The membrane press remains the cheapest and fastest way of removing water from the "cheese" of textiles on a tunnel washer line. As such it can be the key to achieving greater productivity and utility savings.
The market leaders who first started using tunnel washers in the 1970s and 1980s recognised this when they found that reducing cycle times on the washer did not improve output unless they could also shorten the membrane press cycle without leaving any more moisture in the cheese of pressed textiles.
This required tuning the "waiting" times in the computerised sequence to allow more time for the press to exert its maximum pressure and so squeeze out the same amount of water as before.
The same technology is now being used to extract that little bit extra moisture from the linen before it goes to the dryers and so help to meet the UK laundry sector’s commitment to the Climate Change Agreement and reduce the effect of the associated levy on laundry costings.
This is likely to be successful because in most laundries there is still plenty of scope for tuning modern presses with much greater precision.
The potential for savings in fossil fuel in the dryers is very substantial.

Press bursts with percale
The incidence of patterns of tiny holes in fine linen is still a major problem in some plants. The symptom is usually a random pattern of single thread breaks (from five to 10 usually) within the span of a widespread hand.
The problem is greatest with the finest and most expensive fabrics, such as 200 and 300 thread count percale materials. The reason why these fabrics are so sensitive is that the interstices in the weave (the tiny gaps between the threads) are so small that the permeability of the fabric is reduced to a low level.
When the press head comes down and tries to squeeze the water through the fabric and out of the drain holes in the sides and base, it cannot flow through the fabric quickly enough.
Because water is incompressible and the press is very powerful, the textile gives way. Slugs of water cause the fabric to balloon under pressure and eventually five or more threads will break, leading to a localised pattern of holes.
The problem is even worse on new linen. This will still have its fabric dressing, which makes the fabric even less permeable and more prone to failures of this kind.
If the fabric dressing is not washed out in the initial pre-wash before first issue, the problem will persist over the first five to ten washes.
Many launderers reduce the load factor for new percale linens for this reason. However, this is a very expensive cure because it can increase the wash cost by 40%.
A much better solution is to seek help to ensure that the dressing is removed in the first pre-wash, using special chemicals if necessary.

Bursts caused by fast ramps
Modern presses are able to bring down the ram quickly and ramp up the pressure fast enough to cope with very short cycle times.
These work very well with terry towels, which need a high press pressure and as long a time as possible at maximum pressure to produce the shorter drying times and low drying costs required to achieve high towel productivity. However, this can be disastrous with fine sheeting materials, as it may lead to widespread bursting.
One solution is to reduce the slugs in the cheese by tamping – that is, by bringing the press head into contact the work with a low force, then raising it and repeating the "tamp" two or three more times.
This does work but it consumes valuable seconds and relies more on gravity flow to let the slugs of water subside. A better technique, if the control of the tunnel washer allows it, is to start the drain from the last compartment 30 seconds before transfer, leaving just enough water with the textiles to ensure flow down the chute into the press basket.
This will result in far fewer slugs of water and a reduced risk of bursting without the need for tamping.
The process is improved still further if the press controller allows variable rates of increase up to pressure.
A slower ramp-up to full pressure is generally much quicker than tamping and more effective, especially when combined with pre-draining.
The key is to establish the fastest ramp consistent with avoiding textile damage and then tune the wait times to obtain the maximum time at pressure.
With the early 24bar presses this maximum time used to be 60seconds.Now, with press pressures of over 40bar, anything over 30 seconds is economic but the target of maximising the time is still worthwhile.
Those laundries at the forefront of minimising energy costs have realised that this is one of the low-investment routes to doing so.
All it needs is management effort and skilful laundry engineering.

High press pressures – right up to 56 bar – will not cause irremovable creasing, with two provisos. If the rinse water temperature is above ambient, then, as the press pressure goes above 45 – 50C, creases can be set into the sheeting, which can only be pressed out on a well tuned ironer with a good roll-to-bed fit and a good roll-to-roll stretch.
It is above this temperature that creases in terry towelling become so firmly fixed that they cannot be removed in the tumble dryer.
Creasing problems become much more acute with cotton-rich fabrics containing 80% or 70% cotton.
With 50:50 polyester cotton sheeting they can become very severe, especially on old stock where much of the cotton has been rotted away with bleach.
Ironing these successfully needs ironers in tip-top condition.

Shear tears
Membrane presses do need to be kept in perfect working order. If they suffer a "jam-up" with two cheeses under the ram at the same time, then the high forces involved mean that mechanical damage is almost inevitable.
Jam-ups are commonly caused by a cheese sticking to the ram membrane at the start of the up-stroke. If the ram, with the cheese attached, is then brought down onto the next batch in the basket, the pressure behind the ram can be sufficient to distort the basket, so that it is no longer quite circular in cross-section.
The cause of the cheese sticking to the ram is usually air in the membrane, leaking past the washer on the bleed nipple on the up-stroke. As a result the ram is an imperfect fit in the distorted basket. The gaps, which appear around the edge of the ram, allow textiles to slip in between the ram and the basket. This leads to shear tearing of the odd sheet or duvet cover, producing multiple parallel stress tears in the cloth. These stress tears will typically cross the line of the weave so they look different from tears caused when fabric gets trapped in a trolley wheel or jammed in the folder.
Regular checks should be made by looking up onto the underside of the ram (using a torch and camera) from the press mat, to monitor and correct faults of this type before they become an expensive problem.

Moisture retention
The key to minimising drying costs is to achieve the minimum moisture retention in every cheese of pressed textiles, because it is five times more expensive in energy terms to remove moisture in the calender and fifteen times more expensive to tumble dry it out.
This is why buying the most appropriate membrane press and tuning it correctly are vital components in the plant’s overall energy management and cost control strategies.
While early membrane presses did well to achieve 54 – 56% moisture retention, modern units can now get down to 45% for cotton and well below this for cotton-rich and polyester-cotton blends. The shrewdest launderers strive for the optimum performance for every classification and recognise that an extra 0.1% can be worth many thousands of pounds per year.
Of even greater importance is the reduction in tumble dryer minutes that can be achieved by minimising moisture retention.
The increased productivity that this can bring is often worth more in the busy months than the saving in energy, because it can eliminate the "evening towel shift", which cannot be justified in most plants.
All it needs is for the membrane press to be tuned and for the dryers to be brought to peak performance.

Rinsing in the membrane press
The latest tunnel washer design brings the membrane press into the body and allows rinsing in the press itself, doing away with the tunnel washer’s rinse zone.
Although experience with this type of unit is as yet limited, the design can certainly be made to work well.
The secret is to achieve perfect plug flow through the cheese, so all of the soiling and detergent chemistry is forced through by the single charge of clean rinse water.
This requires a compacted cheese and very careful control of the incoming flow, so that there is no channelling and removal is complete. This technology has much to offer and it has not yet realised its full potential.

PRESS BURSTS: Membrane press bursts can be recognised by the pattern of tiny holes, each one a single thread break, within a single hand-span