Desiccant

Desiccant dehumidifiers utilize a desiccant wheel to dehumidify supply air (Figure 4). A desiccant wheel is split into two sectors: supply air and reactivation air, which rotate continuously. Once a wheel absorbs moisture, the moisture is driven off of the wheel during a “reactivation process.” The wheel rotates into a secondary air stream (reactivation air), where water vapor is transferred to heated air and exhausted.

Reactivation temperature can range from 110°F to over 300°F, depending on the application. Generally, the drier the air requirements, the warmer reactivation air must be. As supply air passes through a desiccant, it simultaneously is dried and heated. For example, saturated air entering at 55°F (64 grains per pound) will leave at 97°F (29 grains per pound). Contrary to popular belief, that heat is not carryover from reactivation. The desiccant wheel absorbs moisture at a linear rate for more than 10 min before capacity is reduced measurably. Therefore, dwell time — the time during which a given point is in contact with supply or reactivation air — is approximately 8 min. Rotor speed generally is eight revolutions per hour. Slow rotation speed results in little heat carryover from reactivation air. Most of the heat associated with the drying process is converted latent energy.

FIGURE 4. Desiccant-wheel process.

Note the following:

• The cost of operating a desiccant dehumidifier is approximately 40-percent less than the cost of operating a low-temperature chiller ($9,881 vs. $17,141) (Table 1).

• Desiccant pre-cooling and final post-cooling requirements are met using standard-temperature chilled water.

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  TABLE 1. Comparison of low-temperature-chiller and desiccant-dehumidification approaches.

  Because a desiccant dehumidifier easily can achieve dew points lower than 40°F, only a portion of the total supply air needs to be dehumidified. This allows greater system-design flexibility. In the example of Greensboro, N.C., only outside air is dehumidified. All of the work associated with sensibly cooling supply air to dew point is removed from the equation (Table 1).

• Cooling fluid is warmer, reducing the chance of pipe condensation.

• Cooling fluid does not require brine mixture, avoiding the potential for leaks and removing the cost of maintaining correct fluid chemistry.

• Supply air is not overcooled, dramatically reducing the reheat energy required at each zone (Table 1).

• Conveyed air is not cold and saturated, which reduces the chance of duct condensation and condensation across pressure-loss apparatus within a supply duct.

The desiccant approach does not overcool supply air. Humidity and temperature control are decoupled. Therefore, post-cooling is applied only as needed to satisfy zones with the greatest cooling requirements. Note that for both the desiccant and low-temperature-chiller approaches, multiple-zone service was assumed. For single-zone service, the desiccant approach would not require reheat, while the low-temperature-chiller approach might in the main air handler.

SUMMARY

The health-care industry is advancing at a quick pace, as are dehumidification technologies. With surgical suites requiring significantly drier air, design challenges are magnified. Equipment must positively dry supply air to room set point, account for internal loads, and reheat air. Engineers must evaluate the advantages and disadvantages of different dehumidification approaches and provide the most reliable and energy-efficient solution.

A sales manager for Munters Des Champs, Mike Herwald has spent the last 14 years designing custom energy-recovery solutions across the United States and Canada. He is an expert in the utilization of enthalpy wheels, plates, heat pipes, and desiccants to reduce the energy consumption of commercial and industrial HVAC systems.