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1 3 rd Integrated Seminar 23.3.2005

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2 Cooling Tower Internals Water pass through a nozzle Air-water interface is heat transfer surface Free-fall law, z = 1/(2gθ 2 ) Interrupted tower _ effective first quarters Large air volume and small pressure drop Fig 1 – Free and Interrupted Fall

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3 Internal View of Cooling Tower 1 2 3 4 5 6 1.Fan 2.Drift eliminator 3.Hot water inlet pipe 4.Cold water outlet pipe 5.Spray nozzle 6.Fill Fig 2 – Internal Structure of a Counterflow Cooling Tower

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4 Design Consideration of Counterflow Cooling Tower Performance Base on the following Design Data The total heat load to be removed from several process by cooling tower is 20,000,000 Btu/hr Wet bulb temperature = 65°F Dry bulb temperature = 85°F Hot water temperature = 100°F Approach temperature = 15°F Water flow rate = 1000 gpm A tower 24ft by 24ft has been constructed with a fan capacity of 130,000 cfm Necessary Design Output How many diffusion units must the tower be capable of performing to fulfil the process requirements?

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5 Solving the Problem by Numerical Integration Calculate the Enthalpy of Saturated Air At 65°F the saturation partial pressure of water is 0.3056 psia and latent heat of vaporization is 1056.88 Btu/lb. [From Thermodynamic Properties of Stream – Temperature Table] Humidity, Enthalpy above 0°F,

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6 Calculate the Humidity and Specific Volume Humidity, Specific volume of air, Specific volume of water, Specific volume = air + water = 13.24 + 0.27 = 13.51 ft 3 /lb

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7 Construct the Enthalpies and Humidities of Air-water Vapor-mixtures Table at 14.7 psia Table 1 – Air-water Vapor-mixtures Table

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8 Calculate the Liquid and Air Loading Liquid loading, The air loading was given in the proposal as 130,000 cfm at 65°F wet bulb. The density of the dry air in a cubic foot of mixture is 1/13.51 = 0.074 lb/ft 3. Air loading,

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9 Calculate the Outlet Enthalpy of Air H 1 = 30.4 Btu/lb from Table-1 at 65°F

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10 Construct the Enthalpy Difference Table to be convenient in the Calculation Table 2 – Enthalpy Difference Table T,°FH'H'HH' - H(H' – H) av 80 85 90 95 100 44.1 50.0 56.7 64.2 72.7 30.4 34.73 39.03 43.361 47.72 13.7 15.27 17.67 20.839 24.98 14.485 16.47 19.254 22.909 0.345 0.303 0.259 0.218

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11 Create the Tower Characteristic Curve H 1 = 30.4 H 2 = 47.72 H1'H1' H2'H2' L/G Fig 3 – Solution of Problem

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12 Solving the Problem by the Chebyshev Method L/G = 0.8662 Use the following formula. where,

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13 Table 3 – Air-water Enthalpies Difference using the Chebyshev Method T,°Fh water h air h w - h a T 2 = 80 T 2 + 0.1(20) = 82 T 2 + 0.4(20) = 88 T 1 - 0.4(20) = 92 T 1 - 0.1(20) = 98 T 1 = 100 44.1 46.46 54.02 59.7 69.3 72.7 h 1 = 30.4 h 1 + 0.1(0.8662)(20) = 32.1 h 1 + 0.4(0.8662)(20) = 37.32 h 2 - 0.4(0.8662)(20) = 40.79 h 2 - 0.1(0.8662)(20) = 45.99 h 2 = 47.724 0.069 0.059 0.052 0.042 0.222

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14 Solving the Problem by the Nomograph Method Procedures WB = 65°F, Cooling Range = 20°F, CW = 80°F, L/G = 0.8662 Draw the straight line with wet bulb 65°F, cooling range 20°F, and cold water 80°F. Establish a line drawn through L/G = 0.8662 and parallel to the original line. Read the (KaV)/L value at that place passing through the line.

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15 * * * * (KaV)/L = 1.112 Fig 4 – Nomograph Solution of Cooling Tower Performance

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16 Solving the Problem by the Counterflow Cooling Tower Software Software’s Features This program calculates : The KaV/L (Number of Diffusion Units) of wet counterflow cooling tower based on the Cooling Tower Institute Standard. The evaporation, blowdown, drift losses, and total makeup required by actual mass balance. The fan and pump power required.

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17 Run Identification

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18 Cooling Tower Design Inputs

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19 Cooling Tower Design Outputs

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20 Makeup Water Requirements for a Cooling Tower where, W m = Makeup water, gal/min W e = Evaporation loss, gal/min W d = Drift loss, gal/min W b = Blowdown loss, gal/min Evaporation loss, where, W c = Circulating water flow, gal/min at tower inlet T 1 - T 2 = Inlet water temperature minus out let water temperature, °F

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21 Drift loss, Drift loss is a function of the drift eliminator design, which typically varies between 0.1 and 0.2 percent of the water supplied to the tower. Blowdown loss, Normally, cycles of concentration involved with cooling tower operation range from three to five cycles.

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22 Calculating the Makeup Water Requirement of the Problem Evaporation loss, Drift loss, Blowdown loss, Makeup water,

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23 Comparing the Results MethodKaV/L (Number of Diffusion Units)Total Makeup Water (gal/min) 1.Numerical Integration 2.Chebyshev 3.Nomograph 4.Tower Software 1.125 1.11 1.112 1.1633 23.25 23.786 Fig 5 – Comparison of Output Results of Counterflow Cooling Tower Performance

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24 References “Chemical Engineering Handbook”, Perry, Green, Dom W.et al, 6 th Edition, McGraw – Hill, New York, 1995. “Process Heat Transfer”, D.Q. Kern, 8 th Reprint, Tata McGraw – Hill Book Company, New York, 1997. http://www.engineering-software.com

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