Mixed HVAC System Applications

In certain cases, life-cycle costs for an HVAC system can be optimized with a combination of conventional refrigeration cycle-based cooling and desiccant dehumidi-fication. In many space conditioning applications, desic-cant systems can be integrated into the overall facility HVAC system, providing energy cost savings in addition to environmental health and building protection benefits. This allows for the use of lower capacity refrigeration equipment that can be sized to overcome only the facility's sensible loads.

A typical commercial application design consists of a desiccant system sized to deep-dry minimum outdoor ventilation air requirements. The dry air is ducted through the building and is dried to a point that satisfies the full latent load. This can be integrated into a down-sized (smaller capacity) chiller or DX system with fan coil air handling units (AHUs). This arrangement allows the refrigeration system to operate at the increased thermal efficiency allowed by higher suction temperatures. When airflow can be reduced, duct and fan capacities can be reduced, lowering associated capital and operating costs.

Determination of the economic viability of incorporating a desiccant unit into a mixed system will depend on many factors. Mixed system strategies are more likely to be cost-effective when large quantities of humid outside air are required due to high exhaust rates, or for facilities with high internal latent gains from crowds of people. As general rules of thumb, a mixed system featuring a desiccant unit may prove economical under any of the following circumstances:

• Conventional cooling alone cannot meet year-round dehumidification requirements.

• Electric power costs are high and/or thermal energy costs are low.

• Latent heat loads are high relative to sensible heat loads.

• Latent and sensible heat loads peak at different times.

• The system is required to operate dry with very low RH or no condensation on the cooling coils.

Given one or more of these circumstances, several mixed system configurations may be possible. Figure 39-22 illustrates four different configurations that can be used in mixed systems:

• In System 1, the desiccant is used only to dry pre-cooled makeup air.

• In System 2, the desiccant is used to dry a blend of precooled makeup air and return air.

• In System 3, the desiccant is used to dry a blend of makeup air and return air which is precooled.

• In System 4, the desiccant is used to dry a blend of makeup air and return air that is not precooled.

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