Utilities cooling towers

There are many applications in which it is important to maintain an average basin water temperature at an optimum value. One example is a utility cooling tower application in which an optimum average basin water temperature is required to ensure efficient turbine operation. In this example, the optimum temperature falls between 60° and 75°F. Ice prevention systems should be designed to provide sufficient flexibility to control the basin water temperature between specified limits without significant ice formation for a wide range of heat load and ambient environmental conditions. 

The utihty iadustry utilizes fans typically from 6.7—10 m diameter ia banks of 8 to 12 fans ia wet cooling towers. These towers cool the water used to condense the steam from the turbiaes. Many towers may be needed ia large plants requiring as many as 50 to 60 fans 12 m in diameter. These fans typically utilize velocity recovery stacks to recoup some of the velocity pressure losses and convert it to useful static pressure work. 

After only 4 months of service, the main condenser at a large fossil utility began to perforate. Initial perforations were due to erosion-corrosion (see Case History 11.5). Small clumps of seed hairs entering the condenser after being blown into the cooling tower were caught on surfaces. The entrapped seed hairs acted as sieves, filtering out small silt and sand particles to form lumps of deposit (Fig. 6.24A and B). Immediately downstream from each deposit mound, an erosion-corrosion pit was found. 

The Spray tree is used to distribute water over the wet deck in counterflow cooling towers. It can consist of a single header fitted with spray nozzles or, it can utilize spray branches with nozzles for wider coverage. Spray nozzle designers seek minimal pressure requirements and uniform coverage over wide flow ranges. 

The OSBL costs comprise land, site development, process buildings, auxiliary buildings, Storage buildings, equipment and construction for utilities including cooling towers, construction expenses, waste-disposal facilities, etc. The OSBL costs may be assumed to be about 20-40% of the ISBL costs, with the upper limit applying better to small batch plants. 

Example 17.2 For the process in Figure 16.2, calculate the target for network heat transfer area for ATmi = 10°C. Steam at 240°C and condensing to 239°C is to be used for hot utility. Cooling water at 20°C and returning to the cooling tower at 30°C is to be used for cold utility. Table 17.1 presents the stream data, together with utility data and stream heat transfer coefficients. 

In the United States, about 80% of the 23 million kg of technical PCP produced annually — or about 46% of worldwide production — is used mainly for wood preservation, especially utility poles (Pignatello etal. 1983 Kinzell etal. 1985 Zischke etal. 1985 Choudhury etal. 1986 Mikesell and Boyd 1986 USPHS 1994). It is the third most heavily used pesticide, preceded only by the herbicides atrazine and alachlor (Kinzell et al. 1981). Pentachlorophenol is a restricted-use pesticide and is no longer available for home use (USPHS 1994). Before it became a restricted-use pesticide, annual environmental releases of PCP from production and use were 0.6 million kg to the atmosphere from wood preservation plants and cooling towers, 0.9 million kg to land from wood preservation use, and 17,000 kg to aquatic ecosystems in runoff waters of wood treatment plants (USPHS 1994). There are about 470 wood preservative facilities in the United States, scattered among 45 states. They are concentrated in the South, Southeast, and Northwest — presumably due to the availability of preferred timber species in those regions (Cirelli 1978). Livestock facilities are often constructed of wood treated with technical PCP about 50% of all dairy farms in Michigan used PCP-treated wood in the construction of various components of livestock facilities (Kinzell et al. 1985). The chemical is usually applied to wood products after dilution to 5% with solvents such as mineral spirits, No. 2 fuel oil, or kerosene. More than 98% of all wood processed is treated with preservative under pressure about 0.23 kg of PCP is needed to preserve 1 cubic foot of wood (Cirelli 1978). Lumber treated with PCP retains its natural appearance, has little or no odor, and can be painted as readily as natural wood (Wood et al. 1983). 

Auxiliary processes. Refineries also have other processes and units that are vital to operations by providing power, waste treatment, and other utility services, such as boilers, wastewater treatment, and cooling towers. Products from these facilities are usually recycled and used in other processes within the refinery and are also important with regard to minimizing water and air pollution. 

FIG. 24-65 Mechanical refrigeration TIC systems utilizing chilled water for cooling turbine inlet air, and cooling towers to reject the waste heat into the environment, account for the majority of refrigeration TIC systems sold. 

Cooling towers and utility areas (boilers, air compressors, power generation). 

Cooling towers can be either the victim of an outside ignition source or they can be the culprit causing propagation of a fire to other nearby processing, storage, or utility operations or facilities. In fire protection terms, they may present an "exposure to" or they may suffer "exposure from" other facilities or hazards. Some of the situations and events that have led to fires in cooling towers of combustible construction are ... 

Location of cooling tower with respect to exposure to or from nearby processing, storage, or utility operations or facilities... 

Loss of utilities including electrical power, steam, cooling tower water, instrument air, and nitrogen. 

Noncontact cooling water is normally supplied to several processes from the utilities area. The system is either a loop that utilizes one or more evaporative cooling towers, or a once-through system with direct discharge. 

Wastewater streams from the utility functions include boiler and cooling tower blowdowns and waste brine and sludge produced by demineralizing and other water treatment systems. The quantity and quality of the wastewater streams depend on the design of the systems and the water source. These streams usually contain high dissolved and suspended solids concentrations and treatment chemicals from the boiler and cooling tower. The blowdown streams also have elevated temperatures. 

Outside battery limit (OSBL) defines the work and responsibilities outside the battery limit. The OSBL includes auxiliaries required for the chemical process unit, including tank storage for feedstock and refined products, waste treatment, cooling towers, flares, and utilities that are not included in the battery limit but are required for the unit. Also included are the pipeways, racks, and sleepers, which are used to convey all of the utility s interconnecting process piping to and from the processing facility... 

In the smaller cooling tower installations employed for refrigeration and air conditioning service, induced-draft propeller-type air movers are used. A favored design utilizes the rotary centrifugal squirrel cage blower, which is normally driven by electric motors connected to the rotors by V-belts. Because of lower fan efficiency, these often use more electric power to achieve the same results as propeller fans. 

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 4.11 Power plant installation where multiple tower arrangement is utilized (towers ate operated in parallel). Cooling towers are placed in a row at right angles to the prevailing winds (courtesy of The Marley Company, Mission, KS).

Mechanical-draft cooling towers can be either field-constructed or factory-assembled systems. Installations that are constructed onsite are generally those employed at utility stations. Factory-assembled towers have applications in smaller industrial operations. 

Figure 4.21 Direct, dry-type cooling tower condensing system utilizing a mechanical-draft tower.

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