What should I do if the air pressure in the fan box is insufficient? First confirm the measurement point and pressure definition

A clean room project reported that the fan box had 'insufficient pressure' and the air volume of the high-efficiency air supply outlet at the end was insufficient. During the on-site inspection, the operator pointed to the fan outlet pressure gauge and said, 'Look, it's only 500 Pa. The design is 800 Pa.' However, after careful inspection, he found that the design document stated an external static pressure of 800 Pa, while the pressure gauge measured the fan outlet static pressure. There was a difference in outlet dynamic pressure between the two. The actual external static pressure is only more than 400 Pa, which is indeed insufficient. However, the problem is not that the equipment is small, but that the measuring point position and pressure definition are not aligned from the beginning.

This case illustrates a common problem: before saying 'insufficient wind pressure', you must first confirm which pressure you are talking about. Total pressure, static pressure, dynamic pressure, and external static pressure have different meanings and different measuring points and cannot be confused.

Aili Industry is discussingWhen there is an operation problem with the fan box , it is usually required to provide the location of the measuring point, the type of instrument and the pressure caliber in the design document, and then determine whether the equipment really has not reached the working point.

'Not enough pressure' First ask where the test is

Pressure readings must be viewed together with measuring point locations. For the same system, the pressure measured at different locations may be completely different. Common measurement points include: the fan box outlet, before and after the filter section, before and after key elbows, and the farthest branch. The readings at each location are used differently and cannot be substituted for each other.

The pressure written in the design document is usually 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. This pressure deducts the internal losses of the unit and the outlet dynamic pressure, and is the part actually available to the engineering personnel. If the full pressure at the fan outlet is measured on site, the reading will be greater than the external static pressure; if the static pressure at the fan inlet is measured, the reading may be negative. If you use the wrong caliber, the comparison will be meaningless.

The location of the measuring point also affects the readings. 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.

The definition of pressure should be checked item by item

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. If the external static pressure is written in the design document, but the total pressure is measured on site, and there is one outlet dynamic pressure difference between the two, direct comparison will lead to erroneous conclusions.

Static pressure is the pressure of gas on the pipe wall, which is commonly referred to as 'wind pressure'. 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 the pressure loss of the turns, brackets, mufflers and outlet diffusion sections inside the box. These losses may or may not have been deducted from the sample curve. The supplier should be asked to clarify the measurement boundaries when making an inquiry.

System end configurationFor HEPA/ULPA high-efficiency filters , the initial resistance of the filter and the resistance at the end of use will affect the external static pressure demand. If the calculation is only based on the clean state, 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?

Measure pressure step by step along the air path

After confirming the pressure caliber and measuring point location, measure the pressure segmentally along the air path. Record the static pressure at the fan box outlet, before and after the filter section, key elbows and the farthest end in order, and compare which section the abnormal pressure loss is concentrated in. This method is more reliable than directly replacing a larger fan because it can locate the link where the problem lies.

The pressure difference in the filter section is abnormally high, indicating that the filter is dusty or clogged. The valve is not fully open, which increases local resistance. Too many duct diameter changes or elbows will also consume available pressure. Low speed, dust accumulation on the impeller, or incorrect rotation direction will also cause abnormal performance.

When measuring pressure in sections, the instrument range and pressure measurement method of each measuring point should be consistent. If one section uses a large-range instrument and another section uses a small-range instrument, 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.

Check speed and impeller status

The control side troubleshooting starts with the rotation speed. For fixed-speed fan chassis, it is necessary to confirm whether the power supply voltage and frequency are normal and whether the three phases are balanced. Frequency conversion or DC speed regulating fan box should confirm whether the control signal is in place and whether the analog, PWM or communication signal is consistent with the set value.

Dust accumulation on the impeller will change the impeller balance and airflow passages, reducing fan performance. If the impeller rotates in the wrong direction, the air volume may be very small and the pressure cannot be built up. Manually turn the wheel to confirm that the impeller rotates flexibly without friction or jamming. Before turning on the power, check whether there are any packaging objects, bolts or other objects left around the impeller.

If the speed is normal, the impeller is clean, and the direction is correct, but the pressure is still insufficient, the problem may be that the system resistance exceeds the original design range, and the working point needs to be recalculated.

When do you need to recalculate work points?

If a branch is added, a filter section is added, or the direction of the air duct is changed later in the system, the original air box curve may no longer cover the new requirements. At this time, it is not a device failure, but a change in system requirements. Recalculating the working point is more rational than replacing a larger fan.

Recalculation requires existing air duct diagrams, air volume, segment pressure measurement results and motor input power. Put these data on the same graph, find the new system resistance curve, and intersect it with the fan performance curve to see if the intersection falls within the usable range.

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 equipment and the fan box are different, and the measuring points and pressure calibers should be confirmed separately during the investigation.

What data should be included in the walkthrough record?

A record that can be used for wind pressure investigation should at least include: measuring point location, instrument type and range, pressure caliber (full pressure, static pressure or external static pressure), fan outlet static pressure, pressure difference before and after the filter section, key elbow pressure loss, farthest end static pressure, rotation speed or frequency, control signal value, motor input power and filter status.

Aili Industrial can discuss based on the existing air duct diagram, air volume and segmented pressure measurement results.Fan box adjustment or modification . The motor and driver margins should also be checked before adjustment. 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.