Filter Selection and Sizing Methods

The following information is a general guide to help you select a filtration system to meet your needs. Most process systems will require additional consultation. This guide will also assist you in sizing a system for flow rate. If you wish to size a system for just throughput (volume) or for both flow rate and throughput, contact your local Millipore Applications Specialist.

General Guidelines for Selecting a Gas or Liquid Filtration System

   • Select the filter material that is chemically compatible with the fluid to be filtered and that best fits your application.
   • Determine if you need a single-stage or multistage filtration train. For multi-stage filter systems, select the final filter first. Mulitstage filtration is necessary with plugging streams.
   • When choosing the final filter select the smallest microorganism to retain then choose the appropriate pore size.
   • The prefilter train (clarification and prefiltration) before the final filter is then chosen to give the best combination of retention and capacity. The final prefilter is chosen to have a retention close to that of the final filter.
   • Check the differential pressure to see if it is compatible with the filter and the process. The effective final differential pressure across the cartridge (change-out pressure drop) is a function of the particle/filter interaction. In cases of fluids containing deformable particles, the final differential pressure should be lower than the maximum differential pressure stated in the literature.

Pore Size Selection Guide

Final Filter Choice

Choose the appropriate pore size of the final filter to retain the smallest microorganism to be removed. Then refer to the application guide for the recommended filter.

Clarification and Prefiltration Filter Choice

Clarification and prefiltration stages upstream of the final filter greatly decreases overall filtration costs by removing most of the particles that would clog the downstream final filter(s). Refer to the application guide for the selection of the best protective and highest capacity prefilter train.

Designing the Ideal Liquid Filtration Train

Normal Flow Filtration

Filtration Systems for Normal Flow Filtration (NFF) processing applications generally consist of a series of filter cartridges, with each cartridge in the series protecting and extending the life of the next filter cartridge. Choosing the correct type for each application will lead to an optimized filtration train and reduced overall filtration costs. Due to many different types of feed solutions and filters, choosing the best filter train offers a unique challenge.

Before designing a liquid filtration system, select the design basis. The two most common parameters involved in filter sizing are throughput (also referred to as capacity, expressed in volume of fluid filtered before filter change-out is required) and flow rate. For a place to start see the Quick Sizing Guide below.

The sizing method depends largely on the nature of the fluid to be filtered, and how plugging it will be for the final filter selected based on the microorganism removal requirements previously discussed.

1) Sizing is typically based on flow rate in applications where the fluid has low amounts of particles and colloids, and therefore has low or non-plugging characteristics for the selected filter. It is the case in most bottled water processes, where 0.22 µm or 0.45 µm membrane final filtration is selected for microorganism removal. In such cases, sizing will be calculated by selecting a flow rate value per unit of filter area. This value is selected so that the initial differential pressure created across a new filter at this flow rate is low, typically around 0.07 – 0.15 bar (1 – 2 psid). Filter manufacturers provide charts showing the pressure differentials as a function of flow rate for a specific filter type and surface area. Based on these charts, the flow rate per unit of filter area can be selected, and the total surface area required for the process is obtained by dividing the flow rate of the process line by this flow rate per unit of filter area.

Example: if the flow rate versus differential pressure chart of the chosen filter indicates that a 2,000 liter per hour flow rate will generate less than 0.15 bar (2 psid) across a 30-inch cartridge, a process line with of 6,000 liters per hour flow rate will require a (6,000 / 2,000) = 3 x 30-inch cartridge housing.
Note: when the filtration system directly feeds a filler (no intermediate or storage tank between filter housing and filling machine), fillers demand 30-50% higher flow rate than process capacity. For example, 10,000 bottles per hour for wine: real flow rate needed is 0,75 L x 10,000 x 1.3 = 9,750 L/h. This needs to be taken into consideration when performing sizing calculations.

2) For applications where the fluid will have a medium to large amount of plugging particles (some wines and most beers fall into this category), it will be necessary to study the plugging profile of the filter using small-scale testing in order to perform the necessary filter sizing calculations. Millipore uses the Vmax test, a constant pressure test, for the selection and sizing of filters with such fluids. The speed, reliability and versatility of Vmax testing allow the entire filtration train (from the first prefilter to the final filter) to be designed in a very short time. The basic concept is to apply a mathematical model to the plugging profile observed at small-scale in order to predict plugging at large scale. This method has proven to be applicable to most fluids of organic or biological origin, including wine and beer, where membrane filtration is being used as a microorganism removal technique. Please refer to Millipore Technical Brief AN1025EN00 for more details on the Vmax test and its implementation for sizing purposes.

While Vmax is the preferred sizing technique for the majority of Millipore products, recent work indicates that when using a depth filter, a constant flow test provides a more accurate result. Based on this work, Millipore has developed a constant flow sizing technique, Pmax, that can be used for selecting and sizing depth filters. When distinguishing between the Vmax and Pmax the type of particle-filter media interaction that occurs during the filtration step determines which sizing method to use. Please refer to Technical Note AN1512EN00 for more information on Pmax and Vmax sizing techniques, and how they best apply to particular types of filters and applications.

Access Filtration Services

Vmax testing is one of the Access Filtration Services developed by Millipore to assist you in optimizing filtration efficiency. Access Filtration Services cover areas such as process parameters optimization and operators training programs.

Gas Filtration

In most gas systems the gas is dry and oil free. In situations where particles are hard, size the filtration system according to flow rate. The particles form a relatively permeable cake on the surface of a membrane filter and do not significantly restrict the flow rate. The maximum allowable differential pressure is often used to size a final filtration system. In some cases a face velocity is chosen to optimize retention of certain size particles.

Sizing

Low Pressure Venting (for example, a tank vent filter)
Sterile vent filters are usually sized with the aid of a computer. The rate of steam condensation inside the tank directly affects the filter size needed to avoid implosion as vacuum develops inside the tank after steaming. Tank dimensions (diameter, height, wall thickness and pump-out rate) are needed to safely size the vent filter. These properties dictate the structural strength of the tank and therefore the available differential pressure for filtration.

High Pressure (for example, fermentation intake/exhaust systems)
Fermentation or higher pressure gas filtration is sized using the Ideal Gas Law and the flow rate versus pressure drop curves for hydrophobic cartridges. The maximum differential pressure is often used since the gas is clean and the required filter area is inversely proportional to the allowable pressure drop. Gas inlet pressure, temperature, flow rate, and pressure drop information are required to size a filtration system economically. Typical gas systems are sized at 85 Nm³/h (50 SCFM) per 10-inch (25 cm) cartridge and an initial differential pressure of 0.07 – 0.15 bar (1 – 2 psid).

Steam Filtration

In most of the steaming installations, the sizing of the filtration system is done on the basis of mass flow rate since the particles are hard. The particles form a relatively permeable cake on the surface of the membrane filter and do not significantly restrict the flow rate.

Steam Filter Selection

Stainless steel is the material of choice for steam prefilters. The cartridge is usually selected according to application-dependent criteria such as flow rate versus pressure drop.

Inherently hydrophobic materials such as PTFE are chosen for gas final filters. A device is usually selected using application-dependent criteria including: flow rate/pressure drop, number of sterilization cycles, oxidation resistance, microbiological retention, validation needs, replacement frequency, forward or reverse pressure capability. The Aervent 0.2 µm cartridge filter, for steam sterile filtration, offers superior economics and an extended service life.