Click on the following links:

• What Is Filtration?

• Filter Terminology.

• Depth Versus Screen Filters.

• Hydrophobic Versus Hydrophilic Filters.

• Testing Filters.

• Selecting The Correct Filtration Media and Housing.

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What Is Filtration?

Filtration is the separation of different materials:

• Particles from liquids or gases (air).
• Air from liquids.
• Liquids (not in the vapour phase) from air.
• Micro-organisms from liquids and gases.
• Odour from Air (carbon filters).

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Filter Terminology:

• Filtrate - substance passing through the filter.

• Retentate - substance held back by the filter.

• Upstream - before filtration.

• Downstream - after filtration.

• Driving Force - Pressure available.

• Pressure Drop - Pressure difference between upstream and Downstream.

• Micron - µ or 1000th of a millimetre.

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Depth Versus Screen Filters:

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Hydrophobic Versus Hydrophilic Filters:

• "Hydro" = Water.
  
• "Phobic" = Phobia, afraid, fear, hates, stay away from.
• "Hydrophobic Filters" repel water; but pressure and duration time of exposure is important.  Hydrophobicity is like strength, there are degrees of hydrophobicity,
  e.g. '10 psi for 5 minutes'.
  

• "Philic" = philia, likes, loves, is attracted to.
• "Hydrophilic Filters" attract water and easily 'wet out'.

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Testing Filters:

• Water Breakthrough, Water Entry Pressure (WEP) - usually referring to the pressure required to push water through a hydrophobic filter - measures filter integrity.
• Bubble point - measures the pressure required to remove liquid from the largest pore of the filter - measures filter integrity.
• Flow rate - the flow of liquid or air through the filter at a given pressure. The housing or connectors may have limiting affects.
• Housing Integrity - the pressure that the filter housing will withstand before it bursts.
• Filtration efficiency - a measure of the performance of the filter by comparing the "challenge" with the "filtrate".

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Selecting The Correct Filtration Media & Housing:

1. What is To Be Removed ? - Level Of Filtration Required:

• Clarification or bacteria free.
• Sterile - can not have 99% sterility only 100%

  • use 0.45m for air in non-critical applications.

  • use 0.2m for air & liquids in critical applications.

  • use 0.1m for ultra-critical applications.

• See our online guides:

  • Relative Size of Particles Chart.

  • Filter Membrane Chemical Compatibility Guide.

2. Select Surface Area Of Filter Required

• Flow rate is determined by the pore size of the filter, filter surface area and pressure available.
• As the filter becomes blocked the pressure drop will increase and the flow rate will decrease.
• Increasing the pressure differential will increase the flow rate.
• As surface area of filter increases the pressure drop will decrease or the flow rate will increase.
• Pressures of greater than 15 psi differential should not be exceeded for depth filters.

3. Select Filter Housing Connections

• See our Connector Reference Guide.

4. Can Filters Be Sterilised?

• Autoclave by customer - steam at 121degrees @ 15psi for 20 mins.
• Radiation(Gamma) - by supplier; but some grades of PTFE not stable.
• Ethylene Oxide (EtO) - common but beware of toxic EtO residuals.