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MODULE 6
HUMIDIFICATION AND AIR CONDITIONING
LECTURE NO. 7
6.7 Evaporation loss of water in cooling tower:
Blowdown: During the cooling process of hot water in cooling tower, around 2% water evaporates [1-3]. In the long run, it increases the solid content in the circulating water. Some dust particles also come from the environment and mix with circulating water. But the solid content of the cooled water must be kept under a certain limit to avoid scaling or fouling on the heat exchange equipment. A part of the circulating water is drained from the bottom of the cooling tower to discard the deposited solids from the cooling tower. This is called blowdown. The losses due to blowdown, evaporation, drift and leakage are compensated by adding make-up water.
Water balance in cooling tower M=B+D+E (6.38) where, M is make-up water rate; B is blowdown rate; D is drift leakage loss rate; E is evaporation loss. Solid balance M×C 1 =(B+D) ×C 2 +E×0 (6.39)
( 1 )
r B E D r (6.40)
where, r=C 2 /C 1 ; C 1 is dissolved solid concentration in the make-up water; C 2 is dissolved solid concentration in the circulating water.
Evaporation loss is estimated by a thumb rule as: E=water flow rate (L)×range(ºF)×0.00085 gallon/min
The other design characteristics are pump horsepower, fan horsepower, source of make-up water and drift eliminators.
Nomenclature
a^ contact area/tower volume, m^2 /m^3 L^ Water flow rate, kg/m^2 s cwL Heat capacity of liquid (water), kJ/kg.K V active cooling volume/plan area, m^3 /m^2 Gs Air rate, kg dry air/m^2 s Y/^ Humidity, kg moisture/kg dry air hG Heat transfer coefficient of air film, kJ/m^2 .s.K
Y w /^ Saturation humidity, kg moisture/kg dry air hL Heat transfer coefficient of liquid (water), kJ/m^2 .s.K
z Cooling tower height, m
kG Mass transfer co-efficient of moisture
transport, kg/m^2 .s ( pA )
λw Latent heat of vaporization of water, kJ/kg K Y /^ Overall^ mass^ transfer^ co-efficient, kg/m^3 .s
H Humid volume, m^3 /kg dry
air
References
- Treybal, R. E., “ Mass-Transfer Operations”, 3rd^ Eddition, McGraw-Hill, 1981
- Geankoplis, C.J., “Transport Processes and Separation Process Principles”. 4th^ Edition, Prentice-Hall of India, New Delhi, 2005.
- Dutta, B.K., “Principles of Mass transfer and Separation Processes”. Prentice-Hall of India, New Delhi, 2007.
T
(ºC)
lnPv (Antoine Equation)
pv (bar)
Y'
(kg moist/kg dry air)
H'
kJ/kg dry air 21 - 3.694517 0.024859 0.0156487 60. 23 - 3.572406 0.028088 0.0177391 68. 25 - 3.452184 0.031676 0.0200784 76. 27 - 3.333809 0.035657 0.0226937 85. 29 - 3.217237 0.040066 0.025615 94. 31 - 3.102429 0.04494 0.028876 105. 33 - 2.989343 0.05032 0.0325139 116. 35 - 2.877943 0.05625 0.0365707 129. 37 - 2.76819 0.062776 0.0410932 142. 39 - 2.660049 0.069945 0.0461344 157. 41 - 2.553483 0.07781 0.0517539 174. 43 - 2.448459 0.086427 0.0580195 192. 45 - 2.344944 0.095853 0.0650086 213. 47 - 2.242905 0.10615 0.0728098 235.
Draw a tangent to the equilibrium line through Q. Slope of the tangent is 8.78.
, min
^
slope G (^) s L cWL
=2623 kg/h.m^2 Actual air rate = Gs,min ×1. =2623×1.25 kg/h.m^2 = 3279 kg/h.m 2
Slope of the operating line
Slope ^55003279 ^4.^187 7. 02
L cWL ( TL 2 TL 1 ) Gs ( H 2 / H 2 /) 5500 4. 187 ( 45 30 ) 3279 ( H 2 / 78. 7 )
H 2 / 184 kJ/kg
Now, locate point P (TL2, H 2 / ) (Upper terminal of the operating line) at P (45, 184)
on TL-H/^ plane.
We have, hLā= 0.059 L 0.51 Gs = 0.059(5500)0.51(3279) kcal/m^3 h.K =15637.9 kcal/m^3 hK=15637.9 ×4.187 kJ/m^3 h.K =65,475.9 kJ/m^3 hK
Slope of tie line= kh aa Y
L / 11. 4
^65475.^9
Hi /^ H /^ 0.070^ 0.090^ 0.096^ 0.104^ 0.104^ 0.085^ 0.
0.00 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
( ) / 1 / Hi H
TL (^0 C)
NtG =Area under the curve= (184-78.7)×0.088=9.
k a
H G
Y tG^ / s^ =(3279/5743.5)m = 0.56 m
Tower height= 9.27×0.56 m=5.19 m (Ans.)
