Drift eliminator

Details Drift eliminator Drift eliminator

2000-01-07

2001-11-13

Simmons, David A. (Department: 1724)

Gas separation

Deflector

Parallel and continuous nonplanar members

C055S443000, C055S444000, C096S356000, C261S112200

Reexamination Certificate

active

06315804

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a drift eliminator, sometimes referred to as a mist eliminator, a critical element in both direct and indirect counter-flow and cross-flow cooling towers, spray filled towers, and evaporative condensers, all of which are hereinafter categorically referred to as “tower”. More particularly, the drift eliminator of the present invention has a structure that improves drainage, elevates the drift point, and reduces air pressure drop and therefore power requirements in the equipment in which it is used, resulting in enhanced thermal efficiency.
A drift eliminator has the function of stripping entrained water droplets or mist from the gas (air, typically) stream passing out of the tower. Without drift eliminators, evaporative cooling equipment would be impractical because a majority of the circulating water would be blown right out of the top of the tower. Further, drift is very objectionable because of excess loss of circulating water, loss of water treatment chemicals, and wetting of the space surrounding the tower with its ensuing hazard potentials such as ice formation, staining of surrounding cars and buildings, and spread of bacteria.
A drift eliminator is normally comprised of a plurality of channeling elements having a shape, curved or otherwise, which causes one or more changes in the air direction passing through the channel. The channel shape is designed such that air will pass through the channels with a minimum pressure drop, but the air direction change is too sharp for water droplets entrained in the air stream to pass. The momentum of the drop will cause it to impinge on the surface of the drift eliminator as it changes direction, and the action of gravity will cause the droplets to flow over the surface of the eliminator and fall back into the cooling, tower. Water droplets that make it past the curvature will normally continue to be carried along with the exiting air. This escaping water spray known as “drift” or “carry-over” emanates from the equipment. While this is the primary mode of drift formation, other modes exist. The second most common mode of drift formation occurs when upward air pressure and fluid shear forces reverse the downward gravity force on the water impinged on the drift eliminator surface and push the accumulated water film to the top edge of the eliminator surface. The water film on the top edge of the eliminators coalesces to large drops that are picked up by the passing air and ejected from the tower. Large water drop drift is most commonly encountered when eliminators have become flooded, that is, their impingement surface has been coated with a thick water film.
“Drift rate” relates to the amount of water spray that is carried out of the tower with the air. It is quantitatively measurable and is commonly expressed as a percentage of the circulating water flow in a tower. Drift rates are usually very small, less than 0.01%, and drift eliminators are selected by their drift rate performance which varies with tower application and characteristics that differ based on their manufacturer. The air velocity at which drift eliminators fail to function by exceeding the specified drift rate, thereby passing excessive drift out of the tower, is known as the “drift point” or “spit point”. In a tower, many factors influence both the drift point and the mode of drift formation. These factors include the geometry of the eliminator, the proximity of the eliminator to the tower's water distribution system, the circulating water flow inside the tower, the predisposition of the air pattern approaching the eliminator, water quality, fouling coatings on the eliminator, and the operating time history of the tower. However, for a given eliminator geometry and tower geometry, the controlling factors on the drift point are circulating water flow and air velocity.
FIG. 11
schematically graphs the relationship between circulating water flow and air velocity inside a tower and its effect on the drift point. Circulating water rate is shown as the ordinate, and air velocity is the abscissa. A drift point curve
100
separates a region
102
of acceptable drift rate, which is below the curve, from a region
104
of excessive drift, which is above the curve. The geometry of the eliminator and the geometry of the tower will act to shift the drift point curve generally upward or downward. For modern, high-efficiency drift eliminators, the drift point corresponds to an air velocity typically in the range of 500 to 800 feet per minute (fpm) in the passageway where the drift eliminator is mounted in the tower at circulating water flow rates over the wet deck fill of 3 to 20 gallons per minute per square foot (gpm/ft
2
) of tower plan area. It should also be noted that as the circulating water rate increases, the eliminator tends toward flooding and large droplet drift formation.
Some equipment designs or common maintenance problems predispose drift eliminators to operate at lower drift points than would be desired for the tower's intended purpose. Examples of the aforementioned are towers with high circulating water rates, towers with eliminators directly above but in contact with the circulating water spray system, or towers with poorly maintained water distribution nozzles that force water spray directly into the drift eliminators because these conditions tend to flood eliminators. When eliminators are flooded, their consequent drift point is reduced and the tower's air velocity must be reduced to prevent drift. The result is a loss in overall thermal capability.
It is typical on large counter-flow towers, particularly the field-erected type, to have a substantial plenum space between the spray system and the drift eliminators, moderate water flows, moderate air flows, and regular maintenance attention, all of which create a more ideal eliminator application environment. In contrast, factory assembled towers (condensers) have shipping constraints that require a very compact tower, and the drift eliminators are assembled directly on top of the spray system. Further, the broad demand for energy efficient, compact, factory-assembled towers dictates the use of eliminators in the adverse environments of high circulating water flows (8-20 gpm/ft
2
inside the tower), high air velocity (600-800 fpm), and continual operation with minimal spray system maintenance. The present invention particularly, but not exclusively, targets drift eliminators for these demanding conditions associated with factory-assembled towers.
The current state of the art for counter-flow drift eliminators can generally be divided between parallel blade eliminators and cellular eliminators. Most induced draft towers use cellular eliminators, while most forced draft towers use parallel blade eliminators, but there is no specific requirement for either. Parallel blade eliminators typically have a plurality of parallel curved blades made into sections that can be handled by maintenance personnel. The blades are separated by discrete spacers, which may be separate items or integrally formed in the blade, or the blades may be spaced apart by formed end caps which retain the ends of the blades in a defined spaced relationship.
The most common eliminator used in induced draft counter-flow towers is a cellular drift eliminator. When viewed from the top or bottom, a cellular drift eliminator appears as a plurality of parallel, curved tubes and is distinguishable from an eliminator made of parallel blades. The cellular drift eliminator may be comprised of a plurality of parallel curved surfaces that create the impingement surfaces for water droplets. Between the curved surfaces, a spacing element forms small cells, as in the present invention. Cellular eliminators have also been fabricated from mating corrugated curved blades. The distinctive features of cellular eliminators are that the tubular design adds strength to the eliminator assembly and creates further traps for water migration and droplet impingement. Because of the design, cellula

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