How to choose a fan box? Determine the air volume first, then calculate the system resistance
Direct answer: The fan box selection is not to select the equipment with the largest 'maximum air volume' from the sample, but to first determine the actual design air volume required at the end, and then calculate the resistance that the entire system needs to overcome under this air volume. The two constitute the operating point, and finally find the corresponding operating point on the fan performance curve. Without any step, the equipment may not meet the usage requirements after installation.
Aili Industry is discussingWhen selecting a fan box , it is usually required to provide the design air volume, external static pressure, system usage and operating time, and then check the performance curve and installation conditions. The purpose of this is to separate equipment capabilities from system requirements and avoid using the nominal parameters of a single piece of equipment to replace the actual operating conditions of the entire system.
Design air volume comes from terminal demand
When building a new system, the air supply and exhaust requirements of each area or equipment should be summarized first, rather than inferring the pipeline from the 'maximum air volume' of a certain typhoon chassis. Room usage, heat and moisture load, fresh air volume, air exchange frequency and process exhaust requirements will all affect the terminal air volume. List these requirements item by item, and then consider the simultaneous use coefficient and pipeline air leakage margin to obtain the total air volume of the system.
When the existing system is modified, the current air volume can be measured on the premise of clarifying the valve status, filter status and measuring point location, and the design value can be determined based on the modification goals. If the old equipment operates by turning off the valve for a long time, it means that the original system is over-configured. The new solution does not need to copy the maximum rated value of the old equipment, but should meet the actual use requirements and retain a reasonable margin. Before modification, it is recommended to record the fan outlet air volume, key terminal air volume, filter section pressure difference and motor input power at the same time. These data can reflect the real operating point better than the equipment nameplate.
System resistance cannot only be counted as straight pipes
The external resistance that the fan box needs to overcome includes: filter sections, coils, elbows, reducers, air valves, mufflers, straight sections of air ducts, and the most unfavorable branches. Each of these consumes the pressure provided by the blower. If you only estimate based on the length of the straight pipe and ignore local components and filter sections, the selected fan may have insufficient air flow in the actual system.
When the cleaning or air conditioning system has a filter section, the cleaning status and the allowable pressure difference before maintenance should be understood separately. If the fan is selected only according to the clean filter status, the equipment may not be able to meet the terminal air volume after running for a period of time. The margin cannot be increased indefinitely, otherwise daily operations may require throttling for a long time, resulting in unnecessary energy consumption.the HEPA/ULPA high-efficiency filter should be included in the total resistance of the system; when the process requirements are higher, it can also be evaluatedThe initial resistance and later resistance of Does the ULPA ultra-high efficiency filter meet the requirements?
The working point is the intersection of the two curves
Put the design air volume and the calculated system resistance on the same graph: the fan performance curve represents the pressure that the fan can provide at different air volumes, and the system resistance curve represents the pressure required by the pipeline at different air volumes. The intersection of the two curves is the expected operating point.
When selecting, make sure that the intersection point falls near the high-efficiency area of the fan, and that the capabilities of the motor and driver can be covered. If the intersection point deviates too far from the high-efficiency zone, even if the fan can run, the noise and energy consumption may be high. If a filter section or branch may be added to the system later, it should also be checked whether the minimum usage requirements can still be met after the working point is moved to the direction of small air volume.
The fan law has two practical corollaries here. First, the air volume is directly proportional to the rotational speed. If the rotational speed is reduced by 20%, the air volume will be reduced by approximately 20%. Second, the shaft power is proportional to the cube of the rotational speed. If the rotational speed is reduced by 20%, the shaft power will be reduced to approximately 51% of the rated value. These two relationships determine the energy-saving space of the speed regulation scheme under partial load conditions, and also indicate that the selection must be based on the actual operating point.
Check noise, power supply and installation conditions
After the performance is satisfied, the noise, power supply, cabinet size, air outlet direction, maintenance space and vibration reduction requirements must be checked. The noise value should describe the measurement distance and operating conditions, and cannot be compared with a single decibel number in isolation. The power supply voltage, frequency, and whether a speed control interface is required should also be confirmed during the selection stage.
If the project adopts a DC speed regulation solution, it is also necessary to confirm the speed regulation range, control signal type and driver installation location. Clean room ceiling air supply is comparableFFU fan filter unit ; centralized management of multiple devices can be matchedFFU computer group control system . Protection of single process equipment can be consideredEquipment-side fan filter unit (EFU) . The final filtered air supply at the end of the air duct can beHigh-efficiency air supply outlet (HEPA BOX)。
Selection checklist
A complete selection requirement should at least include:
Design air volume (peak operating conditions and common operating conditions are listed separately)
External static pressure (including all resistance of filter section, coil, valve, etc.)
Air supply or exhaust purpose
Power supply conditions (voltage, frequency, whether to reserve speed adjustment interface)
continuous running time
Air outlet direction and maintenance space
Noise requirements and measurement conditions
Whether interlocking with building control systems or equipment is required
Ai Li industrialThe fan box product catalog serves as a starting point for communication about structure and configuration. For non-standard projects, equipment or computer room drawings should be provided, and then finalized based on the determined working points and on-site interfaces. Selection is not a one-time calculation, but a process of aligning terminal requirements, system resistance, equipment capabilities and installation conditions. Only when these four are matched can the design intention be truly achieved after the equipment is installed.
