What do you think of the ULPA resistance curve?

We often encounter this situation when selecting a model: the supplier gives a sample with a curve drawn on it. The horizontal axis is the air volume and the vertical axis is the resistance. It looks very intuitive, but when it comes to using the system, many people will get stuck in the same place - how does this curve correspond to my actual working conditions?

The resistance curve describes the corresponding relationship between the air volume and pressure loss of the filter under specified conditions. It is not a straight line, nor is it a fixed value. The key to reading the curve is to find your actual working air volume, and then look at the resistance corresponding to that point.

First confirm which model the curve belongs to

When you get a curve, the first thing to do is not to look at the value, but to confirm which product it corresponds to.

The model and specifications must match. Depending on the outer frame size, thickness, filter material type, and sealing method, the resistance curve may be completely different. You cannot use the curve of one product to estimate another.

The test conditions should be clear. Under what conditions was the data on the curve measured? Is the test object a filter material sample or a finished product? Does the test air volume range cover your working area?

Curves should not be shared between different structures. Also marked U15, the curves of glass fiber and PTFE may be different; the structures without partitions and liquid tank seals may also be different. Specific product data is subject to the technical data of the corresponding model. ULPA products can be referenced ULPA ultra-high efficiency filter , please refer to HEPA products Specifications of HEPA high-efficiency filters . Corresponding information for

How to find an actual work location

The operating point is the core of the reading curve. The steps are not complicated, but the system conditions need to be aligned.

Step one: Confirm the actual air volume. What is the design air volume? Will the speed be adjusted during operation? If the system has different working conditions, the work scope and duty working conditions should be listed.

Step 2: Find corresponding points on the curve. Find the actual air volume on the horizontal axis, correspond to the curve upward, and then read the resistance value to the left. Note the unit: the air volume may be m³/h or m³/s, and the resistance is usually Pa.

Step 3: Calculate the system pressure loss together. The resistance of the filter is only a part of the total resistance of the system. Pre-filtration, air ducts, boxes, and terminals all have their own pressure losses. Only by putting the resistance of the filter element into the total resistance of the system can we find the real operating point by comparing it with the fan curve. For specific methods, please refer to the relevant instructions on resistance calculation.

Step 4: Check the final resistance conditions. When the filter element reaches the advanced stage of dust accumulation, the resistance will rise. When selecting a model, you should not just look at the point corresponding to the initial resistance, but also confirm whether the fan still has enough margin under the final resistance condition. This is supported by FFU fan filter unit or EFU equipment-side fan filter units . This is especially critical in systems with

Do not extrapolate outside the curve range.

Some projects will ask: My air volume is a little higher than the maximum value marked on the sample. Can it be estimated by extending the curve?

This is not recommended. Resistance curves usually have test ranges and data outside of the ranges are not verified. When the air volume is too high, the passing wind speed of the filter material increases, the resistance may increase nonlinearly, and the airflow distribution may also change. For working conditions outside the curve, confirmation from the supplier or supplementary testing is required.

Similarly, when the air volume is far below the test range, the data may not be representative. Readings at low air volumes may look good but do not represent actual operating conditions.

What are the restrictions on resistance conversion?

Sometimes the complete curve cannot be obtained, only the data of a few points. Can we use conversion to calculate at this time?

Different media and channels have different flow behaviors, and complete product data cannot be universally replaced by a fixed ratio or square formula. Some information will mention the approximate relationship between resistance and air volume squared, but this is only an approximation under specific conditions and cannot be applied to all products. Especially for products like ULPA, which have dense filter materials and different channel designs, the error in directly applying the formula may be large.

If the resistance under a certain air volume is needed, the safest way is to ask the supplier for the data under that working condition, or read it with a complete curve. Do not extrapolate from a single point of data on your own.

Project judgment example

The design flow rate of a certain equipment is 800 m³/h, and the FFU needs to be equipped with ULPA.

The supplier provided the resistance curve of U15: at 800 m³/h, the initial resistance is about 220 Pa and the final resistance is about 450 Pa. The fan curve shows that when the total system resistance (including prefiltration and air duct) is about 600 Pa, the air volume can still be maintained at about 800 m³/h; when the total resistance rises to 850 Pa (corresponding to the final resistance condition), the air volume will drop to about 720 m³/h.

This means that there is no problem in initial operation, but when the filter element accumulates dust and reaches final resistance, the air volume may drop by about 10%. If the process has strict requirements on air volume fluctuations, you need to consider adjusting the fan configuration during the planning stage, or choose a ULPA structure with a larger effective area.

The core of this judgment process is to look at the filter element resistance curve and the fan curve together, instead of just looking at one number. involving HEPA/ULPA air filters are the same. The method and logic for selecting

Selection and Acceptance Checklist

About Aili Industrial’s resistance curve package

The value of the resistance curve is that it can help the project team determine the system compatibility in advance.

Aili Industrial's manufacturing system covers filters, motors, fans, control systems and complete machine assembly, and can provide ULPA ultra high efficiency filter、HEPA high-efficiency filter and supporting air supply equipment.

When enquiring, provide the design air volume, operating range and system pressure loss conditions to help obtain comparable curve data. Specific performance is subject to the testing data of the corresponding model.