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Gas Technologies: Desiccants

Desiccant systems remove moisture from the air without cooling it, allowing for the separate control of temperature and humidity and saving costly energy. Gas desiccant systems can remove more than twice as much moisture from the air as a refrigeration system. This is because they generally require less than half the air volume required by conventional electric cooling systems to control humidity. Natural gas desiccant systems also complement conventional gas or electric air conditioning systems because drier air is easier to cool. This allows the cooling equipment to operate more efficiently and the system's air handling equipment to be downsized.

Consistent mold coolant temperature must be maintained for optimum results in the plastic injection molding process. Yet rapid cooling of the mold causes water vapor in the surrounding air to condense on the tool surfaces, which can lead to corrosion in addition to watermarks and cracks on the finished part. If coolant temperatures are raised to avoid condensation, cycle times become slower, production decreases, and resin processing parameters change.

Desiccant dehumidification provides year-round control and increased productivity by preventing condensation, eliminating slippery floors, improving refrigeration efficiency plus reducing the cooling load and defrost cycles. Controlling the environment results in lower operating costs and an improved bottom line for most facilities.

By removing moisture from the air, chilled water can be used instead of room temperature water to significantly decrease machine cycle time. As a result, parts cool faster, molding equipment lasts longer, and maintenance and retooling costs are reduced.

Gas-fired desiccant dryers offer the following benefits:

  • Removes High Moisture
  • Reduces Corrosion through the Elimination of Condensation
  • Safer Working Conditions
  • Lowers Energy and Maintenance Costs
  • Fewer Molding Equipment Repairs
  • Produces Faster Cycle Times
  • Reduces Scrap
  • Lower Coolant Temperatures Made Possible

Processes
Desiccant systems can be used in three ways:

1. Spot dehumidifiers deliver dry air directly over the surface of the mold when it opens. This method uses a small dehumidifier and requires ductwork to bring the dehumidified air to the mold cavity.

2. Machine shrouding involves construction of a small chamber to encase the molding machine and limit the infiltration of moist air to the machine.

3. Full-area conditioning eliminates distribution ductwork. However, with a larger space to condition, the unit must be sized to compensate for room openings, people and the presence of other types of equipment.

Common Processes:
Any manufacturing or processing function that is sensitive to humidity, temperature or microorganisms is a natural application for a dehumidification system including:

  • Refrigerated warehouses
  • Plastic injection molding
  • Food process freezers
  • Clean rooms
  • Pharmaceutical encapsulation and packaging
  • Photographic films and papers
  • Glass lamination
  • Heat sensitive drying applications

How They Work
Gas-fired dehumidification uses either liquid or solid desiccants to remove water vapor from the air.

Liquid Desiccants
Liquid desiccant material is a highly stable, nontoxic solution that attracts moisture. As the air to be dehumidified comes into contact with the liquid desiccant, the moisture is absorbed into the solution. The amount of moisture taken out of the air is directly related to the concentration and temperature of the solution. The solution is then boiled off, and the moisture is removed and released into the atmosphere. This restores the solution's ability to reabsorb moisture and begin the cycle again.

Solid Desiccants
Solid desiccants are used in a wheel-type system that resembles a round honeycomb. The holes serve as air channels, each coated with solid desiccant material. Air passes through the holes allowing the desiccant to absorb any moisture in the channels. As the desiccants continue to pick up moisture, their ability to absorb at 100 percent is diminished. This is where natural gas is used. While slowly rotating, 25 percent of the desiccant wheel is dried using a natural gas heat source. The heat evaporates the moisture, releasing it into the atmosphere, while the desiccants regain their maximum absorption ability.