Unesco - Eolss Sample Chapters: Air-Cooled Heat Exchangers and Cooling Towers
Unesco - Eolss Sample Chapters: Air-Cooled Heat Exchangers and Cooling Towers
Unesco - Eolss Sample Chapters: Air-Cooled Heat Exchangers and Cooling Towers
Krger
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
1. Introduction
2. Cooling Towers
2.1. Mechanical Draft
2.2. Natural Draft
3. Air-cooled Heat Exchangers
3.1. Mechanical Draft
3.2. Natural Draft
4. Dry/wet and wet/dry Cooling Systems
Glossary
Bibliography
Biographical Sketch
Summary
In all power plants a large amount of heat has to be rejected in order to sustain the
thermodynamic cycle. In all steam cycles this heat is rejected via the condenser in which
the exhaust steam from the turbine is condensed before being returned as feedwater to
the boiler.
The heat released in the condenser must be rejected to the environment. In regions
lacking large bodies of water this rejection must be to the atmosphere. The requirement
for a small temperature difference between the exhaust steam and the atmosphere, while
rejecting large quantities of heat to air having a low heat capacity compared with water,
poses a challenging technological problem which has resulted in a number of different
engineering solutions.
To ensure that condensation of the steam will occur under low temperature, and hence
high vacuum conditions while maintaining it free from contamination, requires at least
one solid boundary where heat transfer is governed primarily by convection.
Evaporative cooling, however, is very attractive in reducing the overall temperature
difference between the exhaust steam and the atmosphere and is used wherever there is
an adequate supply of water. In arid regions however this may not be an option due to
lack of water and dry cooling systems have to be adopted. Indirect dry cooling systems
have an intermediate closed cooling water circuit while direct systems have the steam
condensed directly by the air flow.
Air flow through the cooling elements may be naturally induced by tall cooling towers
or forced by large fans. The latter provide a more compact arrangement but involve
higher operating costs.
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
In any power generating or refrigeration cycle, heat has to be discharged. This is also
true in many chemical and process plant cycles, internal combustion engines, computers
and electronic systems. The efficiency of a modern automobile engine is such that most
of the energy contained in the fuel is rejected through the exhaust and the radiator. In a
fossil-fired power plant with an efficiency of about 40 percent, more than 40 percent of
the heat input has to be rejected through the cooling system. Even more heat has to be
rejected in less efficient nuclear power plants. Considerably less heat is rejected in a
modern combined cycle power plant.
The hydrosphere has in the past been the commonly used heat sink at industrial plants.
The simplest and cheapest cooling method was to direct water from a river, dam or
ocean to a plant heat exchanger and to return it, heated, to its source. In industrialized
countries, the permissible rise in temperature of such cooling water is often limited, thus
limiting the use of natural water for once-through cooling.
The task of choosing the source of cooling for large industrial plants is becoming
increasingly complex. Dwindling supplies of cooling water and adequate plant sites,
rapidly rising water costs usually at well beyond inflation rates in most industrialized
countries, noise restrictions and other environmental considerations and proliferating
legislation, all contribute to the complexity.
Because of restrictions on thermal discharges to natural bodies of water, most new units
of power generation or large industries, will have to make use of closed cycle cooling
systems. Evaporative- or wet-cooling systems (cooling towers) generally are the most
economical choice for closed cycle cooling where an adequate supply of suitable water
is available at a reasonable cost to meet the make-up water requirements of these
systems. Unfortunately, many cooling towers have in the past failed to meet design
specifications, in part due to outdated design methods.
Air-cooled heat exchangers are found in the electronics industry, vehicles, air
conditioning and refrigeration plants and in chemical and process plants where fluids at
temperatures of approximately 60C or higher are to be cooled. The use of air-cooled or
dry-cooling systems in industry or in power plants is often justified where cooling water
is not available or is very expensive. In certain applications dry/wet or wet/dry cooling
systems offer the best option.
An appropriate and well-designed cooling system can have a very significant positive
impact on plant profitability.
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
2. Cooling Towers
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
Figure 1 shows a typical cooling circuit at a power plant. Turbine exhaust steam
condenses in a surface condenser where heat is given up to the cooling water circulating
through the condenser tubes. The hot water leaving the condenser is piped to the cooling
tower distribution basin from where it flows downward through the fill or packing
which serves to break the water up into small droplets or spreads it into a thin film in
order to maximize the surface contact between the water and the cooling air which is
drawn through the fill by the axial flow fan.
The water, after being cooled by a combination of evaporation and convective heat
transfer, is again pumped through the condenser in a continuous closed circuit. One to
three percent of the circulating water is lost due to evaporation.
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
Over the years the combination of theoretical and experimental studies as well as
extensive practical experience has led to the improved design and operation of such
cooling systems. Unfortunately these developments are not always fully exploited
during the life of a particular system due to poor maintenance and a lack of operating
experience.
2.1. Mechanical Draft
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
Figure 2: Forced draft cooling tower.(a) Cooling tower (b) Temperature distribution
Hot water is introduced through spray nozzles located above the fill and subsequently
flows downward in counterflow with the air-stream. Small droplets that are entrained by
the upward flowing air stream are collected in a drift eliminator where they accumulate
to form larger drops that are returned to the fill. The region below the fill contains
falling droplets and is referred to as the rain zone. The re-cooled water is collected in a
basin from where it is returned to the plant.
Figure 2(b) shows the temperature relationship between water and air as they pass
through the cooling tower. The curves indicate a drop in water temperature and a rise in
the air wetbulb temperature during their passage through the tower. The temperature
difference between the water entering and leaving the tower is defined as the range. The
difference between the temperature of the water leaving the tower and the entering air
wetbulb temperature is known as the approach. Forced draft towers are characterized by
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
relatively high air inlet velocities and low exit velocities and are thus susceptible to
recirculation of the hot, moist plume air.
Figure 3: Induced draft cooling towers.(a) Mechanical draft crossflow (b) Mechanical
draft counterflow
Examples of induced draft towers are shown in Figure 3. They may be of the crossflow
or counterflow type. In a crossflow tower the fill is usually installed at some angle to the
vertical to make provision for the inward motion of droplets due to drag forces caused
by the entering cooling air. Other arrangements of the fill are possible. Plume
recirculation is a smaller problem in induced draft towers than it is in forced draft
towers.
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
However, more fan power is usually required to move the same mass of air because the
air has a lower density (i.e. it is warmer and contains more water vapor than inlet air).
A section through a large single-cell circular induced draft cooling tower is shown in
Figure 4. The fan-drive equipment is located in a chamber isolated from the water
system. Fans having diameters of up to 28 m are employed in such towers.
Mechanical draft towers have traditionally been used in an in-line arrangement of
individual cells to form a rectangular bank as shown in Figure 5(a). A more recent
development is the round mechanical draft tower with multiple fans as shown in Figure
5(b).
THERMAL POWER PLANTS Vol. III - Air-Cooled Heat Exchangers and Cooling Towers - D.G. Krger
U
SA NE
M SC
PL O
E
C EO
H
AP LS
TE S
R
S
Burger, R., Cooling Tower Technology Maintenance, Upgrading and Rebuilding, The Fairmont Press
Inc., Lilburn, GA, 1994. [Description of different cooling towers and their performance characteristics].
Cheremisinoff, N.P. and Cheremisinoff, P.N., Cooling Towers: Selection, Design and Practice, Ann
Arbor Science Publications. Inc., Ann Arbor, Michigan, 1981. [Description of different cooling towers
and their performance characteristics].
Hill, G.B., Pring, E.J. and Osborn, P.D., Cooling Towers, Principles and Practice, ButterworthHeinemann, London, 1990. [Practice and theory of cooling towers].
Kays, W.M. and London, A.L., Compact Heat Exchangers, McGraw-Hill Book Co., New York, 1984.
[Performance characteristics of compact heat exchanger surfaces].
Knirsch, H., Dry Cooling Towers at Biggest Coal Fuel Power Station, Modern Power Systems, Ruislip,
England, July 1991. [Details of the worlds largest direct dry-cooled power plant].
Krger, D.G, Air-cooled Heat Exchangers and Cooling Towers, PennWell Corporation, Tulsa, OK, USA,
2004. [Most comprehensive description of power plant air-cooled heat exchangers and dry- and wetcooling towers and detailed performance evaluations of such cooling systems].
McKelvey, K.K., The Industrial Cooling, Tower, Elsevier Publications Co., Amsterdam, 1959. [An
overview of industrial cooling trowers].
McQuiston, F.C. and Parker, J.D., Heating Ventilating and Air Conditioning, 4th Edition, John Wiley and
Sons Inc., New York, 1994. [A description of heating, ventilating and air-conditioning systems].
Plank, R., Handbuch der Kltetechnik, Springer-Verlag, Berlin, 1988. [A description of air-conditioning
and refrigeration systems].
Biographical Sketch
Detlev G. Krger received his B.Sc. and B. Eng. degrees from the University of Stellenbosch and the
S.M., Mech. E. and Sc.D. degrees in Mechanical Engineering from MIT. He is presently Professor of
Mechanical Engineering and Director of the Institute for Thermodynamics and Mechanics (ITM) at the
University of Stellenbosch in South Africa.
Prof. Krger has authored more than one hundred technical papers in the field of thermal engineering as
well as a book entitled AAir-cooled Heat Exchangers and Cooling Towers@. He has received numerous
awards and is a Fellow of the American Society of Mechanical Engineers. He has served as a consultant
to companies throughout the world and was involved in the development and design of the most modern
and largest cooling plants.