Glass fibers begin as silica glass preforms (shaped as rods or tubes) with fine silica particles deposited on or in them. The particles are consolidated by heating in the presence of chlorine, eliminating traces of moisture and hydroxide ions. Finished preforms typically are 3.3' (1 m) long and 4" (10 cm) in diameter.
Fibers are drawn by heating the preforms in a furnace until they melt. Gravity and surface tension act on the glass to form thin threads -- much as honey drips off a spoon. Each preform creates a fiber that is 7.44 miles (12 km) long and 0.125" (125 km) in diameter. The fibers are wound on a spool at the base of the drawing tower. Spools are 10 to 50' (3.048 to 15.24 m) tall, depending on the type and quality of the fiber made.
Helium plays several important roles in the process. For example, it is used with argon to provide shielding and with nitrogen for inerting. It also carries chlorine to the preforms during heating, helps control preform melting in the drawing process, and cools the newly formed fiber in the cooling towers.
Helium is an excellent cooling medium for the fiber because of its high heat capacity (20.78 J/moloK). During the cooling process, helium flows upward as the fiber travels downward. This countercurrent flow quickly lowers fiber temperature from more than 2,732oF (1,500oC) to between 212 and 392oF (100 and 200oC).
Because trace impurities such as hydrocarbons, carbon, transition metals and moisture can have a disastrous effect on fiber quality, manufacturers demand exceptional gas purity. Specifications for the helium used in cooling optical fibers require purity levels of 99.995 to 99.999%. Depending on production volume, manufacturers typically use 1 million to 100 million ft3 of helium per year to cool fibers during manufacturing.
Distribution SystemsDelivering high purity helium to a plant is just the start. The gas must get to the point of use efficiently in the amounts needed and without contamination. This calls for sophisticated design methods and careful engineering such as a computer-based design and analysis system.
This type of system relies on mathematical models and engineering calculations to define the thermodynamic, fluid dynamic and other behaviors of helium as it moves from on-site storage through the plant. It calculates essential engineering data such as pressure drops, gas flows and thermal effects using variables such as gas velocity and pressure, pipe size, temperature and sources of contamination.
Using computer simulation, engineers lay out pipes of the required size and characteristics, hook them up on screen, and calculate how the distribution system will function in real life. From the simulation, component lists are developed so constructors can enter accurate bids.
Recycle and RecoverEven though helium is a major cost item, most manufacturers release the helium/air mixture in their cooling towers to the atmosphere after it has cooled the fiber. This practice is changing as producers consider recovering and recycling helium to gain efficiency. Helium reuse becomes more critical when a plant expands -- volume needs jump nearly eight-fold when fiber capacity is doubled. In addition, helium is a nonrenewable substance, so recovery helps reduce overall depletion.
Helium emerges from the cooling tower as a 50 to 70% mixture in air. Recycling removes much of the air and boosts helium purity more than 97%. The recycling system collects as much of the helium/air mixture as possible from the cooling tower, separates out the helium and returns the right amount of purified helium to the tower.
Recycling can be done with pressure swing adsorption (PSA) or cryogenic separation. PSA often is preferred. A recovery system using PSA usually needs less power and can be turned on and off more easily than cryogenic separation. This system can recover more than 70% of the helium in a tower.
PSA works by selectively removing oxygen and nitrogen from the helium/air mixture in a series of chambers filled with a molecular sieve. The chambers are alternatively pressured and depressurized. During pressurization, oxygen and nitrogen are adsorbed onto the sieve while the helium passes through. Upon depressurization, the nitrogen and oxygen are released and vented.
Helium recovery and recycling systems can be integrated into existing facilities. In addition to reducing helium purchases, they allow for higher helium flow velocities so the fiber cools more rapidly, which lets manufacturers increase draw speed and productivity. Recovery and recycling systems typically have a one-year or less payback.
Gaseous helium is an excellent cooling medium that quickly reduces the temperature of molten glass and other materials. In addition to high heat capacity, it has a higher thermal conductivity and is less reactive than other gases. These properties not only make helium gas attractive for glass-fiber cooling, but also make helium suitable for other process cooling applications. For instance, it can be used to reduce the temperature of silicon wafers emerging from furnaces during semiconductor fabrication and to control the cooling of metals after they are heat treated with titanium and other rare metals in vacuum furnaces. PCE
Sidebar: Deriving Helum from Natural SourcesWith the number of helium sources limited and scattered around the globe, various forms of recovery and transportation are required.
Helium is derived from natural gas. Only a few natural gas helium sources exist, mostly in the United States in Texas, Kansas, Colorado, Wyoming and Utah. Other sources are located in Poland, Algeria and Russia.
Helium is recovered in a stripping operation that begins with removing water, carbon dioxide and other impurities by scrubbing and chemical means, followed by drying with alumina. Higher boiling hydrocarbons are liquefied and collected. The crude helium (about 70% concentration) is chilled by liquid nitrogen to liquefy most of the remaining gases. Adsorption of other gases on cooled, activated charcoal yields a purity of over 99.995% helium.
Helium is distributed as a gas or cryogenic liquid to bulk users and secondary distribution points worldwide. Gaseous helium is transported in forged-steel or aluminum-alloy cylinders at high pressure, typically 15 MPa (2,200 psi). Larger amounts of the gas are carried in large, horizontal, compressed gas cylinders that are manifolded on truck semitrailers (called tube trailers) or railroad cars.
In addition, large amounts of liquid helium are distributed globally. Liquid helium moves from the point of production near a natural gas field to transfill locations in 15,000-gal liquid trailers or containers that hold as much as 11,000 gal. The liquid then is shipped to customers.