Mist Filters Research Articles

The Behavior of Phobic and Philip Oil Mist Filters Under High Pressure

The Behavior of Phobic and Philip Oil Mist Filters Under High Pressure

Influence of deposited liquid structures on the separation efficiency of thin oleophilic fiber layers of mist filters

The presence of oil mists constitutes a significant hazard to both, human health and the environment. In industrial applications, the effective separation of these mists typically relies on the utilization of multi-layer nonwoven coalescence filters. The underlying mechanisms governing droplet deposition, liquid transport within the filter matrix, and the evolution of the overall differential pressure have been extensively documented in the existing literature. However, limited and contradictory information is currently available regarding the impact of deposited liquid structures under varying saturation levels on the separation efficiency of coalescence filters.In this study, the influence of an increased liquid saturation at different filtration velocities on the fractional and total separation efficiency of thin porous non-woven fiber layers comparable to the first fiber layers of the first filter layer of commercially utilized coalescence filters are evaluated. As the oil load increases to a saturation of S≤ 0.4, the total separation efficiency (number-based) decreases, for all investigated filtration velocities. In addition, a shift of the most penetrating particle size (MPPS) to smaller droplet sizes and a decrease of the MPPS efficiency is observed. This decrease is attributed to a higher interstitial velocity resulting from the accumulation of liquid within the filter matrix, leading to a lower fractional separation efficiency in the diffusion regime. However, at saturation levels S≥ 0.6, the separation efficiency (meaning both, fractional and total separation efficiency) begins to increase due to the formation of partial oil films, leading to a blockage of void space. These films significantly enhance inertial separation. At the highest investigated saturations (S=0.8), the highest separation efficiencies due to a nearly complete coverage of the filter layers by an oil film are observed. The film formation is confirmed through backlit images of the oil-loaded filter material. Furthermore, an increase of the MPPS efficiency for a saturation of S=0.8 with increasing filtration velocity due to increased diffusional separation is recorded, which cannot yet conclusively answered by the authors. Additionally, the differential pressure across the fiber sheets is presented, revealing a more pronounced influence of the oil film on the differential pressure at increasing filtration velocities.

Investigation of oil mist filtration performance of surface modified coalescence filters with asymmetric wettability

Fibrous coalescence filters are widely used to remove oil mists from air and gas streams in industrial applications. The wettability of filter media significantly affects the filtration performance. Although some efforts have been made to improve the filtration performance based on the modification of filter media wettability by surface treatment method, of which the influence on liquid transport mechanism in the filters and the filtration performance is still needed to elucidate. In this paper, oleophilic filter media were treated by fluorocarbon copolymer solution using a spraying method. The effects of the treated area ratio, spraying time and combinations of patterned filter media on the filtration performance were investigated experimentally. The results show that the increase of the treated area ratio contributes to a variation on the wettability from oleophilic to oleophobic, resulting in a decrease in the jump pressure drop and an increase in the filtration efficiency. The treated filter medium under the optimal spraying time has the highest quality factor, which is 2 ∼ 3 times the untreated one. For the dual-layer patterned filters with alternative oleophobic and oleophilic strips, when the number of oleophobic strips patterned in the first layer filter medium is the same, the pressure drop and filtration efficiency of the overlapped arrangement are lower than those of the staggered arrangement. In all configurations, the highest quality factor is found in the dual-layer filter with the overlapped arrangement and four oleophobic strips patterned in the first layer, resulting from the lowest steady pressure drop and relatively high filtration efficiency.

Experimental analysis of droplet coalescence and transport mechanisms on a single vertical fiber

In mist filtration, fiber-based coalescers are an established form of filtering droplets contained in mist. The filtration process can be divided into different process steps, describing the impact of the droplets on fibers, the formation of fluid structures and the liquid transport. In order to investigate mechanisms inside depth filters on a microscopic level, investigations are often reduced to single fibers. In this work, the coalescence and transport mechanisms of axially distributed water droplets on a vertical fiber, subjected to gravity, are reported. This is done with the latest high-speed camera technology commercially available. Automated tracking of droplets is used for a frame-by-frame investigation of droplet position, size, and oscillation. Coalescence mechanisms describe the process of fluid formation. The first observed coalescence mechanism is identified by the coalescence of droplets contained in mist with an adhering droplet at the fiber. The second coalescence mechanism describes the coalescence of two closely spaced sessile droplets on a fiber. As a result, the newly formed droplet oscillates and can begin to drain. Furthermore, the coalescence process of a draining and sessile droplet is reported. Both the draining droplet and the second droplet involved in the coalescence event can be subject to oscillation. The given temporal high-resolution information about the droplet position and deformation improves the understanding of droplet coalescence on fibers and by that also its influence on transport processes and therefore droplet drainage. Observed transport mechanisms which take part in the drainage process are gravitational draining, droplet bouncing, and droplet sweeping.

Modeling and Simulation of Coalescence in the Context of Oil Mist Filtration

Today, aerosols are generated in many industrial processes like machining or in pneumatic compressors. The harmfulness of these droplets often lies in the liquid itself, as it can be toxic or cancerous while exhaled natural aerosols can carry pathogens. For health protection, the filtration of these aerosols is an important task. The wetting properties of the fibers have a large influence on the filtration efficiency and coalescence of droplets on the fibers. Oleophilic fibers are better in capturing the oil particles, while drainage increases using oleophobic fibers. For improved efficiency and drainage properties, a mixed-wet fiber system could therefore be advantageous. Describing droplet capture and coalescence in filter-scale simulations is computationally extremely demanding. We present a new approach for simulating oil mist deposition and droplet coalescence, specifically accounting for the wettability of the fibers, especially for mixed-wet filter media. The aim is to gain a more realistic pressure loss of a fibrous filter structure which takes the presence of the filtered phase into account as well as the coalescence of the liquid phase on the fibers depending on the wettability and the fluid parameters. Our approach is to model the shape of deposited droplets via weighted distance maps, according to the contact angles. Starting from a sphere, the shape is fitted on the solid surface such that the contact angle is met. For validation purposes, volume-of-fluid-based simulations were set up using the same droplet-fiber system as a reference to compare the droplet shape and pressure loss in the presence of oil droplets on the fiber surface.

An effect of fibrous filters modification with MTMS aerogel structure on oil mist filtration dynamics

Various modifications are utilized to improve the oil mist separation efficiency of non-woven filters. Our previous work proposed an oleophilisation method involving coating polypropylene filter fibres with porous organosilica aerogel based on methyltrimetoxysilane (MTMS). The filters with fibers with collector area enhanced by this modification exhibited even three times higher initial separation efficiency for oil mists than the native polymer filters. In order to determine the effect of the amount of aerogel layer deposited on polypropylene fibers it is required to investigate the dynamics of filtration of these filters modified with the sol-gel technique over the whole time of use of the filters. This article describes the aerogel deposition influence on the total efficiency of filters over time, efficiency changes for the most penetrating droplets (MPPS), the evolution of pressure drop and oil transport across the material. The results show that the thicker aerogel layer on fibres results in higher initial separation efficiency and pressure drop, simultaneously elongating an initial and transitional loading stage. Filter performance in steady-state seems unaffected by the amount of deposited aerogel. Also, modification with aerogel makes the filter fibres more oleophilic, which translates into changes in filter behaviour described via Jump&Channel theory.

Investigation of the local oil distribution on oleophilic mist filters applying X-ray micro-computed tomography

Nonwoven coalescence filter media are often used in real-world applications to separate droplet aerosols from a gas stream. While mechanisms like droplet deposition, liquid transport and the evolution of the pressure drop are well understood, information on separated liquid structures formed during filtration is scarce. However, this information is important to understand the initial state of the coalescence filtration process, more specifically, the transition from individual deposited droplets to larger oil structures and subsequently the effect of these oil structures on the separation efficiency.In this work, deposited oil structures in the micrometer region of an oleophilic filter medium of different oil loading stages (saturations) are presented for the first time by utilizing micro–computed tomography (μ-CT) and several post-processing steps. The local (determined via μ-CT) and global (determined gravimetrically) porosities as well as local and global saturations are compared and evaluated. While local (Φlocal=0.9575) and global (Φglobal=0.9588) porosities are in good accordance, significant deviations in the saturation are observed. The local saturation fluctuates significantly at the highest investigated oil loading stage. Especially at high global saturations, local areas are completely saturated with oil. Other areas are weakly saturated, which are particularly relevant for the gas flow to pass through the filter media. Furthermore, a novel routine to distinguish between different coalesced oil objects (e.g. oil droplets or oil sails) is developed and presented. It is shown, that the majority of the deposited oil (>80%) is stored in oil sails between adjacent fibers. Oil droplets surrounding fibers are found to be growing in size with increasing oil loading time. In addition, the volume fraction of the deposited oil in the direction of filter thickness shows, that most of the oil is deposited on the upstream side at the first 80% of the filter.

Modeling oil–mist filtration through coalescence filter media in the presence of gravity-induced flows

This paper jointly addresses numerical and physical challenges related to the modeling of coalescence filtration through vertical filter cartridges, put forth in the following papers by Starnoni, M. and C. Manes:• Starnoni, M., & Manes, C. (2021). A multiphase multicomponent flow and transport model for liquid aerosol filtration in coalescence fibrous filters. Separation and Purification Technology, 266, 118574.• Starnoni, M., & Manes, C. (2022). On the interplay between pressure and gravitational forces in coalescence filters. Journal of Aerosol Science, 105953.In particular, it provides the multidimensional extension of the one-dimensional model presented in Starnoni and Manes (2021) to the general case where internal gravity-induced flows play an effective role. Key to the implementation is the decomposition of the moving oil volumes along the different directions proportionally to the dimensionless number SM, a new quantity introduced in Starnoni and Manes (2022) describing the interplay between the forces dictating the oil dynamics. To validate the implementation, an experimental setup available in the literature is replicated, showing very good agreement in terms of all relevant quantities, namely saturation, pressure drop and drainage volumes. A generalized ‘Network, Channel and Film’ model is also formulated, which extends the scope of application of the original Jump and Channel model by Kampa et al. (2014) to filters with coarser fibrous microstructure. In particular, the existence of an additional flow mechanism termed ‘network’ is postulated, which is characterized by a complex multidimensional oil patterns configuration spanning the entire filter’s height and evolving, in a macroscopic sense, along the direction identified by SM.

How straylight affects driving ability

AbstractWe investigated how an increase in straylight affects the driving capacity of young, healthy volunteers between the ages of 20 and 40 years (i.e., people without cataract) in various real‐life driving circumstances using a driving simulator. This simulator allowed us to assess driving behaviour in a controlled, repeatable environment without risk to life or property, while providing a large set of parameters (speed, brake performance, deceleration, collisions, standard deviation of the lateral lane position, (SDLP), headway distance, etc.). The simulator used a fixed‐base setup with a force‐feedback steering wheel, an instrumented dashboard, brake, and accelerator pedals and with a 135° field of view. Participants had to drive along a certain course with of 6 traffic situations (e.g., crossing pedestrian, obstacle on the way, etc…) while wearing their own spectacle correction, both in the presence of a glare source and without. Next this was repeated while wearing a Tiffen Black Pro Mist (BPM) filter in front of their eyes and a glare source. These filters approximate the optical characteristics of cataract fairly well, where BPM 1 mimics simulates early cataract (which often prompts people to stop driving at night) and BPM 2 simulates serious straylight hindrance. Straylight was measured with the van den Berg straylight meter (C‐Quant).The results showed that increased straylight significant alters driving behaviour, such as a decrease in mean, maximum and minimum speed. The detection time and reaction time to an obstacle on the road was significantly longer with increased straylight. Consequently, straylight is an important factor in traffic safety driving conditions, causing altered driving behaviour and increasing collision risk in certain traffic situations.

Oil Mist Exposure -HSE Hype or Health Hazard?

With rising costs, manufacturers are not making unnecessary investments. Craig Woodward, Filtermist divisional sales director (oil mist & industrial vacuums) says new oil mist filters must be a priority

MTMS-based aerogel structure deposition on polypropylene fibrous filter – Surface layer effect and distribution control for improvement of oil aerosol separation properties

Polypropylene non-woven material with fibres surface modified with methyltrimetoxysilane(MTMS)-based aerogel structure exhibits increased affinity to oil and highly developed collector area, which translates to enhanced oil mist filtration performance, crucial in automotive, commercial and home appliances. This article indicates the filtration improvement ability's dependence on the aerogel structure's deposition and morphology, as previous work raised some critical questions related to surface vs volume deposition's contribution to filtration properties. Modified and native materials were studied on a filtration test bench and via a systematic wettability investigation. The amount of aerogel structure deposit is correlated to the MTMS concentration, and its distribution relay on condensation kinetics, controlled by the catalyst addition rate during the sol-gel process. Though beneficial for separation efficiency improvement, the aerogel structure deposition in filter volume is accompanied by a surface foil layer formation that needs to be removed as it causes up to 200 Pa airflow resistance. The controlled aerogel structure deposition method gave satisfactory results, increasing separation efficiency for most penetrating droplets four times and increasing filter Quality Factor. Modified material is competitive in terms of separation efficiency and airflow resistance with those obtained in the other related works on the subject, giving a post-production method for oil aerosol filter enhancement.

Dynamic pore network model to predict residual saturation and pressure drop in mist oleo-phobic filters

A dynamic pore network modelling (DPNM) framework is used to evaluate pressure drop and residual saturation in oleo-phobic nonwoven media in mist filters. The advanced fully-resolved VOF simulations along with μ-CT measurements are also carried out to substantiate the validity of the DPNM simulations. The results of the study show that PNM could be a beneficial and reliable tool for predicting oil saturation and pressure drop during early-stage design of oleo-phobic filters with nonwoven media. Given the considerably higher efficiency of PNM models compared to CFD-based techniques, some parametric studies were also conducted. Our findings suggest that, for a given set of operating conditions, efficiency/performance can be improved by appropriate choice of filters with optimized properties (fibre diameter, packing density, contact angle, etc.) for which the overall oil saturation and pressure drop in the filter attain minimum, may offer a self-cleaning mist filter. Finally, a correlation was developed for evaluating the performance of oleo-phobic media using the filter specifications and the data of steady state saturation and pressure drop found in open literature. The obtained correlation shows a good conformity with the experimental measurements with an average of 12.1% error for about 93% of data out of 68 data sets collected with a correlation coefficient of (R2) of 0.934.

Influence of 3D printed downstream support structures on pressure drop and entrainment of oleophilic and oleophobic oil mist filters

Industrially applied coalescence filters, e.g. oil mist filters, usually consist of several filter layers in combination with a downstream support grid. While in the past the focus of research on coalescence filters has been e.g. on the filter media itself, information on the influence of support structures on pressure drop and entrainment is scarce. Recent studies on the influence of support structures on differential pressure and entrainment showed, that stainless-steel perforated sheets are particularly well suited for this purpose. Based on these findings, innovative 3D printed support structures with novel printed modifications, like conical holes instead of cylindrical holes or additional roofs above each hole, in comparison to the state-of-the-art stainless-steel perforated sheets, were developed and evaluated in this work. Especially for oleophilic filter media, where the oil film in quasi-steady state is build up on the filter rear side and thus, noticeably affected by support structures, the differential pressure was in some cases slightly reduced, whereas entrainment was significantly reduced (up to −75%) by the developed 3D printed support structures. For oleophobic filter media entrainment was mainly affected by the 3D printed support structures and was significantly reduced.

On the interplay between pressure and gravitational forces in coalescing filters

This paper provides insights into processes governing oil–mist filtration in coalescing filters. In particular, it resolves an apparent inconsistency between different published studies regarding the occurrence (or not) of internal gravity-induced flows. As a result of this, it also clarifies whether in industrially-relevant scenarios such as those pertaining to vertical filter cartridges, non-homogeneous vertical saturation patterns are triggered by these internal flows or are just a result of different oil loading rates. To address these issues, we use Eulerian–Lagrangian CFD simulations, which properly account for the effects of turbulent diffusion of liquid aerosol particles, to replicate an experimental setup available in the literature. In order to interpret results data, we introduce a new dimensionless number, termed SM, defined as the ratio between pressure gradient and gravitational forces, which provides a bulk characterization of the flow and allows to assess whether internal gravity-induced flows, in a given cartridge–oil system, should be expected or not. We show that SM explains well the few experimental data available in the literature and identifies specific behaviors associated with limiting SM values being either very large or close to 1.

Identification of Deposited Oil Structures on Thin Porous Oil Mist Filter Media Applying µ-CT Imaging Technique

The identification of microscale oil structures formed from deposited oil droplets on the filter front face of a coalescence filter medium is essential to understand the initial state of the coalescence filtration process. Using µ-CT imaging and a deep learning tool for segmentation, this work presents a novel approach to visualize and identify deposited oil structures as oil droplets on fibers or oil sails between adjacent fibers of different sizes, shapes and orientations. Furthermore, the local and global porosity, saturation and fiber ratios of different fiber material of the oleophilic filter medium was compared and evaluated. Especially the local and global porosity of the filter material showed great accordance. Local and global saturation as well as the fiber ratios on local and global scale had noticeable differences which can mainly be attributed to the small field of view of the µ-CT scan (350 µm on 250 µm) or the minimal resolution of approximately 1 µm. Finally, fiber diameters of the investigated filter material were analyzed, showing a good agreement with the manufacturer’s specifications. The analytical approach to visualize and analyze the deposited oil structures was the main emphasis of this work.

The influence of layer separation on multilayer mist coalescing filter performance

• Influence of small gaps between layers in a multilayer mist filter “sandwich” confirmed experimentally. • Gap size needed to induce onset of effect studied using pore network modelling. • Important implications for filter performance. • Parametric study to inform future research and design. This work experimentally and numerically examined the influence of micro-sized gaps between layers of filter media on the micro-physics of multilayer porous media commonly used in gas–liquid filter/coalescer applications. It had previously been assumed that if the layers are sufficiently closely packed then they will behave as a monolithic filter with equivalent properties, however no studies have been conducted on the micro-scale interstitial properties of multilayer filters to confirm this. High resolution micro-Computed Tomography ( μ -CT) carried out in this work, determined that even in closely packed “sandwiches” of media, the layers in the filter element behave more like individual filters than a monolithic entity, with each layer having a characteristic liquid saturation profile that repeats throughout the media. To further investigate and understand this phenomenon, simulations using a Dynamic Pore Network Model (DPNM) approach were undertaken to replicate the effect and investigate the influence of layer gap on the onset and magnitude of the effect. The DPNM simulations also revealed the presence of local minima in saturation in the gap, similar to the experiments. Such local reduction in saturation could be interpreted by the concept of capillary bundles. Both experiments and PNM predictions showed that the local minima is more pronounced at the lower velocities. It was also found that the monolith assumption breaks down at a gap thickness of 60 μ m for the micro-glass filter ( d = 4.5 μ m) for the range of parameters considered; however, this was likely larger than the gap width in the experimental cases.

Reduction in oil mist filtration resistance using novel fibrous filters with bioinspired fibrous membrane

Fibrous filters are critical for the removal of tiny oil mists, which is often essential for industrial process and environmental protection. A high pressure drop of the fibrous filter leads to an increase in energy consumption and a tendency to be damaged for fibrous media. The liquid film adsorbed on the surface of fibrous filters owing to the strong capillarity of fine fibers during the oil mist filtration process is one of the main factors resulting in the high pressure drop. Here, we have for the first time reported that a bioinspired fibrous membrane with periodic spindle-knots composited on the oil mist filters can significantly reduce their pressure drop without compromising with filtration efficiency. Coaxial electrospinning was used to prepare the bioinspired fibrous membrane directly on the commercial glass fiber filters. The pressure drop of the composite filter with the bioinspired fibrous membrane was reduced to 2.64 kPa, decreased by 30% and the quality factors were enhanced by 40% for small oil mists and 60% for large oil mists when compared to the control group. Photocuring technology was used to freeze the liquid distribution in filters during the airflow was on and a cluster-like liquid film was observed, suggesting that the change of liquid film morphology is the reason for the reduction in pressure drop. A “migration-instability-holes forming” mechanism was proposed to explain the cluster-like liquid film formation process. Such a novel concept may provide new ideas for fighting against liquid film resistance and the improvement of low energy consumption filters for various applications.

Oleophilic and oleophobic media combinations – Influence on oil mist filter operating performance

For the filtration of fine mists, glass fiber nonwovens are commonly used as filter media, especially in high efficiency applications. For the filters, several layers of these media are usually wrapped around a cylindrical grid; often several types of media are combined to reduce pressure drop and entrainment and improve separation efficiency. The media in use typically differ in their structural and wetting properties. The media configuration, i.e., the sequence of individual media layers in the filters, is generally chosen based on empirical values.In this work, experimental studies with several configurations of filter media were carried out, investigating the operating behavior of the filters. Four media of different structure, two oleophilic and two oleophobic each were combined to determine combinations with beneficial operating properties. It was found that using media with coarse pore structure on the upstream side of the filters could significantly reduce pressure drop, while separation efficiency was slightly improved or hardly affected by the sequence of media. The reason for this improvement was found to be the impact of media sequence on liquid transport mechanisms in the filter media. Two mechanisms were identified to be responsible for the improvement of operating behavior: a.) The propagation of channels from coarse media into subsequent fine layers and therefore adaption of different channel structures and b.) The avoidance of a complete liquid film on the face of fine filter media.

Prediction of residual saturation and pressure drop during coalescence filtration using dynamic pore network model

Microstructure optimization of mist and dust filter media used in many applications including respirators, lubricated machining, compressors and engine crankcases is increasingly becoming reliant on computational simulations of dry and wet filtration processes. Computational fluid dynamics (CFD) based full-morphology simulations with advanced interface capturing techniques offer unique advantages in enabling accurate characterization of the pore-scale transport processes, yet demand substantial computational effort especially for fine filters. In this work, a dynamic pore network modeling (DPNM) framework is developed and evaluated for the prediction of residual saturation and pressure drop during mist filtration, and applied to an oil-air system. Two nonwoven filters (stainless steel and polyester) with different fibre diameters and packing densities are considered. The pore network is extracted using a watershed-based algorithm, from realistic virtual filter geometries (Abishek et al., 2017), and validated for macroscopic properties such as packing density. The validity of the steady PNM model is assessed by comparison of the dry (air) pressure drop against CFD and experimental data in the literature. The validity of the DPNM model is assessed by comparison of multiphase pressure drops and residual saturation against micro-CT and gravimetric experiments carried out in this work, as well as pore-scale CFD simulations using the volume-of-fluid model. It is found that the dynamic pore network model is able to successfully predict the critical properties of the filtration process such as saturation (local and total) and pressure drop to within 15% of experiment and CFD. Considering that the overall computational cost of DPNM is about 2–3 orders of magnitude lower than fully resolved CFD, DPNM stands as a potential tool for the early-stage design and optimization of mist filtration media.

Relevance of downstream support structure design for oleophilic and oleophobic oil mist filter operating performance

Coalescence filters in applications with high separation efficiency requirements, such as compressed air preparation, usually consist of several layers of nonwoven glass fiber media wrapped around a cylindrical metal support grid. The design of this grid has a crucial effect on gas flow inside the filter and therefore on droplet deposition and liquid transport inside the medium. Experimental studies were carried out with flat filter elements in combination with seven different downstream support grids to investigate the impact of the grids on operating behavior of the filters in oil mist filtration. The grids differed in open area fraction, perforation size and distance and were used in combination with either oleophilic or oleophobic filter media. It was found that the choice of the grid affects liquid transport mechanisms in the media, especially the film formation on the filter rear side of oleophilic filters, thus affecting total pressure loss in quasi-steady state, while impact was less significant for oleophobic media. Separation efficiency was not significantly affected by the choice of grid, while entrainment depended on film coverage of the filter rear side and open area fraction of the grid.