Humidity Control Department Editor: Kate Torzewski - Chemical Engineering

Applications
Dehumidification by cooling or dessication has a variety of applications, including:

Preventing moisture regain. Nearly all materials have some affinity for moisture based on surface characteristics and the amount of surface exposed to humid air. Moisture regain occurs when moist particles stick together.

Preventing condensation. Air holds water vapor in proportion to its temperature. Cold surfaces of pipes, vessels, valves and heat exchangers condense moisture unless the air around them is dried to a dewpoint below the temperature of the cold surface.

Preventing corrosion. The exposure of metal surfaces to atmospheric corrosion can be reduced by surrounding the surfaces with dry air. Dehumidifiers also keep humidity low in process control rooms, preventing the corrosion of electrical contacts and sensitive electrical components.

Drying heat-sensitive products. Typically, drying time is reduced by heating a product. If the product is susceptible to damage by heat, drying time can be reduced by using dehumidified air, which reduces the vapor pressure of air above the wet surface.

Cooling

A common method for dehumidification is the use of air conditioning. Figure 1 shows a typical vapor-compression cooling-based dehumidification process. Air to be dried passes through a cooling coil, which lowers the temperature of the airstream below its dewpoint. As the air cools, it loses its capacity to hold water vapor. The water condenses on the cooling coil surface, and falls to the drain pan as liquid. The air is then drier in absolute terms, but it also has a relative humidity close to 100%. If low relative humidity is needed in addition to a lower absolute amount of moisture, the air can be heated after it leaves the cooling coil.

For industrial purposes, cooling-based dehumidification units are optimized for removing moisture rather than removing heat. These units provide deep cooling of small amounts of air rather than slight cooling of large amounts of air, condensing more moisture. Standard refrigeration equipment can produce dewpoints of +40°F (4°C) on a reliable basis.

Dessication
In a desiccant system, the process airstream passes through a desiccant medium. The desiccant adsorbs moisture directly from the airstream. Desiccant dehumidifiers can produce dewpoints below 0°F (–18°C) — a fivefold reduction in the air moisture beyond what can be achieved with a standard grade air conditioning system.

This equipment uses differences in vapor pressure to remove moisture from air by chemical attraction. The surface of dry desiccant has a very low vapor pressure, compared with the much higher vapor pressure of humid air.

Water vapor moves out of the humid air onto the desiccant surface to eliminate the vapor pressure difference, as shown in Figure 2. Eventually, the desiccant surface collects enough water vapor to equal the vapor pressure of the humid air. Then the desiccant must be dried (reactivated) by applying heat before it is recycled to remove more moisture from the air stream.

Cooling vs. Dessication

In most chemical process applications, both technologies work best together. Coolingbased dehumidification handles the moisture load occurring at high dewpoints, and desiccant-based dehumidification removes the moisture load at low dewpoints. The optimal mix of the two technologies depends on the characteristics of the application.

Factors to consider include the following:

Dewpoint control level. When the required moisture-control level is relatively high (above a 50°F dewpoint), cooling-based dehumidification is economical in terms of both operating cost and initial equipment cost. Below 50°F, precautions need to be taken to avoid freezing the condensed water on the cooling coil. Consequently, desiccants are more economical than cooling based systems at lower dewpoints.

Relative humidity sensitivity. When a process needs a low moisture level in absolute terms, but can tolerate a high relative humidity, cooling-based dehumidification without desiccants is cost effective. By contrast, in processes that demand a low relative humidity in addition to a low dewpoint, desiccant systems are used for humidity control, with supplementary cooling systems to keep temperature within acceptable limits.

When a product is sensitive to relative humidity but not to temperature, a desiccant dehumidifier is used without a cooling unit to maintain a constant relative humidity.

Temperature tolerance. If the application can tolerate a wide temperature range, then dehumidification alone may suffice. In most cases, both temperature and moisture must be maintained within set limits, so both cooling and desiccant equipment are used in a combination to maintain control.

References
1. Harriman, L., Don’t Sweat It, Dehumidify, Chem. Eng., August 1997, pp. 80–87.
2. Soleyn, K., Humidity Control: Preventing Moisture Contamination, Chem. Eng., October 2003, pp. 50–51