Filters - Pleated vs. Depth

Which is better for your application?

Pleated versus Depth Filters – Which filter is better for your application?

Pleated filters initially cost more than depth filters but can save on maintenance costs and offer much longer operational time between change-outs, depending on the contaminants to be removed.  Depth filters, on the other hand, initially cost less and in some applications have a longer operational life compared with pleated filters.

In actual field conditions particulate contaminants are rarely in perfect spherical shape as are the glass beads used in laboratories for testing purposes, and may be in elongated or another shape, or they may be deformable gels.  For this reason, consideration must be given to the contaminant particle size distribution and composition when selecting the cartridge for best filtration.  

Pleated filters have many times more surface area as compared with depth type filters.  When the particle size distribution range is not wide a pleated filter can greatly extend the time between cartridge change-outs.  Initial pressure drop across the filtration unit will also be lower with pleated cartridges due to the increased amount of filter area.  The cost of pleated filters, typically higher than depth filters, is offset by the savings from longer operation between cartridge change-outs. 

Pleated filters with thin media are strictly surface filters and retain solid particles as they encounter the media.  Some solid particulates such as iron sulfides may shatter into smaller particles when they encounter surface type medias;  Depth filters retain particles as they pass through the media, slowing down as they are captured, and can be more effective for certain solid contaminants.

Pleated liquid filters are widely used in refinery and chemical plant processes because the processes in plants generally have already been filtered to remove bulk contaminants.  Solid contaminants which occur in the process should be similar in size and nature and can be efficiently removed with pleated surface media.

Depth filters can offer greater ability to retain more solids before plugging and requiring change-out if the particle size range of the contaminants is of varying sizes.  Natural gas filter separators, dry gas filters and gas-liquid coalescers use fiberglass, microglass, polyester or polypropylene depth media to capture particulates and/or coalesce liquid aerosols.  The media is compressed to varying degrees in order to provide graduating porosity for efficient solid particle removal.  Larger size solid particles will be captured in the larger openings in the outermost layer, with smaller particles becoming captured within the filter media as the fluid passes through the gradually reduced porosity of the media fiber layers. 

Pleated filters, typically in fiberglass, microglass, polypropylene or polyester, are also used for natural gas applications.  Gas-liquid coalescers often are furnished with pleated coalescer cartridges; Droplets enlarge as the flow passes through the pleats, growing in size as the pleats spread out, then falling to the bottom of the vessel and to the drain.  Pleated filters are sometimes used in natural gas filter separators if the contaminant particle size is not widely ranged.  Dry gas filters often use pleated filters for the benefit of the additional surface area.

 The following are several examples of filters used in the oil & gas, chemical and refining industries which depend upon depth or surface filtration to remove contaminants:

Wound filters, used in many liquid applications, are depth filters and are a good choice for capturing solid particulates and deformable gels.  Because of their efficiency and economy, wound filters are very often recommended for liquid filtration in the oil & gas, refining and chemical industries.  Wound filters are available in cotton and a range of synthetics.

Bag filters are available in felt material such as polyester or woven material such as nylon, and are generally surface filters.

High flow pleated filters are a good choice for many liquid applications due to the great amount of surface area within the pleats.  An advantage of high flow filters is the inside-to-outside flow path which helps to prevent spillage of contaminants during cartridge change-outs. 

Metal filters are surface filters used for high-temperature and/or caustic applications or backflushing operations and are cleanable and re-usable.  Pleated and non-pleated designs are available in stainless steel to handle many liquid applications where other media types may not be suitable.

Activated carbon filters capture impurities such as dissolved hydrocarbons in amine and glycol processes through the process of adsorption.  The “activated” carbon has been processed typically by exposure to high temperature steam to open pores in each carbon granule creating many times the original surface area.  Activated carbon filters are used for removal of volatile organic compounds, hydrogen sulfide, odors and other impurities.

Liquid-liquid coalescer cartridges are used for separation of two immiscible liquids such as trace water from kerosene or lube oil from an ammonia process.  This type of cartridge uses depth or pleated media to separate a smaller undesired liquid from a larger continuous phase liquid through the process of coalescence.  Small droplets of the discontinuous phase are enlarged in the coalescing filter cartridge and have enough difference in mass to settle out by gravity. 

While many types and styles of filter or coalescing cartridges are used in process industries, most utilize either depth or pleated, or a combination of both. 

Understanding the contaminants in a gas or liquid process is important in selecting the best filter for the service.  The payoff will be in fewer cartridge change-outs, lower operational costs, and most importantly, optimal filtration efficiency.