Filter is a common equipment in industrial production process, its role is to filter the medium to block the impurities in a certain particle size range of materials through valves, meters, equipment, etc., to improve product cleanliness, meet the solid content requirements of the product or downstream process requirements, but also to avoid debris blocking mechanism action, wear clearance surface, so as to ensure the safe operation of these equipment.
The following is a simple description of the filter design and selection of general principles and basic methods.
First, the filter design and selection should be clear before the working conditions
As with all machinery, the design and selection must be clear before the working conditions used by the equipment, the process position in the relevant process, as far as filtration is concerned, the following points must be clear:
1) The type of equipment upstream and downstream of the filter, especially to clarify the requirements of the equipment or process downstream of the filter on the cleanliness of the filtered medium;
2) The type, physical properties and chemical properties of the filtered medium, in particular the viscosity of the medium and the category of possible impurities in it;
3) The place and environment of the filter, the medium temperature, and the working pressure;
4) Filter process connection standard and form
Second, the general method of filter design and selection
1 Determine the inlet and outlet diameters
The inlet and outlet diameter of the filter should not be smaller than the inlet diameter of the equipment protected at the back end, and generally should be the same as the equipment diameter.
2 Determine the nominal pressure
In principle, the pressure level of the filter should be determined according to the highest possible pressure of the process pipeline where the filter is located, but the pressure limit of the system has been fully considered in the design and selection of the equipment protected by the filter, so the pressure level of the filter can be regarded as the same as the pressure of the protected equipment. For designers, the calculation and verification of equipment pressure is essential, here it is necessary to give the calculation method of the maximum pressure of the filter: First of all, it should be clear that the mechanical strength of the metal material and its temperature is related, with the increase of temperature, the pressure capacity of the metal material will decline, the maximum pressure and temperature of the safe operation of the filter can be calculated as follows:
Pmax= Pn-K△T
Pmax- The maximum working pressure that the filter can withstand MPa;
Pn- Nominal pressure of the filter MPa;
T- The maximum operating temperature of the filter is ℃;
△T- temperature difference ℃;
K- Strength decay coefficient MPa/℃; The K value can be selected according to the following empirical values:
a) When the temperature is less than 200℃, K=0;
b) When it is a cast iron filter, K=0-0.004 when the operating temperature is 200 ~ 300℃;
c) When it is a carbon steel filter, K=0.0016-0.008 when the operating temperature is 200 ~ 400℃;
d) When it is an alloy steel filter, K=0.0006-0.006 when the operating temperature is 200 ~ 400°C;
e) When it is a stainless steel filter, K=0.00018-0.006 when the operating temperature is 200 ~ 400°C;
The K value can be determined by the internal difference method between 200-400℃, but the K value changes very little in this interval, so the upper limit of K value is usually taken when the temperature is high, and the lower limit is taken when the temperature is low.
3. Determination of filtration area
The calculation of the filter area is basically not calculated by formula, and the selection is mainly based on experimental data.
According to the basic characteristics of filter flow, the relevant factors affecting the flow rate are effectively analyzed, and the basic principles of filter selection are provided for system designers.
For any particular filter, as the flow increases or decreases, its own resistance will be expressed in the form of the pressure difference between the filter inlet and outlet, and the pressure difference of the filter will increase with the increase of the flow rate, showing the change law of a quadratic curve. In practical applications, due to the strength and energy loss of the filter element, the pressure difference is generally not allowed to be too high, so the conventional design of the product pressure-flow curve, the user can choose a variety of sizes of filters according to the pressure difference to meet the flow requirements.
There are three main factors that affect the filter differential pressure-flow characteristics:
1) Filter area The increase of filter area will increase the flow rate through proportional.
2) Filtration accuracy The impact of filtration accuracy on the flow through the filter is considerable. Due to the influence of the filter structure, filter element production process and other factors, there is no fixed proportional change relationship between the accuracy and the flow, but in general, the higher the filtration accuracy, the smaller the flow through.
3) Medium viscosity changes in the viscosity of the medium will affect the flow rate, because the pressure differential flow characteristic curve provided by the filter manufacturer is made according to the same standard viscosity, so when selecting the filter in this case, it is necessary to carefully consider the impact of viscosity on the flow rate.
Therefore, there are three major curves of filtration: flow differential pressure curve (δp-q), particle size and filtration ratio curve (μ-β), time and pressure drop curve (t-δp). Therefore, the calculation of filtration area should be based on these three curves, the most important of which is the flow differential pressure curve. At present, the most authoritative test method is ISO4572, which has passed the test standard many times.
