How is the pressure drop of a filter calculated? ? ? ?

It depends on the type of filter selected, its density, as well as the volume and content of impurities in the medium being transported.

The relevant information can be found in that section of the book

I checked and found an answer; I’m not sure if it’s correct. It says that filtration can be divided into three types: 1—surface filtration, 2—deep filtration, 3—(I forgot). In terms of overall filtration, these three types don’t occur separately; either two of them occur, or all three. Taking the owner’s filter-type filter as an example: it was initially used for surface filtration, but the impurities in the medium quickly formed a filter cake on the filtering elements; at this point, the filtration process turned into deep filtration, and the resistance also increased... (I can’t remember what happened after that). Due to the dynamic changes in the filtration process, along with factors such as the effective area of the filtering elements, the viscosity of the medium, and the impedance constants of filters of different specifications, I think accurate calculation is very complex, or even somewhat impossible. Generally, in such cases, requirements regarding the pressure loss caused by the filter arise only when the pump is ordered first and it is then determined that the net positive suction head is insufficient. As an estimate, it should be calculated in three parts: the resistance loss caused by inlet and outlet flow, and the resistance loss caused by the filtering elements. Even when calculated in this way, it can only be an estimation of the resistance loss assuming no impurities are present in the medium. When to replace the filter element also depends on the process and the pressure gauge. Can everyone calculate the blocking area of the filter element at which the pressure difference between the inlet and outlet gauges is exactly the pressure difference required by the process to trigger the replacement of the filter element?

I work in the field of filtration; it depends on what type of filter media you use. Moreover, for high-quality filtration products, like those I manufacture, the product instructions include calculations that can serve as a reference for companies.

The pressure drop across the filter screen is dynamic, rising as the amount of contaminants on it increases. It is generally not possible to calculate accurately; empirical values are often used instead, but these values also vary depending on factors such as the model and quality of the filter screen. . . . . . .

Answering your question: ‘At what clogging area of the filter element does the pressure difference between the inlet and outlet gauges exactly equal the pressure difference required by the process to trigger the replacement of the filter element?’ As you yourself said, calculating this is quite complex. When a part of the filtering element is partially blocked, other parts must already have a certain amount of filter cake on them; exactly how much depends entirely on factors such as the solid content of the fluid, particle size, and flow rate. Therefore, it is by no means easy to calculate this amount. Returning to your question, the pressure difference before and after this element is not solely caused by blockage within the element; an increase in pressure drop is also due to the filter cake in the unblocked areas. Moreover, the distribution of the blocked pore sizes also affects the pressure difference before and after. Therefore, it is quite difficult to obtain such a result through calculation. In actual production, the pressure difference before and after the filter screen can be monitored; when this pressure difference reaches a certain value, the filter screen is replaced to ensure the normal continuation of production. Through the summarization of practical production experience, approximate empirical rules regarding the pressure difference and the distribution of the filter cake can be established; these are what we refer to as empirical values. Going a step further, empirical formulas can be derived, but the scope of application of these empirical values and formulas depends on the range taken into consideration by the person who conducts the summarization.

Are there any very detailed reference books or literature? If so, empirical formulas would be ideal

Regarding filters, when selecting them, one needs to consider the type of filter and its density (bulk density)… For the process, only the allowable pressure drop needs to be taken into account… Generally, this pressure drop is very small and can be ignored in the context of the entire process pipeline and equipment system; there’s no need to delve too deeply into it.

Is the pressure drop very small? I don’t think so. But after doing many calculations, I found that the pressure drop is around 10,000 to 20,000 Pa, which is actually quite large

One or two ten thousand Pa is equivalent to 10–20 KPa; in terms of pressure drop, this corresponds to 0.1–0.2 bar (or kg/cm2), which is roughly equivalent to the pressure drop over a 50-meter straight pipe section. I think the pressure drop across the filter is generally around 0.05–0.1 bar, which is considered normal and acceptable. A specific analysis can be done based on your situation; I believe that 0.1–0.2 bar is also a range that can be accepted. The above is for reference only. . .

This pressure drop value is meant to serve as a reference for determining when the filter screen needs to be replaced or cleaned; it is highly dependent on parameters such as the mesh size of the filter screen and the structure of the filter, rather than on the maximum pressure drop permitted by the manufacturing process.