Number of views: 3117 Author: Aili Industry Release time: 2026-09-28 Source: Ai Li Industrial
The first step in fan box selection is not to select equipment, but to determine the air volume. The air volume determines the duct cross-section, wind speed in the duct, resistance along the way, and local resistance, which in turn determines how much external static pressure the system requires. If the air volume is not determined accurately, the air duct size, valve selection, and fan box operating point will all deviate. First determine the air volume, then lay out the air duct, then calculate the resistance, and finally select the air box. This order cannot be reversed.
Air volume is not a number set out of thin air, it comes from the objects the system wants to serve. The air conditioning system must determine the regional air volume based on the heat and moisture load, fresh air volume, and air supply organization requirements of each area; equipment ventilation must be determined by equipment heat dissipation or process exhaust requirements; clean room air supply must meet the requirements for air changes, pressure difference, and self-cleaning time. List these requirements item by item to form a partition air volume balance table, and then consider the simultaneous use coefficient and pipeline air leakage margin to obtain the total air volume of the system.
A common mistake here is to simply add up the nameplate maximum airflow rates of all equipment. In fact, different areas may not necessarily operate at the same time, and the exhaust air of different equipment may not necessarily reach peak values at the same time. The total air volume of the system should be determined based on the partition demand and combined with the simultaneous use coefficient, rather than mechanical addition.
After the air volume is determined, the air duct cross section and reasonable flow rate can be selected. The relationship between air volume, wind speed and pipe diameter is: air volume is equal to wind speed times cross-sectional area. Under the same air volume, the wind speed will be high if the pipe diameter is small, and the wind speed will be low if the pipe diameter is large.
High wind speed will bring three consequences: increased resistance along the way, increased local resistance, and increased noise. When the wind speed is low, the pipe diameter is large and the initial investment is increased, but the operating resistance is small and the noise is low. A balance needs to be struck between the two when designing. If you first lock the equipment according to the nominal air volume of a certain type of fan box, and then reversely estimate the air duct size, two situations may occur: the air volume is estimated to be too large, and the air duct is selected to be thick, and the equipment runs at low load for a long time; the air volume is estimated to be small, and the air duct is selected to be thin, and the resistance will rise rapidly in the later period. Even if the speed is increased, noise and additional power consumption may be caused.
The resistance along the way and the local resistance in the air duct are approximately proportional to the square of the wind speed. As the air volume increases, the wind speed increases, and the resistance increases at a faster rate. The local resistance of elbows, reducers, valves and filter sections also increases with the increase in wind speed. This means that if the air volume is set on the high side, the system static pressure demand will increase significantly, and the selection pressure of the fan box will also increase.
System static pressure is determined by the most unfavorable path. The least favorable path is not necessarily the physically longest path, but the path with the greatest pressure loss. Although some branches are short, they have many elbows and valves or the resistance of the end filter is large, and the pressure loss may exceed that of a long straight pipe. When calculating, the calculation should be carried out section by section along the most unfavorable path, rather than just looking at the length of the total pipe.
If the end of the system is configuredFor HEPA/ULPA high-efficiency filters , the initial resistance of the filter and the resistance at the end of use should be included in the total resistance of the system. If the model is selected only according to the clean state, the terminal air volume may not be satisfied after running for a period of time. When the process requirements are higher, it can be evaluatedDoes the ULPA ultra-high efficiency filter meet the requirements?
When retrofitting an existing system, you cannot place an order directly based on the nameplate of the old machine. The rated air volume of old equipment may not be equal to the actual needs of the production line. The correct approach is to measure the terminal air volume and pressure difference during use in key areas, confirm which branches are open at the same time, and then determine the design air volume.
Common situations at the site are: the old equipment has been running by turning off the valve for a long time, indicating that the original system is over-configured; or the old equipment has been running at full speed and still cannot meet the end requirements, indicating that the original system has insufficient air volume. Both cases require recalculation rather than duplication of the rated parameters of the old equipment. The valve opening, filter status and fan speed should be recorded during measurement, as these conditions will affect the measurement results.
Air volume is not the only parameter. After determining the air volume, the external static pressure must be calculated along the most unfavorable path and the operating point must be checked on the fan performance curve. The working point is the intersection point of the fan performance curve and the system resistance curve. This intersection point should fall near the fan's high efficiency area, and leave an adjustment margin to deal with the increase in resistance such as dust accumulation on the filter.
For systems with a filter section, it is advisable to distinguish the resistance at the initial stage and at the end of use; for systems with large load changes, common partial load conditions should also be listed. The nominal maximum air volume is mostly obtained under specific test conditions and does not necessarily correspond to the external static pressure of the project. The selection table should contain at least three parameters: 'air volume, external static pressure, and machine input power'.
The blower box is responsible for providing the power, and the filtration task is usually performed by a separate device. 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 final filtered air supply at the end of the air duct can beHigh-efficiency air supply outlet (HEPA BOX) . The division of labor boundaries between these equipment and the fan box should be clearly drawn on the system flow chart to avoid confusing power equipment and filtering equipment.
After the air volume is determined, the fan, motor, box structure and control method can be matched around the working point. Aili Industrial has independent supporting capabilities in fans, motors, controls and sheet metal structures, and can check the complete machine configuration based on zoned air volume, most unfavorable path resistance and on-site installation interfaces. This kind of internal collaboration from air volume to the entire machine can reduce air volume, pressure mismatch and interface inconsistency problems that are common in outsourced assembly solutions. Specific 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.
