How to read the wind pressure parameters of the fan box? First confirm the static pressure, total pressure and external static pressure
When looking at the wind pressure parameters of the fan box, the first thing to ask is not 'how many pascals', but 'what pressure is this'. Total pressure, static pressure, dynamic pressure, and external static pressure have different meanings, different measuring points, and different uses. Design documents often write about the external static pressure, that is, the pressure at the outlet of the fan box that can be used to overcome the resistance of the external pipeline; the sample may be marked with full pressure, and the on-site pressure gauge may read static pressure. If you use the wrong caliber, the comparison will be meaningless.
What do total pressure, static pressure, and dynamic pressure mean?
Total pressure is the sum of static pressure and dynamic pressure. The dynamic pressure is proportional to the square of the wind speed. The higher the wind speed, the greater the dynamic pressure. When the wind speed at the fan outlet is high, the dynamic pressure may account for a considerable proportion of the total pressure and cannot be ignored.
Static pressure is the pressure of gas on the pipe wall, which is commonly referred to as 'wind pressure'. It does not include dynamic pressure, but reflects the squeezing effect of air flow on the pipe wall. If only 'static pressure' is written on the fan sample, it usually refers to the static pressure at the outlet section of the fan.
The external static pressure is the available portion of the static pressure minus the internal losses of the unit. The internal losses of the unit include pressure losses in the turns, brackets, mufflers and outlet diffusion sections inside the box. External static pressure is what engineers can really use to overcome the resistance of external ducts, valves, and filters.
The final check of the project is how much external static pressure the whole machine can provide under the target air volume. If you only focus on the total pressure or static pressure and ignore the internal losses and outlet dynamic pressure, the selection may be optimistic.
For the 'rated wind pressure' on the drawing, you need to ask about the caliber.
When the equipment data is written as 'rated wind pressure', the corresponding air volume and test boundaries should be obtained from the supplier. Some manufacturers state full pressure, some state static pressure, and some directly state that external static pressure can be used. The three cannot be replaced at will.
If there is a filter section or silencer section inside the unit, confirm whether its pressure loss has been deducted. Otherwise, the available external static pressure written in the design drawing and the manufacturer's data may point to different locations. For example, if the sample is marked with a total pressure of 800Pa, an internal loss of the unit of 100Pa, and an outlet dynamic pressure of 50Pa, then the actual available external static pressure is only 650Pa. If the design requires 800Pa external static pressure, this equipment will not be enough.
System end configurationFor HEPA/ULPA high-efficiency filters , the initial resistance and resistance at the end of use of the filter should be taken into account in the external static pressure requirements. Calculated based on clean status only, the pressure may be insufficient 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?
Cumulative resistance along the most unfavorable path
From the outlet of the fan box to the most unfavorable end, total the filter section, coil, valve, muffler, elbow, reducer and air duct friction loss one by one according to the design conditions. 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, please note that the pressure values at different measuring points or different air volumes cannot be added directly. Each resistance should correspond to the same design air volume. The filter resistance changes with the usage status, and if necessary, the final resistance should be checked. If the system has multiple branches, the pressure loss of each branch should be calculated separately, and the maximum value should be taken as the external static pressure requirement.
On-site pressure measurement needs to be unified
When the air pressure is insufficient or too high, first indicate the position of the pressure measuring hole, valve opening and fan speed. If the pressure measurement method is different or the instrument has not been calibrated to zero, the reading may not be directly compared with the factory sample.
The position of the pressure tapping port on the air duct wall, the pressure tapping direction, and the zero point of the instrument will all affect the measurement results. If the pressure port is facing the direction of the air flow, the measured pressure may be the full pressure; if it is perpendicular to the direction of the air flow, the measured pressure is the static pressure. During on-site inspection, you should first confirm the pressure measurement method and then read the value.
By measuring pressure along the wind path in sections, abnormal local losses can often be found. Record the static pressure at the outlet of the fan box, before and after the filter section, the key elbow and the farthest end in order, and compare which section the abnormal pressure loss is concentrated in. If the instrument ranges or pressure measurement methods of each section are inconsistent, the data cannot be directly subtracted. Make sure your testing methods are consistent before deciding whether to adjust the valve, clean the filter section, or recheck the fan.
Wind pressure and air volume must be judged in pairs
High pressure does not necessarily mean adequate air supply. If the downstream valve is closed or the filter is clogged, the static pressure near the fan may increase, but the air volume at the end will be insufficient. Slowing down after seeing just one pressure reading can starve critical areas of the wind even more. The air volume, pressure and valve status should be judged together.
Similarly, low pressure does not necessarily mean that the equipment is undersized. Wrong location of the measuring point, miscalibration of the instrument or inconsistent pressure caliber may result in low readings. Confirm the measurement method first, and then judge whether the equipment really has not reached the working point.
See the real external static pressure from the whole machine test data
The reliability of wind pressure parameters depends on whether the test conditions of the whole machine are clear. Aili Industrial has independent supporting capabilities in fans, motors, controls and sheet metal structures. It can calibrate the impeller, motor, driver and box at the same working point, and provide external static pressure data of the complete machine at a specified air volume. This kind of internal coordination from the impeller to the whole machine can avoid the caliber deviation between the sample full pressure, on-site static pressure and the design external static pressure, so that the selection is based on the actual measured data of the whole machine, rather than a simple calculation of the part sample.
If the system is compatible withFor HEPA/ULPA high-efficiency filters , the pressure loss in the filter section should be clearly included in the pressure calculation. 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 pressure characteristics of these devices and the fan box are different and should be clearly specified on the system flow diagram.
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.
