Air Filters: Functions, Types And Working Principles.

Ventilation filters are essential in cleaning the air of dust and other foreign particles that are not needed in the air.


In this article, I will share the functions of filters, types and their working principles.


You should understand that filters are of different types and they work on different principles.


The general thing that we all know about filters is that they filter the air and that’s all.


But there is more to how these filters work and I will discuss that in this article.


Without wasting time, let me start with the functions of these filters.


Before I share the functions of these filters it’s also important you understand that the filters used in ventilation systems can also be used in air conditioning systems.


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Functions of Ventilation Filters.

1. Clean the air from any airborne contaminants.

The filter’s main function is to clean the air entering a space that needs to be conditioned.


As we all know the air that is extracted from the environment, is not free from contamination.


The air may contain dust particles, pathogens, allergens, mist and fog, smoke etc.


Be it an air conditioning system or a ventilation system, the air has to be free from these contaminants for it to be safe for the occupants.


The filters will get rid of these contaminants and ensure that the air is suitable for a required by the occupants.


Also, the filters not only help in ensuring the safety of the occupants but also preserve the parts of the ventilation system or any conditioning system.


If the filters are in place, then this means that the duct system of the air conditioning or ventilation system is going to be safe.


The cleaning cost is going to be reduced significantly which will also help in saving money and the heating equipment used in the system is going to be usable for a long period.


So, I think now you have realized the importance of filters that are used in the air conditioning or ventilation system.


Anyway, filters are not only used in ventilation and air conditioning systems but also in the industrial processes where the air is required to be dust free.


I thought you should know that.


Types of filters;

There are three main types of filters that I’m going to talk about in this article, which are;

  1. Dry filters.
  2. Viscous filters.
  3. Electrostatic filters.


1. Dry filters.

As the name suggests, these filters are dry and this means that the contaminants are collected in the filter medium.


The filter medium can be made from cotton wool, glass fibre, treated paper and foamed polyurethane.


As these filters mainly depend on the medium offered to clean the air, the surface area of the medium matters a lot.


The air has to pass through the cleaning medium for it to be cleaned.


This means, if the air passes through a large surface area of the cleaning medium then the efficiency of that filter will be higher.


So, this brings the other strategies to ensure that these dry filters have higher efficiencies.


To increase the efficiency of these filters the cleaning medium is arranged in a Vformation.


Arranging the filters in a V formation enables the filter to clean a large amount of air since the surface area increases and this also increases the efficiency of the filter.


After a long period of use these filters have to be cleaned to allow air to continue passing through the filters since the filters end up being clogged with contaminants.


Other types of dry filters are electrically operated by a pressure switch that turns on a warning light when the filter medium is dirty.


This is to ensure that the dirty filter is cleaned or removed if it’s beyond cleaning.


The cheaper cleaning medium of the filters may be thrown away and the expensive ones will be vacuumed and cleaned twice or thrice before being thrown away.


Absolute filters; are the dry fabric filter types that are very effective in cleaning the air. This is made possible because of the close packing of the cleaning medium.


This close packing of the cleaning medium provides a very small diameter to allow air to pass through them.


The limitation is that it offers a very high resistance to air passing through these filters.


2. Viscous filters.

These filters have a special coating that aids in cleaning the air by letting the foreign particles from the air stick to it.


And that is why they are called viscous filters.


This special oil that is used makes these filters have a very high capability of cleaning a large volume of air very effectively.


These kinds of filters are mostly used in areas where there is very high pollution because of their cleaning capabilities.


The special oil coated on the filter medium is non-toxic, odourless and non-flammable.


This is to ensure that the air filtered through this filter is safe for the occupants and prevent any fire hazards in the space to be conditioned.


The viscous filters are divided into three namely;

  1. Cell-type viscous filters.
  2. Automatic revolving viscous filters.
  3. Spray-type viscous filters.


Cell-type viscous filters.

This type of filter contains a frame with a wire mesh, metal stamping, metal swarf or combinations of these material that is coated with the special oil inserted in them.


These cells are arranged in V-formation to increase the surface area of the cleaning medium thus increasing the efficiency of the filters.


When the cells are dirty (materials that are coated with oil) are removed from the metal stamping and cleaned with hot water, dried and recoated with clean oil for further use.


The cells can be removed and inserted whenever needed from these frames.


This makes it appropriate whenever there is a need for the ducts to be cleaned or repaired.


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Automatic revolving viscous filters.

This type of filter contains sprockets, a chain, filter plates and an oil bath at the bottom of its housing.


The chain is mounted on the sprockets one at the top and the other one at the bottom where the oil bath is located.


The chain that is mounted on the two sprockets top and bottom, is fitted with filter plates that revolve with the chain.


As the chain revolves together with the filter plates, the plate ends up being coated with oil when it enters the oil bath and this ensures that every filter plate on the chain is coated with oil.


The sprocket, oil bath, chain and filter plates are housed to allow air to pass through one point of the filter and leave through the other.


As the air passes through one point of the revolving filter plates, the foreign particles are trapped on the oil and clean air leaves through the other side.


Revolving of these filter plates through the oil bath ensures that the dirty oil from the filter plates is cleaned and clean oil is recoated to keep on trapping the dirt.


The revolution of this chain is automatic thus it ensures that the cleaning process keeps on happening.


The main disadvantage of this filter is that when the sprockets fail to move the cleaning of the air won’t happen since there won’t be any revolution.


The filters are going to be clogged with dirt, which will increase air resistance passing through the filter and the cleaning of the air won’t happen.


Automatic spray tube viscous filters.

This type of filter contains sparge pipe, a pump, an oil bath, corrugated metal sheets, a cover and oil spray.


Let me share with you how this type of viscous filter works.


Spray-type viscous filter has corrugated metal sheets which are coated with oil to trap foreign particles in the air.


The pump is used to pump oil from the oil bath through the sparge pipe to the spray which is located at the top of the filter that sprays the oil to the corrugated metal sheets.


This ensures that the corrugated metal sheets are coated with clean oil that is used to trap foreign particles from the air.


The oil that drops from the corrugated metal shits flows down to the oil bath and it’s recirculated and the process is repeated.


The corrugated metal sheets are closely packed to ensure that the air passing through the filters comes in contact with the oil to be cleaned.


Also, the passage of air within the corrugated metal sheets is like a maze which allows the air to have maximum contact with the oil and be cleaned effectively.


3. Electrostatic filters.

This type of filter uses electricity to clean the air.


These are the main components of electrostatic filters;

  • Ioniser.
  • Metal collector.
  • Electrostatic power pack.


An electrostatic filter clean the air by using different electric charges.


The contaminants in the air are positively electrostatically charged by the ioniser screen which is the first part of the filter.


This is made possible because the ioniser screen that contains fine wires which possess electrostatic charge produced by a direct current potential of 13KV.


The positively charged contaminants pass through the plates which are positively charged by a direct current potential of 6KV.


So this means the contaminants will be repelled (since the plates and contaminants have the same charges) and will be attracted by the negative earthed plates (grounded collector electrode), thus leaving the air clean when it leaves the filter through the other side.


The grounded collector electrode are usually coated with water-soluble liquid to permit easy cleaning of the plates.


These plates can be removed and cleaned or a sparge pipe can be installed on the top to clean the plates by using warm water.


Electric filters.

In electric filters, the air is ionised by either a positive or negative charge and as they pass through the filter plates they end up being attracted to plates with different charges.


The contaminants in the air end up being trapped in the filter while clean air leaves the filter.


Activated carbon filters.

This filter uses activated granule carbons to remove odour and fumes from the air.


The activated granule carbon absorbs the odour, gas or vapour that decreases the quality of air and leaves the air smelling fresh.


The carbon granules have a high capability of absorbing the gas or vapour from the air.


These carbon granules are usually located in position by a glass fabric on either face of a filter panel.


The panels are arranged in a V formation to increase the surface area thus allowing a large contact of the air with the carbon granules on the glass fabric.


These types of filters are usually fitted in ductwork serving cookers and fish dryers.


Final thoughts.


I have tried my best to summarize the filters used in ventilation systems.


As I said earlier, these filters can be used in air conditioning or ventilation system.


The information I have shared in this article is not thorough, so make sure you do your research before you think of using any filters that I have just talked about in this article.


Thanks for reading, if you have any questions or additional points don’t be afraid to leave your comment.


See you in my next article.


Related articles to check out.

  1. Mechanical Ventilation System Fans: Types, Components & Working Principles
  2. Factors To Consider When Choosing Air Conditioning System
  3. Ventilation System: Purpose, Types, Working Principle And Maintenance
  4. Factors To Consider When Choosing Air Conditioning System

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