Dehumidification drying using refrigeration offers many advantages over conventional heat-and-vent systems or systems using desiccants. Dehumidification systems can reduce energy consumption significantly while also minimizing or eliminating air emissions and providing improved control.
One drawback to dehumidification drying in the past was the limited temperature range of existing technologies. However, modern systems are effective at operating temperatures ranging from 40 to 220°F (5 to 105°C). When combined with supplemental desiccant drying, dewpoints as low as -60°F (-50°C) can be achieved easily with relatively low capital costs and much lower operating costs.
Most conventional lumber dryers (called dry kilns by the lumber industry because they produce kiln-dried lumber) are batch processes using a heat-and-vent system similar to conventional clothes dryers. The systems introduce outside air, heat it to the process temperature, evaporate and absorb water in vapor form, and exhaust it to the atmosphere along with the air necessary to carry the water vapor. In most industrial operations, this process is inefficient and energy-intensive.
By contrast, a dehumidification dryer continuously recycles the air in the dryer chamber and cools a portion of the air, condensing water that is then drained away in liquid form. The heat removed from the air in the cooling process is returned immediately to the circulating air; and more heat is added than was removed, so the temperature in the chamber rises.
Because the water leaves the drying chamber as liquid, no air needs to be exhausted to carry away the water vapor. This design reduces the amount of energy required to heat the air. Also, because the water leaves the chamber in liquid form instead of as vapor, the latent heat of vaporization is recovered fully. Some of the recovered heat is offset by the energy used to run the refrigeration system, but the net result is that only about 40 percent as much energy is used in the process compared to a conventional heat-and-vent system.
In addition to the large energy savings, changing the water vapor from steam to liquid water puts the condensed water in a form that can be treated easily if necessary. The reduction in air emissions can eliminate the need for an air emissions license in most cases. In addition, because the systems rely on electricity, the air emissions from a boiler or burners that might have been used in a heat-and-vent system are eliminated.
An alternate process uses the refrigeration system to cool and dehumidify the air to approximately 36°F (2°C), then passes the air over a desiccant wheel. Desiccant wheels are more efficient when the supply air is cold and, because most of the water is removed by the refrigeration system, they can be much smaller and use less energy than the traditional desiccant method.
Because of the improvements in the dehumidification process, equipment design and use of materials, dehumidification drying is gaining a wider acceptance in markets that increasingly are concerned about energy usage and the environment.