Humidity control improves productivity, cuts waste

Dirt, dust and debris cling to plastic sheets.

Previously-inspected electronic components don’t work.

Wood parts twist and warp.

Paper and cardboard shrink and warp, jamming equipment.

Glue doesn’t cure properly.

What do these problems have in common?

All are commonly caused by low relative humidity. And these problems and others cause machine stops, equipment damage, slow 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 commonly misunderstood. The blame is put on a bad batch of materials, machinery malfunctions or problems with glue, paints and finishes.

Many engineers have only a sketchy idea of how fluctuations in plant humidity impact manufacturing processes, quality and waste as well as employee safety, health and productivity.

Indoor RH varies widely, depending on time of year, indoor and outdoor temperatures and time of day. RH levels can vary widely from building to building in a complex, or in different parts of a building.

In most plants, achieving consistent humidity levels throughout storage areas, manufacturing and assembly, semi-finished goods storage, finishing and shipping can be a key factor in reducing waste and in improving product quality and operating efficiency. Even small changes in RH levels can have major effects.

RH levels are also a major component in improving indoor air quality, reducing the occurrence of allergies, flu and colds and the number of sick days taken.

In most industries, RH levels of 40% to 60% at about 70 F are best for maximum production efficiency, product quality and reduced waste. Plants typically begin to experience dry air problems when RH drops below 30% to 40%.

All organic materials (and some inorganics) contain moisture: kiln-dried lumber contains up to 11% moisture; paper and boxboard up to 9.0% water. When exposed to dry conditions even briefly, materials lose moisture and problems can set in. Although some areas have dry air year-round, problems are most often associated with colder weather and the 'heating season.’ When air is heated, its capacity to carry water is reduced and RH plummets. For instance, when 10-degree F outdoor air with 50% RH is heated to 70-degree F, RH drops to around 8%.

Heating air a few degrees often drops RH to levels requiring adding moisture to the air. For example, maintaining 50% RH at 70 F requires four times as much water in the air as at 32-degree F.

In warmer regions and in hot weather, use of air-conditioning can also dry out air. In warm climates, in addition to solving low RH problems, humidification systems can also be part of an evaporative cooling system.

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 an area with 10% RH, typical assembly activity at a workstation can generate static discharges of 6,000 V. When RH is 40%, the same activity will generate only 800 V.

Static discharge is first felt at about 3,000 V. But even static discharges as low as 500 V can seriously damage electronic components. Static attracts dirt and dust, which cling to plastics, paints and inks and other surfaces.

Static shocks also 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.

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 warp so much they no longer can be run properly through machinery or finishing dies.

Low RH also affects a range of processes. Glue may not cure properly, leading to weak joints and joints popping or breaking. 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.

Proper plant humidity provides a healthier climate for everyone, minimizing respiratory problems and allergies and reducing time and productivity lost to sick days.

Generally speaking, RH of 40% to 60% is best for minimizing bacteria, fungi and viruses, and the incidence of respiratory infections. Allergic rhinitis and asthma will be lowest in that range. Airborne dust is commonly reduced by more than 50%.

If you investigate manufacturing and employee health problems you had last heating season, you’ll probably find that reducing them is easier than you thought. Most dry air problems are relatively easy to remedy through use of a properly-engineered humidification system, and the solutions are usually permanent.

An engineered humidity system isn’t usually expensive; it lasts for years and with the right system for your situation, return on investment is fast: often two years or less.

Manufacturers have developed a wide array of humidification technologies and products aimed at solving dry air problems. Each type converts water into fine droplets, which are evaporated into your plant’s air. In general, the smaller the particles, the faster and more efficiently they increase RH.

Virtually any of the leading technologies can provide adequate humidification to solve problems. But not all methods are 'created equal’ in every situation. Each has its strengths and weaknesses in terms of cost, efficiency and effectiveness, depending on your specific circumstances.

With all types of systems, it is important to use clean, soft water in your system to reduce maintenance costs and to avoid mineral dusting. Depending on water quality in your area, it is usually necessary to consider use of conditioned, reverse osmosis or de-mineralized water.

Steam systems produce very fine droplets (0.3 to 1.0 microns), which evaporate quickly and effectively. But steam systems also produce heat. Most plants don’t welcome the added heat.

Steam systems are typically either 'electric steam’ or 'boiler-fired steam.’ Electricity is an inefficient and expensive way to produce steam, making its use most appropriate for projects in spaces less than 8,000 square feet, where relatively low installed costs can offset high energy and maintenance costs. Energy costs make electric steam less appropriate for larger installations.

Gas-fired systems – especially those requiring new boilers – have high installation and maintenance costs. Buildings with existing boilers enjoy less expensive installation and energy costs, but build-up of chemicals in piping may cause IAQ concerns. Maintenance costs to prevent build-up of corrosive residues on heating elements are also high with all kinds of steam systems.

Centrifugal systems use centrifugal force to atomize water. Units have low installed costs and relatively low operating and maintenance costs. However, this technology often creates larger droplets, which don’t evaporate efficiently, and may lead to excessive condensation and wetness. IAQ and health concerns also have been raised over open water baths, which may promote bacteria growth.

Ultrasonic humidifiers use high-frequency electricity to break droplets away from the surface of a shallow water bath. Ultrasonic systems are more energy-efficient than steam or centrifugal humidifiers. However, air quality and health 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. However they are often a big drag on compressed air capacity, reducing capacity available to run equipment, or forcing plants to increase compressor capacity. Compressor-driven units can be annoyingly noisy and are best placed in areas away from work stations.

High-pressure humidifiers use a high-pressure electric pump to drive water through a fine nozzle at pressures of 900 to 2,000 psi, creating a fine cool mist or 'fog’ of droplets. Since no heat is required, high pressure is an inherently energy-efficient, low-maintenance method, ideal for large spaces and higher demand applications. While installed cost is rarely a problem in larger jobs, it is a relatively high percentage in smaller jobs, making it less appropriate for smaller areas and those with low demand.

For most companies, the biggest selling points for high-pressure humidification are simple installation, low maintenance costs and very low energy costs (about 10% to 15% the cost of compressed air systems, and less than 1% the cost of electric steam). High-pressure systems are usually very low noise, making their use appropriate for use near workstations.

Fog droplets from high-pressure systems are relatively large and require longer distances to evaporate than some other methods. This can be overcome by use of a system with a built-in fan, which distributes moisture faster and more uniformly. An integral fan also allows high-pressure systems to work effectively in spaces with ceilings as low as eight feet.

If you take a hard 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.