Refrigeration – Processes – Circulating external gas
Reexamination Certificate
1999-03-10
2001-03-13
Wayner, William (Department: 3404)
Refrigeration
Processes
Circulating external gas
C062S094000, C062S271000, C096S125000
Reexamination Certificate
active
06199388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field 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 a controlled space.
2. Background
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 have a far greater impact on the health of building occupants than previously suspected. For example, microbial activity (e.g., mold and fungus), which increases at higher indoor humidity levels, has been shown to emit harmful organic compounds. Childhood asthma is now suspected by some researchers to be linked to such microbial activity.
In addition to direct health effects, the odors associated with microbial activity are often cited as a primary reason why indoor air quality is considered unacceptable to occupants. When odors are encountered in a building, building operators often respond by increasing outdoor air quantities in an attempt to eliminate odors. This often 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 and Refrigeration and Air Conditioning Engineers (“ASHRAE”). Several years ago, the ASHRAE issued IAQ Standard 62-1989, entitled, “Ventilation for Acceptable Indoor Air Quality.” This standard emphasizes the need for continuous outdoor air ventilation as well as the importance of maintaining indoor humidity levels. Recently proposed modifications to the standard (62-1989R) have placed further emphasis on humidity control, specifically in hot and humid climates. The proposed standard emphasizes the need for maintaining proper humidity levels during peak and partial loading conditions, and during both occupied conditions (where it is recommended that relative humidity be maintained at no more than 60%) and unoccupied conditions (where it is recommended that relative humidity be maintained at no more than 70%). There is, therefore, a significant need for energy-efficient systems that effectively control the humidity of an indoor space while simultaneously providing high quantities of outdoor air to the space.
Facilities with high occupancy rates or high levels of ingress and egress, such as schools, hospitals, nursing homes and many offices, typically have large amounts of outdoor air introduced to the occupied space and, consequently present a significant HVAC design challenge. Due to the extreme humidity levels and large number of partial-load cooling hours that exist in hot and humid climates, maintaining relative humidity at levels recommended by ASHRAE is extremely difficult and costly if conventional HVAC approaches are to be used for such facilities.
For example, during days when the temperature is moderate but the humidity is high (partial load conditions) a packaged HVAC unit will quickly bring the space to the desired temperature, then turn off its cooling coil. As the outdoor air continues to be provided to the space, the indoor humidity level climbs until the temperature in the space once again causes the thermostat to initiate cooling operation. By this time, the mixed air condition supplied to the cooling coil is elevated in humidity. The elevated humidity level of air reaching the cooling coil results in a high dew point temperature leaving the cooling coil. The space temperature is maintained but humidity control is lost, resulting in elevated space humidity conditions.
One of the most effective design solutions for controlling the temperature and humidity of a controlled space while providing high quantities of outdoor air involves decoupling the latent load from the conventional HVAC units serving the facility. This approach allows the conventional units to be downsized to handle the sensible load only. The dehumidified outdoor air is provided directly to the controlled space via a separate system at a room neutral temperature and humidity (typically 68° F. and 55 grains). This approach allows the desired quantity of outdoor air to be provided continuously while simultaneously maintaining a desirable and consistent relative humidity for the space.
Desiccant-based systems have been used to provide a continuous supply of dehumidified outdoor air. These systems can remove a significant amount of the humidity contained within the outdoor air prior to its introduction to the conventional HVAC system or, as mentioned previously, directly to the conditioned space. This allows the packaged equipment to operate as designed and to better control the space humidity despite increased outdoor air requirements.
Desiccants can be solid or liquid substances that have the ability to attract and hold relatively large quantities of water. In many commercial air conditioning applications where desiccants are used, the desiccant is in a solid form and absorbs moisture from the air to be conditioned. Examples of these types of desiccants are silica gel, activated alumina, molecular sieves, and deliquescent hygroscopic salts. In some cases, these desiccants are contained in beds over which the air to be conditioned is passed. Many times, however, the desiccant is contained in what is known as a “desiccant wheel.”
A desiccant wheel is an apparatus typically comprising a plurality of closely spaced, very thin sheets of paper or metal which are coated or impregnated with a desiccant material. The wheel is frequently contained in duct work or in an air handling system that is divided into two sections. The wheel is rotated slowly on its axis such that a given zone of the wheel is sequentially exposed to the two sections. In the first section, the desiccant is contacted by the supply/outdoor air. In this section, the desiccant wheel dehumidifies the supply/outdoor air stream by absorbing moisture from the air onto its desiccant surface.
In the second section of the desiccant wheel, the desiccant contacts the return/exhaust air being discharged from the space. This return/exhaust air desorbs the moisture from the desiccant that was adsorbed from the supply/outdoor air. By cycling the wheel through these two air streams, the adsorbing/desorbing operation of the wheel is continuous and occurs simultaneously.
The prior art generally includes two types of desiccant-wheel systems: i) the “passive” dual wheel energy recovery preconditioner (DWERP) system; and ii) the “active” thermally regenerated desiccant-based cooling (DBC) system.
As shown in prior art
FIGS. 1A and 1B
, the DWERP system typically combines a desiccant-based total energy recovery wheel and sensible only recovery wheel along with a conventional chilled water or direct expansion (“DX”) cooling coil to cool and dehumidify or heat and humidify the outdoor air supplied to a facility (depending on the ambient conditions).
A typical DWERP system operating in the cooling mode is illustrated in FIG.
1
A. In the cooling mode, the supply/outdoor air stream passes through the desiccant-based total energy recovery wheel where it is precooled and dehumidified, giving up much of its humidity and heat to the desiccant coated wheel media. Next the outdoor air is passed through the cooling coil where it is further cooled and dehumidified until reaching the absolute humidity level required by the occupied space. The cold, dehumidified outdoor air stream is finally passed through a sensible only wheel where it is reheated to the desired temperature, using only the energy contained within the return air stream being exhausted from the space and passed through the secondary side of the sensible only wheel.
The return air stream leaves the sensible only wheel cool and dry, having passed through the transfer matrix cooled by the cold air leaving the cooling coil. This cool, dry air is then passed t
Bryan Cave LLP
Semco Incorporated
Wayner William
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