Humidification in Textile Mills: RH, Systems and Control

Why this article was rewritten

The original version of this post described household humidifiers — wick, steam, impeller and ultrasonic units, and a range of whole-house models marketed for domestic comfort. It used a 30–50% relative-humidity band drawn from that setting, and it carried two unlicensed consumer photographs. That is not how a spinning mill, a weaving shed or a knitwear plant is humidified, and those claims are not repeated here.

This rewrite is for textile-mill and knitwear readers. It explains why humidity is controlled in yarn and fabric manufacture, which relative-humidity figures a published mill-engineering source actually supports, how a typical plant system is arranged, and how that system is controlled and kept clean. The sequence of fibre preparation, spinning, weaving and knitting is set out in Textile Manufacturing Technology: Fibre to Garment. This article does not repeat that process description. It looks at the air around the machines.

Why humidity matters

Relative humidity (RH) is the amount of water vapour in the air, expressed as a percentage of the amount the air could hold at the same temperature. Textile fibres take up and give off moisture as the surrounding air changes, and their strength, extensibility, friction and tendency to hold a static charge change with it. How different fibres behave is discussed in Textile Fibres: Natural, Regenerated and Synthetic Types. The practical point for the mill is that the same lot of cotton or yarn will not run the same way in a dry hall as it will in a stable, correctly humidified one.

In a 2005 article on mill humidification for the Indian Society of Heating, Refrigerating and Air Conditioning Engineers (ISHRAE), M. M. Roy describes the two sides of that balance. From carding through roving, loosely bound fibres in dry, brittle condition are vulnerable to static from friction. That static, and the dryness itself, encourage dust and fibre fly, and they contribute to yarn breakage. Higher fibre moisture lowers electrical resistance and helps to carry charge away, so RH should be held above the lower limit agreed for the process. Above the upper limit, fibres tend to stick and form laps on the rolls, which have to be cleared by hand and stop the machine.

Temperature is part of the same problem. Spinning, weaving and knitting machines, and their motors, release a great deal of heat. Near those machines the air is warmer, so the local relative humidity falls even if the room average still looks acceptable. Roy also notes that the natural wax on cotton softens at those raised temperatures and then lubricates less well, and that modern electronic controls are a reason to keep the department from exceeding about 33 °C. Waste heat is commonly taken out with the return air, often through under-floor trenches, and the split between exhaust and recirculation is one of the ways the inside condition is held.

A short-term swing can do as much harm as a wrong set-point. The aim is a stable band at the machines, not a single impressive reading at a wall sensor.

Typical RH targets — only figures a source supports

There is no single “correct” relative humidity for a textile mill. The fibre, the count, the machine and even the position in the room all matter, and published guides do not all quote the same numbers. The figures below are taken from Roy’s ISHRAE article and from a clearly labelled design example in it. They are starting points to be confirmed for the actual plant, not a contract and not a standard.

A further distinction is important. Laboratories that measure the physical properties of textiles condition specimens in a standard atmosphere. North Carolina State University’s Zeis Textiles Extension physical-testing laboratory, for example, states its conditions under ASTM D1776 as 21 ± 2 °C and 65% ± 5 humidity. That atmosphere is for testing. It is not a process set-point for a blow room or a loom. How fabric tests are specified is covered in Fabric Testing: Methods, Equipment and What Results Mean.

Spinning

Roy’s worked example of a cotton-mill humidification plant gives these desired inside relative-humidity conditions: 65% in the blow room; 55% in carding, combing, drawing and roving; and 60% in ring spinning. Those are the numbers that article uses for design. They show the usual pattern that spinning departments are held in a moderate band, not at the high levels used at the warp on a cotton loom, and that not every room in the mill is set to the same value.

Knitting

Knitting needs a stable humidity so that yarn tension, and therefore loop length and package or fabric density, stay consistent. Roy gives a recommended inside design condition of 55% ± 5% RH for knitting, and, for precise tension control, particularly in hosiery, 25 ± 1.1 °C as well. A knitwear plant that lets the hall drift outside that kind of band will see the effect as tension variation, holes or an uneven fabric, even when the yarn itself has not changed.

Weaving

Cotton weaving is the process that is usually held wettest, but not uniformly throughout the shed. High humidity at the warp increases the yarn’s resistance to abrasion as it passes through healds and reed. Roy states that cotton weaving rooms are designed for 80–85% RH at the warp-sheet level — the “loomsphere” — while about 65% RH is sufficient in the room generally. In the weave-direct arrangement he describes, a separate air-washer and branch ducts brought down to about 1.2 metres above the warp are used to hold 78–80% RH at the warp sheet, with a second system holding about 65% RH in the room. The two figures (80–85% as a design aim, 78–80% as the weave-direct delivery) are close; what matters is that the high humidity is delivered at the warp, not guessed from a single room average.

The figure is a schematic drawn for this article. It groups the spinning, knitting and cotton-weaving figures taken from Roy’s ISHRAE article, and it reminds the reader that a laboratory testing atmosphere is not a mill set-point. It is not a design specification.

Methods and systems

Mills are not humidified with portable room units. The systems Roy describes are central spray air washers. Air is drawn through a chamber of water sprays; water evaporates, the air’s dry-bulb temperature falls and its moisture content rises. That adiabatic, or evaporative, cooling is why humidification in a hot, dry season also takes heat out of the shed, at a lower capital and energy cost than full refrigeration. When the outside air is itself hot and humid, evaporative cooling cannot do the same job, and some mills then spray chilled water in the washer.

A typical washer has several banks of spray nozzles, a tank that collects the water, and an eliminator section to stop droplets leaving with the supply air. Roy notes that a single spray bank is not usually chosen for mill work, because its saturation efficiency is too low, and that a double-bank opposed-spray arrangement is often used. The humidified air is then ducted to the departments, with outlets aimed at motor alleys and other concentrated heat loads, and return air is taken back, commonly through floor grilles and masonry trenches, through rotary drum filters that are cleaned continuously so that fluff does not choke the system. Modern opening, carding and some spinning and weaving machines also have local suction at points of fly generation, feeding a waste-collection system.

Supply-fan speed is not a free variable. Roy’s energy-saving notes allow a variable-speed drive on the washer spray pumps for part-load economy, but they generally advise against slowing the fans, because a lower air flow slows the removal of fluff and can contaminate the yarn or fabric. Clean filters, sound nozzles and a well-maintained washer also help the plant hold its intended saturation efficiency.

Control, water and hygiene

Stable conditions come from measurement and from the dampers, not from a once-a-day adjustment of a valve. Roy recommends digital controls with electronic temperature and humidity sensors in each department, using that feedback to set the positions of the outside-air, exhaust-air, return-air and bypass dampers automatically. The useful reading is the one taken where the process needs it — at the warp sheet in a weave-direct system, for example — as well as a representative room reading.

The water that is recirculated and sprayed has to be managed. Roy gives quality limits of pH 7.5 to 8.5, and a total hardness preferably above 100 mg/l and within 250 mg/l; he warns against using demineralised boiler water in the washer, because moderate hardness helps to form a thin protective coating. As water evaporates, minerals concentrate in the tank, so some water has to be bled off, and the plant has to be cleaned periodically to prevent the growth of algae.

Spray humidifiers and air washers also have a health dimension that the original domestic article did not address in mill terms. The UK Health and Safety Executive lists humidifiers and air washers among water systems that can present a reasonably foreseeable risk of legionella growth where the water may be between 20 and 45 °C, where breathable droplets can be formed, and where the water is stored or recirculated. Its guidance is to assess that risk, to keep the system clean, to avoid conditions that favour growth, and to control the release of spray. Those duties sit alongside the process need for clean, stable humidified air.

FAQ

Is 65% relative humidity the standard condition for a textile mill?

No. 65% RH is widely used as a laboratory atmosphere for conditioning and testing textiles — North Carolina State University’s physical-testing laboratory states 21 ± 2 °C and 65% ± 5 humidity under ASTM D1776 — and Roy also uses about 65% for a cotton weaving room and for a blow-room design example. It is not a universal mill set-point. Ring spinning, knitting and the loomsphere are held at different levels in the same article.

Why is a cotton weaving shed humidified more heavily than a spinning room?

Because the warp is abraded as it weaves. Roy’s reason for 80–85% RH at the warp sheet is to increase the yarn’s abrasion resistance. The rest of the room can be held drier, at about 65% RH, especially if a weave-direct or loomsphere system delivers the higher humidity only where it is needed.

Can a portable domestic humidifier serve a mill or knitwear plant?

No. A mill system has to hold a stated band over a large space, remove heat and fibre fly, and deliver air to the machines, often through ducts, trenches and a washer with recirculated, treated water. Portable room units, and the household 30–50% comfort band in the original post, are a different application.

If the air washer cools the shed, why do some mills still use chilled water?

Evaporative cooling only works when the outside air can still take up moisture. Roy notes that in a hot, humid season some mills spray chilled water in the washer because ordinary evaporative cooling is not feasible. The refrigeration is then a way to keep the inside condition, not a substitute for humidity control.

Does raising the humidity always reduce yarn breakage?

Only within the agreed band. Below the lower limit, dryness, static and fly contribute to breaks. Above the upper limit, Roy warns that fibres start to stick and form laps. More moisture is not automatically better, and a swinging RH is not a substitute for a stable one.

References

• M. M. Roy, “Humidification for Textile Mills”, ISHRAE / Green Business Centre, Issue January–March 2005, ishrae.in/docin/Content/Article_File/e0elz1akol1.pdf.
• North Carolina State University, Zeis Textiles Extension, “Physical Testing Lab FAQ”, textiles.ncsu.edu/zte/physical-testing-lab-faq.
• ASTM D1776/D1776M, Standard Practice for Conditioning and Testing Textiles, store.astm.org/d1776_d1776m-20r24.html.
• UK Health and Safety Executive, “Other systems that can create a risk of legionella”, hse.gov.uk/Legionnaires/other-risk-systems.htm.
• UK Health and Safety Executive, “Legionnaires’ disease — what you must do”, hse.gov.uk/Legionnaires/what-you-must-do/preventing-controlling-risk.htm.