Filter Supplies Blog
December 19, 2018

What you need to know about the most common types of filters

There are many different types of filters. In any decent sized commercial enterprise, you will more than likely have a combination of different types of filters – or all of them.

Understanding the different types of filter ratings:

Before we get too far into explaining the different types of filters, we need to give you some background on the way most filters are rated. 

Micron Ratings

Many filters are given a Micron Rating. What’s a micron you ask? It’s a millionth of a metre. To give you some idea, the diameter of an average human hair is 100 microns in diameter. An average human blood cell is 7 microns across. E Coli Bacteria are around 2 microns. So, if you’re talking about a 1 micron filter, that’s stopping some pretty small stuff.

Micron ratings are further defined by if they are nominal or absolute. Nominal means that the filter will stop 50% of contaminants of that size – 10 micron nominal for example. An absolute rating means that the filter will stop 99% of contaminants of that size.

Further to these ratings are what is known as a Beta Ratio – Baldwin Filters has a particularly succinct description of Beta Ratios here.

"HVAC" Ratings

Heating Ventilation and Air Conditioning (HVAC) & Gas Turbine Filters are rated using the American ASHRAE 52.2:2007 (MERV) and  the European EN 779:2002 and EN 1822:2009 standards (EPA, HEPA & ULPA):

The workings of the various standards and their methodologies are covered in a great article in the International Journal of Rotating Machinery that you can find here for some light bedtime reading. In a nutshell the rating gives a particle size and a retention efficiency. For replacement element purposes a supplier will only need to know the US or European rating in order to supply the correct replacement elements.

1. Oil Filters

Type of Rating: Micron

Oil filters were the first type of filters fitted to modern machines. They fall into 3 main categories:

  1. Full Flow Oil Filter: This type filters 100% of the engine’s oil at full pressure and at a fairly coarse level.
  2. Bypass oil Filter: This type filters a small percentage of the engine’s oil at a time, at a lower pressure, and a finer level.
  3. Combination or Full Flow/Bypass: This type performs both functions of the previous 2 types using internal structures to filter both coarsely and finely at the same time.

Oil filters are either cartridge type which requires an outer canister to be used or a spin-on type that has a canister as part of its construction and attaches to the motor with an internal female thread onto a spigot on the motor.

2. Fuel Filters

Type of Rating: Micron

Depending on the sophistication of the motor they are fitted to fuel filters can either be extremely coarse or very, very fine. As time has gone on the operating tolerances of modern engines means that the filtration fineness of fuel filters has gone down to levels previously only seen in hydraulic applications. Many engines use a combination of coarse and then fine filters to ensure that the delicate fuel pumps and injectors are protected from dirty fuel.

Fuel filters similarly to oil filters are either cartridge or spin-on.

3. Air Filters

Type of Rating: Efficiency

Air filters are designed to be 99.95% efficient from the time that they are first installed. They are tested to ascertain how much dirt they can hold before blocking. Air filters increase in efficiency until they require replacement. Prematurely replacing an air filter does not allow the filter to operate at its most efficient level.

Air filters fall into two main types – primary and secondary. All machines with an air filter have a primary filter. A secondary air filter fits inside the primary, or further down the inlet tract to save the motor should there be a failure of the primary filter.

Air filters come in many different shapes, sizes and types of construction. With very few exceptions.

The most common are:

  • Element Type: Metal ended cylindrical filters, sometimes with a plastic set of fins to move the air around the outside of the filter and deposit the contaminants evenly. These filters use a rubber seal to form an airtight connection with the base of the housing in which they are installed.
  • Radial Seal: These filters are a relatively new innovation in the filter industry. Plastisol rubber ended they form a seal on the inside diameter of the filter onto the housing.
  • Panel: These filters are usually rectangular and seal onto the housing they are installed in using a seal on the outside edge of the filter.

4. Hydraulic Filters

Type of Rating: Micron

Hydraulic filters can be some of the most basic, or the most complicated filters in any fleet. Either Cartridge or Spin-On they can handle as little as 10psi or over 4000psi. Most hydraulic systems have a pressure filter which filters the oil after it has left the pump at full pressure and a return filter which filters the hydraulic oil on the way back to the tank. Some systems also have a suction filter which filters the oil as it leaves the tank.

Most hydraulic filters are metal or nylon ended and form a seal using an encased o-ring or flat seal.

5. Compressed Air Filters

Type of Rating: Micron

Compressed air filters are almost always cartridge style filters that sit inside a housing. They generally filter small flows of air to an extremely fine level to protect instruments, machines and tools further down the line. Typically these filters are some of the finest available – some filtering air down to .01 of a micron or less depending on the application.

6. Process Filters

Type of Rating: Micron

Process filters are used to filter all sorts of liquids and gasses used in industrial applications. They are almost always cartridges. They can be used to filter natural gas, nitrogen, wine, coolant and pretty much any other liquid or gas. The construction of these filters varies by their function and can be made of materials as diverse as stainless steel to nylon or sintered bronze.

HVAC Rating: HYVAC & Gas Turbine Filters are rated using the European normalisation standards:

Coarse, Fine, Semi HEPA, HEPA and ULPA. A commonly used standard a H13 HEPA filter is 99.95% efficient at 0.3 micron.

7. Gas Turbine Filters

Type of Rating: HVAC Rating

Gas turbines are used for power generation in many Oil & Gas and domestic applications. They can be either cylindrical metal ended elements, metal framed panel filters or metal framed pocket filters. 

8. HVAC Filters

Type of Rating: HVAC

HVAC filters can be used in a myriad of applications from commercial air conditioning to hospital operating theatres. Generally panel filters they can be as basic as a G1 rated cardboard framed element that stops 65% of 5 micron contaminants to an U17 ULPA element that stops 99.999995% of contaminant at 0.03 micron.

Final Thoughts

Whilst this may seem like a lot to take in, it is really only a snapshot of the most common varieties of filters available in the market. If you're looking for something more specialised or obscure that doesn't fall into these broad categories, ask a filtration specialist to help.

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