Selection basis
The primary basis for the selection of the purification workbench is the specific requirements of the experimental operation for cleanliness and the airflow organization method in the workbench. Cleanliness is usually based on ISO Class 5 (i.e. 100 environment) in the ISO 14644-1 standard, which means that the number of particles ≥ 0.5 μm per cubic meter of air does not exceed 3,520. The airflow direction is divided into two categories: vertical laminar flow and horizontal laminar flow, the former is supplied from the top to the bottom through the high-efficiency filter, which is suitable for scenarios with high operator protection requirements; The latter is a horizontal supply of air from the back to the front, making it suitable for operations where sample protection is more important. When selecting a model, the cleanliness needs of the experimental object (such as sample, operator, or environment) should be clarified first, and then the corresponding airflow mode should be selected.
Airflow direction
The wind speed range of the vertical laminar flow table is usually 0.3 m/s to 0.5 m/s, which can form a stable downward airflow in the working area, effectively blocking the entry of external pollutants. The wind speed of the horizontal laminar flow table is generally between 0.4 m/s and 0.6 m/s, and the airflow direction is blown from the operator's side to the sample, which is suitable for experiments where the operator's own cleanliness is not high but the sample needs to be highly isolated. To verify whether the airflow direction is reasonable, the smoke test method can be used: a trace amount of smoke is released in the workbench, and the smoke flows in the predetermined direction evenly and without eddy currents. If there is backflow or turbulence, it indicates a problem with the airflow design or installation location.
Cleanliness grade
When determining the cleanliness level, refer to the particle concentration limits in the ISO 14644-1 standard. The following are the common cleanliness grades and corresponding particle count requirements (for ≥ 0.5 μm particles):
| ISO Class 5 | 3520 particles ≤ per cubic meter |
| ISO Class 6 | 35,200 particles ≤ per cubic meter |
| ISO Class 7 | 352,000 particles ≤ per cubic meter |
| ISO Class 8 | 3520000 particles ≤ per cubic meter |
Purification benches are usually required to be ISO Class 5, which can be confirmed by consulting the particle count data in the product factory inspection report during selection. If the experiment involves highly sensitive samples (such as electronic components or cell culture), additional on-site third-party testing can be requested to ensure that the actual use environment meets the requirements.
Wind speed and uniformity
The uniformity of the air speed in the workbench directly affects the cleaning effect. According to ISO 14644-3, the unevenness of the wind speed in the worktable should be less than 20%, that is, the deviation of the wind speed at each measurement point should not exceed 20% of the average wind speed. The wind speed measurement points should be evenly distributed in the working area (such as the leading edge, trailing edge, left and right sides), and each point should be measured 3 times to take the average. If the wind speed is too low, it cannot effectively remove pollutants; Too high may produce turbulence, which in turn destroys the laminar flow state. It is recommended to check whether the product technical parameters contain wind speed uniformity data when selecting the model, and use the hot ball anemometer for re-testing during acceptance.
Filter rating
High-efficiency filters (HEPA) or ultra-high efficiency filters (ULPA) are the core components of the purification bench. HEPA filters have a filtration efficiency of ≥ 99.97% for ≥ 0.3 μm particles, making them suitable for most laboratory scenarios. ULPA filters are more efficient (99.999% efficiency for ≥ 0.12 μm particles≥ making them suitable for precision operations with higher requirements for cleanliness. When selecting a filter, attention should be paid to whether the filter has a replaceable design and whether it provides a leak detection (such as PAO method) report. After the filter is installed, it needs to be tested for integrity to ensure that there is no bypass leakage, and the filter and sealing frame can be detected point by point by scanning method.
Detection and validation
After the workbench is installed, the performance needs to be verified according to standard processes. The following three tests are recommended: First, particle counting test, using a laser particle counter to sample in the center and corners of the working area to confirm that it meets ISO Class 5 or the selected level; second, the wind speed and uniformity test, measure and calculate the deviation according to the above method; Third, the airflow visualization test, using a smoke generator to observe the airflow trajectory. If the test results are not up to standard, check whether the filter seal, fan speed or table placement environment interfere with the airflow. Regular maintenance, such as replacing the pre-filter every six months, maintains consistent performance.
Reference source
1. ISO 14644-1:2015 Clean rooms and related controlled environments - Part 1: Air cleanliness classification
2. ISO 14644-3:2019 Clean rooms and related controlled environments - Part 3: Testing methods
3. Laboratory Equipment Technical Manual Airflow Workbench Chapter
4. Relevant industry specifications and equipment manufacturer technical white papers