Cotton Woven Cotton Filter Bag

A cotton woven filter bag is one of the oldest and simplest filter elements in dust collection. The fabric woven from cotton yarn retains dust in systems operating at low temperature and with dry gas. It is still widely used in units with mechanical shaking, in workshop-type dust extraction systems and in vacuum cleaners. This page describes the bag’s construction, operation, limits, and manufacturing and maintenance conditions.

Construction of the Cotton Woven Filter Bag

The bag is sewn from a fabric obtained by weaving cotton yarn as weft and warp. In felt bags the fibers are randomly distributed, whereas in woven fabric the yarns form a regular weave. This structure gives the bag flexibility and allows it to withstand the shaking motion.

Cotton is a natural fiber. It retains more moisture than synthetic fibers, so static charge build-up on the surface stays lower. In a dry environment its chemical resistance is good, and it also resists abrasion well. However, mineral acids and oxidizers weaken the cotton fiber; it is not used in environments where these chemicals are present.

How Does a Cotton Filter Bag Work?

Cotton bags are mostly used in mechanical shaking units that filter from the inside out. Dusty air enters the inside of the bag, the dust is retained on the inner surface of the fabric, and clean air exits to the outside. Over time a dust layer forms on the inner surface; this layer acts as a second filter that captures fine dust.

When the pressure drop increases, the fan is stopped and the bags are shaken by the hanger mechanism from which they are suspended at the top. The accumulated dust falls into the dust hopper below the bag. This cleaning method is low-energy and suitable for woven fabric. The hard compressed air pulse in jet pulse units, on the other hand, is not designed for woven fabric; felt bags are used in those units.

In woven fabric, the first dust layer determines the efficiency. A newly installed bag may let some of the fine dust through in the first hours; efficiency rises once a thin dust layer has formed on the inner surface. For this reason, shaking is not done so hard or so often that it cleans the bag completely. The duration and frequency of shaking are kept just enough for the pressure drop to return to its previous level. Excessive shaking both lowers efficiency and wears the seams.

Technical Specifications of the Cotton Filter Bag

The properties of cotton woven fabric are summarized qualitatively in the table below. Fabric weight and air permeability vary with the selected weave type; exact values are given at the quotation stage.

Limits of the Cotton Bag

A cotton bag is economical and long-lasting in the right unit. Under the following conditions, another bag type is considered:

• Unit with jet pulse cleaning: a felt bag is required; on most lines a polyester filter bag is used.
• Acid vapor or oxidizing chemicals in the gas: the cotton fiber weakens.
• Gas temperature above 90°C: the fabric yellows and loses strength.
• Humid or sticky dust: the pores of the woven fabric clog, and shaking cannot remove the dust.
• Combustible dust: the low static build-up of cotton is not a substitute for explosion protection; an antistatic filter bag, grounding and the plant’s risk assessment are addressed separately.

Placing a cyclone at the start of the line, so that coarse shavings that may carry sparks do not reach the bag directly, also extends bag life.

Engineering and Manufacturing

The dimensions of cotton bags are often specific to the machine or unit. FLTS sews the bag to the hanger arrangement, the lower connection collar and the bag length of the existing unit. The following points are determined together during manufacturing:

• Fabric: the weave density is selected according to the fineness of the dust.
• Size: the diameter and length are taken from the distance between the hanging point and the lower collar.
• Connection: top hanger loop or lace; clamp, ring or lace at the lower collar.
• Seam: the seam line is made folded so that it does not bear load during shaking.

The fastest way is to send the old bag as a sample. The seam, the connection collar and the length are taken from the sample exactly.

Installation and Maintenance

During installation, the lower collar of the bag is seated fully on the nipple or ring on the dust hopper plate, and the clamp or lace is tightened with even tension. The upper hanger is adjusted so that the bag stays slightly taut. A bag that is too loose folds during shaking and wears at the same spot; one that is too taut opens up at the seam.

For maintenance, check that the shaking mechanism works and that the motor and hanger connections are sound. If the pressure drop does not return to its previous level after shaking, the fabric has begun to clog. If there are holes, thinning or opened seams on the bag surface, it is replaced. The unit is not left standing in a humid environment; running it empty for a while before shutdown helps the fabric dry.

Factors Determining the Price of a Cotton Filter Bag

The cotton woven bag is in the economical group among filter bags. The main items that determine the price are the weave type of the fabric, the bag diameter and length, the connection type and the order quantity. For small-sized bags made specifically for a machine, the unit cost falls as the quantity increases. In the evaluation, the replacement frequency of the bag and the time the unit is stopped during replacement are also taken into account. For other bag types, see the filter bags category.