There has been a steady progression of technical innovations in membrane press operation and in tumble dryer design. Each of these has been relatively small when judged in isolation but the sum total of the economies that can be made now is significant.

The technical advances described can produce marked savings in the drying of towelling and similar products – hitherto the most energy inefficient part of the laundry.

The industry has been rather slow to recognise the importance of tuning the membrane press, perhaps put off by the detailed programming of the press computer that this entails.

The advent of an industry-specific scheme for laundry engineer training, led by Skillfast UK and the Sector Advisory Board, underlines the importance of the laundry engineer and press tuning is one of the key skills needed for success.

When the continuous tunnel washer was introduced into the UK, press stage times of three minutes were common and this met the needs of the sector, which had to adapt its ironer management to cope with twenty loads an hour.

Then it was found that the washer’s output could be increased simply by reducing the press stage time.

Progressive reductions down to 80seconds delivered over twice the design output and detergent suppliers spent the next twenty years developing systems to get the work clean and disinfected at this much higher throughput.

Complex sequence

Many operators overlooked the problems which reduced times caused at the membrane press, because of the complex sequence of steps and safety checks involved in verifying readiness, forming the cheese, bringing down the head, running up to pressure, releasing pressure, raising the head, checking shuttle readiness, discharging the cheese and so on. The number of checks and steps is over twice as many as in the simple sequence described.

The result in many cases was simply a reduction in the time the cheese spent at maximum pressure before it was smartly moved out of the way to make room for the next batch.

A press with a maximum of 27bar was found to need at least 60seconds at full pressure to reduce residual moisture to its optimum level and this was not possible with an 80second or a 90second stage time. The most common compromise was a reduction in time at full pressure to 30seconds, accepting that towels would need an extra few minutes in the tumbler to dry fully.

This was an expensive compromise as the dryers in use were still only 35 – 40% efficient and it was costing fifteen times more in energy terms to remove moisture in the dryer than it would do to squeeze it out in the press.

Membrane press suppliers reacted with a progressive range of designs that reduced residual moisture by raising the pressure on the cheese. Maximum pressures in the range of 50 – 60bar are now common and these have reduced residual moisture levels to below 45%.

The only precautions needed with these high pressure presses are a check to ensure that the incoming electricity supply can cope with the increased demand and care to avoid textile damage.

The other big improvement with high-pressure membrane presses is the fact that they have a much steeper moisture retention curve.

This means that although they might still need 60seconds at pressure to deliver optimum extraction, they will deliver economic levels of residual moisture with 30seconds or less.

Further benefits are anticipated with the latest range of design improvements now being incorporated. The first of these is the rate at which the press can be brought up to pressure.

Many seconds can be saved in each cycle if the hydraulic system can be brought towards the design pressure quickly, without bursting the textiles.

To avoid the tiny holes which can be created in weak fabrics, the hydraulic volume must be increased early in the cycle and then brought carefully under intelligent control for the last stages.

The second improvement concerns the rate at which water drains from the cheese and every designer has spent time and effort in optimising the basket construction, surface friction, hole-design and layout to maximise the outwards flow.

At the same time, the base of the press has to be made both very strong (to withstand the higher pressures) and very permeable (to maximise the downward flow). The lower face of the membrane itself has for some time been designed to promote radial flow across the rubber surface.

By incorporating these improvements into washing lines with press stage times of 80seconds, it is possible to achieve significant savings in residual moisture.

These translate into direct savings in gas in the dryers – or even greater savings for the less efficient steam dryers still in use.

The savings are only realised if the press is correctly tuned so that its cycle corresponds exactly with that of the tunnel washer it serves. The acceptance trials at commissioning should be carefully carried out and monitored to verify the savings.

The process should involve the metering of gas used for drying before the change and multiple measurements of the residual moisture achieved with the old press.

The order for the new press should then be placed against a written guarantee of the predicted savings. If identical metering trials and residual moisture measurements are then made after installation, the financial and energy saving can be readily verified. Most reputable suppliers of laundry machinery will be happy to help acceptance trials of this type, especially if they are allowed to use the results.

The advent of direct gas-fired dryers went some way to solving the problems of lower moisture extraction due to reduced stage times. The gas dryer is more efficient than the steam-heated one, as all the products of gas combustion go into the dryer and are used for heating, whereas if the gas is burnt to raise steam, a total of 30% of the energy is lost up the flue, in distribution and in condensate collection.

Gas also solved the problem of longer drying times, because it allowed higher temperatures than could be achieved with 8bar or even with 10bar steam. Provided the dryer did not singe the tips of the terry loops on the towels, the results were acceptable, although the towels were probably harsher, especially in hard water areas.

Now the sector is demanding much higher efficiencies in drying.

Radical redesign

Dryer manufacturers have addressed this with radical re-design of drying concepts. The first of these improvements has been the burner, which now generates uniform flames across the full width of the drum. This gives a drying air stream which dries the goods uniformly and shortens, overall times.

The second burner improvement has been modulation of both the air and the gas streams, so that as the demand for input heat falls towards the end of the drying cycle, the gas rate and the air rate are reduced in proportion. This gives a reduced gas input to produce the lower volume of heated air towards the end of the cycle.

The third improvement is the variation now possible in inlet air temperature and the ability of a modern dryer to vary this over the cycle so that the dryer is truly programmable. Producing a softer towel relies on a slightly lower temperature at the start of the cycle, building up to a maximum during the central constant rate of drying period and then reducing towards the final end-point to avoid hardness and harshness.

The initial lower temperature period is designed to open out the terry loop to give the desired softness, before this is blasted with very hot air in its compressed state.

The fourth improvement concerns the rotational speed of the cage.

Just as the producers of crease-free polyester-cotton garments found it necessary to fine-tune the speed of rotation in the washer–extractor to avoid roping creases, so too there is an optimum speed in tumble drying a towel.

At the start of the cycle the lifters can get hold of the work fairly easily to give a good “lift and drop” action to open out all of the towels very quickly.

Towards the end of the cycle a slightly higher speed is needed. Hence in tumble drying the optimum speed varies through the cycle. The best modern dryers have intelligent controllers which detect the dryness and vary the speed automatically.

This is particularly important when processing printed polyester curtaining which is generally better tumbled than calendered.

Fifthly, the latest dryer computers can work out the maximum amount of exhaust air to re-circulate into the air inlet. As soon as the outlet air is less than saturated, it pays to recycle some of it to make use of the residual drying capability. If this is done in an intelligent controlled manner, the amount of gas needed reduces but the drying time is virtually unchanged.The secret lies in the logic of the controller and precise, continuous positioning of the recycle damper.

Finally, the modern efficient rental dryer must have an automatic dryness end-point controller. This usually works by infra-red sensing of the surface temperature of the towels. Only by the use of an automatic terminator can the launderer cope with variations in towel type, load weight, residual moisture and temperature.

These improvements make it financially justifiable for modern rental plants to consider replacing dryers that were state of the art five years ago but now need upgrading.

All diagrams courtesy of Kannegiesser