Candle Filter Basics and Components

Candle Filters are also known as Backwashing Tubular Filters. This pressure filter is ideal as for either polishing or thickening. Candle Filters do not have any moving parts and can process multiple batches without direct operator supervision. This filter has three components, the vessel, the candles, and the discharge mechanism.

The Vessel operates under pressures between 6-10 bar. If the filter is designed for polishing its bottom is conical to assist in cake discharge. Thickener filters usually have a flatter bottom. Both configurations have are fed from the bottom to prevent solids from settling, and have a safety valve for overflow at the top. All the candles plug into headers that carry liquid filtrate out near the top of the filter.

Cake Discharge occurs by a controlled release of trapped air or gas and manipulating the slurry inlet, filtrate outlet, and sludge drain.    

Candles are named based on their shape. These filter elements are usually hollow tubes with a cap on the bottom and a threaded connector on top that attaches to a header. The outer face of the tube is usually made of punch plate, expanded metal, sintered metal, or porous ceramic to provide support for the filter media.

One filter media is stainless steel woven wire cloth, it is great for withstanding pressure and provides excellent cake release.

Another filter media is wedge wire, a triangle shaped wire that offers excellent abrasion resistance. 

Fabric media is the most versatile; polypropylene, nylon, teflon, polyester, and cotton textiles can be woven in a variety of style to allow filtration ranging from 1-900 micron. Fabric media also allows higher flow rates.

Candle Filter Operational Sequence

In its simplest set up, a Candle Filter can operate in three stages: Filling, Filtration, and Discharge. Depending on the process, the operation can include all or most of these steps listed below:

Precoating- If your solid is the product, then precoat is likely not for your operation since it mixes filter aid into the cake. If your slurry is sticky or if you need very clear filtrate right at start-up then running diatomaceous earth, perlite, or another filter aid prior to filtration may be a step you should take.

Filling- Slurry enters the vessel from the bottom, compressing the air or gas in the vessel above the candles and headers.

Filtration- Increasing pressure forces filtrate through the filter media on the candles, through the headers, and out the filtrate outlet. Solids are collected on the filter media and form a cake.

Washing- Spray bars near the headers can clean cake down the length of the candles.

Drying- Steam or gas replaces slurry and is forced through the cake and candles until there is no more filtrate to capture

Heel Removal- Air or gas fills the vessel and presses the remaining slurry out a pipe on the bottom and back into the feed tank.

Discharge- Both the slurry inlet and the filtrate outlet are closed. The sludge drain on the bottom of the vessel opens. The condensed air or gas trapped in a bubble above the candles and headers gets forced out the sludge drain. This backwash breaks the cake off the candles and it is discharged. The unit is now ready to begin filling again.

Troubleshooting and Industrial Filter Media Selection

New door gasket is leaking. If a replacement gasket was cut with straight ends, it may not create a perfect seal. Cut another with angled ends and install.

Torn or bent candles can be the result of excessive body feed or running the filter too long between cleaning cycles.

Cloudy filtrate or premature blinding. Most of National Filter Media’s experience troubleshooting candle filters has been in helping to determine the correct filter media for your application and operating conditions. Our textile engineer and industry specialists helped Shane develop a textile selection questionnaire that gives us all the information we need to recommend a better filter bag. Just contact Shane through the form in the Navbar to begin troubleshooting your filter media today.

Call NFM for Replacement Bags, Candles, and New Filters

National Filter Media has been fabricating replacement bags for Backwashing Tubular Filters for decades. We now offer a line of filters built to meet any code necessary. Contact Shane today for a new filter, candles or gaskets, or replacement bags. We supply parts for any OEM's filter.

Industrial Filter Media Identification

National Filter Media has the analytical equipment and expertise to identify fabric. Based on our findings you may choose to replace with the same filter media or upgrade to improve performance, reduce down time, or cut costs. Click the link to learn more.

Thanks for taking a look at  Shane has just begun building this filtration troubleshooting and basic information guide. If the article you are looking for isn't up yet, or your problem is beyond the basics, you can contact him anytime at Shane is happy to help or direct you to NFM's field service representative in your area.

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Industrial Filter Media Identification