Cooling towers operation

Not only may the cooling-tower plume be a source of fog, which in some weather conditions can ice roadways, but the plume also carries salts from the cooling water itself. These salts may come from salinity in the water, or may be added by the cooling-tower operator to prevent corrosion and biological attack in the column. 

Cycles of concentration involved with cooling-tower operation normally range from three to five cycles. Below three cycles of concentration, excessive blowdown quantities are required and the addition of acid to limit scale formation should be considered. 

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

In normal operations, continuously recirculating water picks up waste heat from a refrigeration compressor or process heat exchanger, and the hot water is pumped to the top of the tower and dropped over the cooling tower. Evaporative action removes the heat from the water and adds it to the air. The hot, moist air is ejected from the fan stack, and the cooled water returns to the compressor or exchanger to pick up more heat. Figure 1.1 illustrates the cooling tower operation. 

This section reviews some basic definitions and formulas in thermodynamics. These definitions will be used to develop energy balances to describe cooling tower operations. In our discussions we will use the following terms system, property, extensive and intensive properties, and... 

Before developing specific relationships to describe cooling tower operations, it is worthwhile to review some elementary principles in developing material and energy balances. In addition, we need to review heat and mass transfer analogies before tackling design problems. The more experienced reader may wish to proceed to Chapter 4 or try the example problems at the end of the chapter as a refresher. 

We will apply these definitions in Chapter 5 to analyzing cooling tower operations. At this point the reader should examine some of the problems at the end of this chapter to review some of the concepts discussed thus far. 

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. 

A cooling tower operation is designed without any recycle stream. Approximately 700,000 lb/hr of hot process water at 140°F is to be cooled and returned to the process operation. Moist air is used as the cooling medium and is fed at a rate of 5.5 X 106 ft3/hr. The dry- and wet-bulb temperatures of the incoming air are 80°F and 60°F, respectively. The air leaves the tower with an estimated wet-bulb temperature of 95°F and a dry-bulb temperature of 100°F. Estimate the temperature of the water returned to the process operation. 

The cooling tower operation is illustrated in Figure 5.5. Absolute humidities for incoming and exiting air streams can be obtained from the psychrometric chart (Figure 2.5) ... 

Let us now develop material and energy balances directly applicable to cooling towers. The idealized cooling tower operation is illustrated in Figure 5.6. The cooling tower operates with some type of heat source (a con-... 

It should be noted that Lewis number, Le, is only a prediction. In reality, Le is closer to 0.9. The manner in which Equation 5.29 was derived produces an error only in the convective heat transfer coefficient. In normal cooling tower operation, convective heat transfer is generally less than 20% of the total heat load. For now, Equation 5.29 represents the centerpiece of our... 

Figure 5.12 Important design parameters for the countercurrent cooling tower operation.

A cooling tower operates in the countercurrent mode as illustrated by Figure 5.13. Entering air has a 5% wet-bulb temperature of 65°F. Hot process water enters the tower at 118°F and cold water leaves at a 15° approach to the wet-bulb (i.e., at 80°F). The cross-sectional area of the tower is 676 ft2. Determine the number of transfer units (Ntu ) required to meet the process requirements. Air is supplied to the tower by a blower having a capacity of 250,000 cfm and the water loading is 1500 lb/(hr)(ft2). 

Polyphosphates are also used in cooling systems to attain sufficient corrosion control. Cooling towers are operated in a pH range of 6.0 to 7.5 to provide optimum stability for the polyphosphate. The feasibility of cooling tower operation at higher pH levels, in which the potential for corrosion is decreased, has increased the popularity of low-chromate programs. 

There is, however, a danger for ice formation in the fill if too much water is bypassed. In the majority of cooling tower operations, standard practice is to open and close the bypass valves in a cyclic fashion to maintain a desired average basin water temperature while minimizing fill ice formation. 

Dynamic Plume Model for the Prediction of Atmospheric Effects Associated with Cooling Tower Operation... 

Given are two examples which show different methods that are used to reduce the quantities of waste water from cooling tower operation. In one case, the treatment technique of an existing facility was modified to reduce waste water production. In the order case, a new plant design included facilities for the minimization of the requirements for waste water disposal. 

Cooling Tower Operations with Air/Water Interface and Energy Considerations... 

Environmental Aspects of Cooling Tower Operation. Accumulation and Escape of Dissolved and Undissolved Substances... 

Fog and Ice Formation During Cooling Tower Operation Bach, H. 

The Use of Membrane Processes in Cooling Tower Operations Cecil, L. K. 

Cooling Tower Operating Problems/Solubility of Calcium Sulfate Glater, J. 

Salt Water Cooling Tower Operation (in Relation to Ecology)... 

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