Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
2000-03-02
2003-02-11
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
C261SDIG001, C261SDIG007
Reexamination Certificate
active
06517058
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cooling towers and other direct contact heat and mass transfer devices utilizing fill media. More particularly, the invention relates to an improved film fill for use in cooling towers.
2. Description of the Related Art
Fill is the material over and through which fluids in heat and mass transfer devices flow. Cooling towers utilizing fill are constructed so that films of the liquid to be cooled flow over and coat the fill surfaces while the cooling medium flows through the fill. The liquid to be cooled and the cooling medium flow in different directions and directly contact each other.
Generally, the liquid to be cooled is sprayed over the fill, with the fill being contained within an enclosed space. The cooling medium, e.g., air, is supplied by means of a natural, induced, or forced draft. The draft may be horizontal (i.e., cross current) or vertical (i e., counter current). The liquid to be cooled flows down, coating the fill, and directly contacts the counter or cross current cooling media. Generally, in counter current cooling, the air enters at the bottom of the tower and travels upward. In general, the greater the surface area of the liquid to be cooled contacting the cooling media, the more efficient the cooling tower will be.
Generally, such cooling towers include a housing through which air is admitted and exhausted by suitable means such as, e.g., exhaust fans. The liquid to be cooled (e.g., water) is distributed throughout the housing by a water distribution system (e.g., sprinklers) located above the fill. The water falls by gravity to a basin located at the base of the housing.
Fill can take many forms, including multi-cell blocks, multiple sheet configurations, and multiple plate configurations. Fills can also be made of many different materials including, but not limited to, plastics such as PVC, wood, metals, ceramics and fibrous cement.
Film fills should preferably exhibit the following characteristics:
1. Deploy a large surface area in a relatively small volume, thereby maximizing heat and mass transfer.
2. Allow air and water to pass over and through the fill pack and come into contact with each other with little airflow resistance.
3. Minimize the accumulation of solids, i.e., fouling of fill surfaces.
4. Provide a long service life, preferably in excess of 25 years.
5. Be inert to various water chemistries and insusceptible to UV damage.
6. Be able to withstand freeze-thaw cycling without damage.
7. Be able to operate at water temperatures in excess of 135° F. without loss of physical integrity or mechanical strength.
8. Be of rugged construction with the ability to withstand foot traffic on the top surface of the fill without damage or loss of shape.
9. Be non-flammable.
10. Be low in cost.
11. Be lightweight, thereby minimizing structural support requirements.
12. Be comprised of materials that are non-toxic, non-hazardous and suitable for easy and safe disposal at the end of service life.
Film fills currently utilized in cooling towers include multi-plate types comprised of asbestos-cement or fibrous cement plates, multi-sheet types comprised of plastic sheets, and ceramic multi-cell block types. Each of these fills exhibits a number of the aforementioned desirable characteristics while suffering from various drawbacks.
Multi-Plate Asbestos-Cement and Fibrous Cement Fills
Multiple vertically placed asbestos-cement or fibrous cement plates have been extensively employed in cooling towers as film fill. The plates are flat and rectangular, approximately {fraction (3/16)} inch thick and are spaced in the range of ½ to 2 inches apart. Multiple layers of the plates are typically deployed in cooling towers, carried either directly on beams, or, alternatively, suspended from the beams.
These fills exhibit limitations with respect to service life, attack by various water chemistries and damage caused by freeze-thaw operation. They are also heavy and high in cost. Use of asbestos-cement fills is extremely problematic because they are hazardous and present disposal problems.
These plate type fills have straight top and bottom edges. If alternate layers are stacked parallel to each other in a cooling tower it becomes necessary to install special transverse spacers between layers to maintain separation and provide adequate structural support. The spacers also serve to minimize the flow restrictions occurring at the interface between layers. The parallel stacked arrangement is disadvantageous since internal mixing of the water and air flowing over and through the fill media can take place in one plane only, thereby diminishing the thermal performance of the fill. Additionally, extra costs are incurred relative to the supply and installation of the spacers.
If alternate layers are stacked at right angles to each other, good mixing of both air and water is promoted. However, air side pressure drop is significantly increased because of the restriction in cross-sectional area that occurs where alternate layers contact each other. Spacers between layers can be utilized to avoid the multiple restrictions at the layer to layer interfaces. This, however, increases the cost of the fill.
Fill plates of this type are relatively thick and are flat on both sides. The considerable thickness of the plates increases the size of the obstruction created at contact points, reduces cross sectional flow area and consequently increases air side pressure drop. The thickness of these fill plates additionally increases the weight of the fill with an attendant increase in structural costs. Air side pressure drop is detrimental to cooling tower efficiency since it necessitates increased power consumption to create adequate air velocity.
In some cases, plates of this type are assembled into multi-plate assemblies, also referred to as fill packs. In those cases, plastic spacer combs are utilized to join multiple plates to form a fill pack prior to installation in cooling towers. Assembly is accomplished by inserting combs through slotted openings in the plates and then rotating the combs by 90°. The spacer comb design has several shortcomings including the limited service life of the plastic comb material, reduction in strength at elevated temperatures and restrictions in available comb pitch settings.
Plate type fills made of asbestos-cement or fibrous cement absorb significant amounts of water when in service. Absorption of water subjects plates of this type to freeze-thaw damage with an attendant decrease in useful service life. Additionally, the cementitious makeup of these plates and the use of cellulose fibers in fibrous cement precludes the use of these plates in certain water chemistries due to the reactive nature of these materials.
Multi-Sheet Plastic Fills
Multiple vertically placed plastic sheets are frequently employed in cooling towers as film fill. The sheet material most commonly used is polyvinyl chloride (PVC). The sheets are usually corrugated with the flute angle of the corrugations inclined typically about 60° from the horizontal. Adjacent sheets are cross-stacked with respect to each other so that the corrugations serve as a spacing means for the sheets. Other spacing configurations and sheet topographies are also employed depending on the application. The sheets are typically attached to each other with a bonding material at the locations where they contact each other thus forming fill packs. The sheets are very thin, typically in the range of 0.008 to 0.015 inches.
Multi-sheet assemblies formed of plastics such as PVC provide only a limited service life, are flammable, have low structural strength, and can present disposal problems. Additionally, plastic fills cannot be operated at high water temperatures, i.e., in excess of 135° F. without risk of structural deformation unless high temperature plastics such as CPVC are utilized. Use of high temperature plastics such as CPVC can result in as much as a doubling of fill costs. Fills of this type can be made rugged enough to withstand foo
Engh Ann L.
Fay Herman P.
Baker & Daniels
Bushey C. Scott
Sandkuhl Clay Works, Inc.
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