Cooling towers tower fill

Problems associated with biofilms. Once bacteria begin to colonize surfaces and produce biofilms, numerous problems begin to arise, including reduction of heat-transfer efficiency, fouling, corrosion, and scale. When biofilms develop in low-flow areas, such as cooling-tower film fill, they may initially go unnoticed, since they will not interfere with flow or evaporative efficiency. Over time, the biofilm becomes more complex, often with filamentous development. The matrix provided will accumulate debris that may impede or completely block flow. 

Place 200 ml. of absolute ethyl alcohol in a 500 ml. aU-glass wash bottle, and cool to — 5° by immersion in a bath of ice and salt. Pass a slow stream of ammonia, derived from a cylinder and dried by passage through a tower filled with small pieces of quicklime, into the alcohol until the... 

Theoretical possible heat removal per pound of air circulated in a cooling tower depends on the temperature and moisture content of air. An indication of the moisture content of the air is its wet-bulb temperature. Ideally, then, the wet-bulb temperature is the lowest theoretical temperature to which the water can be cooled. Practically, the cold-water temperature approaches but does not equal the air wet-bulb temperature in a coohng tower this is so because it is impossible to contact all the water with fresh air as the water drops through the wetted fill surface to the basin. The magnitude of approach to the wet-bulb temperature is dependent on tower design. Important factors are air-to-water contact time, amount of fill surface, and breakup of water into droplets. In actual practice, cooling towers are seldom designed for approaches closer than 2.8°C (5°F). 

FIG. 12-14 Sizing chart for a coiinterflow induced-draft cooling tower, for induced-draft towers with (1) an iipspray distributing system with 24 ft of fill or (2) a flume-type distributing system and 32 ft of fill. The chart will give approximations for towers of any height. (Ecodyne Carp.)... 

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

Figure 9-104. Spray-filled counterflow induced draft cooling tower. Used by permission of The Marley Cooling Tower, a United Dominion Co.

The cooling tower cools hot water tvith cool air by countercurrent (or cross-current) fiow of the tw o fluids past each other in a tower filled with packing. This involves both mass and heat transfer. The water surface that exists on the tower packing is covered with an air film assumed to be saturated at the water temperature. The heat is transferred between this film and the main body of air by diffusion and convection. Detailed presentations of the development of cooling tower theory are given in References 39 and 46. 

The fill illustrated in Figures 9-1 lOA and B is typical of many cooling tower heat transfer evaporative cooling surfaces. The wooden splash type is the oldest in terms of length of usage, while the film types (some fabricated of plastic) have been in service about 40 years [148]. 

Figure 9-110A. Commercial cooling tower fill packing. Reproduced by permission of the American Institute of Chemicai Engineers, Keily, N. W., cind Swenson, L. K., Chemical Engineering Progress, V. 52, No. 7 (1956) p. 263 aii rights reserved.

Guidelines for Cooling Tower Recirculating Water (Normal Limits for Using Film Fill)... 

Passive fire protection for cooling towers involves increasing spacing distances and using noncombustible materials of construction. For cooling towers of totally noncombustible materials of construction, there are no fire protection requirements. Noncombustible means that the cooling tower s structure, fan and distribution decks, louvers, and fill materials must all be noncombustible materials. 

Some manufacturers of internal cooling tower components, specifically fill material and drift eliminators, have products produced from less easily ignited plastic that have been tested by a nationally recognized testing laboratory and determined to have sufficient fire resistance or reduced flame spread ratings that when, and only when, used in an otherwise noncombustible cooling tower, do not require fixed automatic fire protection. 

In July of 1997, a cooling tower at an ammonia and urea plant, originally constructed in 1968, caught fire and was destroyed. The plant produced 1,450 tons/day (1,315 tonnes/day) of ammonia and 240 tons/day (218 tonnes/day) of urea. The coolingtowerwasa 5-cell, induced draft, cross flow unit. It was constructed of redwood with steel supports and fiberglass fill. The capacity of the cooling tower was 50,000 gallons (190,000 liters). 

The fill material in natural draft cooling towers is frequently asbestos cement. Erosion of this fill material may result in the discharge of asbestos in cooling water blowdown. In a testing program for detection of asbestos fibers in the waters of 18 cooling systems, seven of the 18 sites... 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

Some HTU data for cooling tower packing have been published, for example, those summarized on Figure 9.16. Other data appear in the additional literature cited for this chapter. Several kinds of tower fill made of redwood slats are illustrated in Figure 9.17. The numbers N of such decks corresponding to particular NTUs and (L/C)s are given by the equation... 

Figure 9.17. Kinds of fill made of redwood slats for cooling towers, and factors for determining the required number of decks with inlet water at 120°F (Cheremisinoff and Cheremisinoff, 1981).

Cooling towers are classified according to the method by which air is introduced to the tower. The principal types are atmospheric spray, natural-draft, mechanical-draft, deck-filled, spray-filled, coil shed and hyperbolic towers. Most industrial cooling tower installations are field-erected units designed for specific thermal characteristics. 

In an atmospheric spray tower the air movement - is dependent on atmospheric conditions and the aspirating effect of the spray nozzles. Natural-draft cooling tower operation depends on a chimney or stack to induce air movement. Mechanical-draft cboling towers utilize fans to move ambient air through the tower. Deck-filled towers contain tiers of splash bars or decks to assist in the breakup of water drops to increase the total water surface and subsequently the evaporation rate. Spray-filled towers depend only on spray nozzles for water breakup. Coil shed towers are comprised of a combination structure of a cooling tower installed on top of a substructure that contains atmospheric section coils. Hyperbolic natural-draft cooling towers are typically large-capacity systems. 

Figure 4.10 Various geometries employed in constructing redwood fill for cooling towers.

Figure 4.20 Cooling tower design developed by Baltimore Aircoil Co. The system is designed to operate without fill packing.

Cooling tower packing material is commonly referred to as fill. The two most common configurations used are illustrated in Figure 5.3. The purpose... 

Figure 5.3 Cross-sectional view of commonly used cooling tower fill arrangements.

A direct-contact gas cooler system operates as follows Approximately 35,000 lb/hr of bone-dry air is passed over hot trays. The air is heated from 150°F to 325°F as it passes over the trays. It exits from the unit with a due point of 105°F. The hot air is sent to a direct-contact cooler, where its temperature is reduced back to 150°F. During the cooling stage, the air is dehumidified with water that is heated frpm 75°F to 105°F. The unit is rated at 3.5 inches of water pressure drop (a) Determine the number of diffusion units needed for this operation and (b) Establish the required dimensions for the direct-contact cooling tower (Hint Use standard low-pressure-drop data from the literature. Some of the older literature give pressure drop data for simple fill. See Sherwood, T. K. and C. E. Reed [6]. 

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