Buyer’s Guide to Filters

Here at Microtech Filters we are looking to lead the way with innovative ideas in the design and manufacture of a wide range of industrial filters. We have produced this buyer’s guide to enable you to become more familiar with the terms and processes of filtration and the products and services available through our company.

We are committed to filtration manufacturing, with over 30 years of experience and ever updated technology, there is no-one better qualified to compile an overview of Filtration.

Hello and Welcome

We are all aware that the process and practice of Filtration can be complicated and often boring. So here we have your easy to use guide, to replace those over-complicated textbooks, or having to spend hours searching the internet.


Filters surround us and keep us healthy – “A filter is basically a device for separating one substance from another, solids, liquid or gas. – hence filtration is simply a process of separation” (Filters and Filtration Handbook 4th Edition T. Christopher Dickenson, 1997).

Principles of Filtration

The process of filtration and separation falls into four categories:

  • Solids-Gas
  • Solids-Liquids
  • Liquids-Liquids
  • Solids-Solids

Each type of separation and its function performs a specific form of separation according to its purpose in industry. Below is a list of variables which enable manufacturers to use the most suited components to serve their environmental purpose: Filtration variables:

  • Flow rate
  • Differential pressure
  • Viscosity
  • Contaminant
  • Flow conditions
  • Compatibility

Main industry users:

  • Water and Sewage – Purifying drinkable water and sanitising sewage
  • Chemical – Separating useful chemicals for production
  • Power Generation – Reducing high level pollutants
  • Food and Beverage – Purifying flavours, eg Smirnoff Vodka is triple distilled
  • Pharmaceuticals – Accuracy is essential to enable no contaminants are found in remedies
  • Mining – Air filters are a necessity to protect human life when confined below ground
  • Paper – To purify the paper content and to protect workers from harmful fumes
  • Plant Machinery – Cleansing fuel output and to prolong life and efficiency

Something that goes unrecognised is that filtration products keep us healthy! by keeping the air in our working and social environments clean.

Filters are often purposely made to be bio-degradable where possible in order to protect our environment and decrease levels of landfill.

“Environmental issues are of growing concern for government and industry” (Filters and Filtration Handbook 4th Edition T. Christopher Dickenson, 1997).

Here at Microtech Filters we look to re-use materials and products to further protect our environment, one of our many services is to backwash and cleanse your filter to prolong its life and effectiveness, a process which can be undertaken on most of our filters, please call us to find out more information or any other queries you may have.

ENVIROMENTAL ISSUES The Microtech team is committed to saving our environment through production, operation and disposal of filters.

Manufacturing – Low energy papers, air pressurised machinery, degradable materials are used where possible.

Operation – A large range of our products are applied to operations whereby pollution is reduced or air is purified in the working environment, a catalyct converter in a car and air conditioning units are two examples.

Disposal – Filters such as automotive air filters are not constructed from metals, in an attempt to reduce landfill levels.

In the next chapter – Filter Media, we look at the medium of the filter, the component which allows the filter to separate particles and the varying differentials of each medium.

Filter Media
Filter product media

Filter product media

“It is rightly said that the heart of any filter is the filter medium”, Handbook of Filter Media, D.Purchas 1996. Unless it is fitted with the correct medium, a filter cannot perform its process.

Right: is a photograph of a selection of filter mediums.

Each type of medium is suitable to different environments and conditions. Every form of media can be enhanced by choosing an optional extra, which increases both effectiveness and stability to suit its function.

The number and variety of mediums is huge, they range from metal sheets to paper to plastics and beyond. The type of medium selected depends upon the size of the catchment particle and the conditions (eg temperature) the filter is exposed too.

A successful medium must incorporate many factors to carry out a positive function. These can be placed into three headings, listed below:

1. Machine orientated properties – conditions the filters medium is exposed to, which restrict the use of particular mediums in certain filter types.

  • Rigidity
  • Strength
  • Resistance to abrasion
  • Stability to vibration

2. Application orientated properties – the compatibility of the medium to its environment.

  • Health and Safety
  • Absorptive Characteristics
  • Adsorptive Characteristics
  • Static
  • Disposability
  • Cost
  • Chemical stability
  • Thermal stability
  • Dynamic stability

3. Filtration specific properties – the ability to achieve its task.

  • Retention efficiency
  • Resistance to flow
  • Dirt holding capacity
  • Smallest particle retention

All three headings and their incorporates act as test of function. Testing of the medium is critical, for example when filtering dust the medium must have a high dirt hold capacity, and be made from fabric so capable of removing sub-micrometre dust and fume particles.

Below are a selection of medium’s which enable filter production here at Microtech Filters:

Stainless Woven Mesh
Sintered stainless steel
Metal sheets -
perforated steel
Metal sheets - expanded steel
Porous – Carbon
Non woven - Needle felts
Non woven -
polyester spun bonded
Glass Fibre
Cellulose - Paper

All filter media is measured in thickness (micrometer) and weight (gram per square cm) in order to achieve efficient separation in its working environment.

The table below depicts the main applications, advantages and optional extras of the filter mediums we use, here at Microtech Filters:

MediumTypical ApplicationAdvantagesOptional Extras
Stainless Woven MeshStrainer/DiscsRigid, hard wearing,
High efficiency thermal stability, HDHC*
- multi-pleated layers, adds rigidity and efficiency
- Sinter mesh, gives a greater bond of fibres
- 5-1000 micron
Sintered Stainless Steel/Perforated Steel/Expanded SteelDustDurable, resistance to abrasion, thermal and dynamic stability- construct from fibre strands
- epoxy resin and heat cure, achieves further rigidity
- 3-50micron
CarbonOdourHDHC*, resistant to chemical reaction, thermal stability
Absorbs waste, statically charged
- compact carbon to increase dirt hold capacity
- use impregnated activated carbon
Needle feltsIndustrial CompressorsFlexible and versatile, efficient, large surface area- form around inner scrim to enhance rigidity
- 3-10 micron
Spun Bonded Polyester Tea BagEfficient, retention efficient, cost effective, disposability- multi pleated layers, adds rigidity and increases efficiency
- 3-50 micron
Glass fibreHydraulic/FuelLow energy media, HDHC*, re-useable, small particle abstraction, efficient, high retention rate- Three alternative fibres; Multigrade, Glass Microfibre and Quartz Microfibre
- 1-40 micron
PaperAirRe-useable, HDHC*, efficient, disposability, large surface area, cost effective- impregnate paper with a resin to give extra strength
- curing, adds strength and increases dirt hold capacity
- 5- 25 micron

* HDHC = High Dirt Hold Capacity

How to make a filter

The principles of making a filter are similar in process, but vary to meet its processing needs.

Measure & CuttingEnd Caps
Rolling & WeldingPerforated/Expanded Steel
Slitting & PleatingMedia
Final AssemblyPot Moulding
Heat PressHeat Press
Quality Control & BoxPackaging


Below we explore the standard process of filtration production.

1. Measure & cut perforated steelmeasure_perforated_steel2. Roll & spot weldrolling-mesh3. Slit & Pleatslit-pleat
4. Seam media using adhesiveseam-adhesive5. Inner cylinder with mediainner-cylinder6. Pot mould end capsfilter-end-caps
7. Heat pressheat_press8. Quality Controlquality_control9. Package in boxpackaging

A Typical Finished Product – below is a diagram of the manufactured filter and its components.

typical_filter_structure We hope this chapter has broadened your knowledge of how to make a filter, the process does vary according to specifics, yet the stages of production are generally the same

Types of Filter

There are many different types of filter produced here at Microtech Filters, these can be separated into nine separate categories; below we have depicted their main characteristics and uses:



  • Public Health, Mining, Power Generation
  • Air conditioning units, extractor units
  • Cellulose paper, synthetic and cotton fibres
  • Low maintenance, cost effective, high dirt hold capacity, long life, hard wearing, thermal stability, reduces levels of pollutant air and harm to humans. Protects component parts from damaging pollutants
  • The British Filter Industry produces 2.7 million air filters per year


  • Chemical, Food and Beverage, Mining
  • Beer fermentation, Cigarettes
  • Spun bonded carbon
  • Highly efficient, non-reusable, low maintenance, absorbs deposits, statically charged
  • Smirnoff vodka is filtered through carbon, filtering out impurities and giving better colour and taste


  • Vacuum manufacturing, Cement Works
  • Vacuum cleaners, Air purifiers
  • Cellulose paper, glass fibre, non-woven fabrics
  • Long-life, cleanable, cost effective, durable, large surface area


  • Agricultural, Ministry of Defence
  • Tractors, Tanks, Aircraft
  • Nylon cloth, cellulose paper, mesh
  • Thermal efficient, re-useable, durable, small particle abstraction, chemical stability
  • Before 1910, new cars were sold with a piece of chamois leather which filtered the fuel before being passed to the fuel tank


  • Gas works, Mining, Chemical
  • Air cleansing machinery
  • Spun bonded nylon, glass fibre
  • Health and environmentally efficient, long-life, durable, high dirt hold capacity
  • Kleenex tissues were originally used as filters in gas masks


  • Plant machinery, Oil Refineries, Aerospace
  • Oil purifiers
  • Glass Micro fibre, woven wire cloth, paper
  • Disposable, rigid, extended life, efficient,
  • Ernest Sweetland invented the modern oil filter in 1923; he named the product “Purolator” – 
a combination of the words: “PURE OIL LATER”


  • Food and Beverage, Chemical, Water and Sewage
  • Coffee percolator, Sewage separators
  • Media must adhere to health and safety regulation specifics; wedge wire screens, perforated steel, woven wire mesh
  • Re-usable, cost effective, easily maintained, pump protection, rated to 50 micron – 4mm
  • In 1865, coffee percolators appeared in Europe and America thanks to inventor James Mason

SEPARATOR (Coalescing)

  • Fuel, Chemical, Oil Refineries
  • Diesel Polishing
  • Glass Micro fibres, needle felts, synthetic material
  • Particles adhere to fibres and not passed through filter, efficient, flow rates from 20-1000 litres/min
  • Coalescing filters are one of the most recent filters to be produced, first tested in the 1980’s


  • Aquatics, Chemical
  • Fish tank, Water purifiers (eg Brita)
  • Glass micro fibre and intertwined mesh
  • Back washable, efficient, cost effective, highly beneficial to sustaining life in the Third World
  • In a 100-year period, the average water molecule spends 98 years in the ocean, 20 months as ice, about 2 weeks in lakes and rivers and only the lucky ones pass through a filter!!!

All filter types are made up of different components, dependant upon their working environment.

The components used differ in order to satisfy thermal, rigidity and chemical properties as well as to suit level of dirt hold capacity and future re-usability.

Remember all filter types are environmentally friendly, both through their use and their disposability.

Closing Statement

This buyer’s guide has looked to explore the purpose of a filter, its components, how to construct and its types in a clear easy to use format. We hope you have learnt about the processes of filtration and their properties and have enjoyed the experience.

If you should have any queries regarding your filtration requirements please do not hesitate to call, email or fax us here at Microtech Filters where a dedicated member of staff will be pleased to assist and cater for your needs.

We specialise in the production of both standard and non-standard filters, with over thirty years of experience we have seen it all!


Absolute Rating is a value associated with a filter that represents the size of the smallest particle completely retained.

Absorptive the taking in, incorporation or reception of gases, liquids, light or heat.

Adsorptive the retention of gas, liquid or solid on a surface due to positive interaction (attraction) between the surface and the molecules of the adsorbed material.

Air to Cloth Ratio is the flow rate divide by the volume of the filter.

Beta Rating the measurement of filter retention efficiency. Ratio of particles exposed to a filter (as feed stream) to particles present in the filtrate.

Chemical stability is defined as the ability of a filter’s materials of construction to resist chemicals so that the filter’s function is not adversely affected, and the filter material does not shed particles or fibres, or add extractables.

Contamination – A substance that is either present in an environment where it does not belong or is present at levels that might have detrimental consequences upon its function.

Dirt holding capacity – Amount of dirt or debris retained by a filter in grams per unit area of the filter medium.

Filter Efficiency is the ratio of particles trapped by a filter over the total number of particles found in the air upstream of the filter. A count of the downstream particles is often used to determine the number of particles trapped by the filter.

FLO Rate is a measure of the amount of air that flows through a filter. It is related to the degree of contamination, differential pressure, total porosity, and filter area.

Micrometre “micron.” It is a 1/1,000,000 of a metre. 60µm = approximately the diameter of a human hair.

Pore Size Rating is the pore size of the filter determined by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. Pore size ratings are usually stated in Micrometers (µm). Ratings can be stated as either nominal or absolute.

Retention efficiency – Ability of a filter to retain particles suspended in a gas or liquid. Expressed as a percent of particles originally present.

Thermal stability is the ability of the filter media and device components to maintain integrity and functionality at elevated temperatures.

Viscosity – A fluid’s resistance to flow. Viscosity is used to describe friction of fluids.