Triumph of man’s ingenuity

One might have thought that, in more than 30 years, tunnel washer design would have become well stabilised and that machines being offered in 2001 would display few, if any, additional benefits. Fortunately, man’s ingenuity has risen to the challenge and current tunnel washer designs display some startling innovations.

Construction

A purchaser investing £250,000 in a new tunnel washer line is looking for 12 to 20 years trouble-free operation. To achieve this requires good welding, correct stress relief, adequate safety factors built into each component and so on. As a result, suppliers are now moving progressively towards stainless steel for every component in contact with the wash liquor. Kannegiesser’s Powertrans, available from Ducker UK, extends this to cover the pipework and the drum support rings, which are reportedly in solid stainless.

A tunnel washer is basically a rotating tube supported at two or three points. With repeated oscillation or rotation, lines of maximum stress will open up to form stress fractures or corrosion fractures, whichever weakness appears first. So the purchaser who rightly expects the life of the machine to extend well beyond its warranty will want reassurance over metal thickness, and key stress points.

Iron in tunnel washers has long been a problem because it rusts easily under laundry conditions. Iron oxide particles flowing into the wash zone with the incoming water will encourage chemical breakdown of cellulose in cotton by catalysing and accelerating the damage caused by chlorine bleach, especially if this gets into the hot wash zone itself. The move to stainless piping is overdue.

In a top transfer tunnel washer, the work is lifted and partially drained before being moved forward to the next compartment. A bottom transfer machine simply shunts the work and the liquor associated with it along the bottom of the machine into the next compartment. In the UK, 95% of machines supplied are of the bottom transfer type, whereas in France the reverse ratio applies.

Bottom transfer

The attraction of a bottom transfer machine is that it keeps the detergency in the main wash zone by moving this against the counterflow at the transfer point. This makes the machine particularly economical with medium to light soiled hotel work.

Top transfer machines are suited to heavy soil applications and they also, theoretically, give better rinsing.

However, the latest changes involving careful design of the draining arrangements in the transfer chute mean it is possible to tailor a top transfer machine quite precisely to produce optimum wash results with minimum consumption of water, steam and chemicals.

Heat recycling

There were various attempts made during the Eighties and Nineties to recover waste heat from the tunnel washer drain stream between the pre-wash and the main wash. But the results were not particularly good, because the internal recycling arrangements mean that this stream is at a relatively low temperature so the economics of heat recovery are poor. Even when sufficient heat can be recovered to make a heat exchanger worthwhile, it was found that using this to warm up the rinse water also resulted in an increase in the front end temperature of the machine. Higher pre-wash temperatures then caused setting of protein staining and soiling, and the overall wash quality declined after heat recovery was installed.

This was unfortunate because water extraction in the membrane press improves remarkably if the rinse temperature is raised. In addition, tumble drying gets off to a flying start with early evaporation leading to reduced tumbling times. Reduced quality and the capital costs of the heat exchanger meant that few were actually installed.

New detergent systems now make it possible to use hot rinsing without front end stain setting. Carefully developed systems are now available from detergent suppliers and from tunnel washer manufacturers. These are being sold partly on energy benefits, but the major attraction is the increase in throughput they offer for the high volume textile rental plant. This outweighs the considerable savings in water and steam because it overcomes the bottleneck at the press and tumble dryer. As a result, tumble dryer “holds” are virtually eliminated so that output increases without reducing stage time.

Improvements in press pressure

Most membrane presses in the UK still operate at around 24 bar pressure, producing good results on a 120 second cycle, but struggling to extract adequate water when stage times are reduced below this. The use of hot rinsing helps to offset reduction in press performance caused by inadequate time at pressure.

However, the greatest benefit is achieved with a modern press capable of achieving 47 bar and then applying hot rinsing to this. One operator has reported tumble dry times (for terry towels) falling from over 18 minutes to below 15 minutes, eliminating tumbler holds and raising line output by more than 10%.

The improvements in water economy offered by a tunnel washer over a washer- extractor (reducing consumption from between 20 and 30 litres per kilogram down to eight litres per kilogram) has had such a dramatic impact on rental costs that many believe that further substantial reductions are unlikely.

Rinse water recycling

However, the latest designs involve splitting of the rinse and press recycle streams and injecting a flushing recycle into the beginning of the rinse zone (to take away the initial heavy chemical contamination from the wash zone). As a result many rental operators are now approaching five litres per kilogram, and some claim better. These improvements have been mirrored on the continent with some laundries claiming figures consistently below four litres per kilogram.

These are quite credible claims with the revised flow systems, but maintaining economy and quality calls for good laundry management. In particular, it is essential to monitor alkalinity at the press, and many plants now scan this automatically and use the result to control a variable sour stream. This ensures that any occasional excess alkalinity is neutralised so that it cannot cause galling in the tumbler or the calender.

Another consequence of this rethinking of the rinse water flows is that rental laundries are consistently beating the magic 10 litres per kilogram water consumption target averaged over the entire plant.

Processing of mixed work

Suppliers of tunnel washers have always claimed that they can process every type of article from lightly soiled cotton sheets to oily overalls. However, exaggeration of early claims has seriously damaged credibility. So now, although improvements in designs mean that it is genuinely possible to process work formerly only handled by dedicated extractors, the tunnel washer sales teams still have an uphill battle on their hands.

In order to achieve replacement of a washer-extractor-based laundry processing mixed work with a single tunnel washer, it is necessary to design the unit with proper imperforate separation and superb control of soil and colour contamination in the recycle pipework and recycle tanks. Modern computer systems can handle this with ease, so on units designed with imperforate baths connected by their weir boxes it is now possible to achieve what were the over-optimistic claims of 20 years ago.

For this to be of commercial value it has to be achievable in a relatively small machine handling 25kg or 35kg batches, and the latest designs can do just that. So the medium-size mixed laundry, which would not have entertained a tunnel washer five years ago, is now going to be at a commercial disadvantage processing in anything else.

A tunnel washer is now the routine choice for garment processing. Washer-extractor-based plants are going to be at an increasing commercial disadvantage and unable to compete profitably given the very tight margins on some of the largest commercial contracts currently in force.

Two lines

The best economies come if the dedicated garment rental plant plans for two processing lines – one white, one coloured. This simplifies both machines and avoids the costs associated with bad separation and the net result is always better than trying to process everything through one large machine, even if it is correctly batched.

As a glance at the latest Lavatec design shows, the tunnel washer comes into its own with controlled cooldown so that thermal shock creasing can be minimised cheaply and easily as the batch passes from stage to stage, without penalty in time or water consumption.

The best modern designs also employ lifter and beater systems within each compartment which give the different types of wash action. With good lifter design it is possible to get a mixing action on the clockwise rotation and a squeezing action on the anticlockwise to produce just the mix needed for optimum and uniform soil release. The design of the perforation in the transfer chutes is also critical on garment machines, particularly on those designed for removal of heavy soil.

Pressing problem

Pressing has always been a problem on garment lines and the best results are achieved using one or two centrifuges (depending on the output). Even with intelligent computer control at the membrane press, it is unlikely to give the same consistency of minimum pressure creasing when compared with a well tuned centrifuge.

Of course, the greatest benefits on garment production come from design of the entire line. Those operators far sighted enough to incorporate rinse water heat recovery find that, provided garments are removed and hung from the centrifuge as soon as the cycle finishes, then excellent crease removal is obtained in the tunnel finisher and the tunnel finisher speed itself can be raised because of the warm start and lower moisture retention.

This does not work if warm garments are simply left in a barrow for half an hour because no tunnel finisher is going to remove the resulting pressure creases. Success calls for both good planning and good management, as always.

So where have we got to with tunnel washer design and how many more opportunities are still out there?

First, the design advances of the last three to five years mean that every rental operator, both flatwork and garment, has to take a careful look at current throughputs and current operating costs. In price sensitive markets the economies achievable with the latest systems are going to make a difference that will certainly approach one penny per piece and in some areas might approach double this.

In order to implement this new technology, you first need to know how good you are at the moment in terms of wash quality, weight throughput, litres of water per kilogram, kilowatt hours of drying energy per kilogram and market requirements.

Detailed look

You then need to take a detailed look at your existing equipment to find out whether you are getting the best out of it in terms of the parameters just mentioned. A long hard look here will almost certainly reveal potential benefits that can be achieved with little or no expenditure. The next step then is to identify the target parameters you must achieve in order to make a reasonable return, bearing in mind that these have to be achievable with justified investment where necessary.

So for lightly soiled rental flat work you should be aiming to achieve decent quality in terms of stain and soil removal and whiteness retention without excessive chemical damage using a stage time below 90 seconds in a 14-stage machine. And you should be looking at doing this with a target water consumption around five litres per kilogram and a washing energy consumption below 0.4 kilowatt hours per kilogram.

To do this without significant investment in new dryers you will need to be pushing your moisture retention from the press down below 50% for both sheets and towels, and you will need drying times that are significantly below 18 minutes.

If you are processing garments in washer extractors then you need a strategy for conversion to batch washing over the next four or five years – otherwise you could find your costs make your position in the market non-viable. It could well take five years to bring a development plan properly into action so it is vital to start thinking about it now and to identify positive benefits before getting frightened by the capital cost.

The equipment supply sector and the detergent suppliers have together given the industry some tremendous opportunities. But it is the rental operators that have to turn these into hard cash. The market leaders recognise this and have their plans in place, but the opportunities are there for all.