Contec liquid filtration

Microfiltration is a pressure-driven membrane process that primarily retains particles and microorganisms on the basis of their size. Typical pore sizes range from approximately 0.1 to 10 µm; the distinction from ultrafiltration is not defined in exactly the same way in all specialist sources. The term 'ultrafiltration' is generally used when the pore sizes are below approximately 0.1 µm.

Depth filters trap particles within a porous, multi-layered structure and are often suitable for a wide range of particle sizes and gel-like contaminants. Surface filters primarily trap particles on a defined surface; pleated designs offer a large surface area while maintaining a compact design. The classification depends on the design, and not every pleated filter is automatically a surface-only filter.

The beta value βx is the ratio of the number of particles of size x µm and larger upstream of the filter to the number downstream of the filter. The separation efficiency is calculated as η = (βx − 1) / βx × 100 per cent. Examples: β10 = 10 corresponds to 90 per cent, β10 = 200 corresponds to 99.5 per cent and β10 = 5,000 corresponds to 99.98 per cent separation efficiency for particles 10 µm and larger – in each case under the specified test conditions.

A nominal filter fineness describes a manufacturer-defined, non-complete separation at the specified particle size. An absolute filter fineness indicates a specified, high separation efficiency; however, it must always be considered in conjunction with test procedures and the retention rate or beta value. Data from different manufacturers can therefore only be compared if the underlying test conditions are known.

The starting point is the largest particle size still permitted in the filtrate. In addition, particle distribution, the desired retention rate, process risk and downstream components must be taken into account. An unnecessarily fine design often leads to increased initial pressure loss, more frequent replacement and higher costs, without bringing about a corresponding improvement in process performance.

The filter fineness describes the particle size to which the specified filtration efficiency of a filter element refers. A lower µm value indicates finer filtration. However, the numerical value alone is not sufficient: what matters is whether the fineness is specified in nominal or absolute terms, and which test method or beta value is used to substantiate it.

The correct filter fineness depends on which particles need to be removed and the required purity of the filtered medium. When making a selection, therefore, particle size, process requirements, flow rate and permissible pressure drop should all be taken into account. It is advisable to make use of the advice offered by filter manufacturers.

The filter area required depends, in particular, on the flow rate, the filter medium, viscosity, particle load and the permissible pressure drop. The filter area should therefore be designed on the basis of the specific process data and the characteristic values of the respective filter element. It is advisable to make use of the advice offered by filter manufacturers.

In addition to the required filter fineness, the filter medium, temperature, operating pressure, flow rate and chemical resistance of the filter material are crucial. The dirt-holding capacity and the desired service life should also be taken into account.

The filter material must be chemically and thermally resistant to the medium to be filtered. Depending on the application, materials such as stainless steel, polypropylene, polyester, polyamide, PTFE or other suitable filter materials are used.

The differential pressure Δp is the difference in pressure between the filter inlet and the filter outlet. It consists of the initial pressure drop in the clean system and the resistance that builds up during operation due to accumulated contaminants. That is why Δp is an important criterion for condition monitoring, but not the only one.

As filtration progresses, particles accumulate on the filter element and increase the flow resistance. This increases the pressure difference between the filter inlet and outlet.

The key factors are the maximum differential pressure permitted by the manufacturer, the required flow rate and the required filtrate quality. The element must be replaced or cleaned at the latest when any of these criteria are no longer met. A universal conversion factor in bar is not reliable, as the design, medium, temperature and process conditions vary.

Appropriate measures include ensuring a sufficiently large filter area, multi-stage pre-filtration and fine filtration, depth or surface filtration appropriate to the type of particle, a low initial pressure drop, and avoiding unnecessarily fine filter elements. Process variations, precipitates and foreign matter should also be investigated. The most cost-effective solution is determined by service life, filtrate quality and total cost of operation.

The dirt-holding capacity describes the amount of contaminants that a filter element can retain up to a defined end point. This limit could, for example, be a specified differential pressure or a situation where the separation efficiency is no longer sufficient or the flow rate is no longer adequate.

Key factors include the medium, flow rate, operating and design pressure, temperature, viscosity, filter fineness, solids content, material and seal compatibility, connections, installation location and access for maintenance. In addition, relevant legal requirements and customer specifications must be checked.

As viscosity increases, flow resistance increases and the achievable flow rate at a constant pressure decreases. That is why viscosity and temperature must be considered together. Data relating to water cannot be applied to oils, paints or other viscous media without adjustment.

Yes, provided that all components in contact with the medium are suitable: housing, filter medium, support structure, adaptor, seals and, where applicable, coating. The permissible values of the weakest component are decisive. Chemical resistance must always be checked in relation to the medium, concentration, temperature and duration of exposure.

Yes, provided that the materials used and the design meet the relevant hygiene and conformity requirements. Depending on the process, other factors such as cleanability, sterilisability, minimal dead space, surface quality, traceability and microbiological retention may also be relevant. Authorisation must be granted on a case-by-case basis.

A bypass is defined as an undesirable flow path through which fluid flows past the filter medium. Possible causes include incorrect adaptors, unsuitable or damaged seals, dimensional deviations, incorrect assembly or a faulty filter element. A suitable component and careful installation are therefore just as important as the filter fineness.

Yes, suitable filter housings and filter elements are available for the food and beverage industry. In this context, materials, seals, design and cleanability must comply with the relevant hygiene requirements.

Activated carbon is a natural product based on natural carbonic raw materials. Its porous structure gives it a very large inner surface, allowing it to bind chemical compounds and molecules. 

Activated carbon filters are used to reduce dissolved organic contamination (color, odor, and taste contaminants, chlorine, pesticides and medication residue). It does not remove all pesticides, pharmaceutical residues or dissolved substances across the board. Suitability and capacity must be assessed for the specific substance and process.

For example:

• water treatment
• chlorine separation
• drinking water
• pesticide separation
• pharmaceutical products
• organic material separation
• food and cosmetics
• color and odor removal