scope:

INTRODUCTION

ANSI/ASHRAE Standard 62-2001 (ASHRAE 2001), Ventilation for Acceptable Indoor Air Quality, increased ventilation requirements in all commercial buildings and is currently referenced by many model building codes. These increased ventilation requirements imposed greater heating and cooling loads on HVAC equipment and in many climates increased the dehumidification load which in turn impacted the ability of HVAC systems to control humidity within conditioned spaces. ASHRAE Research Project RP-1254 completed Phase 2 of a two-phase effort to evaluate several means to improve an HVAC system's dehumidification performance and provide better humidity control in commercial buildings. The Phase 1 effort (Brandemuehl et al. 2001) developed a plan to guide the evaluation effort which was carried out as part of Phase 2. This technical paper presents the results of Phase 2 which was targeted at:

• Comparing various unitary air conditioning system humidity control configurations for application to commercial buildings in terms of humidity control performance, operating costs, and life cycle costs to each other as well as to conventional unitary equipment, and

• Developing guidelines to help HVAC engineers and practitioners identify the important application characteristics and climate factors that determine which option is most appropriate.

For unitary products, many options exist today for better dehumidification technologies including:

• Evaporator coils with more rows and lower airflow rates,

• Lower airflow rates,

• Air-to-air heat exchangers to precool and reheat air entering and leaving the cooling coil,

• Condenser reheat coil in series with evaporator,

• Prevention of evaporation from wet cooling coil by cycling the supply air fan,

• Bypassing a fraction of the airflow around the cooling coil,

• Pre-drying using active desiccant prior to the cooling coil,

• Enthalpy recovery wheel using exhausted return air,

• Dedicated preconditioning DX system for outside air,

• Separate outside air conditioning dual path system,

• Enthalpy recovery wheel with a separate outside air conditioning system,

• Air-to-air heat exchangers with a separate outside air conditioning system,

• Separate outside air conditioning with desiccant system, Reducing ventilation by using carbon dioxide monitoring, and

• Separate outside air conditioning with reducing the ventilation quantity by carbon dioxide monitoring.

Given the large number of options, it is difficult to choose which enhanced unitary technology is appropriate for a given application. This ASHRAE research project was initiated to improve our knowledge about these choices by providing a quantitative study across many of the available options. By examining these applications from the perspective of what is the most cost effective way to control indoor humidity levels, designers will be able to use the guidance provided to assist in selecting the equipment configuration for a specific building.