Loading is the first consideration. Tunnel washers are designed to handle full loads economically and most operators achieve this for table linen, sheets and pillowcases.
Towels pose a particular problem because they arrive in the laundry with varying degrees of moisture from 0 – 75% of their weight so the operator cannot be sure how much of the load weight is water.
The average is known to be over 20%, which means that one-fifth of the washing and drying energy for towels is probably being wasted.
Processing towels requires a raw energy use of 0.4kWh/kg and drying energy use of 1.2kWh/kg so that 20% represents over 0.3kWh/kg.
Wise operators will have checked with the tunnel manufacturer to find the over-capacity tolerance of their particular machine. As 20% is quite typical, loading a 50kg tunnel washer to 60kg will not cause any greater risk of blockage but will raise the average dry towel batch size above the usual 47 – 50kg. The improvement in towel productivity in many plants is worth more than the resultant energy saving.
Cycle termination
A batch of dry towels weighing less than 50kg will probably dry faster than the designed drying time for a 50kg dryer. This can be capitalised upon if the dryer knows that the batch is dry before the end of the normal programmed time.
An automatic cycle terminator will monitor the actual temperature of the drying textiles (using its built-in infra-red pyrometer) and will override the dryer timer to stop the cycle when the towel temperature reaches a pre-set level.
This saves both time and energy. Tumble drying is the least energy-efficient process in the laundry so the benefits are disproportionate and include an increase in productivity and lower energy use as well as reducing greying in two- and three-year old towels.
Water levels
Laundries using tunnel washers can set water consumption precisely and consistently but more effort is needed to achieve economies with washer-extractors. So water and energy use for these washers varies widely from laundry to laundry. Plants that have tuned the dips down to the levels (which vary with cage diameter) recommended by the British Launderers Research Association many years ago have achieved water consumption ratios below 20litre/kg dry work.
Un-tuned plants typically deliver 30 – 45litre/kg. Energy use ratios vary correspondingly from 0.7 up to 1.1kWh raw energy/kg dry textiles.
Tuning the washer-extractor is time consuming. Start with the pre-wash and main-wash. When these are reduced to the correct settings wash quality usually improves because the mechanical action increases and the detergent dose can also be reduced to maintain the original concentration.
Taking out unnecessary rinse levels reduces water use, effluent production and process time. These savings are valuable, even in tunnel washer-dominated plants, because the washer-extractors use so much energy and water per kg.
Steam trap monitoring
Reducing energy use in steam-heated dryers and ironers relies on perfect entrapment and discharge of the liquid condensate and removing air from beds and heater batteries perfectly.
Modern steam traps achieve this when new but require fortnightly monitoring to maintain this performance. This can be done by an in-house engineer using a hand-held leak-detector and who is meticulous in doing so. Plants that maintain traps in good condition generally use about 10% less raw energy in raising steam compared with those that only check traps once a year. Poor trap maintenance not only leads to live steam leaking through the trap it can also lead to corroded air vents that also leak live steam.
Hydro-extraction tuning
Drying work on the calender is five times more expensive in energy use than removing water in the washer-extractor or membrane press and drying in the tumbler is 15 times more costly.
So each washer should be tuned to extract the maximum amount. The optimum spin cycle may increase from six minutes to nine but the savings will more than justify the extra time.
Membrane press efficiency is measured by the moisture retention of the pressed work. A modern press capable of achieving 40 – 60bar should yield a moisture retention below 45% for cotton sheets, depending on the press temperature.
Recent pressure on stage times has produced problems for membrane presses as a 2.5minute cycle may not allow enough time to achieve maximum pressure and retain it long enough to squeeze out all the water. The solution is to tune the “wait times” in the computer sequence to the minimum to make the time at pressure as near to 60seconds as possible. Resist the temptation to speed the ramp-up to pressure if the sheet cannot withstand it. A pattern of small holes over the spread of a hand-span is a sure sign of a press burst and much ruined stock.
Circulating re-wash
Linen marked for re-wash should only be sent through the original process if it is creased or over-dry. Stained re-wash will not come clean if washed in the same process. A re-wash process is needed. It should then be inspected and either accepted or rejected and put aside as potential scrap. If a pile of potentially scrap textiles accumulates, this should be treated for mildew or rust marking, inspected and either put back into circulation or put straight into the waste skip.
Productivity
Energy management means managing money and time and is especially important during slack times. Then it is essential to maximise the pieces per operator hour (ppoh) so the boiler house can shut early and staff can go home. The unconscious slowing down when the pressure is off increases the kWh/kg sold ratio. Production managers used to achieving 150 ppoh under pressure should be encouraged to maintain this during slack periods to lower overheads and avoid losses when sales reduce.
Compressed air
The air compressor is driven by one of the largest and least well-managed motors in the plant, probably 20kW or more. It runs continuously, clocking up 60,000 kWh in a typical 3,000hour year. Up to half of this can be wasted through air leaks so air leakage measurement and control is the first step in managing this consumption.
Air compressors with a good load-shedding capability will consume much less energy if they are only supplying the process load and the effort will be reflected in the meter reading every month. Air leaks are cheap to find and fix and sometimes shrewd maintenance of this type will postpone the need to replace the compressor for several years.
Insulation
Most laundries still do not have insulation throughout the boiler-house – on all metal surfaces at steam temperature and on hot water pipes and tanks. The payback on in-house insulation is usually about three months. The engineer needs a stock of resin-impregnated fibreglass with aluminium backing foil, a reel of lagger’s tape and a large knife/pair of scissors. Tackling 1m2/day gives rapid benefits.
Boiler house
Maximising ppoh will minimise boiler hours and this is probably the largest single contribution that production managers can make to maximising energy efficiency.
Organising in-house insulation of the boiler house is almost as important and worth the modest investment in materials. In any area that does not have perfectly soft water, free of impurities, management of boiler blowdown is critical. The best systems deliver a trickle of blowdown from the boiler that is just sufficient to maintain the total dissolved solids (TDS) in the boiler water below 3,500 parts per million. This can be achieved without expensive investment, although automatic systems are available.
Areas of poor water quality will require high blowdown but it is generally cheap and effective to recover and recycle the waste heat in the blowdown and it is best to do this as part of an integrated condensate and boiler management system. This will certainly involve making use of flash steam and conserving condensate heat.
Investment strategies
Some of the steps recommended here will need modest investment but even for a large laundry the total is unlikely to exceed £15,000.
Most plants will be able to improve energy efficiency by around 30% as a result of making these incremental improvements.
However, this level of increased efficiency will demand considerable management and engineering time over a period in excess of twelve months. Allowing time and the modest spend necessary for such improvements should be the first step in any investment strategy. Once the basics have been achieved, larger investments can be considered.
