ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
"INTRODUCTION Safety factors are commonly used by engineers when designing various types of HVAC systems for use in all types of buildings. Excessive use of safety factors in the design process can result in larger-than-necessary equipment@ inflated installed costs@ and sometimes excessive energy use. This is especially true when safety factors are used during several steps of the design process@ which ""compounds"" their impact along the way. However@ many of the decisions made during the design process are based on incomplete information@ assumptions that may turn out to be invalid@ or valid assumptions that may no longer be valid one@ five@ or ten years after the system or equipment is installed. Therefore@ safety factors are important tools for design engineers@ allowing the HVAC system to be designed with ""reserve capacity"" to accommodate unexpected loads and the need for increased airflow or dehumidification capacity. A dedicated outdoor-air system (DOAS) uses a separate piece of equipment to condition (filter@ heat@ cool@ humidify@ dehumidify) all of the outdoor air brought into the building for ventilation. This conditioned outdoor air is then delivered either directly to each occupied space or to local HVAC units serving those spaces. Meanwhile@ the local units (such as fan-coils@ water-source heat pumps@ PTACs@ small packaged units@ VAV terminals@ chilled ceiling panels@ or chilled beams) located in or near each space provide cooling and/or heating to maintain space temperature (Coad 1999@ Shank and Mumma 2001) Treating the outdoor air separately can make it easier to verify that sufficient ventilation airflow reaches each occupied space and can help avoid high indoor humidity levels. The latter is accomplished by dehumidifying the outdoor air to remove the entire ventilation latent load and most (or all) of the space latent loads@ leaving the local HVAC units to primarily handle space sensible cooling loads. Some types of local HVAC equipment@ such as chilled ceiling panels or chilled beams@ must operate dry and avoid condensation. This limits their duty to handling sensible loads only. Figure 1 shows several example DOAS configurations. Some deliver the conditioned outdoor air (CA) directly to each zone (Mumma 2008)@ while other configurations deliver the air to the intakes of local@ single-zone units (such as fan-coils@ water-source heat pumps@ dual-duct VAV terminals@ small packaged rooftop units@ or single-zone air handlers) or to centralized@ multiple-zone units (such as floor-by-floor VAV air handlers or self-contained units). In addition@ there are many types of dedicated outdoor-air equipment available (Figure 2). Dehumidification is usually provided by direct-expansion (DX) refrigeration@ a chilledwater coil@ a desiccant-based dehumidification device@ or some combination of these technologies. Often@ the dedicated outdoor-air unit includes an exhaust-air energy recovery device (such as a total-energy wheel@ fixed-plate heat exchanger@ coil runaround loop@ or heat pipe)@ which can reduce energy use and allow for downsizing of the cooling and heating equipment. In fact@ ASHRAE Standard 90.1 requires the use of an exhaust-air energy recovery device for many DOAS applications (ASHRAE 2007)."