Cooling Tower Selection And Sizing 674w67

Chapter VI Cooling tower design

Lecturer: Chakkrit Umpuch Department of Chemical Engineering

Faculty of Engineering Ubon Rachathani University

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What you will learn in this chapter is here.

5.1 Introduction to cooling tower 5.2 Vapor pressure of water and humidity 5.3 Cooling tower theory 5.4 Cooling tower design

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5.1Introduction What is cooling tower? Cooling tower is the heat rejection device, which extracts heat waste to atmosphere through the cooling of water stream to a lower temperature.

Figure 1: Closed Loop Cooling Tower System

The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling. 3

Types of Cooling Towers

Figure 2: Mechanical Draft Counterflow Tower

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Figure 3: Mechanical Draft Crossflow Tower

5.2 Vapor Pressure of Water and Humicdity Vapor Pressure of Water Pure water can exist in three different physical states: solid ice, liquid and vapor. The physical state in which it exists depends on the pressure and temperature. AB line is liquid and vapor coexist in equilibrium. AC line is ice and liquid coexist in equilibrium. AD line is ice and vapor coexist in equilibrium.

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5.2 Vapor Pressure of Water and Humidity Steam Table

For example: At 100°C (212°F) the vapor pressure of water is 101.3 kPa (1.0 atm). At 65.6°C (150°F) the vapor pressure of water is 25.7 kPa (3.72 atm). At 25.7kPa and 65.6°C, water will boil. 6

5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart 1. Definition of humidity The humidity H of an air-water vapor mixture is defined as the kg of water vapor contained in 1 kg of dry air.

(6.1) where pA is partial pressure of water vapor in the air. P is the total pressure (101.325 kPa, 1.0 atm abs, or 760 mmHg) Saturated air is air in which the water vapor is in equilibrium with liquid water at the given conditions of pressure and temperature. Hence, the saturation humidity Hs is

(6.2)

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5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart 2. Percentage humidity The percent humidity Hp is defined as 100 times the actual humidity H of the air divided by the humidity Hs if the air were saturated at the same temperature and pressure. (6.3) 3. Percentage relative humidity The amount of saturation of an air-water vapor mixture is also given as percentage relative humidity HR using partial pressures. (6.4)

Note that HR ≠ Hp: 8

(6.5)

Ex 6.1 The air in a room is at 26.7°C (80°F) and a pressure of 101.325 kPa and contains water vapor with a partial pressure pA= 2.76 kPa. Calculate the following. (a) Humidity, H. (b) Saturation humidity, Hs, and percentage humidity, Hp. (c) Percentage relative humidity, HR. Solution: (a) From the steam tables at 26.7C, the vapor pressure of water is pAS = 3.50 kPa (0.507 psia). Also, pA = 2.76 kPa and P = 101.3 kPa (14.7 psia). For part (a), using eq. 6.1,

H

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18 .02 p A 18 .02 (2.76 )   0.01742 kgH2 0 / kgair 28 .97 P  p A 28 .97 (101 .3  27 .6)

Solution: (b) From part (b), using eq. 6.2, the saturation humidity is

Hs 

18 .02 p As 18 .02 (3.05 )   0.02226 kgH2 0 / kgair 28 .97 P  p As 28 .97 (101 .3  3.05 )

The percentage humidity, from eq. 6.3, is

H P  100

H 100 (0.01742 )   78 .3% Hs 0.02226

For part (c), from eq. 4, the percentage relative humidity is

H s  100

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p A 100 (2.76 )   78 .9% p As 3.05

5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart 4. Dew point of an air-water vapor mixture The temperature at which a given mixture of air and water vapor would be saturated is called the dew-point temperature or simply the dew point. For example, At 26°C (80°F), the saturation vapor pressure of water is p AS = 3.5 kPa (0.507 psia). Hence the dew point of a mixture containing water vapor having a partial pressure of 3.50 kPa is 26.7°C. If an air-water vapor mixture is at 37.8°C and contains water vapor of p A = 3.50 kPa, the mixture would not be saturated. On cooling to 26.7°C, the air would be saturated, i.e., at the dew point.. On further cooling, some water vapor would condense, since the partial pressure cannot be greater than the saturation vapor pressure. 11

5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart 5. Humid heat of an air-water vapor mixture The humid heat cs is the amount of heat in J (or kJ) required to raise the temperature of 1 kg of dry plus the water vapor present by 1 K or 1°C. (SI) (English)

(6.4)

(6.5)

6. Humid volume of an air-water vapor mixture The humid volume v H is the total volume in m3 of 1 kg of dry air plus the vapor it contains at 101.325 kPa (1.0 atm) abs pressure and the given gas temperature. Using the ideal gas law, (6.6)

(6.7) 12

5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart 7. Total enthalpy of an air-water vapor mixture. The total enthalpy of 1 kg of air plus its water vapor is Hy J/kg or kJ/kg dry air. (6.8) (SI)

(6.9)

(English)

(6.10 )

8. Humidity chart of air-water vapor mixtures. A convenient chart of the properties of air-water vapor mixtures at 1.0 atm abs pressure is the humidity chart. In this figure the humidity H is plotted versus the actual temperature of the air-water vapor mixture (dry bulb temperature).

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5.2 Vapor Pressure of Water and Humidity Humidity and Humidity Chart

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Ex 6.2 Air entering a dryer has a temperature (dry bulb temperature) of 60°C (140°F) and a dew point of 26.7°C (80°F). Using the humidity chart, determine the actual humidity H, percentage humidity Hp, humid heat cs, and the humid volume vH in SI and English units. Solution: The dew point of 26.7°C is the temperature when the given mixture is at 100% saturation. Starting at 26.7°C, Fig. of humidity chart and drawing a H = 0.0225 kg H2O/kg dry air is read off the plot. This is the actual humidity of the air at 60°C. Stated in another way, if air at 60°C and having a humidity H = 0.0225 is cooled, its dew point will be 26.7°C. In English units, H = 0.0225 lb H2O/lb dry air. Locating this point of H = 0.0225 and t = 60°C on the chart, the percentage humidity HP is found to be 14%, by linear interpolation vertically between the 10 and 20% lines. The humidity heat of H = 0.0225 is, from eq.(6).

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cs  1.005  1.88 (0.0225 )  1.047 kJ / kg dry air  K

or 1.047 10 3 J / kg.K

cs  0.24  0.45 (0.0225 ) cs  0.250 btu / lbm dry air. F The humidity volume at 60ºC (140ºF), from eq. 7 is

vH  (2.83 10 3  4.56 10 3  0.0225 )( 60  273 )  0.977 m3 / kg dry air In English units,

vH  (0.0252  0.0405  0.0225 )( 460  140 )  15 .67 ft 3 / lbm dry air

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5.2 Vapor Pressure of Water and Humidity Adiabatic Saturation Temperature

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5.2 Vapor Pressure of Water and Humidity Adiabatic Saturation Temperature Total enthalpy of the entering gas mixture = enthalpy of the leaving gas mixture (6.11)

(SI)

(English)

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(6.12 ) (6.13 )

Ex 6.3 An air stream at 87.8°C having a humidity H = 0.030 kg H2O/kg dry air is ed in an adiabatic saturator with water. It is cooled and humidified to 90% saturation. (a) What are the final values of H and T? (b) For 100% saturation, what would be the values of H and T? Solution: For part (a), the point H = 0.030 and T = 87.8ºC is located on the humidity chart. The adibatic saturation curve through this point is followed upward to the left until it interests the 90% line at 42.5ºC and followed upward to the left until it intersects the 90% line at 42.5ºC and H = 0.0500 kg H2O/kg dry air. For part (b), the same line is followed to 100% saturation, where T = 40.5ºC and H = 0.0505 kg H2O/kg dry air.

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5.2 Vapor Pressure of Water and Humidity Wet Bulb Temperature

For air-water vapor mixture, the adiabatic saturation lines can also be used for wet bulb lines with reasonable accuracy. Hence, the wet bulb temperature determination is often used to determine the humidity of an airwater vapor mixture. 20

Ex 6.4 A water vapor-air mixture having a dry bulb temperature of T = 60°C is ed over a wet bulb as shown in slide 17, and the wet bulb temperature obtained is TW = 29.5°C. What is the humidity of the mixture? Solution: The wet bulb temperature of 29.5°C can be assumed to be the same as the adiabatic saturation temperature TS, as discussed. Following the adiabatic saturation curve of 29.5°C until it reaches the dry bulb temperature of 60°C, the humidity is H = 0.0135 kg H2O/kg dry air.

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Theory and Calculation of Water-Cooling Towers Pressure gradient Water vapor diffuses from the interface to the bulk gas phase with a driving force in the gas phase: Hi – HG kg H2O/kg dry air No driving force in liquid phase, since water is pure liquid

Temperature gradient The temperature driving force is TL – Ti in the liquid phase and Ti – TG K or °C in gas phase.

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Theory and Calculation of Water-Cooling Towers L = water flow , kg water/s.m2 (lbm/h.ft2) TL = temperature of water, °C or K (°F) G = dry air flow, kg/s.m2 (lbm/h.ft2) TG = temperature of air, °C or K (°F) H = humidity of air, kg water/ kg dry air (lb water/ lb dry air) Hy = enthalpy of air-water vapor mixture, J/kg dry air (btu/lbm dry air)

(SI)

(6.14 ) (English) (6.15 )

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Theory and Calculation of Water-Cooling Towers Total heat balance for the dash line box

(6.16 )

(6.17 )

(6.18 )

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Design of water-cooling tower Using Film Mass transfer Coefficients The tower design is done using the following steps. 1.

The enthalpy of saturated air Hy1 is plotted versus Ti on an H versus T plot as shown in Fig. in slide 24 This enthalpy is calculated with eq. using the saturation humidity from the humidity chart for a given temperature, with 0°C (273K) as a base temperature. Calculated values are tabulated in Table in slide 27

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Knowing the entering air conditions TG1 and H1, the enthalpy of this air Hy1 is calculated from eq. 6.8. The point Hy1 and TL1 (desired leaving water temperature) is plotted in Fig. in slide 24 as one point on the operating line. The operating line is plotted with a slope LcL/G and ends at point TL2, which is the entering water temperature. This gives Hy2 . Alternatively, Hy2 can be calculated from eq. 6.16.

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Knowing hLa and kGa, lines with a slope of –hLa/kGaMBP are plotted as shown in Fig. in slide 24. From eq. 6.18 point P represents Hy and TL on the operating line, and point M represents Hyi and Ti, the interface conditions. Hence, line MS or Hyi -Hy represents the driving force in eq. 6.17.

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The driving force Hyi - Hy is computed for various values of TL between TL1 and TL2. Then by plotting 1/(Hyi -Hy) versus Hy from Hy1 to Hy2, a graphical integration is performed to obtain the value of the integral in eq. 6.17. Finally, the height z is calculated from eq. 6.17.

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Ex 6.5 A packed countercurrent water-cooling tower using a gas flow rate of G = 1.356 kg dry air/s.m2 and a water flow rate of L=1.356 kg water/s.m2 is to cool the water from TL2 = 43.3 °C (110 ° F) to TL1=29.4 °C (85 °F). The entering air at 29.4 °C has a wet bulb temperature of 23.9 °C. The mass-transfer coefficient kGa is estimated as 1.207x10-7 kgmol/s.m3.Pa and hLa/kGaMBP as 4.187x104 J/kg/K (10.0 btu/lbm. °F). Calculate the height of packed tower z. The tower operates at a pressure of 1.013x105 Pa.

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