NPTEL – Chemical – Mass Transfer Operation 1
MODULE 6
HUMIDIFICATION AND AIR CONDITIONING
LECTURE NO. 6
Key points in the design of cooling tower:
(I) An increase or decrease in wet-bulb temperature of entering water (mainly
due to atmospheric condition) cannot change tower characteristic
KY/ a V
.
L
(II) An increase in ‘cooling range’ can not change tower characteristic
KY/ a V
. It increases ‘approach’ only.
L
KY/ a V
(III) A change in L/G can change tower characteristic .
L
Fill height (FH) depends on tower characteristic, L/G and correlated by the
following equation:
n
KY/ a V L
C FH
L G
where, C and n are constants and solely dependent on tower fill.
Step-by-step design procedure of cooling tower
1. Specify the inlet and outlet temperatures and flow rate of warm water.
2. Select the design value of dry-bulb and wet-bulb temperatures of air (at
the proposed geographical location).
3. Draw the ‘equilibrium line curve’ i.e., saturation humidity curve [H/ vs T].
The enthalpy data are calculated using vapor pressure equation for water
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NPTEL – Chemical – Mass Transfer Operation 1
and physical properties of air and water vapor
[ H / 1.005 1.88Y / (TG T0 ) 2500Y / kJ/kg]. T0 is 25ºC.
4. Locate the lower terminal of the operating line, ‘B’ on TL-H plane by the
point (TL1, H1/ ). This point indicates the condition at the bottom of the
tower.
5. Draw a tangent to the equilibrium line through the point ‘B’. The slope of
the tangent gives the ratio of the liquid and minimum gas flow rate. Hence,
minimum air rate is calculated. Actual air rate taken is usually 1.25 to 1.5
times the minimum [not required if air rate is given].
6. The upper terminal of the operating line is located by the point ‘A’ (TL2, H 2/
). It is the point where the operating line of the slope determined in step 5
meets the vertical line through TL2. It can also be located by calculating the
top end enthalpy H 2/ from Equation (6.18) as LcWL (TL 2 TL1 ) Gs ( H 2/ H1/ ) .
H 2/
dH /
7. Evaluate the integral in Equation (6.27) N tG / ( H i/ H / ) , number of gas-
H1
phase enthalpy transfer units and calculate height gas-phase enthalpy
Gs
transfer units, HtG as H tG /
. kY/ a and hL a are required. A set of
kY a
hL a
parallel lines (tie lines) of slope is drawn between the operating line
kY/ a
and equilibrium line. H/ and H i/ are taken from terminals. Integral is
TL 0
dTL Lc
calculated numerically or graphically. [ N tG (H
TLi iH )
/ /
and H tG / WL ].
kY a
8. If the overall enthalpy transfer coefficient KY/ is known and used, ‘tie lines’
are vertical. For a given value of H/, value of H*/ is given by the point on
the equilibrium line vertically above it. The integral of Equation
H 2/
dH /
( H */ H / ) NtoG gives the number of overall transfer units.
H1/
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NPTEL – Chemical – Mass Transfer Operation 1
Gs Lc
9. The height of a transfer unit H t 0G /
or H t 0G /WL is calculated.
KY a KY a
The packed height is the product of height of transfer unit and number of transfer
units.
Approach: It is the difference between cooling water temperature leaving
cooling tower and wet-bulb temperature of inlet air which is approach to wet bulb
temperature (ºF), (TL1-Tas). For getting small approach, cooling tower height must
be increased. To achieve zero (0) approach theoretically, infinite packing height
is needed.
Range: ‘Cooling range’ or purely ‘range’ is the difference in the inlet hot
water and outlet cooled water temperature (ºF) (TL2-TL1).
Approach to wet bulb Cooling range(ºF) Packed height
temperature (ºF) (TL1-Tas) (TL2-TL1) (ft)
15-20 25-35 15-20
10-15 25-35 25-30
5-10 25-35 35-40
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