COOLING TOWER

By. Engr. Yuri G. Melliza

Hot water 1m1

t1

h1

Cold water 2t2, h2, m2

t3, h3, m3

Make-up water 3

B Air OuthB,WB, ma

A Air inhA,WA, ma

Fan

Catch Basin

Cooling TowerA Cooling tower is a wind braced enclosure or shell usually made of wood, concrete or metal with fillings on the inside to aid water exposure. The water to be cooled is pumped into a distributing header at the top of the tower from which it drops in sprays to the filling. The water spreads out in the filling thus exposing new water surfaces to the air circulating through the tower. The cooled water drops to the bottom of the tower called the catch basin. The air circulating through the tower becomes partially saturated with moisture by evaporating some amount of water. This evaporation is mostly what cools the water.

Fundamental Equations:1. Actual Cooling Range (ACR)

ACR = t1 - t2

2. Cooling Tower Approach (A)A = t2 - tWA

3. Theoretical Cooling Range (TCR)TCR = t1 – tWA

4. Cooling Tower Efficiency

100% xtttt

eWA1

21

5. Vapor Pressure PV = PW - PA(td - tW)

where: A = 6.66 x 10-4 (For tW of equal or greater than 0C.

A = 5.94 x 10-4 (For tW of less than 0C)6. Specific Humidity or Humidity Ratio

kgdakgm

PvPPv 0.622

W

7. Relative Humidity

% 100 xPdPv

Φ

8. Enthalpyh = 1.0045td + W(2501.3 + 1.86td) KJ/kgda

9. Specific Volume

kgdam

Pv)(P

273)0.287(t 3d

υ

10. Degree of Saturation

PvPPdP

Φμ

11. By moisture balance in the tower: a) With make up water, m1 = m2

m3 = ma(WB - WA) kg/sec

b) Without make up water available, m1 m2:

m1 - m2 = ma (WB - WA)

kg/sec

12. By Energy Balance in the tower a. Considering make up water

KJ/kg

m)hW(W)h(hmhm

h

kg/sec )hW(W)h(h

)h(hmm

1

3ABABa112

3ABAB

211a

b. Without considering make up water

KJ/kg )W(Wmm)h(hmhm

h

kg/sec )hW(W)h(h

)h(hmm

ABa1

ABa112

2ABAB

211a

13. Driving Pressure

KPa

1000ρ-ρgH

ΔP iod

14. Mass Flow rate of air and vapor mixture m = ma(1+W) kg/sec

m = ma + mv

15. Cooling water flow rate related to Brake Power of an Engine L/hr

t-tPower Brake

904.3m21

w

where:m1 - mass flow rate of water entering tower in kg/secm2 - mass flow rate of cooled water in kg/secm3 - make up water in kg/sec h1 - enthalpy of hot water in KJ/kg h2 - enthalpy of cooled water in KJ/kg h3 - enthalpy of make up water in KJ/kg hA - enthalpy of air entering tower in KJ/kgda hB - enthalpy of air leaving tower in KJ/kgda WA - humidity ratio of air entering tower in kgm/kgda WB - humidity ratio of air leaving tower in kgm/kgda ma - mass flow rate of dry air in kg/sec td - dry bulb temperature in C tw - wet bulb temperature in C

t1 - temperature of hot water, C t2 - temperature of cooled water, C t3 - temperature of make up water, CH - tower height, meterso - density of outside air and vapor mixture, kg/m3

i - density of inside air and vapor mixture, taken at exit of the fill, kg/m3