If production people are lined up to complain about jammed equipment and rising defect rates, it must be heating season. As one operations manager says, "When they start coming, I know it's fall."
When heating systems crank up in early fall, often running through early spring, plants of all types are plagued by problems tied to dry air. When in-plant relative humidity (RH) falls below 30-40% problems start.
- Static electricity discharges destroy electronic components;
- Die-cut paperboard packaging blanks shrink and curl in 1-2 hours exposure to dry air, jamming robotic folding and gluing lines;
- In a plastic bottle blow-molding operation static electricity causes line stops and jams in the palletizing line when static discharges knock over bottles;
- Wood splits or cracks after a day or two in dry air in a kitchen cabinet factory;
- Dust and dirt cling to plastic sheets at a converter, due to static.
All these problems and others cause machinery stops, equipment damage, slower production speeds, wasted materials, and lower efficiency in plants of all types. But because the problems are usually seasonal, and often sporadic, their causes are frequently misunderstood. The blame is put on a bad batch of materials, machinery malfunctions, or problems with glue, paints, and finishes or other unrelated causes.
Indoor RH varies widely, depending on the time of year, indoor and outdoor temperatures, and time of day. RH levels often vary widely from building to building, or in different parts of a building. Even small changes in RH levels can have major effects. And the damage can come quickly; sometimes in as little as 1-2 hours of exposure to dry air.
In most plants the key to reducing waste and improving product quality and operating efficiency is consistent humidity control throughout the entire cycle of storage, manufacturing and assembly, semi-finished goods storage, finishing and shipping.
In most industries, RH levels of 40-60% at about 70°F are best for maximum production efficiency, product quality, and reduced waste.
All organic materials (and some inorganics) contain moisture: kiln-dried lumber contains up to 11% moisture; paper- and boxboard up to 9% water. When exposed to dry conditions even briefly, materials lose moisture and problems can set in, including:
- Static electricity
- Shrinkage/warping
- Brittleness/cracking
- Wrinkling and curling
- Inconsistent drying of paint, ink, and surface finishes
- Viscosity changes
- Unpredictable glue drying/curing
- Increased dust
- Dry eyes, lips, nose
- Increased allergies/flu/colds.
When air is heated, RH plummets. For instance, when 10°F outside air with 50% RH is heated to 70°F, RH drops to 8%.
Heating air just a few degrees often requires that moisture be added. For instance, maintaining 50% RH at 70°F requires four times as much water in the air as at 32°F. When heating systems run, most plants will need to add hundreds or thousands of gallons of water to the air every day to achieve desirable RH levels.
Static Electricity
The most common, easiest-to-diagnose dry-air problem is static electricity. As RH drops, the frequency and voltage of static discharges increases. Static causes plastic sheets, paper, and cardboards to stick together, causing misfeeds and stops in printing presses, cutting machinery, and other equipment.
In a work area with 10% RH, typical assembly activity can generate static discharges of 6,000 V. When RH is 40%, the same activity will generate only 800 V. At 55% RH, the charge is reduced to 400 V.
You first feel a static discharge at about 3,000 V. But static discharges as low as 500 V can seriously damage electronic components. Moreover, static attracts dirt and dust, which cling to plastics, paints, and inks and other types of surfaces.
Shocks can cause employee discomfort necessitating static cuffs or even causing serious injuries from slips and falls as workers jerk away from the shock and lose their balance.
Dimensional Changes
Dry air also causes problems with dimensional instability in materials ranging from boxboard to plastic to wood. Materials often shrink: wood veneers become brittle and shatter; wood components warp or twist; plastic, paper, and cardboards shrink or curl.
Processes Affected
Glue may not cure properly, leading to weak joints and joints popping. Laminating processes may fail, leading to rework or waste. Paint, ink, and surface finishes such as varnishes may bubble or fail to adhere properly. In screen or offset printing, print detail or color accuracy may be affected.
Indoor Air Quality (IAQ)
RH levels are also a major component in improving indoor air quality (IAQ), reducing the incidence of allergies, flu, and colds and the number of sick days taken.
RH of 40-60% is usually best for minimizing bacteria, fungi and viruses, and respiratory infections. Allergic rhinitis and asthma will be lowered. Airborne dust is commonly reduced by 50% or more.
A Simple Solution
Most dry air problems can be permanently remedied through use of a properly engineered humidification system. Most systems aren't expensive, they last for years, and with the right system for your situation, ROI may be fast -- often less than two years.
There are several humidification technologies and products. Each converts water into fine droplets, which are evaporated into the air. In general, the smaller the particles, the faster and more efficiently they work.
All of the leading technologies can provide adequate moisture to solve most problems. But not all methods are "created equal" in every situation. Each has its strengths and weaknesses in terms of cost, efficiency, and effectiveness, and their appropriateness depends on the specifics of your situation.
With all systems, it is essential to use clean, soft water in your system to reduce maintenance costs and to avoid mineral dusting. It is usually wise to use conditioned, reverse osmosis (RO) or demineralized water.
Common Humidification Solutions
Steam systems produce very fine droplets which evaporate quickly and efficiently. But steam systems also produce additional heat, which may not be welcomed.
Steam systems are either "electric steam" or "boiler steam." Electricity is an inherently inefficient and expensive way to produce steam, making its use most appropriate for projects in spaces under 8,000 sq ft, where relatively low installed costs can offset high energy and maintenance costs. Energy costs make electric steam least appropriate for larger installations.
Gas-fired systems, especially those with new boilers, have high installation and maintenance costs. Buildings with existing boilers enjoy lower installation and energy costs, but buildup of chemicals in piping may cause IAQ concerns. Maintenance costs to prevent buildup of residues on heating elements are also high with all steam systems.
Centrifugal systems use centrifugal force to atomize water. Units have low installed costs and relatively low operating and maintenance costs. However, they often create larger droplets which don't evaporate efficiently, leading to excessive condensation and wetness. IAQ concerns also have been raised over bacteria growth in water baths.
Ultrasonic humidifiers use high-frequency electricity to produce fine mist droplets from the surface of a shallow water bath. They are more energy efficient than steam or centrifugal humidifiers, but air quality concerns also have been raised over bacteria in water baths.
Compressed air systems use high velocity air to blow water through a nozzle into fine droplets. Since water is not heated, maintenance requirements and costs are generally low. They can be a big drag on compressed air capacity, reducing capacity available to run equipment, or forcing plants to increase compressor capacity.
Compressor-driven humidifiers produce a high-pitched hissing sound and nozzles are best placed in areas away from work stations.
High-pressure humidifiers use high-pressure electric pumps to drive water through a fine nozzle at pressures of 900-2,000 psi, creating a cool mist or "fog" of droplets.
Since no heat is required, high pressure is an energy-efficient, low maintenance method ideal for large spaces and higher demand applications. While installed cost is rarely a problem in larger jobs, it may be an issue in smaller areas and those with low demand, making these systems less appropriate for those applications.
The biggest selling points for high-pressure humidification are low maintenance costs and very low energy costs (10-15% as much as compressed air systems; less than 1% as much as electric steam). High-pressure systems are usually low noise, making them appropriate for use near workstations.
Fog droplets are relatively large and slower to evaporate than those from some other methods. This can be overcome by use of a built-in fan that distributes moisture faster and more uniformly. This also allows some high-pressure systems to work effectively in spaces with ceilings as low as eight feet.
If you look at the production and health problems you had last heating season, you'll find many were caused by dry air. Luckily, through the many methods of humidity control available to you, the solutions can be simple, fast, easy, permanent, and relatively inexpensive.
