Avoiding burst damage to flatwork

The first six months of 1999 saw a marked increase in damage to rental flatwork which manifests itself as a pattern of small holes in sheeting and table linen. Pillowslips and towels are relatively unaffected. Examination of a large number of items has indicated common faults which, when put right by the engineer and the wash-house manager, produce a cure.

The underlying problem associated with most incidences of multiple holes continues to be unnecessarily high levels of bleaching with sodium hypochlorite coupled with processes which allow this bleach to get into the hot wash zone. The laundry industry still relies too heavily on chlorine bleaching for stain removal and the damage this causes can be made ten times worse if even a small amount of bleach gets into a main wash at temperatures above 60ºC. Even at 60ºC the damage can be rapid, weakening the cotton yarns and resulting in a low tear strength.

When bleach damage causes overall weakening of sheets and tablecloths the items do not necessarily tear or go into holes straight away—there is quite a wide window of tolerance and even severely damaged material can still have a useful life of several months. However, this relies on the items not encountering any excessive stresses when they are so weakened.

The stage in processing where the greatest tensions are applied to the fabric is usually in the extraction press after the tunnel washer. If a slug of water gets trapped in a ‘bag’ of fabric then sometimes the only way it can escape is to force itself to flow through the fabric itself. When a pattern of holes is found in a weak sheet with skeins of unbroken warp thread or unbroken weft thread across the holes this is entirely characteristic of burst damage in the press. If examination of the damaged area reveals no particular incidence of slubs or knots or other faults in the original cloth and there is no sign of damage from a sharp object, then that is almost certainly the problem.

Typical maximum pressures of 24 bar some years ago for a standard two-stage membrane press are now being superseded by pressures of 45 bar and higher for a modern single shot press.

There are two features to burst damage. The first concerns the presence of slugs of water in the batch of linen. The second concerns the rate of pressure increase relative to the time needed for the water to drain safely out of the work without giving rise to the problem.

In order to minimise the risk of large slugs of water being carried through into the press and trapped in the centre of the load, it is obviously important that each batch to the continuous batch washer is correctly sized with no overloading or excessive underloading. It is equally important that between batch transfer and a press building up to pressure there is adequate time for as much loose water as possible to drain away safely before the load is contained in the membrane or the basket.

A continuous batch washer is designed to operate continuously not intermittently. If the machine has been held artificially, waiting for a tumble dryer to become free for example, then a period of rotation is required prior to transfer. Otherwise there is always a risk of discharging forward an uneven mixture of textiles and water, regardless of whether the continuous batch washer has top transfer or bottom transfer—although top transfer poses a lower risk because it gives better drainage.

Slowly does it

As already indicated, the second point to take into account is the rate of build-up of pressure when the press goes into its extraction cycle. Obviously the slower the build-up the better because this will reduce the risk of trapped slugs of water causing rupture holes—but many continuous batch washer cycle times are quite short and do not allow much time at maximum pressure so there is some urgency about achieving this as quickly as possible.

The best advice to give on plants where multiple holes are a problem and where much of the linen in circulation is weakened by chemical damage is to slow the build-up of pressure from the pressure pump using a suitable throttle and at the same time to tune out an equivalent number of seconds from the various waiting times in the computer controller for each stage of the press cycle.

How can tumble dryers and calenders compensate for any reduction in moisture retention from the press?

Brand new tumblers are usually at peak efficiency and capable of drying their design load of cotton towelling in 18 minutes. The best would probably achieve 16 minutes. After two years of average laundry maintenance—or lack of it—the times will probably have increased.

But it is usually possible to re-tune a tumble dryer so as to give cycle times close to the ‘as new’ performance. For a gas-heated unit this means ensuring maximum air flow with correct temperature control which may require attention to thermostats and lint screens. For steam-heated dryers the maintenance needs to extend to steam dryness, steam trapping, air venting and fouling on the steam side.

Once the towel drying time in the tumble dryers is minimised attention can be turned to sheet conditioning. Ideally a rental sheet should require no longer in the tumble dryer than the time necessary to break the ‘cheese’. The calender itself is far more energy efficient at drying a sheet from 50% moisture retention. The rolls must be a perfect fit for the bed and this means checking roll diameter after each reclothing using the special metal tapes which should have been supplied with the new calender.

It is disturbing that many laundries have lost these and as a consequence are operating with calender capacities only 50% to 60% of the design throughput.

If sheets are starting to display patterns of holes, with groups of four or five holes occurring in localised areas, and if the tear strength of the damaged sheeting is noticeably lower than that of the new sheets being bought, then it is necessary to take two separate sets of actions.

Firstly, consider a reduction in bleaching levels to no more than 1 gram available chlorine per kg of work, equivalent to under 7 ml of concentrated bleach (as delivered) per kg of work. The ideal bleaching temperature is 50ºC or below. Even this level should be reduced if there are no problems with unremoved vegetable dye staining. Protein stains should be removed by better washing—there is no point in trying to burn them off the fabric by over bleaching.

Secondly, while the bleaching is being put right, attention should be turned to the membrane press—if burst damage is suspected, any changes necessary should be made to tunnel rotation following a tumbler hold, and pressure build-up in the press should be reduced.