Heavily soiled work contracts are a major challenge for the laundry manager but they can also have considerable engineering implications as well. In this article we examine the cleaning of heavily contaminated food, fish or abattoir work which needs a good wet wash process.

Drycleaning of heavily soiled, oily workwear also gives rise to problems that the engineer can assist with. The secret for the successful removal of protein soiling lies in the first wash. Loose debris must be flushed away with a medium dip but it is essential to run this at a temperature below 38°C for at least the first five minutes to avoid setting proteins onto the fabric in such a way that they become virtually impossible to wash off, however severe the main wash is made.

Once the proteins have been correctly softened—and most food industry work contains proteins of some sort—then the main wash must follow the established principles of good laundering: strong alkalinity, high temperature and high mechanical action.

Creasing of polyester cotton workwear can be minimised by loading the machine to 80% capacity but with cotton goods the best mechanical action is obtained with 100% loading. If there is any doubt then the load factor can be calculated on the basis of 20 litres per cage capacity per kg of dry cotton work.

Polyester cotton is best processed in an open pocket machine with large lifters to discourage rolling and roping creases. Pressure creases will be minimised if the diameter to depth ratio for the cage dimensions is as close to unity as possible.

The cooldown for polyester cotton work should not exceed 4°C per minute and, if the laundry has a hot water system available, then cooldown might be achieved most easily on the first rinse. This should be discussed with the laundry engineer.

Best practice The garments need to be rinsed so that the final rinse liquor contains less than 2g alkali per litre. If this needs three rinses rather than two then it makes sense to reduce the rinse dip.

Rinses use a great deal of water and the volume required for two 375 mm dips would give far better rinse performance if it is spread out over three rinses. This can be critical if the laundry is paying high charges for water and effluent or if water is in short supply.

The major problem the laundry engineer faces is complaints from its local water authority that the highly alkaline hot wash dump causes corrosion of Portland cement sewer pipes. This can be minimised by putting in sufficient holding capacity in the drain system to blend the hot wash liquor with the rinses, thereby cooling the discharge as well. Water authorities are getting very critical on discharge alkalinity but it is possible to minimise the problem before resorting to acid neutralisation.

Drycleaning solvent is the natural choice for removal of oil and grease because this does not wash off easily in water. It would require an aggressive detergent to promote emulsification and this would give rise to potential problems with effluent quality as well. From an environmental point of view, it is far better to transfer oily greasy soiling to the still of a drycleaning machine than to put it down the drain—and then pay the effluent company for its treatment.

Most industrial drycleaning units are equipped with a sludge cooker as well as a still so that dirty still bottoms can be run out into the sludge cooker before they create hard deposits on the still walls. A sludge cooker is designed with easy cleaning in mind and typically employs removable heater batteries to facilitate this. These would usually be cleaned on a weekly basis, either by production or engineering departments.

With a still/sludge cooker arrangement and a recovery head in good condition, it is usually possible to clean work to a good standard and recover all of the solvent at the end of the cycle. Good solvent recovery relies on rapid cooling water circulation at the recovery head, a low cooling water temperature from the cooling tower, clean heat transfer surfaces within the recovery head and good air flow around the drying circuit with no blocked filters. It is important that the filters themselves are kept in good condition with no holes. If not, lint and other debris blows through and blocks the heat transfer matrix in the recovery head—thereby creating a difficult and unnecessary cleaning operation.

So why do problems arise? The main problem, from the laundry engineer’s viewpoint, is the tendency for an industrial drycleaning machine to become slightly acid as a consequence of perchloroethylene solvent breaking down in the still itself.

This occurs if hot spots are allowed to develop on the still heating surfaces or in the sludge cooker. When this happens perchloroethylene breaks down to produce hydrogen chloride and other obnoxious chemicals.

Hydrogen chloride is the worst because it dissolves in any moisture in the still to create hydrochloric acid. This will progressively dissolve the machine components and can make the garments themselves slightly acid, encouraging the fabrics to rot. More seriously, some wearers have sensitive skins and acid fabric tends to encourage dermatitis, especially with garments that chafe against the skin in hot weather.

So how can this be avoided? The best way by far is to control the steam pressure to the still and sludge cooker using an appropriate pressure reducing valve. In this way hot spots are minimised—the lower the steam pressure the lower the steam temperature. It may be necessary to gradually increase the steam pressure over the working week as the heat transfer surfaces become contaminated but, even if this is done, it is possible to limit acid formation to an acceptable level.

It takes a few seconds to check the acidity in the machine on a weekly basis using a simple pH paper dipped into the water discharged from the separator. Ideally the pH should be between 6.5 and 7.5. Below this figure the machine becomes acid and can produce pungent odours and corrosion. Above this, the machine becomes alkaline which tends to encourage sewer smells and other foul odours.

The big attraction of drycleaning is the tremendous power it has to remove heavy oil and grease contamination. It is also able to concentrate the soiling removed into still residue, usually for sale as a premium fuel. The disadvantages of drycleaning are the high capital cost of the machinery and the continuing problems of coping with ever more stringent environmental legislation vis-á-vis solvent emissions.

Wetcleaning, on the other hand, is the only sensible way of removing waterbased soiling and this includes most fish, abattoir and food industry contamination, plus soiling which derives from the skin of the wearer.

The way forward for wet washing workwear must be in the use of continuous tunnel washers because only with these can the industry achieve chemical, water and steam economies. Even then, effluent is always going to be very expensive—one effect of water economy is to concentrate contamination in the effluent.

The major contributions a laundry engineer can make to a successful workwear business lies in correctly designing the machinery used for processing polyester cotton, having a solid understanding of the interaction between cooldown, effluent volume and effluent quality and a good control of water engineering including softening.

As far as drycleaning of workwear is concerned, the engineer has even more to offer: precise control of distillation conditions, superb maintenance of the recovery circuit and correct design of still residue handling.