How to measure the velocity and uniformity of air filters in clean rooms

Test Purpose

Airflow is the most important factor in controlling cleanliness, temperature and humidity, and it also has some impact on noise. Therefore, Wind speed measurement is always the first step in the clean room test. The purpose of Wind speed measurement is to confirm that the airflow sent by the filter meets the design specifications, and secondly to confirm the uniformity of the airflow. In some occasions, due to site restrictions, the indoor ventilation must also be converted by multiplying the wind speed by the outlet area. Many single-flow clean rooms are designed as vertical laminar flow, so the uniformity of wind speed is very important. Only uniform vertical laminar flow can effectively eliminate particulate pollution. 

For non-single-flow clean rooms, since the concept of particle control is dilution, not immediate elimination, its ventilation is generally more important than wind speed, so most of the time it only needs to measure the Air volume. In a single-flow clean room, the location of wind speed measurement can be specified by the owner, generally on the surface of the wave net or at the working height. However, it is necessary to indicate whether it is the wave net wind speed measurement (filter surface) or the clean room indoor wind speed measurement (working height).

Instruments used

The IEPA/ULPA high-efficiency filters used in clean rooms usually have their airflow speed controlled within 0.5/s, so the anemometer used must be a low-speed type. For wave net wind speed measurement, a single-point anemometer such as an Electronic pressure gauge can be used in conjunction with a skin private or hot wire anemometer. Multi-point anemometers such as Shortridge Velgid 16-point anemometers can also be used. Vane anemometers are usually not used in clean rooms due to their different use ranges. Although hot wire anemometers have good high-frequency response, their accuracy is very low at low speeds (below 0.5m/s), so they are not very suitable. Generally, the thermal response anemometer commonly used is the Rhermeister anemometer. The rotary anemometer is also not suitable for low speeds due to its own weight problem. The requirement for drive B is <0.25 m/s to 0.6 m/〉 between 50fpm and 120fpm, with an accuracy of ±5 percent of the reading. 

The test instrument must be calibrated and still within the validity period. Qualified calibration documents must be attached before the formal test and when submitting the report.

Test steps

1. Record the size, quantity and number of air filters on the drawing.

2. The sampling point is 75-150mm below the air filter.

3. Under each Air Filter, if a single-point anemometer is used, one point is taken for every 1 square foot; if a multi-point anemometer is used, one point is taken for every 4 square feet. Taking a 600mmx1200mm filter as an example, a single-point anemometer should measure 8 points; a multi-point anemometer should measure 2 points.

4. A 5-second average must be taken for each measurement point.

5. Analyze all wind speeds according to the acceptance criteria, calculate the average, standard deviation, relative standard deviation, and mark the unqualified test points.

6. Record all original values and the values after analysis.

The test steps for wind speed measurement are the same in single-flow or non-single-flow clean rooms. It is important to pay special attention to the anemometer during sampling when testing in non-single-flow clean rooms to avoid affecting the accuracy.

Data analysis

Wind speed measurement is simple, but uniformity must be determined by data analysis. Uniformity is represented by relative standard deviation, and its calculation steps are as follows.

1. Average: Take the arithmetic average of all wind speed measurement values.

2. Standard deviation: Calculate the standard deviation of all wind speed measurement values.

3. Relative standard deviation: The relative standard deviation is a percentage. Divide the standard deviation by the average and multiply by 100 to get the percentage.

Wind speed measurement is simple, but uniformity must be determined by data analysis. Uniformity is represented by relative standard deviation, and its calculation steps are as follows. From the above explanation, it can be seen that the relative standard deviation is actually just the standard deviation without unitization. Its physical meaning is the same as the standard deviation, which is the degree to which the data deviates from the average value. Therefore, the higher the relative standard deviation, the more uneven the data is. In the case of vertical laminar flow, of course, the lower the relative standard deviation, the better.

Acceptance criteria

The acceptance criteria must be set by the owner. The design team designs the Air conditioning system according to the owner's requirements, and the testing unit tests according to the specifications and provides it to the owner as the basis for acceptance. Generally speaking, there are three acceptance conditions:

• The difference between the wind speed values of two adjacent points cannot be greater than 20.

• The measured average wind speed should be within the design specification.

• The relative standard deviation should be within ±15%.

Regarding the first provision, a difference greater than 20 indicates that there are sufficient conditions for the generation of regional turbulence in this area, which must be adjusted. Regarding the second provision, in the clean room, the average wind speed of a single filter is usually set within 10 of the design specification, and the total is within 5. For general air conditioners, the number of individual air outlets is within 20, and the total number is within +10.