Tower Selection

000 gpm at 90/80/70. The conditions correspond to a rating factor of 1.00, so from the rating factor equation 10,000 gpm (1.00) = 10,000 tower units of rated area. 

Example 2. If the conditions are changed to 110/85/70, the corresponding rating factor is 1.422. This represents a greater degree of difficulty, so 10,000 (1.422) = 14,220 tower units of rated area. 

Example 3. For a tower of given design, the plan area required will vary directly with the rated areas as calculated above. Thus, the tower requiring 

000 tower units may occupy 24 x 36 ft., or 864 sq.ft, of plan area. If conditions are changed to require 14,222 tower units, the plan area must be increased to 864 (14,220/10,000), or 1,229 sq.ft, of plan area. 

Example 4. If the tower having 854 sq.ft, of plan area operated at 110/85/70, what gpm will it cool  

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Cheremisinoff, N. P. and P. N. Cheremisinoff, Cooling Towers, Selection, Design and Practice, Ann Arbor Science Publishers, Inc. (1981). 

Pfeiffer, E. L., Preliminary Cooling Tower Selection, Foster Wheeler Corp., Bui. CT-49-6, reprinted from Chem. Eng (date not given). 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

INDUSTRIAL Water Society Guide to Mechanical Draught Evaporative Cooling Towers Selection. Operation and Maintenance (London, 1987). 

Knuesch, T. Environmental Aspects of Cooling Tower Selection, Process Eng. (November 1978). 

Figure 4.18 Upper right figure shows a forced-draft or blowthrough tower, which has a fan at the bottom for driving air through the fill above, Tower selection for smaller units can be made from the accompanying curves and table for a cold water temperature of 85°F (this is generally the water basin discharge temperature for small towers). As an example, enter at 104°F hot water temperature to a wet bulb value of 75°F, then drop vertically to the water flow selected (580 gpm). This falls between curves that designate the manufacturer s distinct model size. Select the next larger size, i,e., the curve immediately below, and follow across to the recommended tower model).

In 1974 the Atlantic City Electric Co. placed Unit 3 of its B L England Station into commercial operation. Condenser cooling for the unit is provided by circulating sea water in a closed-cycle, natural-draft system. The cooling tower selected for the site was a hyperbolic, counterflow unit. The thermal test instrumentation procedures and test data as well as drift measurement results are given. The paper indicates that the tower operates within design specifications for thermal performance and that it meets the environmental criteria regarding the drift. 

COOLING TOWERS—Selection, Design Practice—Nicholas P. 

Typically, the air-stripper manufacturer will supply liquid flow ranges acceptable for a particular tower. Selecting an air stripper for which the design flow is at the lower end of the tower s rated capacity will produce high contaminant removal rates, but may not optimize power requirements. For large-scale systems where significant operational costs may be incurred by overdesigning the system, the use of pressure-drop curves and calculations such as Eqs. (1)-(13) are required. 

Thns, for a valne of G/L of 4.0, the valnes of Z/HTU for XJX = 0.1 are 1.49 for 85°F and 1.95 for 75°F. The height of the tower at 85°F is 1.48 x 23.5 = 34.8 ft, whereas at 75°F it becomes 1.95 x 23.5 = 45.8 ft. This shows that setting the GIL ratio to 4.0 instead of 2.0 wonld resnlt in a shorter tower. The amount of air requirecf, however, is not doubled 34.8 times 2 divided by 49.5 = 1.40. Thus, at this higher air loading rate, only 40% more air is reqnired. To find the optimum GJL ratio, the entire design mnst be priced and the minimnm cost tower selected. This requires repetitive calculations using a computer and incorporating reasonably accurate cost data as well as mass transfer, enthalpy transfer, and pressure drop characteristics on the detailed analysis. 

The problem of cooling tower selection is not merely to determine the required size and cost of a tower to meet a given set of conditions, but also to select the optimum des conditions on the basis of overall plant economy. The following examples show how the charts (Figures 3-2 through 3-5) are used and deal with the various problems involved in selecting the optimum tower. 

A decade ago it was not uncommon to specify 85° F cold water and to select exchangers for 88 to 90° F cold water. To some engineers it seemed logical to include a small 3°F safety factor in the calculations. However, this 3°F safety factor quite often turned out to be a 50% safety factor as far as the price and size of the cooling tower were concerned. Table 3-5 indicates that a tower sized to cool 28,500 gpm from 118 to 88° F with an 80° F wet bulb would cost 361,920, or 12.70 per gpm. A cooling tower selected to cool 28,500 gpm with the 3°F safety factor or from 115 to 85° F with an 80°F wet bulb would cost 532,480, or 18.68 per gpm. This is approximately 50% more in cost, length of concrete basin and fan horsepower. 

Design wet bulb temperature is determined by geographical location. Usually the wet bulb temperature selected is not exceeded more than 5% of the time in any area. Wet bulb temperature is a factor in cooling tower selection. The higher the wet bulb temperature the smaller the tower required to give a specified approach to the wet bulb at a constant range and gpm. 

A cooling tower selected to cool 28,500 gpm of water from 118 to 88° F at 70° F wet bulb would be smaller than a cooling tower selected to cool 28,500 gpm from 118 to 88°F at an 80°F wet bulb. In spite of the fact that the wet bulb temperature has increased, the reduction of approach from 18 to 8°F dictates that the tower must be larger. Table 3-9 indicates the effect of wet bulb temperature in sizing a tower when the gpm and range remain constant. 

Selection for both oil and protein has resulted in significant progress. For example, the sum for oil and protein for Tower selected for both oil and protein is approximately 2.9% higher than the sum for Midas selected primarily for oil content (Stefansson and Kondra, 1975). The improved protein content has permitted some Canadian rapeseed crushers to merchandize canola meal with 36 or 38% instead of the previously guaranteed 34% protein content (at 8% moisture). However, much work remains to be done to bring the sum of oil and protein in the cultivars of turnip rape up to the standards now available in rape and to maximize the production of these two components of the seed in rape and turnip rape. 

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