Filter elements for fog purification towers” 

Critical Parameters for Selection of Filter Elements for Mist Treatment Towers

Choosing the right onefilter element for mist purification towerdetermines not only the efficiency of aerosol capture, but also the economic feasibility of the entire ventilation system in the long term. In our practice of working with metallurgical plants and galvanizing shops, we have repeatedly encountered a situation where customers saved on the initial purchase of demisters, choosing cheap analogues without EAC or GOST certificates, which led to the failure of fans and compressors after 6-8 months of operation. The real cost of owning equipment does not depend on the purchase price, but on the frequency of replacements, pressure drops and energy consumption. If you are looking for a reliable solution for liquid-gas separation, you need to understand the differences between wire mesh, fiber mat and plate structures, and their impact on the flow resistance of the system.

This article was written by engineers who have personally tested various types of mist eliminators in aggressive environments with temperatures up to 150°C and acidic vapor concentrations above standard values. We will not use general phrases about “high quality”. Instead, we will provide specific data on separation efficiency (up to 99.8% for particles as small as 3 microns), permissible gas flow rates and actual material service life depending on the chemical composition of the technological process. Correctly selectedfilter elementis able to reduce emissions of harmful substances below the maximum permissible concentrations (MPC) established by Rosprirodnadzor, and avoid fines that are many times greater than the cost of the equipment itself.

Technological features and classification of materials

The main task of anyfilter element for mist purification tower— coalescence, that is, the process of merging small drops of liquid into larger ones under the influence of inertial forces when passing through a porous structure. The efficiency of this process directly depends on the geometry of the channels, the specific surface area of ​​the material and the wettability of the fibers or wire. On the industrial equipment market in Russia and the CIS countries, three main types of structures are most common: wire mesh bags, fiber mats made of synthetic or glass fibers, and corrugated plate blocks. Each of them has its own niche of application, and an attempt at universalization here often leads to technical failures.

Wire meshes, usually made from stainless steel AISI 304, 316L or special alloys such as Hastelloy, are the standard for high temperatures and pressures. In our latest project at a petrochemical plant, we used braided mesh with a wire diameter of 0.2 mm and a mesh size of 0.6 mm. This configuration ensured stable operation at a gas speed of 2.5 m/s without the phenomenon of “flooding”, which is typical for denser structures. However, it is important to remember that metal elements are sensitive to mechanical vibrations; one of our clients experienced package destruction due to the resonant frequencies of the fan, which could have been avoided by correctly calculating the rigidity of the support grid.

Fiber mats made from polypropylene, polyester fibers or fiberglass demonstrate superior efficiency in trapping submicron particles (less than 1 micron) that metal meshes often miss. These materials are ideal for sulfuric acid production and pickling processes where dense mists are generated. The key parameter here is the packing density, measured in kg/m³. Too high a density (>180 kg/m³) creates excessive pneumatic resistance, forcing the fan to work at maximum loads and consume excess electricity. Density too low (<120 kg/m³) leads to fog breakthrough. The optimal range, confirmed by our tests for most chemical scrubbers, is 140-160 kg/m³.

Plate elements, often called zigzag type drift eliminators, operate on the principle of changing the direction of flow. They are less effective for very small drops, but are irreplaceable where there is a risk of clogging the pores with solid particles, since they are easier to rinse with water under pressure without dismantling. The choice between these types should be based on the particle size distribution of the aerosol. If the flow is dominated by droplets larger than 10 microns, the plates will be more economical. For fine mists formed during intense bubbling or spraying, deep fibrous or mesh layers are indispensable.

Calculation of hydraulic resistance and selection of standard size

A mistake many designers make is to treat the filter as a static element. Actuallyfilter element for mist purification toweris a dynamic system whose parameters change during the accumulation of liquid. Hydraulic resistance (pressure drop, ΔP) is a critical parameter that influences the choice of fan power and, therefore, the operating costs of the plant. The dependence of the pressure drop on the gas velocity is nonlinear: with an increase in the flow rate by 2 times, the resistance can increase by 4-5 times. Therefore, accurate calculation of the operating point is extremely important.

When designing, we always allow for a margin of cross-sectional area. The standard recommendation is to calculate the gas velocity in the free section of the apparatus in the range of 1.5–3.0 m/s for mesh elements and 2.0–4.0 m/s for plate elements. Exceeding the upper limit of this range leads to secondary entrainment of liquid: drops that have already settled on the element are carried off by the flow and again enter the gas line. This phenomenon is called “secondary entrainment” and negates the entire work of the scrubber. In one case at a fertilizer plant, ignoring this factor led to corrosion of the pipelines after the scrubber, as a concentrated acid solution began to circulate where clean gas should have flowed.

To select the correct size, you need to know not only the gas volume flow (m³/h), but also its temperature, pressure and viscosity. The density of a gas changes with temperature, which directly affects the mass flow rate. For example, when a gas is heated from 20°C to 80°C, its density drops by approximately 20%, which requires recalculation of the speed in the working section. Our engineers use the following formula to preliminarily estimate the required filtration area: S = Q / (3600 * Vopt), where Q is the flow rate in m³/h and Vopt is the optimal speed in m/s. The resulting value is rounded up taking into account a safety factor of 1.15-1.20.

The height of the filter material layer should also be taken into account. To achieve separation efficiency of 99% or higher, the height of the wire mesh stack should usually be at least 100-150 mm. Increasing the height improves the quality of cleaning, but proportionally increases the resistance. There is a limit after which further increase in height becomes impractical. In our calculations, we rarely use layers thicker than 200 mm for single-stage systems, preferring to cascade installation of elements with different densities if ultra-deep cleaning is required.

Chemical resistance and compliance with GOST and EAC standards

An aggressive environment is the main enemy of any gas cleaning equipment. Materialfilter elementmust be absolutely inert to the components of the process gas. The most common mistake is using standard AISI 304 stainless steel in environments containing chlorides or strong oxidizing agents at elevated temperatures. This inevitably leads to pitting corrosion and rapid destruction of the structure. For such conditions, we strongly recommend the use of high molybdenum alloys (AISI 316L, 317L) or titanium meshes, despite their higher initial cost.

Polymer materials such as polypropylene (PP) and polyvinylidene fluoride (PVDF) cope well with acids and alkalis at temperatures up to 90-100°C (for PP) and up to 140°C (for PVDF). However, they have limitations in mechanical strength and can deform under their own weight on large tower diameters without proper support. Teflon-impregnated fiberglass (PTFE) is a versatile solution for a wide range of chemicals and temperatures up to 260°C, but is fragile and requires care during installation. The choice of material should always be confirmed by the chemical compatibility tables of the raw material manufacturer.

On the Russian market, the presence of certificates of conformity is a mandatory requirement for the legal operation of equipment. Any suppliedfilter element for mist purification towermust have a declaration of compliance with the Technical Regulations of the Customs Union (TR CU 010/2011 “On the safety of machines and mechanisms”, TR CU 032/2013 “On the safety of equipment operating under excess pressure”). The absence of the EAC marking on the product or in the accompanying documentation gives the supervisory authorities the right to stop the enterprise and impose a fine. In addition, specific industries may require safety data sheets and sanitary-epidemiological reports.

We recommend requesting from the supplier corrosion test reports carried out in accredited laboratories. Often suppliers claim “high durability” without documentary evidence, which in real conditions results in accidents. A reliable manufacturer is always ready to provide samples of material for independent examination before concluding a large contract. This is standard practice in our company, which eliminates the risk of declared characteristics not meeting actual operating conditions.

Typical installation errors and methods for diagnosing faults

Even the highest quality filter element can fail prematurely due to installation errors. The most common problem is uneven distribution of gas flow across the cross section of the tower. If gas enters the filtration zone unevenly (for example, due to the absence of straightening grids or poor inlet pipe design), local zones of excess velocity occur. In these areas, fog breaks through and material is quickly carried away, while the rest of the filter operates ineffectively. The solution to this problem lies in the aerodynamics of the device, and not in the filter itself.

The second critical point is the lack of drainage. The filter catches the liquid, which should flow down freely under the influence of gravity. If the housing design does not provide sufficient space under the filter bag or the drainage holes are clogged, liquid accumulates inside the pores. This leads to a sharp increase in resistance and a “barrage” effect, when the filter turns into a solid liquid plug that stops the gas flow. When installing, always check the angle of inclination of the support grids and the diameter of the drain pipes; they must correspond to the expected volume of condensate.

Diagnosis of the condition of the filter during operation is possible using indirect signs. The first alarm signal is an increase in the pressure drop on the differential pressure gauge. If ΔP exceeds the calculated value by 30-40%, this may indicate contamination of the pores with solid particles or salinization with salt crystals. In such cases, flushing is necessary. The second sign is the appearance of visible fog or droplets at the outlet of the pipe. This indicates either mechanical damage to the element (rupture of the mesh, formation of fistulas in the mat), or the exceeding of the design performance of the device.

In our practice, there was a case when a client complained about the constant carryover of acid, although the filter was recently installed. During on-site diagnostics, it turned out that the installation crew damaged the sealing gasket around the filter perimeter when tightening the mounting bolts. The gas simply bypassed the filter medium through the slot, following the path of least resistance. This emphasizes the importance of quality control of installation work and the use of high-quality sealing materials that are resistant to the working environment. Regular visual inspection (through manholes) and instrumental monitoring of parameters should be included in the maintenance schedule.

Economic justification and payback period

When estimating project costs, many buyers focus solely on unit price. This approach is wrong for industrial equipment. Costfilter element for mist purification towerconstitutes only a small fraction of the total costs of eliminating the consequences of its failure. Production downtime due to a clogged filter, repair of corroded fans, payment of fines for exceeding maximum permissible concentrations and the cost of hazardous waste disposal are the real expense items. Investment in quality material with a long service life pays off through reduced operating costs (OPEX).

Let's look at an example of calculating savings. Replace a cheap polypropylene mat every 6 months versus a quality PTFE material that lasts 3 years. At the price of a cheap mat X and an expensive one 3X, in 3 years you will buy 6 cheap mats (6X expense) versus one expensive one (3X). Savings on purchases will be 50%. Add to this the cost of the replacement team (downtime, labor costs, access to the hazardous area), and the benefits of a quality solution become obvious. In addition, the stable low resistance of a quality filter reduces fan power consumption by 5-10%, resulting in additional annual savings.

An important factor is the possibility of regeneration. Metal and plate elements can often be washed in place or removed for ultrasonic cleaning, restoring their properties almost completely. Fiber mats, as a rule, cannot be restored when deeply contaminated with solid particles and require complete replacement. When choosing a technology, it is necessary to evaluate the composition of the pollution: if it is pure condensate without suspensions, the fiber filter will last a long time. If the gas contains dust or reaction products that can crystallize, preference should be given to self-cleaning designs.

We offer our clients to audit existing gas cleaning systems. It often turns out that the wrong type of filter is the cause of chronic environmental problems at the enterprise. Replacing one unit can solve the emissions problem once and for all. Our company is ready to provide a technical and commercial proposal with a payback period (ROI) calculation for your specific case, taking into account current electricity tariffs and Russian legislation.

Frequently Asked Questions

How often do filter elements in the cleaning tower need to be changed?

The service life depends on the type of material and gas contamination. Metal mesh lasts 5-10 years, subject to periodic washing. Fiber mats made of polypropylene in aggressive environments require replacement every 12-24 months. The main indicator of the need for replacement is the inability to restore the original hydraulic resistance after washing or visible destruction of the material structure. Don't wait for complete failure, plan for replacement during planned production shutdowns.

Can filters be washed with pressure water?

Yes, but with restrictions. Plate and metal mesh elements withstand pressure washing with water up to 50-80 bar to remove salt deposits and dirt. Fiber mats cannot be washed under high pressure - this will destroy their structure and worsen their filtering properties. For them, only gentle rinsing with a weak neutralizer solution or a soft low-pressure wash is allowed. Always check the material manufacturer's instructions before starting work.

What material to choose for sulfuric acid with a concentration of 93%?

For concentrated sulfuric acid at temperatures up to 80-90°C, polypropylene (PP) or PVDF elements are the optimal choice. Stainless steel corrodes quickly in such an environment. If the temperature exceeds 100°C, PTFE elements or special graphite materials must be used. It is also important to consider the presence of impurities: if the acid contains oxidizing agents, the choice is narrowed to PTFE and tantalum. Consultation with your process technologist is required before ordering.

Does gas humidity affect filter performance?

Droplet eliminator filters are designed specifically for operation in wet gases saturated with liquid vapors. Dry gas does not require the installation of such elements, since there is nothing to capture. However, if the gas is supersaturated and intense condensation occurs directly in the filter body, this can lead to its flooding. It is important that the filter operates in the mode of capturing droplets that have already formed, and not in the mode of steam condensation. To prevent this, gas pre-coolers are sometimes installed in front of the treatment tower.

How to check the tightness of the filter installation?

The tightness test is carried out by visual inspection of the places where the filter adheres to the tower body during operation (through inspection windows) or using a thermal imager (if there is a temperature difference). A more reliable method is to measure the pollutant concentration immediately downstream of the filter and compare it with calculated values. If the actual emission is significantly higher than the calculated one if the filter cloth itself is in good condition, it means that there is a flow of gas past the filter through leaks in the seat. Filling the gaps with sealing materials solves the problem.

Conclusion and recommendations for choosing a supplier

To summarize, we can say that competent selectionfilter element for mist purification toweris a complex engineering problem that requires taking into account many interrelated factors: from the chemical composition of the gas to the aerodynamics of the device. There is no universal solution suitable for all cases. Attempts to save money at the design or procurement stage inevitably lead to significant losses during operation. Trust the choice of equipment to professionals who have experience in implementing similar projects in your industry.

Selecting a reliable partner is critical to ensuring the longevity of the system.Wuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.specializes in the development and production of high-tech solutions for the oil and gas and chemical industries. Although the company's primary focus is heat exchange equipment (titanium shell-and-tubes, ASME high-pressure heat exchangers, 316 stainless steel corrugated tube bundles, C46400 marine brass and N06625 nickel alloys), our in-depth experience with corrosion-resistant materials makes us experts in selecting components for aggressive environments. We manufacture carbon steel, stainless steel, alloy steel, titanium, copper and nickel alloy products certified to stringent international PED and ASME standards. This expertise in metallurgy and material strength allows us to guarantee the highest quality components and custom solutions that can be integrated into gas cleaning systems, ensuring their resistance to high pressures, temperatures and chemical influences.

Do not risk the environmental safety of your enterprise and the health of your employees. Contact our technicians today for a free consultation and equipment estimate. We will help you find the optimal price-reliability ratio that will ensure the smooth operation of your production for years to come. Go to sectioncatalog of filter elementsto get acquainted with the detailed characteristics of our solutions, or fill out the application form on the website to contact the manager.