Polypropylene Felt Bag Filter

Name: Polypropylene Felt Bag Filter
Brand: Prosestim

1–200 µm, pH 1–14 Chemical Compatibility and High Dirt-Holding Capacity

The polypropylene felt bag filter is a needle-punched felt PP filter that provides depth filtration in industrial liquids, with chemical compatibility and high dirt-holding capacity.

Diameter

100–180 mm

Boy

230–810 mm

Precision

1–200 µm

Max. Operating Temperature

90 °C (194 °F)

Polypropylene Felt Bag Filter

What Is a Polypropylene Felt Bag Filter?

Polypropylene felt is a nonwoven fabric produced by the needle-punched method. As liquid passes through it, particles are retained not only at the surface but throughout the depth of the filter fabric. This gives felt bag filters a higher dirt-holding capacity than many cartridge filters.

Thanks to their low cost, wide chemical compatibility and high efficiency, PP felt bag filters are widely used in chemicals, food, water treatment and metalworking. To minimise surface fibres coming loose into the process, “singed” or “glazed” (calendered) surface treatments can be applied.

Product Identity and Technical Summary

Polypropylene felt bag filters are made from 100% PP fibres, free of binders and resins; the bag is joined by ultrasonic welding or stitching. With a micron range of 1–200 µm and chemical resistance across pH 1–14, they are used on industrial, food and chemical process lines. The maximum operating temperature is 90 °C. The raw material is suitable for food contact; please consult our technical team for compliance documents.

Note: The values given are for standard production. Please consult our technical team for special application conditions.

Key Technical and Structural Features

Material: 100% pure polypropylene (PP) needle-punched felt; nonwoven, uniform depth fibre structure.
Micron Rating: 1 µm · 5 µm · 10 µm
25 µm · 50 µm · 75 µm
100 µm · 150 µm · 200 µm
Max. Operating Temperature: 90 °C (194 °F) continuous operating temperature; high dimensional and chemical stability.
Recommended Change-Out Pressure: Bag replacement is recommended when the inlet–outlet pressure difference (ΔP) reaches 2–2.5 bar.
Surface Finish: Singed (prevents fibre migration)
Glazed (Calendered)
Seam Construction: Fully Ultrasonically Welded (Leak-Tight)
5-Thread Industrial Stitching
pH Range & Chemical Resistance: High chemical resistance to acidic and alkaline solutions in the pH 1–14 range; process compatibility should be confirmed on hydrocarbon and solvent-based lines.
Food Compatibility: Made from raw material suitable for food contact; can be used in food, beverage and pure water processes. Consult our technical team for the required compliance documents.
Specific Gravity: Lightweight fibre architecture of 0.91 g/cm³ with a high pore volume.
Binder Content: 100% pure mechanical needling with no binders, resins, adhesives or silicone.

Filtration Architecture and Working Principle

Polypropylene felt bag filters pass dirty liquid from the inside of the bag to the outside in a three-stage treatment process. First, coarse particles are retained on the inner surface, then finer particles in the depth of the felt; particles down to the selected micron rating are thus removed from the system.

Filtration is based on the inside-out flow principle. The treated liquid collects inside the filter housing in which the bag filter sits and is removed from the system through the outlet connection. Making the bag filter longer than the basket height helps prevent the stretching, deformation and possible mechanical damage that differential pressure can cause.

STEP 01

Surface Filtration

Dirty liquid enters the bag and meets the inner surface. Large particles above the specified micron threshold are retained in the inner layer of the bag and cannot pass the filter wall. This first filtration stage stops coarse contaminants from reaching the rest of the system, protecting pumps, pipelines and equipment from wear.

PROCESS BENEFIT Stops coarse contaminants before they reach equipment. Protects pumps and pipelines from wear.

STEP 02

Small Particles

Smaller contaminants that get past the inner surface enter the deep fibre network of the polypropylene felt wall. As the liquid moves from inside to outside, tortuous micro-pores trap the particles. The uniform structure of PP fibres offers high dirt-holding capacity even under heavy particle loads.

PROCESS BENEFIT Captures micro-particles invisible to the eye. Safeguards product purity and consistent process quality.

STEP 03

Differential Pressure

Contamination building up inside the bag gradually makes it harder for liquid to pass out. The difference (ΔP) between the high pressure inside and the outlet pressure outside rises; when the 2–2.5 bar change-out threshold is reached, the bag is replaced to maintain process continuity.

PROCESS BENEFIT Replacing at the right time reduces unnecessary cost and the risk of system failure.

Advantages of the PP Felt Bag Filter

The polypropylene felt structure goes beyond surface filtration, retaining particles deep within the fibres. With a wide chemical resistance profile, low operating costs and applications ranging from food to chemicals, PP felt bag filters offer a long-term, reliable solution on industrial filtration lines.

Wide Chemical Resistance: Outstanding resistance to inorganic chemicals, acids and alkalis; technical confirmation is recommended for specific solvent applications.
Depth Filtration: Retains particles not only at the surface but throughout the thickness of the fabric.
Low Operating Costs: High capacity means fewer change-outs and a low unit price.
Food Compatibility: Raw material suitable for food contact.
High Flow Rate: Allows operation at high flow rates with minimal pressure loss.
Silicone-Free Production: No risk of cratering in paint and coating applications.
Ultrasonic Welding: A joint with no needle holes or thread, reducing the risk of leakage, fibre breakage and thread-related contamination.
Easy Installation: Collar options that fit standard housings perfectly.
Bypass Protection: With PP ring and snap ring collars, the collar seats in the housing under pressure, reducing the risk of edge leakage; models with handles make the bag easy to remove.

Bag Diameter and Length Options

PP felt bag filters fit a wide range of housings with standard 100 mm and 180 mm diameter options. Four standard sizes, from short to long, meet different flow and volume requirements while offering direct compatibility with your existing system.

100 mm Diameter

Suitable for small-scale systems and laboratory-type filter housings.

180 mm Diameter

The industry-standard size and the most widely used size for high-flow systems.

Custom Manufacturing

Bags can be made to sample for older or imported special equipment.

180 × 430 mm: Standard Large Diameter • Short Length (~0.25 m² Filter Area)
180 × 810 mm: Standard Large Diameter • Long Length (~0.50 m² Filter Area • Most Common)
100 × 230 mm: Compact Small Diameter • Short Length (~0.08 m² Filter Area)
100 × 380 mm: Compact Small Diameter • Long Length (~0.14 m² Filter Area)

Note: Filter area values are approximate; the exact area depends on bag length and the weld/seam allowance.

Micron Options and Precision

We offer a wide range from 1 micron to 200 microns according to the clarity your application requires. Efficiency in felt filters is nominal; the retention rate for particles of the stated micron rating is about 70%. The exact rate varies with the nature of the particles, the flow rate and the dirt load in the bag.

Micron (µm)	Precision Level and Typical Application
1 – 5 Microns (µm)	Precision Level: Very Fine Typical Application: Final filtration, clarification
10 – 25 Microns (µm)	Precision Level: Fine Typical Application: Fine chemicals, drinking water
50 – 75 Microns (µm)	Precision Level: Medium Typical Application: Process water, pre-filtration
100 – 200 Microns (µm)	Precision Level: Coarse Typical Application: Cooling towers, coarse particles

Collar Types and Adapters

A safe, leak-tight connection of the bag filter to the system starts with choosing the right collar. Six collar types, from PP rings to steel snap rings and from fabric flanges to drawstrings, can be selected according to the design of your existing housing, the operating pressure and the requirements of the chemical environment.

Critical Note: If you are not sure which collar type your filter has, you can send us a photo or ship a sample bag to our facility and ask us to produce an exact replica.

TECHNICAL ASSESSMENT BASED ON FIELD EXPERIENCE

Engineer's Note

The most common problem we see in the field with polypropylene felt bag filters is bypass caused by choosing the wrong collar type. Even if the filter medium has the right micron rating, filtration quality may fall short if the bag mouth does not seat fully on the housing’s sealing surface. The second most common problem is choosing a bag the same length as the basket. In that case the bag cannot sit fully on the bottom of the basket; as the pressure difference rises, the load falls not on the basket but on the bottom and seams of the bag, the bag tears, and the feedback from the field is that “the filter burst”. The bag should be chosen about 10 cm longer than the basket so that it sits on the bottom of the basket under pressure and the basket carries the load. When to choose it: in processes with a high particle load where the aim is to extend the filter change interval, a coarse + fine stage combination significantly extends the life of the main filter. When not to choose it: polypropylene felt should not be put into service without confirming chemical compatibility; swelling or mechanical degradation can occur with solvent-based liquids.

Caution In systems without differential pressure monitoring, bags may be changed too early or too late. Installing inlet/outlet pressure gauges quickly pays off by protecting filtration quality and avoiding unnecessary consumable costs.

Application and Selection Criteria

Collar Type and Flange Fit: Choose the collar type based on a photo or sample of your current filter; relying on catalogue dimensions alone is not enough.
Bag and Basket Length: Choose a bag about 10 cm longer than the basket. A bag the same length as the basket cannot reach the bottom and will tear at the base under pressure.
Stage Combination: In processes with a high particle load, combining a coarse and a fine stage can significantly extend the life of the main filter.
Chemical Compatibility (Solvents): Polypropylene felt can swell or degrade in solvent-based liquids; it should not be put into service without confirming chemical compatibility.

Industry Applications

With their chemical resistance, wide micron range and food-compatible materials, polypropylene felt bag filters work without interruption in very different industrial processes. From paint and coating lines to food processing plants and from water treatment systems to petrochemical processes, they can be integrated into any application with the right micron and collar combination.

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Performance Parameters

The field performance of a polypropylene felt bag filter depends on three main parameters considered together: flow capacity, the balance of service life against micron rating and surface area, and sealing. The depth structure of needle-punched felt spreads particles through the thickness of the fabric, slowing the pressure rise even with a high dirt load and extending bag life.

High Flow: Maximum liquid throughput at low differential pressure thanks to the large surface area and optimised pore structure.
Micron / Surface Area / Service Life Balance: Bag life is determined as much by filtration surface area as by micron rating. When enough surface area (the right bag length and number of bags) is chosen for the flow and solids load, the pressure difference rises more slowly; the right micron rating delivers the required clarity.
Sealing: The risk of bypass (leakage) is minimised, especially in ultrasonically welded models.

Filter Selection Technical Checklist

The wrong filter choice means early clogging, system failure and unnecessary cost. Choosing the right PP felt bag filter requires eleven critical parameters to be considered together, from the chemical nature of the fluid to the operating temperature and from the flow rate to the solids load. The checklist below is designed to help you make a fast, reliable choice on site and in technical discussions.

Chemical Nature of the Fluid: Choose the material according to the acid/base content (PP offers wide chemical compatibility with acids and alkalis).
Operating Temperature: If the liquid temperature is above 90 °C, Polyester (PE) should be chosen.
Required Cleanliness Level: Choose the coarsest micron rating that will not harm the process. At fine levels such as 1–5 µm, choose an ultrasonically welded bag to prevent needle-hole leakage.
Flow Rate: The flow rate in m³/h determines the filter length and number.
Filter Size and Number (Surface Area): Provide enough filtration surface area for the flow, viscosity and solids load. Bag length and the number of bags working in parallel are determined by this area; insufficient area means early clogging and short life.
Viscosity: As liquid viscosity increases (like the difference between water and honey), increase the filtration surface area rather than lowering the micron rating: a longer bag or more bags.
Collar (Ring) Type: It must seat fully in the existing filter housing seat.
Food Approvals: If there is food contact, confirm that the product is made from raw material suitable for food contact and check the compliance certificate.
Maximum Pressure: Check the pressure difference that the body and bag can withstand.
Ease of Change-Out: A handle/loop on the bag speeds up operation.

Points to Watch During Use

Getting maximum efficiency and life from a polypropylene felt bag filter depends on correct installation, the right collar choice and timely replacement. Sudden pressure fluctuations, incorrect installation, exceeding the 90 °C limit or going past the change-out threshold can permanently damage the felt structure, cause bypass and reduce filtration efficiency.

Initial Start-Up: Sudden pressure surges should be avoided when the filter is first put into service.
Timely Replacement: Replace the bag when the pressure difference (ΔP) between the inlet and outlet gauges reaches 2–2.5 bar. Exceeding this threshold can compact the felt, tear the bag and cause bypass.
Collar and Housing Fit: The collar type must match the sealing surface of the filter housing exactly.
Washing and Reuse: Whether a bag filter can be washed and reused must be assessed separately according to the nature of the retained particles, the filtration precision and process safety requirements.
Correct Placement & Installation: The bag must be placed correctly in the filter basket and the housing.
Basket and Seat Check: If the old bag is deformed, torn or bypassing at the edge, check not only the bag but also the filter basket and the sealing surface.
Ring and Gasket Protection: The PP ring, snap band or snap ring must not be damaged during installation.
Even Lid Tightening: The housing lid should not be overtightened; deformation of the collar creates bypass.

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Your Questions Answered

Frequently Asked Questions

At Prosestim, we are committed to answering your questions and providing technical support for our filtration solutions.

When should I replace the bag filter?

Replacement is generally recommended when the pressure difference between inlet and outlet reaches 2 to 2.5 bar.

Can PP bags be washed and reused?

Because felt bags provide depth filtration, dirt retained inside the fibre structure cannot be completely removed by washing, and the fibre structure can be damaged during washing. Reuse must therefore be assessed separately according to the nature of the retained particles, the required filtration precision and process safety. A new bag is recommended for precision and critical applications.

Why does bag length matter?

The longer the bag, the larger the filtration area, giving higher flow and longer service life.

What is the difference between welded and stitched bags?

In stitched models, needle holes can let unfiltered liquid through. In ultrasonically welded models, the felt edges are fused by heat generated by vibration, so there are no needle holes or thread; this prevents leakage through holes and contamination from thread.

Do you have food-contact-compliant PP filters?

Yes, we manufacture from raw material suitable for food contact. You can consult our technical team about the compliance documents required for food and beverage applications.

What is the maximum temperature it can withstand?

Polypropylene felt bags withstand up to 90 °C. For higher temperatures, polyester felt (up to 135 °C) or PTFE (up to 250 °C) should be chosen.

Can it be used with acidic liquids?

Yes. Polypropylene shows high chemical resistance to most acids; confirmation of chemical compatibility is recommended for highly concentrated oxidising acids.

What does a singed surface do?

It is a heat treatment that stops fibres breaking off and getting into the cleaned liquid.

Do you manufacture custom sizes?

Yes, we can manufacture to sample or technical drawing for all non-standard bag filter housings.

How do I know which micron rating to choose?

Our technical team will recommend the most suitable micron rating based on the type of contamination in your process and the clarity you need.

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