How dry cleaning works
By Marshall Brain
http://www.JewishWorldReview.com | (MCT) >Everyone has seen a washing machine. You throw in a load of clothes, add soap and turn it on. Gallons of water pour in to wash and rinse the clothes. It's not that different from the days of washing clothes in the river.
Dry cleaning, on the other hand, is a mystery. You drop off your clothes, and then what? How can you clean something without using water?
The reason it is called "dry" cleaning is not because it is dry. It's because the process uses something other than water as the solvent. In the early days of dry cleaning, the solvent might have been kerosene or some other petroleum-based solvent. Today the most common solvent is called perchlorethylene, or perc. There are two advantages to avoiding water. First, a solvent like perc works better than water on oily stains. Second, most natural fibers that would shrink or fade in water don't shrink or fade in a "dry" solvent.
Because a dry cleaner uses a solvent, however, the "washing machine" is much more complex. A typical dry cleaning machine is massive and can hold up to 100 pounds of clothes. It circulates and reuses the solvent, so there are pumps and filters in the machine to keep the solvent clean. A typical machine might pump 1,500 gallons of solvent per hour through its filters, spraying clean solvent on the clothes continuously.
Once the wash cycle finishes, pumps move all the solvent to a storage tank. The same machine dries the clothes. Warm air flows through the clothes and evaporates the solvent. Cold coils condense the solvent in the air back to a liquid and recycle it so that the solvent does not end up in the environment. There is a tiny amount of solvent left in the cloth, and that is what you smell when you pick up your clothes from the cleaner. The good news is that perc smells better than the kerosene used in the past.
There is more to dry cleaning than just the washing. When you drop off your clothes at the cleaner, the first step is tagging. Each garment you bring gets a numbered tag that helps keep track of your order. Then the clothes are checked for stains and damage, and anything left in the pockets gets removed. Stains are pre-treated with chemicals. The clothes go into the dry cleaning machine, come out and get inspected again. Then they go to a set of rather surprising ironing machines. There are machines specially shaped so they can quickly iron sleeves, collars, shirts and pants. Then your order is assembled, bagged and hung.
Even though 85 percent of the 30,000 dry cleaners in the United States use perc, there is a movement to phase it out because it has several environmental problems. Perc is toxic, and when spilled it gets into the air and water and can affect worker health. There is evidence that perc causes cancer in rats and people. Perc also off-gases in your home and some people are very sensitive to it. So dry cleaners have been trying to find a better, less toxic alternative.
One possibility is liquid carbon dioxide. If you pressurize pure carbon dioxide gas, it turns into a liquid that makes a very good solvent. A dry cleaning machine that uses liquid carbon dioxide is similar to a perc dry cleaning machine. The main difference is the pressurization. The liquid carbon dioxide circulates through the clothes using pumps and filters. The advantage of liquid carbon dioxide is that it is harmless to the environment. If spilled, it turns immediately to gas and dissipates into the air. There is no solvent left in your clothes to out-gas either.
There are other possibilities as well. Oil companies are trying to develop better petroleum solvents. Silicone liquid is a possibility. There is even some experimentation using water and ultrasonic waves, much like the system used to clean jewelry at a jewelry store. With all of these different dry cleaning systems, the goal is the same: to clean your clothes thoroughly, without shrinking or fading the fabric.
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© 2007, How Stuff Works Inc. Distributed by McClatchy-Tribune Information Services.