Post on 13-Jul-2015
transcript
Outline
Overview of heat transfer through evaporation
Overview of relevant terms related to cooling operations
Introduction to cooling towers
Basic working principle of a cooling tower
Classification of cooling towers
Major components of a cooling tower
Design of natural draft cooling tower with numerical example
Strategies to improve cooling tower performance
Importance of cooling towers in Chemical Process Industry
References
Evaporation
Evaporation is the conversion of a liquid phase of a material orsubstance to a vapor phase at a specific temperature and pressure.
At normal temperature and pressure, all liquids posses a liquid andvapor phase, which are in equilibrium with each other, if thetemperature and pressure conditions remain the same.
Rate of evaporation of a liquid is affected by the change inambient temperature and pressure conditions.
The change affects the vapor pressure of the liquid, causing anincrease or decrease in rate of evaporation.
Why evaporation causes cooling ?
Evaporation results in vapor escaping from the surface of the liquid.
The vapors require energy to escape.
That energy is heat.
As vapors escape the surface, they carry with them some heat
content of the liquid, and thus cause the liquid surface to cool.
Terminologies relevant to cooling
operations Humidity – Amount of vapor associated with a unit mass of dry gas.
Relative humidity – Ratio of partial pressure of vapor in gas to the partialpressure of vapor in same gas at saturation.
Humidification – Process of increasing the amount of vapor in a gasstream.
De-humidification – The opposite of humidification i.e. the process ofdecreasing the amount of vapor in a gas stream.
Wet-bulb temperature – The lowest temperature that watertheoretically can reach by evaporation.
Factors affecting rate of
evaporation
Some major factors include
Temperature and pressure of the liquid being evaporated.
Temperature and pressure of the ambient gas which will accept the
vapors from incoming liquid.
Humidity of the ambient gas.
Flow conditions for liquid and gas.
Weather conditions in case of open-air evaporation generally
describing the temperature, pressure, and velocity of ambient air e.g.,
water evaporating from a pond.
Cooling towers
As per the definition of Cooling Technology Institute (CTI), USA
“A cooling tower is a heat rejection device,
which extracts waste heat to the atmosphere
though the cooling of a water stream
to a lower temperature.”
Cooling towers are used in process industries to cool off effluent water from various heat transfer
equipment e.g., condensate from a condenser.
Cooling towers, in general, cool the water to a temperature below the dry-bulb and above the
wet-bulb temperature of air at the present conditions.
The cooled water is sent back to the process for reuse, thus emphasizing conservation of water.
Basic working principle of a cooling
tower
Considering an example of an air-water system, the basic working
principle of a cooling tower can be listed as,
1. Hot water and relatively cool ambient air enter the cooling tower.
2. Heat transfer between the air stream and the water stream occurs.
3. Hot water transfers its heat to the ambient air and becomes cool.
4. Cool water is removed from the cooling tower and sent back to the
process plant.
5. The resulting hot air rises and is, generally, removed from the top of the
tower by virtue of its low density.
Classification of cooling towers
Cooling towers are generally classified based on the following
factors,
1. Method by which air is introduced into the tower.
2. Flow configuration inside the tower.
3. Method of heat transfer / heat removal.
1. Air Introduction Method
NO. TYPE DESCRIPTION
1 Natural draft cooling
towers
Air movement is regulated without any help of
a mechanical fan or regulator and is
dependent on the height and size of the
tower
2 Mechanical draft
cooling towers
Air is regulated by means of mechanical fans.
It has two further types based on the
positioning of the fan
Induced mechanical draft where the fan is
positioned on the top side of the tower
Forced mechanical draft where the fan is
positioned at the bottom side of the tower
2. Flow configuration
NO. TYPE DESCRIPTION
1 Cross flow configuration The air stream enters the tower in a direction
perpendicular to that of flow of water e.g., air
entering from the sides of the cooling tower in
association with the water stream entering
from the top of the tower
2 Counter-current flow
configuration
The air stream and water stream flow in
parallel but opposite direction inside the
tower e.g., air entering from below and water
entering from the top of the tower
3 Co-current flow
configuration
The air stream and water stream flow in
parallel and same direction inside the tower
3. Method of Heat Transfer
NO. TYPE DESCRIPTION
1 Dry cooling towers Transfer heat through a surface which
separates the working fluid from ambient air
e.g., tube to air heat exchanger. No
evaporation occurs in dry cooling towers
2 Wet cooling towers Transfer heat on the principle of evaporative
cooling e.g., hyperbolic natural draft cooling
towers used in power plants
3 Wet-dry hybrid cooling
towers
Combination of an air-cooled heat
exchanger and a wet cooling tower to cool
off the required fluid
4 Evaporative condenser
cooling tower
Principle of wet cooling tower is applied to
cool a process fluid which remains isolated
from the cooling tower fluids (usually air and
water)
Major components of a cooling
tower
NO. NAME OF
COMPONENT
FUNCTION MATERIAL OF
CONSTRUCTION
1 Frame and
casing
Supports exterior enclosures SS 316/304,
Concrete, Fiber glass
2 Fill Increases contact between air
and water, facilitating heat
transfer. Has two types; Splash fill
and Film fill
PVC, wood,
Polypropylene
3 Cold water basin Receives water at the bottom of
the tower
SS 316, Concrete
4 Drift eliminators Reduce loss of water due to
windage/drift
PVC, Polypropylene
Major components of a cooling
tower (contd.)
NO. NAME OF COMPONENT FUNCTION
5 Air inlet Enables air to enter the tower
6 Louvers Louvers equalize air flow into the fill and
retain the water within the tower
7 Nozzles / Spray tree Distributes water to wet the fill
8 Fans Regulate air flow in case of mechanical
draft towers
Design of a cooling tower
Design procedure of a hyperbolic natural draft cooling tower is
taken as reference.
Design of a cooling tower (contd.)
Important parameters prerequisite to design,
CLASS NO. NAME SYMBOL UNITS
MEA
SU
RED
PA
RA
METE
RS
1 Wet bulb
temperature of air
𝑇𝑤 K
2 Dry bulb
temperature of air
𝑇𝑑 K
3 Inlet water
temperature
𝑇𝑖𝑛 K
4 Outlet water
temperature
𝑇𝑜𝑢𝑡 K
5 Water mass flow
rate / Water load
𝑊𝐿 kg/sec
6 Enthalpy change
(air passing through
tower)
∆𝐻′ kJ/kg
Design of a cooling tower (contd.)
CLASS NO. NAME SYMBOL DESCRIPTION UNITS
PER
FO
RM
AN
CE P
AR
AM
ETE
RS
1 Range ∆𝑇 ∆𝑇 = 𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 K
2 Approach ∆𝑇∗ ∆𝑇∗ = 𝑇𝑜𝑢𝑡 − 𝑇𝑤 K
3 Effectiveness 𝐸𝑐𝐸𝑐 =
∆𝑇
∆𝑇 + ∆𝑇∗∗ 100
%
4 Cooling capacity 𝑄 𝑄 = 𝑊𝐿 ∗ 𝐶𝑝 𝑤𝑎𝑡𝑒𝑟∗ ∆𝑇 kW
5 Performance
coefficient
𝐶𝑡 Value is usually 5.2 or
lower
--
Design of a cooling tower (contd.)
CLASS NO. NAME SYMBOL DESCRIPTION UNITS
DESIG
NP
AR
AM
ETE
RS
1 Duty coefficient 𝐷𝑡 𝑊𝐿
𝐷𝑡= 0.00369 ∗
∆𝐻′
∆𝑇∗ (∆𝑇∗ + 0.0752∆𝐻′)
0.5 --
2 Base area of tower 𝐴𝑏𝐷𝑡 =
19.5 ∗ 𝐴𝑏 ∗ 𝑧𝑡0.5
𝐶𝑡1.5
m2
3 Height of tower 𝑧𝑡 Assumed values are used during
calculations to confine to a height to
diameter ratio of 3:2; in case of
hyperbolic natural draft towers
m
4 Diameter of tower 𝑑𝑏𝑑𝑏 =
4 ∗ 𝐴𝑏𝜋
m
Design of a cooling tower –
Numerical example Numerical example, with reference to Coulson & Richardson, Chemical
Engineering Vol.1, 6th Edition.
Q. (a) Determine the diameter and height of a hyperbolic natural draftcooling tower handling 6500 kg/s of water under the followingconditions.
Inlet water temperature = 318 K
Outlet water temperature = 313 K
Dry bulb temperature of air = 301 K
Wet bulb temperature of air = 295 K
(b) Determine the effectiveness and cooling capacity for the specifiedtower.
Design of a cooling tower –
Numerical solution
Available values Required values
𝑇𝑤 = 295 K ∆𝐻′
𝑇𝑑 = 301 K ∆𝑇
𝑇𝑖𝑛 = 318 K ∆𝑇∗
𝑇𝑜𝑢𝑡 = 313 K 𝐷𝑡
𝑊𝐿 = 6500 kg/s 𝐴𝑏
𝑧𝑡
𝐸𝑐 𝑄
Design of a cooling tower –
Numerical solution (contd.)
1. Range = ∆𝑇 = 𝑇𝑖𝑛 − 𝑇𝑜𝑢𝑡 = 318 – 313 = 5 K
2. Approach = ∆𝑇∗ = 𝑇𝑜𝑢𝑡 − 𝑇𝑤 = 313 – 295 = 18 K
3. Mean temperature of water = 0.5 ∗ (𝑇𝑖𝑛 + 𝑇𝑜𝑢𝑡) = 0.5*(318 +313) = 315.5 K
4. Using a humidity-enthalpy chart, we shall calculate values of enthalpy of air
at the mean temperature of water and at the dry bulb temperature
Design of a cooling tower –
Numerical solution (contd.)
5. The corresponding values of enthalpies at mean water temperature and dry
bulb temperature are approximately 150 kJ/kg and 83 kJ/kg
6. Enthalpy change of passing air = ∆𝐻′ = 150 – 83 = 67 kJ/kg
7. Duty coefficient = 𝑊𝐿
𝐷𝑡= 0.00369 ∗ ∆𝐻
′
∆𝑇∗ (∆𝑇∗ + 0.0752∆𝐻′)
0.5= 27388
Design of a cooling tower –
Numerical solution (contd.)
8. Assuming height 𝑧𝑡 by hit and trial method and taking 𝐶𝑡 as 5.2, the
base area, diameter, and conformity of the height to diameter
ratio of the tower is calculated as,
No. Height
𝑧𝑡 (m) 𝐴𝑏 =𝐷𝑡 ∗ 𝐶𝑡
1.5
19.5 ∗ 𝑧𝑡0.5
(m2)
𝑑𝑏 =4 ∗ 𝐴𝑏𝜋
(m)
Height to diameter ratio
𝑧𝑡 / 𝑑𝑏 ≈ 1.5
1 95 1709 46.6 2.04 ≠ 1.5
2 90 1755 47.3 1.90 ≠ 1.5
3 85 1806 47.9 1.77 ≠ 1.5
4 80 1862 48.7 1.64 ≠ 1.5
5 75 1923 49.5 1.51 ≈ 1.5
Design of a cooling tower –
Numerical solution (contd.)
9. The acceptable value of height and diameter for the specified
tower is 75 m and 49.5 m, respectively
10. Effectiveness of tower = 𝐸𝑐 =∆𝑇
∆𝑇+∆𝑇∗∗ 100 =
5
5+18∗ 100 = 21.74%
11. Cooling capacity
𝑄 = 𝑊𝐿 ∗ 𝐶𝑝 𝑤𝑎𝑡𝑒𝑟∗ ∆𝑇 = 4500 (kg/s) * 4.1815 (kJ/kg K ) * 5 (K)
𝑄 = 94083.75 kW
Strategies to improve cooling tower
performance When designing a cooling tower, always use the highest wet-bulb temperature
as reference.
Monitor the range and approach carefully during the design.
High range and low approach leads to good performance.
Improve the quality of water to be cooled by the tower resulting in low utilization of make-up water.
Regularly monitor the tower for scale build-up and biological impurities.
Regularly monitor the flow of water and air inside the tower
Importance of cooling tower in CPI
Cooling towers are used to cool industrial processes and
applications to ensure that the correct temperature of the
environment and the process are maintained during manufacturingor large industrial processes.
Natural draft cooling towers require no power and are of key
importance in power plants.
References
Literature:
Dr. N.P. Cheremisinoff, Handbook of Chemical Processing Equipment, 1st
edition, Butterworth-Heinemann, USA, 2000.
J. M. Coulson, J.F. Richardson, J.H Harker, J.R. Backhurst, Chemical
Engineering Volume 1 – Fluid Flow, Heat Transfer and Mass Transfer, 6th edition, Butterworth-Heinemann, USA, 1999.
San Diego County Water Authority, Technical Information for Cooling
Towers Using Recycled Water, San Diego, USA, 2009.
Training Session on Energy Equipment, Cooling Towers, UNEP, 2006.
Websites / URL’s:
http://www.cti.org/whatis/coolingtowerdetail.shtml
http://www.engineeringtoolbox.com/
http://www.deltacooling.com/resources/principles-of-cooling-towers/