Desiccant System Performance

Desiccant system performance depends on several factors. These include:

• The sorption/desorption characteristics of the particular desiccant used.

• The amount of desiccant exposed to the reactivation and process airstreams by the system.

• The process and reactivation air velocity through the desiccant.

• The process air moisture and temperature levels.

• The reactivation air moisture and temperature levels. Each type of system will offer varied performance depending largely on the operating conditions. Some systems are more effective at higher dewpoint temperatures, while others perform better at very low dewpoint temperatures. The best desiccant for a particular application will depend on the range of water vapor pressures that will occur in the air, the temperature level of the available regeneration heat source, and the moisture sorption and desorption characteristics of the desiccant operating with in those parameters.

The range of vapor pressures that may be encountered varies widely. For example, consider the range of vapor pressures for a constant dry-bulb (db) temperature of 70°F (21°C). At 100% RH, the dewpoint temperature is 70°F (21°C) and the corresponding vapor pressure is 0.74 in. Hg (2.5 kPa). At 10% RH, the dewpoint temperature is 12°F (-11°C) and the corresponding vapor pressure is 0.07 in. Hg (0.24 kPa). Each desiccant has a different sorption characteristic that affects system performance. With liquid absorption solutions, vapor pressure is directly proportional to its temperature and inversely proportional to its concentration.

A graphical representation of the relationship of moisture capacity to RH when desiccant and air are at the same temperature can be shown as a capacity isotherm. Figure 39-18 shows representative capacity isotherms of four desic-cants at 77°F (25°C). Capacity is expressed as the weight of water retained as a percentage of the dry weight of the des-iccant. As shown, at 20% RH, molecular sieve holds about 20%, while LiCl holds 35%. Note that large variations from these isotherms occur because manufacturers use different optimization methods for a given desiccant, depending on the particular application. Also note that these capacity figures assume that the desiccant has enough time to absorb the moisture. In practice, capacities are lower due to the need to

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