Cotton Ginning Basics

Cotton January 04, 2008|Print

Machinery in the Saw Ginning System

Quality preservation during ginning requires the proper selection and operation of each machine in a ginning system. These decisions are based on the quality of cotton coming into the gin and the amount of trash and moisture content. The more uniform these parameters, the more consistent the ginning process. The generally utilized machinery sequence for spindle-picked cotton is as follows:


Typical gin equipment processes for picker harvested cotton.
Typical gin equipment processes for picker harvested cotton.

Ginning Machinery Sequences:

1. Module Feeder

2. Feed control

3. Tower drier

4. Cylinder cleaner

5. Stick machine

6. Tower drier

7. Cylinder cleaner

8. Impact cleaner (optional)

9. Extractor feeder

10. Gin stand

11. Lint cleaner(s)

12. Bale Press

13. Roller Ginning

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Inside the Cotton Gin


Contents

Feed Control and Pre-Cleaners

Gin machinery operates more efficiently when the cotton flow rate is constant. In early gins the flow rate was often erratic because of the variable work rate of the person unloading the wagon. The automatic feed control was developed to solve this problem by providing an even flow of cotton to the gin’s cleaning and drying system. The module feeder also performs a similar function and may be used to feed seed cotton directly from a module into the gin. The green boll trap is important for removing green bolls, rocks, and other heavy foreign matter from rough cotton. These large, heavy materials should be removed early in the ginning system to prevent damage to machinery and to preserve fiber quality. Green boll traps use sudden changes in flow direction and/or reduce air velocities to separate heavy foreign materials from seed cotton

Driers

The most important factor in preserving quality during ginning is the fiber moisture content. At higher moistures, cotton fibers are stronger, but trash is harder to remove and cleaning machinery is less efficient. At low moisture, fibers are easily broken. Consequently, controlling fiber moisture content is a compromise between good trash removal and quality preservation. For most conditions, cotton should be ginned at 6 to 7 percent lint moisture. Practically all of the gins are equipped with at least one stage of seed cotton drying and most ginning systems have two stages. The temperature of the conveying air is regulated to control the amount of drying. To prevent fiber damage, the maximum temperature in the drying system should be kept below 350 °F.

Cylinder Cleaners

Cylinder cleaners consist of six or seven revolving spiked cylinders that turn about 400 rpm. These cylinders convey the cotton over a series of grid rods or screens, agitate the cotton, and allow fine foreign materials such as leaf trash and dirt to fall through openings for disposal. In many gins, two cleaners are installed in parallel (split stream), with each one cleaning half the seed cotton. Cylinder cleaners break up large wads and generally get the cotton open and in good condition for additional cleaning and drying. Cylinder cleaners may also be used to remove seed cotton from the hot air line as it comes from the drier. They may be used in either a horizontal position or inclined at an angle of about 30 degrees (inclined cleaners).

Stick Machines

The stick machine was developed to remove the extra foreign matter taken from the plant by mechanical harvesters. Stick machines use the centrifugal force created by high-speed saw cylinders to sling off foreign material while the fiber is held by the saw. Inside a stick machine, seed cotton is wiped onto the saw teeth by stationary wire brushes. Grid bars or stationary wire brushes are located around the saw cylinder to reduce the amount of seed cotton that is thrown off the cylinder. The seed cotton which is thrown off with the foreign matter is picked up by reclaimer saws and put back into the seed cotton stream. Reclaimer saw cylinders are similar to main sling-off cylinders, but usually run slower and have more grid bars. The foreign matter that is slung off the reclaimer feeds into the trash handling system.

Extractor-Feeders

The primary function of an extractor-feeder is to feed seed cotton uniformly to the gin stand at controllable rates. Seed cotton cleaning is a secondary function. Feed rollers, located at the top of the extractor-feeder and directly under the distributor hopper, control the feed rate of seed cotton to the gin stand. These feed rollers are powered by variable-speed motors controlled manually or automatically by various interlocking systems with the gin stand.

Gin Stand

The gin stand consists of a set of saws rotating between ginning ribs. The saw teeth pass between the ribs at the ginning point. Here the leading edge of the teeth is approximately parallel to the rib to pull the fibers from the seed rather than cutting them. The actual ginning process (separation of lint and seed) takes place in the roll box of the gin stand. When all the long fibers are removed, the seeds slide down the face of the ginning rib between the saws and fall onto a conveyor under the stand. Lint is removed from the saw by a rotating brush. It is then conveyed to the next machine in the ginning system, usually a lint cleaner.

Lint Cleaners

Gins typically use two types of lint cleaners, air jet and saw. The air-jet lint cleaners are directly behind the gin stand and use centrifugal force to remove trash from the lint as it makes a sharp turn in the duct work. In a saw-type lint cleaner, a condenser removes the fiber from the conveying air stream and forms it into a batt. The batt is fed to a saw cylinder which normally rotates at approximately 1,000 revolutions per minute. The saws carry cotton over grid bars, which, aided by centrifugal force, remove immature seeds (motes) and foreign matter. The cleaned lint is removed from the saw by a rotating brush which also provides air to convey it to the next machine. Lint cleaners can improve the grade of cotton by removing foreign matter if the cotton has the necessary color and preparation characteristics. Lint cleaners may also blend light spotted cotton so that it becomes a white grade. But fiber length and several other important quality factors can be damaged by excessive lint cleaning, especially when the cotton is too dry (<5% moisture content).

For average machine-picked cotton, the first stage of saw-lint cleaning can remove 20-30 pounds of lint and foreign matter from each bale. The second lint cleaner would be expected to remove an additional 10-12 pounds and the third stage about 6 pounds. Determining the number of lint cleaners that gives maximum bale value is a compromise between increased grade and reduced length, turnout, and other fiber quality factors important to textile manufacturers. The price differentials for grade and staple length have a great influence on this decision. Under most circumstances, one or two saw-type lint cleaners will give the best economic returns. Consequently, ginning systems should be designed so that all saw-type lint cleaners after the first stage can be by-passed.

Bale Press

Cotton must be baled and packaged to protect it from contamination during transportation and storage. The U.S. textile industry has required that bales be packaged a standard dimension and density of 28 pounds per cubic foot. This standard bale supports storage, handling and process uniformity thoughout the marketing and processing system. Bale coverings and ties should meet the specifications developed by the Joint Cotton Industry Bale Packaging Committee.

Process Control

Computer control of the ginning process is one way to ensure that the appropriate drying and cleaning are done to the fiber. Process control uses instruments to determine trash, color and moisture content of the seed cotton and fiber throughout the ginning process. From this information, machine adjustments are continually made to the feed rate, the drying temperature and number of drying stages, the number of lint cleaners, and finally the moisture content of the fiber as it is packaged in the bale. Using market economics and standard machine operational parameters, seed cotton is processed to optimize its value to both the grower and the textile processor.

Roller Gins

Roller gins are used to preserve the quality of extra long staple (Pima) cottons grown in the western United States . Although only a small percentage of the U.S. cotton crop is Pima, it is a highly valued specialty crop which demands a premium price. Roller gins have a lower capacity than saw gins and consequently, the cost of roller ginning is higher than saw ginning.

History

The first mechanical gin was a roller gin. In 1840, Fones McCarthy invented a more efficient roller gin which consisted of a single leather ginning roller, a stationary knife, and a reciprocating knife which pulled the seed from the lint as the lint was held by the roller and stationary knife. Although the McCarthy gin was a major improvement over the Churka-type gin, machine vibration due to the reciprocating knife along with maintenance problems prohibited high ginning rates. In the late 1950s and early 1960s, a rotary-knife roller gin was developed by the USDA Southwestern Cotton Ginning Research Laboratory and adopted by the ginning industry. The ginning roller and stationary knife were retained from the McCarthy gin while a rotary knife replaced the reciprocating knife, eliminating the lost time of the backstroke of the reciprocating knife and reducing the vibration. The rotary knife roller gin stand allowed increased ginning rates and is currently the only roller-type gin used in the United States.

Roller Gin Equipment

Roller ginning systems normally include similar seed cotton cleaning as used for machine picked upland cotton. Cleaning equipment includes cylinder cleaners, stick machines, and revolving screen (impact) cleaners. Tower dryers and hot-air cylinder cleaners are commonly used for seed cotton drying. Optimum fiber-moisture content for roller ginning is 5 to 6 percent. Drying fiber lower than 4 percent can cause increased static-electricity problems and fiber breakage.

The ginning roller is the most important and expensive component in the gin stand. Roller-covering material is made from 13 layers of plain-woven cotton fabric cemented together with a white rubber compound. The fabric lays on the bias so that neither the warp or fill yarn are parallel to the direction of cutting; this prevents the material from unraveling from the roller surface. The roller material mounts on to the roller core with the cut edges of the fabric layers serving as the ginning surface.

Rotary-knife roller gin stands separate fiber from seed by using the frictional forces between a moving roller and fixed stationary-knife surface. During normal ginning, the roller-to-fiber force is greater than the stationary-knife-to-fiber force; therefore, the fiber sticks to the roller surface and slips on the stationary knife surface. Cotton is ginned as fibers adhered to the roller surface slip under the stationary knife which holds the seed.

The rotary knife clears the stationary knife edge of accumulated seed cotton and ginned and partially-ginned seed. Partially ginned seed are either pulled back to the stationary knife or swept along with the seed and later reclaimed. At the ginning point, seed cotton trash is separated with about 45 to 50 percent going with the lint and the remainder with the seed. The carryover reclaimer removes un-ginned and partially-ginned cotton from the seed flow and returns them to the distributor for ginning.

Lint cleaning in roller gins is different from saw gins and varies among locations. Traditionally, the mill-type opener/air-jet lint cleaner combination was used to remove motes, broken seed, entanglements caused by the machine pickers and pin trash not removed in seed cotton cleaning. Many of the mill-type openers have been replaced by cylinder and revolving screen (impact) cleaners used in combination with air-jet cleaners. Currently, the most common lint-cleaning sequence utilizes incline, impact, and air-jet cleaners. The PIMA bales are packaged using the same equipment as Upland cotton.