped with differential pressure sensors. During debugging, it was found that the control effect was not good and the rotation speed fluctuated. After inspection, it was found that the sensor was installed close to the elbow, where the air flow generated eddy currents and the readings were unstable. The controller keeps chasing this fluctuation signal, and the speed changes back and forth.
The solution is not to adjust the control parameters, but to change the location of the measuring points. The sensor should be installed in a location where the air flow is stable and the straight pipe section is long enough, avoiding elbows, reducers, air valves and fan outlets. If site conditions are limited and the ideal location cannot be found, you may consider installing a current equalizing device or switching to other measurement methods.
There is another situation: the sensor itself is fine, but the control target is set unreasonably. For example, the air volume is required to fluctuate within ±2%, but the resistance of the system itself changes beyond this range, and the controller can only keep adjusting. The control accuracy must match the actual conditions of the system, and goals that are too high cannot be set away from reality.
Debugging should cover boundary conditions
When debugging the closed-loop control system, you cannot just try it under the design working conditions. At least the following states should be covered: filter cleaning and near-final resistance, key valves fully open and partially closed, working mode and duty mode, and multiple branches simultaneously opened and partially closed.
Record sensor readings, control output, actual air volume and rotational speed in each state. If the control effect is not ideal under certain conditions, the control strategy needs to be adjusted or logic added. For example, when the filter resistance is close to the final resistance, if the rotation speed is close to the upper limit, it means that the design margin is insufficient, and it may be necessary to replace a larger fan or optimize the pipeline.
At the same time, the allowable speed range must be set. The minimum speed is determined by the end demand, not the minimum output of the drive. The maximum speed is limited by the motor, impeller and drive. When the control output exceeds this range, there should be an alarm or limiter, and the target cannot be pursued indefinitely.
What does the equipment do when the sensor fails?
How the equipment should behave when a sensor fails or the signal is lost must be agreed upon before commissioning. There are three common options: maintain the current speed, return to the preset safe speed, and stop.
Systems with minimum air supply or differential pressure requirements should generally not be shut down. For example, a clean room needs to maintain positive pressure. If the fan stops after a sensor failure, the pressure difference in the room will quickly get out of control. At this time, it should return to a safe preset speed, maintain basic air supply, and issue an alarm at the same time, waiting for manual processing.
Systems with upper pressure differential requirements should slow down or shut down to avoid excessive pressure differential. The specific choice depends on process requirements and safety assessment. These logics should be specified in the control scheme and set in the drive parameters. During debugging, sensor disconnection should be simulated to verify whether the actual action meets the requirements.
The division of labor with terminal equipment must be clear
The fan box is adjusted according to the air volume and serves the entire system, not a single end. If the end of the air duct isHigh-efficiency air supply outlet (HEPA BOX) , the total air volume control also needs to be coordinated with the air volume balance at each end. The air volume at the end is uneven, and no matter how accurate the total air volume is, it may still not be enough in some areas.
System end configurationWhen using HEPA/ULPA high-efficiency filters , the initial resistance of the filter and the resistance after use will affect the working point. Control strategies should be able to accommodate this range of variation. When the process requirements are higher, it can be evaluatedDoes the ULPA ultra-high efficiency filter meet the requirements?
Clean room ceiling air supply is comparableFFU fan filter unit ; centralized management of multiple devices can be matchedThe FFU computer group control system realizes zone speed regulation and status monitoring. Protection of single process equipment can be consideredEquipment-side fan filter unit (EFU) . The division of control between these devices and the fan box should be clearly stated on the system flow chart.
Control implementation from the perspective of whole machine matching
Adjusting the air volume ultimately depends on the matching of the entire machine. Whether the driver can receive sensor signals, whether the control parameters can cover the system resistance variation range, and whether the fan curve and pipeline characteristics allow sufficient speed adjustment range, all need to be confirmed during the selection stage. Aili Industry has independent supporting capabilities in fans, motors, controls and sheet metal structures. It can put the driver, motor, impeller and box at the same working point for verification, and match the speed range and feedback method according to the project control logic. This kind of internal collaboration from the power end to the control end can reduce the common problems of signal incompatibility, control parameter mismatch and insufficient speed range coverage in outsourced assembly solutions. The project only needs to provide the target air volume, sensor position, operating mode and fault action requirements. The control scheme and driver parameters can be set according to the point table before leaving the factory.
Specific structure and configuration options can be found inIt is confirmed in the fan box product catalog that non-standard projects should provide equipment or machine room drawings, and then shape them based on the determined working points and on-site interfaces.
