A candle filter element is a tubular filtration component used to separate solids from liquids or gases.

This guide explains the working principle, structure, filter media, applications, and key features of candle filter elements to help you better understand this filtration solution.

What Is a Candle Filter Element?

A candle filter element is a tubular filtration component used to separate solids from liquids or gases.

The candle filter gets its name from its long cylindrical shape, which looks similar to a candle.

Multiple candle filter elements are usually installed inside a pressure vessel. During filtration, fluid passes through the filter media, while solid particles are retained on the surface and form a filter cake.

Candle filters are commonly used for materials such as:

Application Material	Examples
Industrial chemicals	Fine chemicals, titanium dioxide, china clay
Mineral processing	Red mud
Gas and liquid treatment	Flue gas, brine
Other industries	Pharmaceutical, food processing

How Does a Candle Filter Element Work?

The filtration process of a candle filter element includes filling, pre-coating, filtration, washing, cake drying, heel expulsion, and discharge.

Step 1: Filling

Initially, the slurry enters the filtration vessel from the bottom and gradually fills the chamber.

As the slurry level rises, it pushes and compresses the air or gas remaining inside the upper part of the vessel.

The bottom inlet design helps keep solid particles suspended in the slurry, allowing them to distribute evenly on the surface of the candle filter element before filtration begins.

Step 2: Pre-coating

During pre-coating, the suspension circulates through the candle filter elements and creates a pre-coat layer on the filter media surface.

Pre-coating is required in two main situations:

When high-quality filtrate is required immediately after filtration starts.
When filtering high-viscosity slurry that may cause filter media blockage.

The pre-coat layer acts as an additional filtration barrier and helps prevent cloth blinding.

During cake discharge, the layer between the pre-coat and filter cloth can break away, leaving the filter media cleaner.

Step 3: Filtration

During filtration, the pressure inside the vessel pushes the slurry through the candle filter media.

The fluid enters through the filter media and flows into the internal headers before leaving through the filtrate outlet.

Meanwhile, solid particles are retained on the filter surface and gradually form a filter cake.

For applications with fine particles that are difficult to filter, a certain amount of filter aid can be added to improve cake permeability.

Note: When the filter cake itself is the desired product, filter aid is usually not required.

Step 4: Washing

After filtration, the remaining cake needs to be removed from the candle filter element.

A gas flow is introduced through the filter to remove residual liquid and push the filter cake away from the candle surface.

The solid cake then falls to the bottom of the vessel and is discharged through the discharge pipe.

Step 5: Cake Drying

After cake removal, hot air or steam can be introduced into the vessel.

This process removes the remaining moisture or filtrate trapped inside the filter cake, helping to achieve a drier final product.

Step 6: Heel Expulsion

After cake drying, air or gas is introduced into the vessel again to remove the remaining slurry or liquid residue.

The residual material is pushed out through the bottom pipe and returned to the feed tank.

Step 7: Discharge

During discharge, the slurry inlet and filtrate outlet are closed, while the bottom discharge port is opened.

The compressed air or gas inside the vessel flows out through the discharge port, creating a backwashing effect that helps remove the remaining cake from the candle filter elements.

Candle filter system

However, some cakes are hard to discharge.

For cakes that are difficult to remove, a vibration system can be added to shake the candle filter assembly and improve discharge performance.

Before then, ensure you interlock the outlet with a pressure sensor to prevent the outlet from opening when pressure builds up.

Finally, the unit would be ready for another set of the filling process.

What Is the Structure of a Candle Filter Element?

Candle filter structure

The candle filter element comprises three main components: the vessel, filtering elements, and cake discharge system.

▶ Vessel

The vessel configurations are of two types:

The conical shapes bottom vessels to enhance the filtration of cake and polishing.
Vessels with dish bottom shaped for thickening slurry.

The vessel operates at a pressure ranging 6-10 bar.

The filter feed is at the lower section of the vessel to enhance the flow of the injected slurry.

That kind of flow is essential in maintaining quick settling solids in a uniform suspension while ensuring that homogenous cakes form.

An overflow connection is at the dished top, which controls the flow velocity.

The filtrate outlet from each row of candles connects the horizontal header.

Subsequently, all the headers supply the filtrate via valves to the collection tank directed for extra processing.

A sight glass on every header allows the monitoring of the quality of the filtrate

Candle filters are suitable for both polishing and thickening of the filtrate to identify torn cloths.

▶ Candle Elements

The candle elements consist of the filtrate core and the filtering medium.

Filter elements are hollow tubes with a cap at the bottom and a threaded connector at the top connecting the header.

The filtrate core allows filtrate flow while supporting the filter medium.

The core may consist of perforated tubes or ball-like packing material inside a coarse mesh screen sleeve.

The filter medium is available in a wide range of types.

Common filter media include:

Stainless steel woven wire cloth
Wedge wire
Polypropylene
Nylon
PTFE
Cotton textiles
Polyester

▶ Cake Discharge System

Cake discharge takes place by either snap blow or vibration mechanism.

Snap blow from the backside of the medium is useful for releasing cakes that can be easily discharged.

The vibration mechanism shakes the whole pack of candles to release sticky cakes that are difficult to remove.

For more information about cake discharge methods, see How Can You Discharge Cake on Candle Filter Element?

What Are the Key Features and Benefits of Candle Filter Elements?

Candle filter elements are widely used in industrial filtration applications because they provide stable filtration performance, efficient cake separation, and flexible operation options.

Different shapes of candle filters

Feature	Description
Large filtration area	The tubular design provides a large filtration surface area. The filtration area can be 3–4 times larger than cylindrical screens, providing higher dirt holding capacity and stable filtration performance.
Fine filtration performance	Candle filter elements can remove fine particles from liquids and gases according to the selected filter media and micron rating, making them suitable for high-quality filtrate and polishing filtration applications.
High pressure resistance	Candle filters can withstand high pressure conditions, with some applications such as marine filtration requiring pressure resistance of more than 10 MPa.
Reusable filter media	Stainless steel and sintered metal candle filter elements can be cleaned through backwashing and reused, extending service life.
Easy installation and maintenance	Bolt or threaded connections make installation, disassembly, and cleaning easier.
Magnetic insert option	In large-volume oil filtration applications, magnetic inserts can be added to improve metal particle removal.

What Are the Applications of Candle Filter Elements?

Candle filter elements are used in various industries for liquid and gas filtration.

Industry	Application
Water Treatment	Removal of suspended particles and contaminants from water
Pharmaceutical Industry	Product purification and process filtration
Oil Processing Industry	Oil purification and removal of impurities
Chemical Industry	Filtration of chemicals and removal of unwanted substances
Food & Beverage Industry	Liquid clarification and purification
Automotive Industry	Engine oil and lubricant filtration

What Are the Filter Media Options for Candle Filter Elements?

The filter media is the real place in the candle filter, where the separation of the suspensions and the pure fluid takes place.

During filtration, the porous filter media allows the fluid to pass through while retaining suspended particles and forming a filter cake on the surface.

Candle filters features

Candle filter elements use different types of filter media depending on the application requirements, including:

Filter Media	Description
Porous ceramics	Suitable for applications requiring fine filtration and high temperature resistance.
Expanded sheets	Provide good mechanical strength and stable filtration performance.
Sintered metal tubes	Suitable for high pressure, high temperature, and reusable filtration applications.
Woven wire mesh / synthetic filter cloth	Available in different materials, weaves, and micron ratings for various filtration requirements.

The selection of suitable filter media depends on several factors, including:

Particle size and characteristics
Filtration accuracy (micron rating)
Chemical compatibility
Operating temperature and pressure
Cake formation and discharge requirements

Choosing the right filter media is an important step in candle filter design. The selection usually depends on application experience, testing results, and filtration requirements.

One common evaluation method is the pressure leaf test, which helps analyze cake formation behavior and filtration performance before selecting suitable filter media.

For more information about choosing suitable filter media and candle filter elements, see What Should You Look for When Buying Candle Filter Element?

What Is the Micron Rating of Candle Filter Element?

Micron rating indicates the filtration accuracy of the candle filter layer.

The filter layer, such as wire woven mesh, glass fiber, or filter paper, allows the filtrate to pass through while the filter cake remains trapped.

The micron rating depends on the required filtration precision for the application.

Common filtration ratings include:

5 micron
10 micron
20 micron
30 micron
40 micron
80 micron

For example, a 5-micron filter element means that the filter material only allows the passage of particles with a maximum dimension of approximately 5 microns.

Are Candle Filter Elements Efficient and How Do You Determine Their Efficiency?

Yes, candle filter elements are highly efficient in the filtration of liquids, colloidal substances, and other applications.

The efficiency of a candle filter element can be determined by checking the filtration accuracy, such as the micron rating.

Besides, the pressure difference of the filter can also be used as an evaluation factor. A higher pressure difference indicates a higher level of particle retention during filtration.

Are There Any Disadvantages of Candle Filter Elements?

Although candle filter elements provide reliable filtration performance, there are some limitations that should be considered before selection.

Limitation	Description
Maintenance space requirement	Sufficient overhead space is required for removing and replacing candle filter elements.
Cake discharge requirements	Some sticky or highly adhesive cakes may require additional discharge methods, such as vibration.
Proper filter selection required	Incorrect filter media or filter design may affect filtration efficiency and service life.

For applications with special operating conditions, factors such as cake characteristics, pressure conditions, and filter media compatibility should be evaluated before choosing a candle filter system.

When Should You Choose Candle Filter Elements?

You need candle filters in the following scenarios:

When you require minimum working floor space for your filtration application.
When you are dealing with highly volatile fluids that are not safe for use in vacuum filtration.
When you need excellent pure filtrate for polishing applications.
When your application involves saturated brines at extreme temperatures, a steam-jacketed tank may be required.
When you need to discharge either a dry cake or viscous slurry.
When dealing with products that are environmental pollutants, such as flammable, toxic, or volatile cakes, unique discharge mechanisms may be required.

However, attention should be paid during selection in the following cases:

If the cake becomes too thick, dense, and dry but the pressure is insufficient to keep it attached to the candle.
When coarse mesh is used, pre-coating may be required because it helps retain cakes with fine particles.

Candle Filter Element: Working Principle, Applications, and Key Features