Blog

Steam Boiler

In a nonwovens plant a common method of providing heat for process applications is through the use of a boiler that generates steam. The steam may be used to heat ovens, calenders, area heaters, and rotary drying cans. Whatever the use, it is important for production employees to know what a boiler is and to be aware of safety issues involved with the generation and use of steam. The picture below shows a cutaway view of a modern steam boiler. Boilers primarily use oil or natural gas to produce the flames that heat the water in the boiler. The heating of the water produces steam which is piped to wherever heat is needed. Water turned into steam has the ability to hold a large amount of heat. Since steam is easy to convey through pipes it is very convenient to use in a manufacturing environment.

Most steam boilers use a closed system to conserve energy and water. The boiler heats water and turns it into steam. The process generates steam under pressure, usually in the range of 50 to 125 pounds per square inch. The pressurized steam pushes through pipes to the point of use. As the steam heats fabric or air, it gives up its heat and turns back into hot water. This water is pumped to a receiving tank near the boiler and when the boiler runs low on water, the water from the tank is pumped into the boiler to be turned into steam again.

Boilers contain numerous safety systems to insure that they operate properly. It is extremely important that a boiler does not become over pressurized as this could cause it to rupture and destroy itself and the equipment and building around it. No one should operate a boiler without proper instruction and training. Boilers must be properly maintained. They should also be inspected at least yearly.

Click for photo credit

Click for photo credit

Crush Cut Knife

Since there is such a variety of nonwoven fabrics, we use different types of knives to slit different types fabric. One type of knife used primarily in light weight and thermoplastic nonwovens is the crush cut knife. An older name for this type of knife is score cutter. The knife uses a semi sharp circular blade that is pneumatically loaded against a hardened anvil roll. The blade crushes the fibers and causes the fabric to split. Since it crushes the fibers and does not shear them, it does tend to leave some fuzz on the edge of the material. It works the same way a pizza cutter works. The anvil roll is actually covered with hardened sleeves that can be replaced. The sleeves are harder than the knife blade steel. The knife holders usually mount on a dovetail steel bar and are easily removed and repositioned.

As you can see in the picture, there are various types of blades that can be mounted in the knife holder. An advantage to this type of knife is that they are very thin and the knife holders can be mounted close together for cutting narrow strips of material. Another advantage is that the blades can quickly be changed when they get dull.

It is important to not that the blades have to be sharpened properly. Score cut knives are not sharpened to a razor sharp point. Rather, they have a rounded point usually of A (.002” to .004” radius), B (.006” to .008” radius) or C (.010” to .012” radius).

Fiber Blend

To achieve the desired characteristics such as softness, strength, thickness, color, etc. in a non-woven fabric made with staple fibers, it is quite often necessary to blend several types of fibers. For example, it may require that rayon be blended with polyester, or two or three different col-ors of polypropylene be blended together. The result of combining these fibers together, what-ever they may be, is called a fiber blend.

To achieve a consistent fiber blend requires that the fibers be carefully measured before being mixed together. It is almost like measuring flour and sugar to make a cake. The most common machines used to measure the fibers are weigh-pan hoppers. Fiber falls into a weigh-pan attached to load cells on the hopper. The weigh-pan fills until a preset weight is reached. When all the weigh-pans in the system reach their desired weight, they open and dump the fiber onto a conveyor and the process starts over again. The fibers are conveyed to other machines that will further open and mix the fibers.

A newer type of system uses small conveyors attached to the hoppers instead of weigh pans. These conveyors are mounted on load cells and can continuously weigh the fiber on them. Since they operate continuously instead of start-stop, they can process more fiber in a given amount of time.

These systems are controlled by computers that monitor the operation and insure that the amount of each fiber in the blend remains consistent. The computers also generate reports such as pounds per hour , total number of drops, and coefficient of variation.

Packing Ratio

Nonwoven fabrics range from soft and thick to hard and thin. The production of large mill rolls on a winder is a science in itself. One aspect of this process that a production employee needs to know is the packing ratio. Many of the winders used in nonwovens are the two drum surface winders like the one in the picture. The fabric enters between the two drums (rolls). The fabric comes in to contact with the first drum, is wound onto the fabric roll, and then comes into contact with the second drum. The second drum is often referred to as the packing drum. It usually revolves at a speed faster than the entry drum. The difference in speed between the two drums is called the packing ratio. The higher the packing ratio, the faster the packing drum goes with respect to the entry drum.
Most winders made today have individual motors for each drum to allow for instantaneous and infinitely variable packing ratio adjustments.

There are several things to watch for.

  1. A packing ratio too high on soft materials will cause loss of loft and possible tearing of the fabric.
  2. Usually the drums are covered with rubber or some type of lagging. If this surface is not kept clean and in good repair, the fabric can slip on the packing drum and the wind of the fabric roll will not be as tight as desired.
  3. It is possible to have a ratio too high on slick materials and the fabric will slip on the packing roll.
  4. If the winder has a top riding roll, the wind of the fabric roll is a combination of the packing ratio and the top roll pressure.

Derived Weight

Derived weight is the average weight per area measure within a roll of fabric. In the United States this is most commonly expressed in ounces or grams per square yard. It is calculated by weighing a roll of fabric and subtracting out the weight of the core. This net weight is then divided by the number of square yards of fabric on the roll. Squared yards is calculated by measuring the width of the roll in inches, dividing by 36 to convert to yards and them multiplying by the length of the roll in yards.

Here is an example:

The roll weighs 717 lbs. We know the core weighs 4.7 lbs. Subtracting we get the net weight which is 712.3 lbs. From the counter on the winder, we know the roll is 650 yards long. We measure the roll and it is 58.5 inches wide. We divide 58.5 by 36 inches per yard and we find the roll is 1.625 yards wide. Multiply the width in yards by the length in yards and we come up with 1,056.25 square yards of fabric on the roll. Convert the pounds the roll weighs (712.3) to ounces by multiplying by 16 since there are 16 ounces in a pound. This gives us 11,396.8 ounces. Divide by the square yards and we come up with a derived weight of 10.79 ounces per square yard.

Note that this is an average weight. There could be variations within the roll. Calculating the derived weight roll to roll gives a good indication how accurately the machine is maintaining its weight setting.

Hump Magnet

Loose pieces of metal in a fiber system can be destructive. They can cause sparks which in turn can cause fires. They can also tear up the wire on a card. It is imperative that metal is prevented from going through a fiber system. One of the primary defenses in the battle against this is the hump magnet. Magnets will attract ferrous metal, that is any metal containing iron or steel. Magnets will not attract aluminum or bronze.

Hump magnets are located between the balefeeds and the next piece of machinery which is usually some type of reserve. Fiber is blown through the hump magnet. The hump magnet actually contains two powerful magnets. Because of the magnet’s shape, the fiber is first blown against one magnet and then the other. Pieces of ferrous metal will stick to the magnets if they are not imbedded too deeply in the fiber.

The magnets are on hinges allowing them to be opened and cleaned when the fiber system is shut off. They should be cleaned daily to prevent a buildup of trash that would make the magnets ineffective. The magnets are very powerful so be careful not to get your watch or cell phone near them. The magnets can destroy a watch or phone.

Downtime

The most important resource in any manufacturing plant is time. It is one resource that cannot be bought and you only get a certain amount every day. Therefore, the wise use of time is critical. Machines cannot operate 100% of the time. When a ma-chine is not in operation producing product we say that machine is incurring down-time. Some downtime is positive and some is negative. Downtime must be scheduled to maintain and clean machines to keep them operating at peak efficiency. We call this planned downtime. It is placed in the production schedule and everyone prepares for planned downtime by having everything ready to make the best use of the time while the machine is down. The amount of planned downtime should be as little as possible, but as much as needed. If enough planned downtime is not included in the schedule, the machine will start shutting down unexpectedly.

The negative downtime is called unplanned downtime. Unplanned downtime is any stoppage of the machine that is not in the production schedule. Examples are electrical or mechanical breakdowns, power outages, lack of raw materials, operator errors, and quality problems. The goal for any manufacturing facility should be zero percent unplanned downtime. However, this goal cannot be achieved unless all employees realize the importance of preventing unplanned downtime.

Employees cannot concentrate on reducing unplanned downtime unless all downtime is measured and analyzed. The best method for measuring is to have the machines automatically record downtime into a computerized database program. Once the downtime is captured, the operators can explain the causes and plans can be formulated to eliminate those causes. Constantly being on the look out for impending downtime will go a long way to reducing it. The keys to this are constant patrolling of the production equipment and a good preventative maintenance program.

Magnehelic, Photohelic

In a nonwovens plant, staple fiber such as rayon, polyester, polypropylene and natural fiber such as cotton is often either blown or sucked through ductwork. The pressure or vacuum in the ductwork must be maintained at a certain value for the system to operate properly. Two very common devices for measuring and maintaining pressure and vacuum are the magnehelic and photohelic. These devices come with two ports to connect them to ductwork using a piece of plastic tubing. One port measures vacuum and the other pressure. They can be obtained in a wide variety of pressure/vacuum ranges and with various units of measure such as inches of water, Pascals, pounds per square inch, and inches of mercury.

The magnehelic is simply an indicator of pressure or vacuum. It is useful for monitoring what is happening in the fiber system.

The photohelic is not only an indicator but is also a switch with adjustable high and low limits. The red hands on the photohelic are adjustable and can be set anywhere on the dial. When the black indicator hand is between the two red hands, nothing happens. When it falls below the left red hand a switch closes. When it goes above the right red hand a different switch closes.

A typical use for the photohelic is to sense the fiber level in a fiber reserve. When the fiber level is low, air will escape from the reserve and the black hand of the photohelic will drop be-low the left red hand. This will signal the system to feed more fiber. As the reserve fills, the backpressure will build and the black hand will go above the right red hand. This will stop the flow of fiber.

Variable Speed Motor Controller

Nonwoven machines are powered by electric motors. Many of the motors on these machines can be sped up or slowed as needed. This is done using variable speed motor controllers. Variable speed motor controllers are electronic devices, often called “drives”, that can change the speed of a motor either manually, when an operator turns a potentiometer, or automatically as when a motor changes speed in response to a change in tension. Normally machine operators do not adjust the controllers themselves as they are shut up in control cabinets. The most common type of controller used is an AC inverter driving an AC motor. However, there are still many DC motors and controllers in service. In a large nonwoven line, there can be 30 or more motor drives that communicate with each other over networks. This communication enables the drives to keep all sections of a nonwoven line operating at the correct speed.

Safety Interlock

Operator safety is a major concern in a nonwovens plant. To prevent operators form getting into running machinery, safety interlocks are employed to keep the doors shut until the machinery has come to a complete stop. The interlock contains an electric solenoid that will not re-lease the tang until the machine is stopped. The interlock is usually placed on the machine and the tang on the door. Additionally some interlocks may have secondary safety devices. The picture at the top right shows an interlock with a screw that must be backed out before the inter-lock will open. Since the screw is long and the threads fine, it takes a couple minutes to open the interlock thus allowing, in this instance, a doffer roll to come to a complete stop. The bot-tom right picture shows an interlock combined with a push button. The button lights when certain electrical conditions are met. Once the button is lit, it can be pushed and the interlock will open.

No one should ever attempt to electrically or mechanically defeat a safety interlock.