HEPA filter selection
Current HEPA filter efficiency from 99.9% ~ 99.999999% are available for adoption. Rational use of HEPA filter face velocity brings efficiency and overall economic effect.
The same material filters, high efficiency and its resistance is also large, high prices have reduced and vice versa, if the wind is resistance to change, efficiency and volume changes required by the specific circumstances of a reasonable choice.
(1) the general principle is that the higher the efficiency of HEPA or ULPA optional cleanliness requirements to achieve high and low cleanliness requirements lax use less efficient HEPA filter.
(2) a high amount of dust, the filter efficiency changes, cleanroom little effect, so less demanding cleanroom cleanliness should not use high efficiency HEPA filter.
(3) low dust content, the higher the efficiency of the HEPA filter at low wind speeds for cleanroom has obvious advantages, therefore, requires strict clean room in the selection of high efficiency filter at the same time reducing its welcome and wind speed.
Filtration Efficiency Measurement Methods
(1) Sodium Flame Method:
Aerosols generated from atomized NaCl solution are used as the dust source, with particle sizes ranging from 0.02 to 2 μm and a median particle size of approximately 0.6 μm (measured as about 0.5 μm in China). Measurement is conducted using a photometer. This method is no longer used internationally. Although it has not yet been officially abolished in China, it no longer meets current requirements.
(2) Fluorescence Method and Oil Mist Method:
These methods are no longer in use.
(3) U.S. Military Standard (Mil-Std 282) Method:
The dust source consists of 0.3 μm monodisperse DOP particles, and measurement is performed using a specialized instrument, the Q-107 Penetrometer. This method can also measure filter resistance. However, it is now considered outdated.
(4) Method from U.S. IEST Guidelines (IEST-RP-CC-007):
A discrete particle counter (OPC or CNC) is used to measure the penetration rate of specified particle sizes through the filter and determine its efficiency. The dust source is a DOP aerosol, though it is not limited to DOP as long as it meets certain optical characteristics. The aerosol can be monodisperse or polydisperse.
(5) European Standard EN 1822 (1998) Method:
This method also uses a discrete particle counter (OPC or CNC) to measure the penetration rate at the Most Penetrating Particle Size (MPPS) and determine efficiency. Hence, it is often referred to as the MPPS method. A movable probe is used for line-by-line scanning to obtain local penetration rates and local efficiencies, which are then used to calculate the overall efficiency. The dust source is a DOP aerosol or alternative substance, with requirements largely similar to those in method (4).
Since the MPPS of a filter depends on the fiber diameter of the filter media, packing density, airflow velocity, etc., the MPPS of the filter media must first be determined (generally between 0.1–0.3 μm) before scanning measurements are conducted. The scanning process also allows for simultaneous leak testing of the filter.
Evaluation:
Both methods (3) and (4) utilize particle counters for measurement and can employ either monodisperse or polydisperse aerosols, as long as their optical characteristics meet the requirements of the particle counter. This represents the current trend. Method (4) incorporates scanning, which also addresses leak detection, making it advantageous. However, the practical significance of using MPPS may not be substantial.
