PTT 253/3 (RY01/RY20) HEAT TRANSFER Cooling Tower Sriyana Abdullah Dept. of Chemical Engineering Technology Sem II 2017/2018

Cooling Tower Use to cool the hot water (i. e the used cooling water) comes from heat exchangers or condensers Within the area of contact between water and air is called an interface Cooling of warm water through evaporation by passing cold air in opposite direction (counter-current) -reduce water temperature Packing/cooling fill/slats installed inside the cooling tower to increase heat and mass transfer performance - Humidify of the air due to the evaporation of the water-increase air temperature

Natural draught cooling tower Types of cooling tower Mechanical draught cooling tower Natural draught cooling tower

Cooling Tower Theory & Calculation L= water flow, kg water/s·m2 TL = water temperature, °C or K G = dry air flow rate, kg/s·m2 TG = air temperature, °C or K H = Humidity of air, kg water/kg dry air* Hy = enthalpy of air-water vapor mixture, J/kg dry air o=latent heat of water, J/kg cS= humid heat = cL + cG*H Driving force: Driving force: (b) (a) (c) (a)=(b)+(c) Figure 10.5-1: Temperature and concentration profiles of upper part cooling tower (Geankoplis, C. J., 2014) The enthalpy, Hy given by: Eq 1.0 *Humidity, H can be retrieved from the humidity chart

Cooling Tower Theory & Calculation Consider an adiabatic, countercurrent packed cooling water tower; 2 1 3 Perform the energy/heat balance; Heat emitted=Heat absorbed 1: G(Hy-H1)=LcL (TL-TL1) Eq. 1.1 2: G(Hy2-Hy1)=LcL(TL2-TL1) Eq. 1.2 3: LcLdTL=GdHy Eq. 1.3 Total sensible heat transferred from bulk fluid to interface; LcLdTL=GdHy=hL·a·dz(TL-Ti) Eq. 1.4 a=A/V (m2/m3) Liquid phase volumetric heat transfer coefficient *Flow rate of gas and liquid water is assumed constant since only a small water evaporated (1-5%). *cL is assumed constant at 4.187103 J/kg·K Figure 10.5-2: Continuous countercurrent adiabatic water cooling (Geankoplis, C. J., 2014)

Cooling Tower Theory & Calculation As from Fig. 10.5-1, Eq. 1.4 becomes; Sensible heat in liquid to interface (a)= latent heat to bulk air + sensible heat to bulk air (c) Eq. 1.5 Eq. 1.6 Eq. 1.7 Eq. 1.8

Cooling Tower Theory & Calculation Eq. 1.9 Eq. 1.10 Eq. 1.11

1. Cooling Tower Design- using film mass transfer coefficient Step 1: Draw the equilibrium line in the Hy-TL plot (The plot is plotted using data from Table 10.5-1) Step 2: Draw the operating line using the given / calculated data of TL1, Hy1 TL2, and Hy2 ,its slope represents LcL/G Table 10.5-1: Enthalpies of saturated air-water vapor mixtures (Geankoplis, C. J., 2014) Step 3: From Eq 1.11, by knowing the slope of –hLa/MBkGaP, point P (Hy, TL) and point M (Hyi,TLi) is a line with that slope, hence MS (Hyi-Hy) is the enthalpy driving force Step 4: The driving force (Hyi-Hy) can be computed at various values if Hy and TL between TL1 and TL2. Figure 10.5-3: Temperature enthalpy diagram and operating line for water cooling tower (Geankoplis, C. J., 2014)

Example Example10.5-1 (Geankoplis, C. J., 2014) A packed adiabatic countercurrent water-cooling tower using a gas flow rate of G=1.356 kg dry air/m2 ·s and a water flow rate of L=1.356 kg water/s·m2 is to cool the water from TL2=43.3°C to TL1=29.4°C. The entering air at 29.4°C with humidity, H is 0.0165 kg water/kg dry air. The mass-transfer coefficient kGa is estimated as 1.207 x10-7 kg mol/s·m3 ·Pa and hLa/kGaMBP as 4.187x104 J/kg·K. Calculate the tower height, z. The tower operates at 1 atm. Solution: Use data in Table 10.5-1 to plot an equilibrium line in Hy-TL plot. Use Eq. 1.0 to calculate enthalpy of entering air, Hy1 Use Eq. 1.2 to calculate Hy2 G(Hy2-Hy1)=LcL(TL2-TL1) Draw the operating line.

Example 10.5-1 5. Draw several line with the slope of hLa/kGaMBP = -4.187x104 6. Construct a table which tabulated the Hyi, Hy thus, Hyi-Hy and 1/(Hyi-Hy) 7. Solve the integral using graphical method then, use Eq 1.10 to calculate z, Eq. 1.10

The integral is the area under the curve, trapezoidal area Figure 10.5-4

2. Cooling Tower Design- using overall mass transfer coefficient Is applicable when only kGa is available to determine z Eq. 1.12 Thus, instead of retrieving Hyi from figure, the enthalpy at equilibrium, Hy* is retrieved.

Determine the minimum value of air flow