The main performance indicators of the filter are filter accuracy, pressure drop characteristics, scale capacity, in addition to the working pressure and temperature and other parameters.
1. Filtration accuracy
Filtration accuracy refers to the ability of filters to remove particulate pollutants of different sizes, which is evaluated by absolute filtration accuracy, filtration ratio and filtration efficiency. The filtration accuracy can be divided into four grades: coarse (d ≤ 0.1mm), ordinary (d ≤ 0.01mm), fine (d ≤ 0.005mm) and superfine (d ≤ 0.001mm).
(1) Absolute filtering accuracy refers to the size of the hardest contaminated particles that can pass through the filter, expressed in microns. It can be determined by experimental method.
(2) Filtration ratio (β value) refers to the ratio of the number of pollutant particles Nu per unit volume larger than a given size xin the upstream oil of the filter to the number of pollutant particles Nd per unit volume larger than the same size in the downstream oil.That is to say, for a contaminated particle of a certain size x, the expression of its filtration ratio βx is as follows：
βx= 75, most of the contaminated particles larger than x are filtered out, so the value of x can be considered as the absolute filtering accuracy of the filter. The filtration ratio has been adopted by the International Organization for Standardization (ISO) as a performance index for evaluating the filtration accuracy of filters.
(3) Filtration efficiency Ec reflects the ability of filters to remove contaminated particles from oil, which can be expressed by the following formula
The relationship between Ec and β is shown in the following table:
The relationship between filtration efficiency Ec and filtration ratio β
filtration ratio β
filtration efficiency Ec（%）
2. Pressure drop characteristics
The filter in the hydraulic circuit is a kind of liquid resistance to the oil, so the pressure drop is inevitable when the oil passes through. Generally speaking, when the size of the filter element and the oil flow rate are fixed, the higher the filtration accuracy of the filter element, the greater the pressure drop. When the flow rate is fixed, the larger the effective filter area of the filter element or the smaller the viscosity of the oil, the smaller the pressure drop.
The maximum pressure drop allowed by the filter element should be based on the principle that the filter element will not be destroyed structurally. Usually, the initial pressure drop of filters for aviation and naval vessels should not exceed 0.25 MPa; the pressure drop of filters for machinery should not exceed 0.08 MPa~0.15 MPa.
The problem of contaminants retained by filters before pressure drop is greater than the prescribed limit is called contamination capacity, expressed by weight (g).
The larger the scale capacity of the filter, the longer its life, so it is an important indicator reflecting the life of the filter. The larger the effective filter area is, the larger the scale capacity will be.
The design area of the filter is
In the formula, q is the rated flow rate（L/min） of the filter.；η is the dynamic viscosity (Pa·s) of oil filter ；Δp is pressure drop（MPa）；α is the oil permeability per unit area （L/cm2） of filter element，determined by experiment。When oil temperature is 20℃，for special strainer α=0.003~0.006 ；Paper filter element α=0.035；Gap filter element α=10；General mesh filter element α=2。
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