Cooling towers approach

Natural-draft cooling towers are extremely sensitive to air-inlet conditions owing to the effects on draft. It can rapidly be estabUshed from these approximate equations that as the air-inlet temperature approaches the water-inlet temperature, the allowable heat load decreases rapidly. For this reason, natural-draft towers are unsuitable in many regions of the United States. Figure 10 shows the effect of air-inlet temperature on the allowable heat load of a natural-draft tower for some arbitrary numerical values and inlet rh of 50%. The trend is typical. 

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). 

Figure 30-lC is distinctly different from the first two in the type of SO2 control processes used and the sequence of the particulate matter and SOj controls. It is a promising approach for up to 90% SO2 control of western United States coal, and there is a single waste product. Other features include the collection of particulate matter at temperatures below 90°C and the possibility for spray dryer cooling tower water integration. This. system may or may not include a catalytic NO unit. 

This equation is good if the air temperature is 50°F or above, the cooling tower s approach to the wet bulb temperature is 5°F or above, and Hog is within a range of about 0.1 to 8. 

Cooling tower 10°F approach to design wet bulb temperature. 

The effects of wet bulb, approach and range on mecbanical draft cooling tower size is indicated in Figure 9-118. 

Paige. P. M. Chem. Eng.. 74(14) (1967) 93. Costlier cooling towers require a new approach to water-systems design. 

Alternative 1 consists of preliminary treatment for heavy metals removal with the primary concern being iron removal (Figure 8.3). The levels of iron observed in the groundwater at this site would be very detrimental to the downstream treatment processes. This pretreated water would then be used for cooling tower makeup water followed by biological treatment. This approach would be the easiest and cheapest alternative. This combined process should provide effective removal of BTEX. 

For economic reasons, equilibrium conditions cannot be approached closely. In a cooling tower, for instance, the effluent air is not quite saturated, and the water temperature is not quite at the wet bulb temperature. Percent saturation in the vicinity of 90% often is feasible. Approach is the difference between the temperatures of the water and the wet bulb. It is a significant determinant of cooling tower sizfe as these selected data indicate ... 

The approach has a significant effect on the tower size, as shown in Figure 5.11. For a given heat load, gpm and wet-bulb temperature, the cooling tower size increases as the approach decreases, and the closer the cold water temperature approaches the wet-bulb temperature, the greater the increase in the cooling tower size. 

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). 

Cooling towers are capable of operating over a wide range of water rates, air rates and heat loads. Variations are reflected in the approach of the cold water to the wet-bulb temperature. The available tower coefficient is not a constant but varies with operating conditions. 

Tower performance is specified in terms of the cooling tower s range, approach, wet-bulb temperature and water rate. The rating of a tower is established by developing a series of charts that relates these variables. 

A cooling tower has been designed to handle 7650 gpm of hot water at a 15°F range and a 10°F approach to 70°F wet-bulb temperature. Determine the tower units of rated area. 

In the first design, a small L G ratio must be used. Since G cannot be increased beyond certain limitations (because of economics), L should be small. Experiments on cooling towers have indicated that their characteristics break sharply when L approaches a critical point varying with design. The... 

There are several approaches to sizing cooling towers outlined in the literature. Most are empirical or semiempirical in nature. The following outlines... 

A cooling tower has a cross-sectional area of 25 X 25 ft. The total heat load to the unit is 27,500,000 Btu/hr. The locality has a 5% wet-bulb temperature of 75°F. Water exits the tower with a 12° approach to the wet-bulb temperature (i.e., 87°F).The hot process water enters the tower at a temperature of 125°F, and the water equivalent to this range is 1800 gpm. The systems fan capacity is 150,000 cfm (a) Determine the number of diffusion units that the tower must be capable of performing to meet process requirements (b) the tower manufacturer provided the following data for overload and underload conditions for the tower ... 

It is important that drive shafts be properly balanced. Imbalance not only causes tower vibration but also induces higher loads and excessive wear on the mechanical equipment coupled to the shaft. With drive shafts approaching speeds of 1800 rpm in most cooling tower applications, it is necessary that the shafts be dynamically balanced to reduce vibrational forces to a minimum. 

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