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The need for air filtration in pneumatic applications is a highly discussed topic in the industrial world. General-purpose filters or compressed air streams can perform well to a certain level by capturing large contaminants. Air dryers are one such application where water is effectively removed from the stream of compressed air. But they fail to capture extremely small (≥ 0.1 micron) solid particles, aerosols and, liquid droplets of water and suspended in the air. This can cause huge problems downstream. Hence coalescing filter were developed to overcome this problem

What is Coalescing Filter?

A coalescing filter is generally used in a compressed air system to remove liquid and oil aerosols where they can be used to remove liquid and gas phase contaminants including oil aerosols from compressed air as well as liquids.

Which Contaminants can be filtered out using Coalescing Filters

Liquid phase separation.

  • Remove organic contaminants in the liquid phase
  • Remove moisture particles from different organic streams

Gas-phase separation

  • Eliminate liquid moisture from a moisture-laden stream of air
  • Eliminate oil mists from a stream of air

Types of Coalescers

There are two types of coalescers used for industrial purposes.

  • Mechanical coalescers
  • Electrostatic Coalescers

Electrostatic Coalescers

Electrostatic coalescers utilize electrical charges (AC or DC or a combination of both) to induce droplet coalescence in emulsions to increasing the size of the droplet. Generally, Water droplets have a higher permittivity than the surrounding oil, and water with dissolved salt shows good conducting properties. What happens when an uncharged droplet subjected to an AC electric field? The field itself polarizes the uncharged droplet creating an electric field all around the droplet to act against the external field. As stated above water droplet has very good conductive properties. Because of that, the induced charges will reside on the surface. Further, the droplet doesn’t show a net charge, but a negative and one positive side with zero electric fields inside the droplet. When a such couple of droplets with induced dipoles get closer to each other, they will be attracted with a pulling force until they get merge showing coalesce property.  Electro coalescers contain large settling tanks with electrodes. These electrodes used in electro coalescers are insulated in order to prevent short-circuiting. Further droplet coalescence is attained by applying AC electric fields (50–60 Hz) to water-in-oil emulsions under turbulent-flow conditions where the turbulence boosts the collision frequency between water droplets.

Mechanical Coalescers

Mechanical coalescer utilises physical alteration methods such as coalescing filters or baffle walls to sort out condensate formed by condensation of water or hydrocarbon to coalesce them into larger molecules. Mechanical coalescers are used in separating liquid water and oil in the compressed air by using coalescing effect. Apart from gaseous phase contaminants, mechanical coalescers can additionally remove particles. Borosilicate micro-fiber is the most commonly used medial in mechanical coalescers to remove contaminants in the fluid.

How Does a Coalescer Work?

Coalescing filter elements remove contaminants using three different mechanisms:

Direct interception

Filter elements contain matrices of defined pores. If the particles (0.1 to 1 µm in size), are larger than the pore sizes then they fail to go through the filter element, and this process of removing particles from the stream of air is called Direct Interception.

Direct Impaction

The process of removing particles in 1 micron and a larger range. What happens in direct impaction is particles that come along with the stream of air collide and are trapped by the fibers. Generally, air enters the housing of coalescing filter and flows through the media from the inside to the outside of the filter element. Oil and water droplets that get subjected to coalescing effect get collected on the filter fibers as when they grow large in size and they get shifted downward due to gravity. The cleaned air exits the housing through the outlet while the liquid gets collected in the base and then drained away.

Diffusional interception

Baby particles (less than 0.1 µm in size), subject to Brownian motion with rapid random movements about a nominal trajectory. This causes the particles to come into contact and get collected on the pore walls and fibers of the filter element. Particle removal efficiency in this mechanism increases with higher Brownian motion.

Why Install Oil Coalescing Filters in Pairs?

Installing oil coalescer filters in pairs will give you significant benefits.

Installing pairs of coalescing filters will help to it protects the fine filter by reducing the pressure loss where first filter act as a general-purpose filter and protect the second one. Here the second filter works as a very efficient filter when the stream of air contains bulk contamination. However, both filters eliminate the contaminants at varying degrees to achieve purity. This arrangement will also help to lengthen the operating life of the element. Hence you can achieve significant improvement in the filtration process while cutting down operational expenditure and reducing pressure loss.

When should you Replace Coalescing Oil Filter?

After using the coalescing for a long time, contaminants get clogged and as a result of that pressure drops start to rise. It gives an early warning Whenever the pressure drop rises from 8 psi to 10 psi, by notifying that is the end of life and the filter needs to be replaced.

When contaminants started to get clogged you will be able to notice various types of problems such as Increase in the differential pressure within the coalescing filter elements. The velocity of the gas does not show same as usual.

How do you Clean Oil Coalescing Filter?

When you use the coalescing filter for a longer excessive accumulation of contaminants on the element will occur by reducing the volume of filter left for the filtration.

Therefore, in order to get rid of this issue, you need to remove the accumulated pollutants. Withdrawing the accumulated pollutants can be achieved by remove the sponge support of the element and wash it thoroughly. Further clean the filter at an interval of 1-2 months will also help to enhance the performance of the coalescing filter.

Filter cleaning can be done by removing the steel housing of the filter and then washing it with a jet of water. Care must be given when you remove bottom sediments, loose material. Cleaning of inlet and outlet tunnels also should be done carefully.

You should note that cleaning frequency will depend upon the quantity of oil and grease in the air stream. Further steps must be taken to never use poisonous or toxic chemicals in the cleaning process.

Pressure and Temperature

Coalescing filters shows its performance at its best when it is located at the highest pressure in the compressed air system and the lowest temperature. It is advisable to locate coalescing filters as close to the point of use as possible in order to ensure no additional condensing of liquids takes place.

How does Coalescing Filtration Differ from Particulate Filtration?

Particulate filtration can completely separate the solid particles (dirt, rust, dust, pipe scales, etc.) from a liquid phase or gas phase. While Coalescing filters separate two ‘phases’, such as removal of water aerosols and droplets from a gas

Simply, if you need to remove oil and water, the best choice would be coalescing filter while if you need to remove solid particles it’s best to go for a particulate filter.

To undertake particulate filtration, and coalescing filtration you need to pick any kind of Particulate filter element and a coalescing element.

Coalescing Elements:

Coalescing elements are extremely fine fiber meshes that act as a separation device for liquids and are frequently mistaken for filters.  These coalescing elements project compressed airflow from the inside of the element to the outside. Water, oil, aerosols, and fine droplets collide together to form heavier, larger droplets due to coalescing effect and they are gravitationally drained away. these elements can be used in a variety of industrial applications and they play a major role in protecting compressed air dryers and components used in downstream of the process.

Features of a coalescing element

  • Ideal to eliminate the Sub-Micron Contaminants
  • Ability to operate under high flow rates and low-pressure drops
  • Completely disposable
  • Ability to function well in a wide range of corrosive applications
  • Ideal for various adsorbent gases

Particulate Elements:

Particulate elements help to protect downstream air and processes from dust, dirt, and other solid particles as well as loose metal particles that enter the system with the stream of air. These filters have an outside to inside flow where particles hit and trapped at the outside of the filter, as the scrubbed compressed air travels through the element to the inside and downstream. Usually, Particulate elements are installed after a compressed air dryer, to prevent a variety of particulates, desiccants, rust scale, and a multitude of other contaminants inhaled by the compressor to migrate downstream.

How do you Optimize the Efficiency of Oil Coalescing Filter?

  • Reducing the Operating Temperature
  • Reducing the Viscosity of Process Fluid
  • Increasing Fluid Velocity
  • Using a Smaller Pore-sized Filter Media

At FLTR, we offer highly efficient coalescing filters and particulate filters. Contact us now for all your filter replacements

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