Current terry towel processing offers some of the most valuable opportunities for the modern launderer. The drying process itself is fundamentally inefficient, but the laundry operator could be doing so much more. We take an in-depth look at how to achieve significant cost and quality improvements, with a surprising productivity bonus.
Moisture retention is the starting point.
The closer you can get to 40% retention on 100% cotton towels the quicker the drying cycle can be, plus the optimum amount of energy will be consumed. We have in recent times seen levels as high as 65% on undersized batches therefore ensure any monorail bag system or step conveyor weighing points are regularly calibrated, to ensure optimum loading.
Don’t forget that towels are wet when they arrive at the sorting room, so there is the opportunity to slightly overload to consider the moisture in the towels. This should be checked regularly to ensure the risk of a blockage is eliminated. Also, LTC research determined that increasing an undersized batch progressively by 1 Kg improved the moisture retention from a 40bar press by 1% per additional kilo, in this case study up to 7%.
Thus it is vital, to keep tumbler times to the minimum and to routinely check the membrane press settings. Every new press should come with a pressure gauge showing the force being exerted on the cheese of wet textiles. The check is sadly neglected, and the result is widespread wastage of energy and time in the tumblers. Squeezing moisture out in the press uses only one fifteenth of the energy needed in the dryer, per litre of moisture removed. A 40-bar press needs to be at its full 40-bar pressure for at least 30 sec to dewater towels adequately (generally the longer the better), which may require careful tuning to accommodate this within the time cycle of the batch washer (possibly with fine adjustment of the ‘wait times’ for each step in the press programme).
Greying continues to be a limiting factor, for towel life, which is only being tackled successfully in a handful of plants. Towel greying has two main components – in the wash and in the dryer.
Chemical suppliers can eliminate greying in the wash, with modern suspending agents which wrap around every dirt particle or micelle of saponified skin sebum, neutralising any micro-electronic attraction to prevent re-deposition onto the clean textile. Greying in the wash can become a problem if the towel ground weave has an element of polyester and the chemical supplier is unaware of this, as polyester has an electro-chemical affinity with fats and oils therefore, the chemistry should be tailored to overcome this.
The big problem is greying in the dryer, which occurs whenever any part of a textile becomes bone dry. When a dry terry loop brushes repeatedly against the dryer cage, the tiny static charge generated attracts every minute dirt particle in the drying air stream, leading inevitably to greying, which normally becomes very apparent in the second year of the towel’s life.
Greying in the dryer can be measured using a test piece.
Browning of towels, which is often confused with greying can be a big problem in areas with water quality issues. Iron salts tend to degrade into iron oxide (rust) in the wash process, which dyes the cotton slightly, but enough to look unsightly. It can be a big problem in hard water areas (such as the south and southeast of England). The maximum amount of iron for laundering is only 0.1 part per million (ppm). Anything above this causes poisoning of the resin in the water softener and accelerated rotting of cotton if chlorine bleach is still being used. It is possible to test if a textile is contaminated with iron and a test piece also available to test for this. Premium detergents would be expected to contain sufficient chelating agents to capture and neutralise excess iron in the raw water, but high levels of iron really merit installation of a small iron removal unit.
Harshness is also associated with overdrying; as soon as a towel is bone dry then any continuation of the tumbling process will make it harsh to the touch. In stark contrast, 2-4% residual moisture will make it demonstrably softer, with much less tendency to greying.
Automatic dryness terminators can solve the problems of greying and harshness.
They generally work by infra-red assessment of the degree of dryness in the tumbling textiles themselves, and they can be set to terminate the drying process at 2-4% moisture, to give soft, white towels.
The sensors need to be kept clean with a ppm routine that is rigorously adhered to.
Productivity
The cost of automatic cycle terminators is usually recoverable by the energy savings over a 12-month period, but the true value is often represented by the quality improvement and the increased throughput. Productivity can also be maximised by loading the CBTW batches to a weight which reflects the moisture content of the used towels as previously mentioned. This is frequently in the range 10 – 15% of the dry weight, but this does vary, so it pays to do check weighing on a plant-by-plant basis. Once you know the average moisture content, you can increase the batch weights for used towels cautiously (to avoid any increase in the risk of tunnel blockages). You should be able safely to increase used towel throughput up to 10% above the nominal batch weight and many plants achieve much more.
The risk of all the tumble dryers being occupied with full-dry work can be reduced further by instituting careful sequencing of wash batches. Each full dry load needs to be separated by a minimum number of short-tumble loads. In this way the risk of ‘tumbler holds’ can be virtually eliminated.
Some plants pride themselves on achieving the maximum number of loads per hour, consistently.
Spa towels are believed to be behind some recent laundry fires. The essential oils in some spa products are not removed easily (they need the correct emulsifier) and any residual oils left after the wash will break down in the dryer. They are oxidised by the oxygen in the drying airstream. This reaction is exothermic (it gives off heat) and it can continue in the pile of finished, folded spa towels.
If the heat generated cannot dissipate, the pile can warm to its auto-ignition temperature, causing an explosive fire and flinging blazing textiles in all directions.
This is believed to have been the cause of many otherwise unexplainable fires (with some good CCTV evidence), affecting two or three large laundries (and many smaller ones) each year. The risk is minimised by designing the correct wash process for spa towels and making sure it is used consistently.
Even if the spa oils do not cause ignition, the by-products from the decomposition tend to be acidic, causing accelerated rotting of the cotton and very short textile life. The decomposition also produces odours which either reflect the original scented essential oils or (eventually) foul and rancid by-products. The best solution for monitoring the effectiveness of the spa towel wash is to sniff every batch of spa textiles as it comes out of the dryer.
Towel design
It is widely acknowledged that incorporating polyester into the textile reduces the moisture retention at the membrane press and increases the strength. It does not make sense to blend polyester into the terry loops, because this would affect the harshness and could also increase the risk of greying.
However, research has been carried out with polyester in the ground weave which appeared to show useful advantages in dewatering and increased strength. As mentioned, the polyester would tend to attract and retain more oily soiling than cotton, but use of a suitable detergent should counter this.
Dryer design
There has been much R&D investment in recent years concentrating on the thermal efficiency of the latest dryers to minimise the use of the increasingly expensive gas energy being consumed. These have included automatic cycle terminators mentioned earlier, followed by the automatic lint screen. The latter maintains maximum airflow throughout the drying cycles by clearing lint continuously, which in turn minimises drying time. Significant improvement in thermal efficiency has also been sought using the intelligent recycling of exhaust air within each dryer. These systems manage the exhaust airstream of saturated air going to atmosphere, to minimise energy usage whilst maintaining productivity.
With the development of an AI driven system, we now have the potential to vary the recycle continuously, with a recycle rate calculated automatically to utilise fully the time available for drying.
An AI driven system could take into account the pattern of classifications currently in the CBTW (and the dryers) and calculate the maximum drying time available to each load of towels. When this is translated into thermal efficiency, one would expect further improvements to be achieved and maintained.
Conclusion
There are several areas of untapped savings here and the current poor thermal efficiency of towel drying, along with premature greying, harshness and low productivity make any improvement doubly rewarding. Please let us know your successes.
