Refrigeration – Processes – Circulating external gas
Reexamination Certificate
2002-06-28
2004-06-22
Jones, Melvin (Department: 3744)
Refrigeration
Processes
Circulating external gas
C062S271000, C062S332000
Reexamination Certificate
active
06751964
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to the field of heating, ventilating, and air conditioning (“HVAC”). More particularly, this invention relates to systems and methods for controlling the temperature and humidity of an enclosed space.
The quality of indoor air has been linked to many illnesses and has been shown to have a direct impact on worker productivity. New research strongly suggests that indoor humidity levels may have a significant impact on the health of building occupants. For example, microbes such as mold and fungus, which proliferate at higher indoor humidity levels, have been shown to emit harmful organic compounds. In addition to direct health effects, often the primary air quality complaint of building occupants is unpleasant odors associated with microbial activity. Building operators often attempt to eliminate odors by increasing outdoor air quantities. This usually exacerbates the problem because increasing outdoor air quantities often results in higher indoor air humidity levels, which, in turn, fosters further microbial activity.
The HVAC industry has responded to these indoor air quality (“IAQ”) concerns through its trade organization, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (“ASHRAE”). ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air Quality, sets minimum ventilation rates and other requirements for commercial and institutional buildings. Meeting these standards generally requires systems capable of providing an increased supply of outdoor air to the conditioned space while maintaining acceptable humidity levels within the space. A large body of research supports the need for continuous ventilation in accordance with ASHRAE 62-1999, while maintaining the relative space humidity between 30% and 60%. IAQ problems including unacceptable odors and microbial infestation often occur when HVAC systems fail to meet these design criteria.
Commercial and institutional facilities often use “packaged” units, which combine air conditioning, heating and sometimes air handling equipment in a single housing. Such systems are generally designed to provide inexpensive heating and cooling. Such packaged units are generally installed outside the building envelope, frequently at ground level or on the building roof. A typical packaged unit includes a supply fan and filter, a return air fan, a heating source (typically an indirect gas fired heater or electric heating coil), an outdoor air intake, and a mechanical refrigeration system consisting of a compressor, cooling coil, and a condensing coil with a fan that rejects heat to the outdoors. Typically a small fraction of outdoor air is mixed with a much larger fraction of return air from the building, conditioned by the unit then circulated through the building by means of a system of supply and return ductwork. The advantages of such packaged equipment include low purchase cost, simplicity, familiarity, and compact design. More than 80% of all air-conditioning systems sold to the commercial marketplace involve compressorized package equipment.
A significant shortcoming of such packaged HVAC units is that they are typically designed to utilize minimal outdoor air, and, as such, are frequently incapable of handling the increased continuous supply of outdoor air necessary to comply with ASHRAE 62-1999 guidelines. This is especially true in applications where the need for 100% outdoor air systems exist, such as makeup air to restaurants and hotel facilities. It is also true for applications like schools, movie theatres and other facilities where a high occupancy density results in the need for very high outdoor air percentages being provided by the HVAC system.
To meet the increased outdoor air requirements of the ASHRAE standards, HVAC professionals have attempted to use oversized packaged equipment to match the increased cooling load associated with higher outdoor air percentages. However, such oversized systems generally suffer from sub-par performance and are expensive to operate. As importantly, the oversized cooling capacity required to meet peak outdoor air load conditions proves excessive at the more common part-load conditions, and creates serious performance problems ranging from over-cooling the space and lost humidity control due to reduced compressor cycle times to freezing up coils and shortened compressor life. Therefore, providing outdoor air continuously presents a tremendous challenge to conventional packaged HVAC equipment.
For example, on mild, humid days (part-load conditions) an oversized packaged unit will quickly cool the space to a set temperature and then shut off the compressor. If the evaporator fan is kept running to maintain a continuous flow of outdoor air to the space, the indoor humidity level will usually climb due to the humidity level of the outdoor air being introduced. This increase in humidity will continue until the space temperature rises to the point that the thermostat once again calls for cooling. By this time, the humidity of the return air entering the cooling coil of the packaged HVAC system is elevated. The elevated humidity of the return air results in an elevated dew point temperature leaving the cooling coil. Typically, the system can maintain space temperature, but humidity control is lost, resulting in uncomfortable, cold, clammy conditions. Occupants will often respond by lowering the thermostat setting, causing the space relative humidity to further increase. If such high humidity conditions persist, microbial growth and other moisture-related IAQ problems may arise.
Another problem associated with oversized packaged equipment selected to process outdoor air on a continuous basis results from the re-evaporation of moisture that has condensed on the evaporator coil. Henderson et al. (1998) and Khattar et al (1985) both have confirmed the phenomenon, often observed in the field, where the actual moisture removed by a packaged HVAC unit is significantly less than anticipated based upon published performance data. Their research shows that this reduction in dehumidification capacity is attributable to moisture condensed on the direct expansion (DX) coil evaporating back into the supply air stream when the coil is cycled off but the fan continues to operate. Henderson (1998) has shown that evaporation of moisture condensed on the DX coil can reduce actual latent heat removal to less than 50% of the unit's capacity at part load conditions. (1) Henderson, H. 1998. The Impact of Part Load Air Conditioner Operation on Dehumidification Performance: Validating a Latent Capacity Degradation Model. Proceedings ASHRAE IAQ 98. (2) Khattar, M et. al. 1985. Fan Cycling Effects on Air Conditioner Moisture Removal Performance in Warm, Humid Climates. Presented at the International Symposium on Moisture and Humidity, Proceedings. April, 1985, Washington D.C. (3) Henderson, H. 1990. An Experimental Investigation of the Effects of Wet and Dry Coil Conditions on Cyclic Performance in the SEER Procedure. Proceedings of USNC/IIR Refrigeration Conference at Purdue University, West Lafayette, Ind. July, 1990.) These and other limitations present significant problems when packaged rooftop systems are forced to handle high percentages of outdoor air volume, particularly if operated as 100% outdoor systems. When applying a conventional packaged rooftop system to handle all outside air, the cooling tons required at peak conditions are far greater than the cooling output available at the rated airflow of the conventional unit. This occurs because standard conventional packaged cooling equipment currently available on the marketplace by the major HVAC equipment manufacturers is generally designed to accommodate only a relatively small portion of outdoor air, typically 10-20%.
For example, a typical packaged gas/electric rooftop unit available on the market today may have a rated cooling performance at 95° Fahrenheit (F) ambient, 80° F. coil entering dry bulb, 67° F. coil entering wet bulb in accordance with the ARI Standard 2
Bryan Cave LLP
Hayes Christopher J.
Jones Melvin
LandOfFree
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