What is Dry Cleaning 

Dry cleaning uses non-water-based solvents to remove soil and stains from clothes. The potential for using petroleum-based solvents in this manner was supposedly discovered in the mid-19th century by French dye-works owner Jean Baptiste Jolly, who noticed that his tablecloth became cleaner after his maid spilled kerosene on it. He subsequently developed a service cleaning people's clothes in this manner, which has became known as  "dry cleaning". Early dry cleaners used petroleum-based solvents, such as  petrol and kerosene. Flammability concerns led William Joseph Stoddard, a dry cleaner from Atlanta, to develop Stoddard solvent as a slightly less flammable alternative to gasoline-based solvents. The use of highly flammable petroleum solvents caused many fires and explosions, resulting in government regulation of dry cleaners. 

After World War1, dry cleaners began using chlorinated solvents. These solvents were much less flammable than petroleum solvents and had improved cleaning power. By the mid-1930s, the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), colloquially called “perc”, as the ideal solvent. It has excellent cleaning power and is stable, nonflammable, and gentle to most garments.  

Traditionally, the actual cleaning process was carried-out in centralized factories. High street Dry Cleaning shops received the garments from customers. They then sent them to the factory to have them processed (cleaned and pressed), then had them returned to the shop, where the customer could collect them. This was done mainly due to the risk of fire or dangerous fumes created by the cleaning process. 

Modern Dry Cleaning machines are fully enclosed. In these enclosed machines, solvent recovered during the drying process is returned, condensed and distilled, so it can be reused to clean further loads, or safely disposed of. The majority of modern enclosed machines also incorporate a computer controlled drying sensor, which will automatically sense when all possible traces of perc have been removed from load during the drying process. This system ensures that only the smallest amount of perc fumes will be released when opening the door at the end of the cycle. This is further minimized by the use of an exhaust fan causing a lower pressure inside the cage. This means that when the door is opened the lower pressure inside the cage causes the air outside to flow into the cage rather than the reverse.  

The Process 

To put the process into simple terms, a dry-cleaning machine is similar to a combination of a domestic washing machine, and clothes dryer. Garments are placed into a washing/extraction chamber (referred to as the basket, or drum), which is the core of the machine. The washing chamber contains a horizontal, perforated drum that rotates within an outer shell. The shell holds the solvent while the rotating drum holds the garment load. The basket capacity is between about 10 and 40 kg (20 to 80 lb). 

During the wash cycle, the chamber is filled approximately one-third full of solvent and begins to rotate, agitating the clothing. The solvent temperature is maintained at between 20 - 30 degrees Celsius. On refrigerated machines this temperature can be further lowered to as low as 14degrees Celsius for delicate fabrics and dyes. During the wash cycle, the solvent in the chamber (commonly known as the 'cage') is passed through a filtration chamber and then fed back into the 'cage'. This is known as the cycle and is continued for the wash duration. The solvent is then removed and sent to a distillation unit comprising a boiler and condenser. The condensed solvent is fed into a separator unit where any remaining water is separated from the solvent and then fed into the 'clean solvent' tank. The ideal flow rate is roughly 8 litres of solvent per kilogram of garments per minute, depending on the size of the machine. 

Garments are also checked for foreign objects. Items such as plastic pens will dissolve in the solvent bath and may damage textiles beyond recovery. Some textile dyes are "loose" (red being the main culprit), and will shed dye during solvent immersion. These will not be included in a load along with lighter-color textiles to avoid color transfer. The solvent used must be distilled to remove impurities that may transfer to clothing. Garments are checked for dry-cleaning compatibility, including fasteners. Many decorative fasteners either are not dry cleaning solvent proof or will not withstand the mechanical action of cleaning. These will be removed and re-stitched after the cleaning, or protected with a small padded protector. Fragile items, such as feather bedspreads or tasseled rugs or hangings, may be enclosed in a loose mesh bag. The density of perchloroethylene is around 1.6 g/cm³ at room temperature (60% heavier than water), and the sheer weight of absorbed solvent may cause the textile fibre or fabric to fail under the normal forces during the extraction cycle unless a mesh bag provides mechanical support. 

Many people believe that marks or stains can be removed by dry cleaning. Not every stain can be cleaned just by dry cleaning. Some need to be treated with spotting solvents; sometimes by steam jet or by soaking in special stain remover liquids before garments are washed or Dry Cleaned. Also, garments stored in soiled condition for a long time are difficult to bring back to their original color and texture. Natural fibers such as wool, cotton, and silk of lighter colors should not be left in dirty or soiled condition for long amounts of time as they absorb dirt in their texture and are unlikely to be restored to their original color and finish. 

A typical wash cycle lasts for 8–15 minutes depending on the type of garments and degree of soiling. During the first three minutes, solvent-soluble soils dissolve into the perchloroethylene and loose, insoluble soil comes off. It takes approximately ten to twelve minutes after the loose soil has come off to remove the ground-in insoluble soil from garments. Machines using hydrocarbon solvents or the new Silicone based “Green Earth” require a wash cycle of at least 25 minutes because of the much slower rate of solvation of solvent-soluble soils. A dry-cleaning surfactant  (soap) must also be added. 

At the end of the wash cycle, the machine can go through a second wash cycle. In the second wash  cycle the garment load is rinsed with fresh distilled solvent from the pure solvent tank and a detergent is added. This pure solvent wash prevents discoloration caused by soil particles being absorbed back onto the garment surface from the "dirty" working solvent. 

After the rinse cycle, the machine begins the extraction process, which recovers dry-cleaning solvent for reuse. Modern machines recover approximately 99.99% of the solvent employed by both extraction and evaporation processes. The extraction cycle begins by draining the solvent from the washing chamber and accelerating the basket to 350 to 450 rpm, causing much of the solvent to spin free of the fabric. Until this time the cleaning is done in normal temperature, the solvent is never heated in dry cleaning process. When no more solvent can be spun out, the machine starts the drying cycle. 

During the drying cycle, the garments are tumbled in a stream of warm air (60-63°C) that circulates through the basket, evaporating any traces of solvent left after the spin cycle. The air temperature is controlled to prevent heat damage to the garments. The exhausted warm air from the machine then passes through a chiller unit where solvent vapors are condensed and returned to the distilled solvent tank. Modern dry cleaning machines use a closed-loop system in which the chilled air is reheated and recirculated. This results in the high solvent recovery rates and reduced air pollution.  

After the drying cycle is complete, a deodorizing (aeration) cycle cools the garments and removes the last traces of solvent, by circulating cool outside air over the garments and then through a vapor recovery filter made from activated carbon and polymer resins. After the aeration cycle, the garments are clean and ready for pressing/finishing. 

Solvent Processing  Processing 

Working solvent from the washing chamber passes through several filtration steps before it is returned to the washing chamber. The first step is a button trap, which prevents small objects such as lint, fasteners, buttons, and coins from entering the solvent pump. 

Over time, a thin layer of filter cake (called muck) accumulates on the lint filter. The muck is removed regularly (commonly once per day) and then processed to recover solvent trapped in the muck. Modern machines use "spin disc filters," which remove the muck from the filter by centrifugal force while it is washed with solvent. The clean solvent is then returned to the working solvent tank. 

To enhance cleaning power, small amounts of detergent (0.5%-1.5%) are added to the working solvent and are essential to its functionality. These detergents help dissolve hydrophilic soils and keep soil from re-depositing on garments. Depending on the machine's design, either an anionic or a cationic detergent is used. 

Since the solvent recovery is less than 100%, and because dry-cleaning does not remove water-based stains well, entrepreneurs have developed the wet cleaning process, which is, in essence, cold-water washing and air drying, using a computer-controlled washer and dryer. In general, wet cleaning is regarded as being in its infancy, although low-tech versions of it have been used for centuries. 

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