Purification (Clean) Air Conditioning Engineering Air Filter
Purification (Clean) Air Conditioning Engineering Air Filter is a very important link. It directly determines the qualification of the purification project. It can be said to be the core work of the purification project.
Design standards for clean workshops in the pharmaceutical industry" (GB50457-2019); "Design specifications for clean workshops in the electronics industry" (GB50472-2008); "Technical specifications for construction of hospital clean surgery departments" (GB50333-2013) and other industry design specifications also have Special requirements regarding the concentration of suspended particles in the air in clean rooms (operating rooms).
It can be seen that the concentration of suspended particles in the air has a crucial impact on the clean room.
So in purification and air-conditioning projects, what means do we use to control the concentration of suspended particles in the air? And how to achieve the control effect step by step? This is inseparable from the air filter we are going to talk about today.
In purification air-conditioning projects, the air mostly passes through the coarse (primary) efficiency filter and medium-efficiency filter in the combined air-conditioning unit, and then is sent to the clean room through the high-efficiency filter at the end of the ventilation duct. Through these three levels of filtration, the air is Most particles larger than 0.5um can be filtered out.
For clean rooms with high cleanliness requirements, sub-high-efficiency filters are sometimes installed in combined air-conditioning units to meet the requirements for higher filtration accuracy.
So how does the air filter filter?
Characteristics of air filters: surface speed, filtration speed, efficiency, transmittance, resistance and dust holding capacity, etc.
The so-called surface speed refers to the speed of air flow (m/s) on the filter section, which reflects the ability of the filter material to pass the air flow. Usually determined by the discriminant: u=Q/(Fx3600).
Where u is the filter surface velocity (m/s); Q is the air volume passing through the filter (m³/s); F is the filter cross-sectional area (㎡).
The filtration speed refers to the speed of air flow over the area of the filter material (cm/s), which reflects the ability of the filter material to pass the air flow. Usually determined by the discriminant: V=0.028Q/f.
Where V is the filter velocity (cm/s); f is the net area of the filter material (㎡). This feature is an important indicator for judging filters. The filtration speed of high-efficiency and ultra-high-efficiency filters we often say is generally 2-3cm/s, and the filtration speed of sub-high-efficiency filters is 5-7cm/s.
----------------------------The efficiency of the filter is divided into weight efficiency and counting efficiency----------------------------
When the dust concentration of the filtered gas is based on the gravimetric concentration, it is the gravimetric efficiency; when the dust concentration of the filtered gas is expressed in the counting concentration, it is the counting efficiency.
The common expression of counting efficiency is: n=(N1-N2)/N1=1-N1/N2.
Among them: N1 and N2 are the dust particle concentration (particles/L) in the airflow at the inlet and outlet of the filter.
What corresponds to filtration efficiency is: transmittance.
The expression is: K=N2/N1=(1-n)x100%.
K is the transmittance of the filter. As the name suggests, the transmittance refers to the rate of dust particles passing through the filter.
----------------------------filter resistance----------------------------
The filter has a resistance value. In actual projects, we will check the resistance value of the filter to determine whether the filter should be replaced. It can be said that this is a very important parameter basis.
The resistance of the filter is composed of the resistance of the filter material and the resistance of the filter structure. The usual expression is:
P=P1+P2=CV(mth power)
P1=AV
P2=Bu(nth power)
Among them, P1 is the resistance of the filter material, △P2 is the structural resistance of the filter, V is the filtration speed, u is the surface speed, P is the total resistance of the filter; A, B, C, n, and m are coefficients.
From this, we can draw a conclusion that the resistance of the filter material is proportional to the square power of the filtration speed, and the total resistance is exponentially related to the filtration speed.
----------------------------Dust holding capacity----------------------------
Normally, we take the weight of dust deposited on the filter when the running filter reaches its final resistance (mostly 2 times the initial resistance) as the dust holding capacity of the filter.
In actual projects, the selection of the final resistance of the filter must be determined based on the nature of the project and a technical and economic comparison. For the same Purification (Clean) Air Conditioning Engineering Air Filter, if the selected final resistance is high, the dust holding capacity will be large. If the same filter has different sizes, the dust holding capacity will also be different.
In actual engineering design, it is important to consider how to correctly select the final resistance of the filter.
Normally, when designers design, the recommended values for the initial and final resistance of the filter are:
Coarse filter (G4): initial resistance ≤50pa, final resistance ≤100pa.
Medium efficiency filter (F8): initial resistance ≤120pa, final resistance ≤250pa.
High efficiency filter (H14): initial resistance ≤220pa, final resistance ≤450pa.
Service life of the air filter: The time it takes for the filter to reach its rated dust holding capacity is the service life of the filter.
When the filter reaches the rated dust holding capacity, non-woven coarse-efficiency filters and medium-efficiency filters can be cleaned (except for biosafety experiments). They can be used again without damage after drying. High-efficiency filters, sub-high-efficiency filters are not acceptable and must be replaced.
The expression of service life is: T=P/(N1x10(-3rd power)Qtn)
Among them, T is the service life of the filter (d); P is the dust holding capacity of the filter (g); N1 is the dust concentration of the air in front of the filter (mg/m³); Q is the air volume of the filter (m³/h) ;t is the working time of the filter in one day (h); n is the gravimetric efficiency of the filter.
In actual projects, due to the uncertainty and non-uniformity of many parameters, it is difficult to use filter service life to measure filter replacement and cleaning time. Therefore, a filter pressure difference detection device is generally used to detect whether the filter resistance value has reached the designed final resistance, so as to determine the basis for filter replacement and cleaning.
In addition, when the air volume in the clean room decreases, the cleanliness decreases, and the concentration of viable bacteria exceeds the standard, the coarse-efficiency and medium-efficiency filters should be cleaned in time and the high-efficiency filter should be replaced.
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