An evaluation of applicable materials for an industrial cooling tower is presented in this study.
Advantages and disadvantages of different sets of materials including reinforced concrete and FRP
(Fiber-Reinforced Polymer Composites) for cooling tower structure are discussed. After evaluating
each material characteristic, the one case study of cooling tower is considered for cost estimation. The
results showed that the FRP is best structural material for cooling tower construction mainly due to its
superior performance in sea water corrosive environment. From the economical point of view, although
the construction cost FRP structure is a little higher, this can be easily balanced by less maintenance
costs of FRP structure considering its high durability in hostile environments.
Key words: Fiber-reinforced polymer composites (FRP) cooling tower, concrete cooling tower, polyvinyl
chloride (PVC) fills.
INTRODUCTION
Current paper covers a review of applicable materials for
an industrial cooling tower. Cooling towers are usually
exposed to severe internal operating conditions such as
high temperature, wet, corrosive and abrasive
environments and sustained loading. After many years of
utilizing redwood in cooling towers because of its natural
tendency to inhibit decay, the quality of redwood
diminished and Douglas fire was introduced to the
market. However, the negative effect of Douglas fire was
that it deteriorated rapidly in comparison to the redwood.
Various methods of pressure treatment and incising were
developed to offset the micro-organisms that attacked
and eventually depleted the wood. In addition to the wood
being supplied and utilized by the tower market, other
materials such as galvanized steel, stainless steel,
concrete, and in some cases asbestos cement board
casing panels were utilized on field erected towers.
During the l970s, the environmental movement caused
several industries to be scrutinized. The chemicals used
to pressure treat the wood were viewed as possible
hazards, therefore resulting in tighter controls and new
formulations to be applied. The end result was an
increase in the material cost of wood. Asbestos was also
under scrutiny and ultimately dropped from the industry
due to the threat it posed of potential health hazards.
Through the s and into the early l970s, various
existing cooling tower companies as well as newly
formed organizations were looking for alternative building
materials that would offer comparable if not greater
strength to the materials being utilized while remaining
competitive.
FRP materials have been employed in cooling towers
as secondary components (including pipes and fan
stacks) for over 30 years, the primary structure
traditionally being constructed from wood, concrete or
steel. However, FRP composites are now prevailing as
the most suitable primary structural material in view of
their superior performance in hostile environments and
other beneficial properties. Consequently, the cooling
tower industry has seen a rapid uptake of FRP towers in
recent years. The design flexibility of FRPs has allowed
new types of cooling tower to be developed which are
more efficient and cost effective than previous designs
and materials. The modular, cellular construction systems
provide structures of high integrity that can be rapidly
installed. The desirable environmental properties of FRP
Boroujeni. 153
materials also help the structures meet the increasingly
stringent legislation imposed on them.
In order to recognize the advantages and
disadvantages of different applicable materials as cooling
tower structural members, a brief review of these sets of
materials are presented subsequently. The configuration
of cooling tower is shown in Figure 1.
REINFORCED CONCRETE COOLING TOWERS
The complete structure including exterior walls, fan deck,
partitions and windscreen are designed in order to be
executed in reinforced concrete material with all the
specific requirements of this particular application. The fill
consists of modules designed with vertical flutes, 20 mm
opening, for optimum cooling and minimum fouling
characteristics. The fill comprises of vacuum formed PVC
(polyvinyl chloride) sheets, bonded to form modules 500
mm high by 500 mm wide with a nominal length of
mm. Fill is supported from below by tower structural
beams and covers the entire internal plan area of the
tower. Hot water is introduced to the tower through
ground headers, valves and risers provided by others.
Tower headers have one outside flanged connection per
cell. The main header consists of a concrete flume. PVC
distribution pipes are fitted into the flume and uniformly
cover the plan area of the tower; these pipes are securely
fitted with spray nozzles. The main header consists of
concrete flume. The fan consists of multiple, manuallyadjustable blades attached to a steel hub. The fan deck is
accessed by a caged ladder and/or concrete stairway.
154 J. Mech. Eng. Res.
Hand railing is provided around the perimeter of the fan
deck. Access to the inside of the tower is through a
lockable hatch in the fan deck, with a ladder leading
down to the drift eliminator level for inspection of the
cooling tower internals. From there, removable FRP
grating allows access to the whole plan area and to a
second ladder leading up to the gear reducer. The fan
stacks, as standard, are constructed of heavy, ribbed
fiberglass panels bolted together.
FRP COOLING TOWERS
FRP materials have many key properties which make
them suitable for use in cooling tower applications. Their
inherent corrosion, moisture and temperature resistance
significantly increases the durability and service life of the
structure, as well as reducing the need for maintenance.
FRP structures also exhibit superior dynamic response to
high wind loads in comparison with conventional
structural materials. Maximizing the glass volume not
only enhances the material strength and stiffness
properties, but reduces creep and hydrothermal effects
due to the lower resin content. FRP parts offer more
flexibility of shape than steel or timber. Components can
therefore be manufactured with features that enable rapid
connection and modular construction, minimizing the
material content whilst providing the required buckling
strength. The modular design methods associated with
FRP structures are quicker and easier. A standard range
of field erected towers can be formulated efficiently from
the initial design. Suitable limit-state design methods
account for the variability of all the material parameters -
allowing production of safe but efficient designs.
Although comparable to conventional tower structure
materials in initial cost, FRP materials offer significance
through life cost savings. They have longer service lives,
lower replacement frequency and require little
maintenance. The lower replacement frequency also
reduces the significant process downtime costs
associated with structure replacement. Less raw material
use in the overall structure brings associated cost
savings and gains are made from the rapid installation,
which is much less labor intensive due to the lightweight
components. Transportation costs are also reduced as
less, lighter weight material is required.
FRP is preferable to wood in instances where
environmental issues are a factor since it contains no
preservatives that could leach into the water being
cooled. FRP materials can aid compliance to legislation
regarding discharge to rivers. Greater cooling capacity
means that the water released can closely approximate
the temperature of the river as stipulated in regulations. It
has also been proved that composite tower structures
offer reduced noise emission due to their preferable
dynamic behavior. It is worth-mentioning that the
acceptance of pultruded FRP towers has become so
widespread that it is estimated over 70% of new and
replacement field erected towers in the USA are specified
with pultruded FRP structures. Pultruded FRP cooling
towers are in service today in numerous applications.
Type II, III, IV pultruded shapes are acceptable with a
synthetic polyester fibre-surfacing veil with a minimum
effective thickness of 10.0 ml minimum to provide long
term UV (ultraviolet) protection.
Grade 1 or grade 3 resins are acceptable for the
structure with a flame spread rating of 25 or less per
ASTM E84 flame spread test (CTI STD 137, 94). The
resin must be high quality and chemical resistant. The
resin shall be an isothalic polyester, vinyl ester or
urethane type resin system.
The glass reinforcing may be continuous roving,
continuous strand mats; woven or non-woven fabric,
unidirectional fabric or a combination of these. The
reinforcing shall be made from type C or type E glass
fibers.
Additives to the resin mix may be used to improve
performance characteristics of the final composite.
Typical additives are UV inhibitors, antimony trioxide as
an improved flame retardant and a minor percentage of
fillers. Any mold release that is used must not reduce the
long-term strength of any epoxy joint that may be used in
the tower structure.
In general, advantages and disadvantages of the FRP
materials can be noted as follows:
Advantages:
1. High specific strength.
2. Good in-plane mechanical properties.
3. High fatigue and environmental resistance.
4. Adjustable mechanical properties.
5. Lightweight.
6. Quick assembly/ erection.
7. Low maintenance cost.
8. Highly cost-effective.
Disadvantages:
1. Lightweight (problematic in wind resistant design).
2. Brittle.
3. High initial costs.
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4. Low to moderate application temperature (-20 up to
80°C).
5. Low fire resistance (sometimes with unhealthy gases).
Most structural profiles are produced in conventional
profile shapes similar to metallic materials. Being
somehow similar in geometry and properties, however no
standard geometry, mechanical and physical properties
are used by all manufacturers.
A variety of continuous and woven reinforcement types
are commonly used in fiberglass pultrusions. The four
major types are E-Glass, S-Glass, Aramid, and Carbon.
Boroujeni. 155
Table 1. Typical properties of fibers used in pultruded structural profiles.
Property E-Glass S-Glass Aramid Carbon
Density (lbs/in3
) 0.094 0.090 0.053 0.064
Tensile strength (psi) 500,000 665,000 400,000 275,000 - 450,000
Tensile modulus (106 psi) 10.5 9.0 9.0 33-35
Elongation to break (%) 4.8 2.3 2.3 0.6-1.2
Table 2. Typical properties of resins used in structural pultrusions.
Property Polyester Vinylester Epoxy
Tensile strength (psi) 11,200 11,800 11,000
Elongation (%) 4.5 5.0 6.3
Flexural strength (psi) 17,800 20,000 16,700
Flexural modulus (106 psi) 0.43 0.54 0.47
Heat distortion temperature (°F) 160 220 330
Short beam shear (psi) 4,500 5,500 8,000
The most commonly used reinforcement is E-Glass.
Other reinforcements are more costly, and therefore are
used more sparingly in construction. Table 1 provides
some physical properties of the four reinforcing fibers CTI
(CTI STD 137, 94).
FRPs are produced usually by pultrusion method.
There are two types of reinforcing fibers in FRP materials
called continuous strand mat and continuous strand
roving
Continuous strand mat
Long glass fibers intertwined and bound with a small
amount of resin, called a binder. Continuous strand mat
provides the most economical method of obtaining a high
degree of transverse or bi-directional strength
characteristics. These mats are layered with roving, and
this process forms the basic composition found in most
pultruded products. The ratio of mat to roving determines
the relationship of transverse to longitudinal strength
characteristics.
Continuous strand roving
Each strand contains from 800 to 4,000 fiber filaments.
Many strands are used in each pultursion profile. This
roving provides the high longitudinal strength of the
pultruded product. The amount and location of these
&#;rovings&#; can, and does alter the performance of the
product. Roving also provides the tensile strength needed
to pull the other reinforcements through the
manufacturing die. Since pultrusion is a low-pressure
process, fiberglass reinforcements normally appear close
to the surface of the product. This can affect appearance,
corrosion resistance or handling of the products. Surface
veils can be added to the laminate construction, and
when used, displaces the reinforcement from the surface
of the profile, creating a resin-rich surface. The two most
commonly used veils are E-Glass and polyester. Resin
formulations typically consist of polyesters, vinyl esters,
and epoxies, and are either fire retardant or non-fire
retardant.
Resins are another important component of FRP
materials. Polyesters and vinyl esters are the two primary
resins used in the pultrusion process. Epoxy resins are
typically used with carbon fiber reinforcements in
applications where higher strength and stiffness
characteristics are required. Epoxies can also be used
with E-glass for improved physical properties. Typical
physical properties of resins used in pultruded structural
shapes are given in Table 2.
Various fillers are also used in the pultrusion process.
Aluminum silicate (kaolin clay) is used for improved
chemical resistance, opacity, good surface finish and
improved insulation properties. Calcium carbonate offers
improved surfaces, whiteness, opacity and general
lowering of costs. Alumina trihydrate and antimony
trioxide are used for fire retardancy. Alumina trihydrate
can also be used to improve insulation properties. Resin
formulations in a pultruded fiberglass structural shape
can be altered to achieve special characteristics as
dictated by the environment in which the shape is
intended for use.
FRP CASE STUDIES
A case study design of FRP cooling tower is considered
156 J. Mech. Eng. Res.
here. The tower is a FRP structure with PVC fills. The
scope of the project was to furnish and install a multi-cell
induced draft counter flow FRP structure cooling tower,
custom designed to be field erected within a contractorsupplied reinforced concrete basin. The tower structure
was field erected from pultruded FRP structural members
that were designed specifically for cooling tower
application.
The FRP members were constructed of a fire-retardant,
self-extinguishing resin system with a flame spread rating
of 25 or less. The FRP members were also protected
from UV degradation by the use of surfacing veils and UV
stabilizers incorporated in the resin system. The tower
structure was designed in accordance with CTI STD 137
(94) to withstand the following dead and live loads as per
the following:
1. Wind load: Per applicable building code. Wind load is
to be applied to tower walls and fan stack. Tower casing
shall not be considered as sacrificial when calculating
tower structure loads.
2. Seismic load: Per applicable building code, to be
applied to total operating weight of the tower.
3. Deck dead load: Weight of deck materials.
4. Deck live load: 60 PSF (280 kg/m2
) equally distributed
load over entire usable roof deck.
5. Fill support dead load: Dry weight of fill material plus
water hold up weight plus 15% additional allowance for fill
clogging.
6. Fill support live loads: 300 lbs (140 kg) of concentrated
load for temporary maintenance foot traffic.
7. Eliminator dead and live load: Dry weight of drift
eliminators.
The strength of the FRP members was de-rated for long
term temperature exposure. The maximum operating
temperature exposure for design purposes was 40°C.
When designing connections, the minimum service
factor for dead loads allowed for a connection is 4.0. The
service factor for connections with temporary loads due
to wind, seismic, etc. may be reduced to 2.5. Either a
mechanically bolted joint or combination of mechanical
and adhesive (epoxy) joints may connect the union of two
or more FRP components. Either joint is acceptable when
properly designed and installed. When connecting hollow
type structural members by the use of bolted joint, the
service factor for bearing dead loads must be 4.0
minimum and 2.5 minimum for live and dead loads.
Bearing hole elongation of 4% or greater is considered
failure when stress is applied to any joint. On bolted joints
of hollow tube members, 304 stainless washers are
required to keep the connections tight as well as protect
the FRP members from over tightening and cracking the
FRP (CTI STD 137, 94).
REINFORCED CONCRETE CASE STUDIES
A case study design of concrete cooling tower is
considered here. The concrete tower structure was
designed in accordance with ACI codes (ACI 318, )
to withstand the ASCE 7 dead and live loads (ASCE 7,
). Earthquake load in this study is calculated based
on ASCE 7 (Ultimate level) therefore earthquake load
used in the load combinations should be divided by 1.4 to
decrease it to service level.
COST ESTIMATION
In order to compare construction costs of concrete and
FRP structure cooling towers, cost estimation is
conducted based on structural analysis and design for
the cooling tower under study. The construction cost of
FRP structure is about 10% higher than the reinforced
concrete one, which is due to the fact that FRP products
are more expensive than common structural materials
like structural steel and reinforced concrete. But
considering less maintenance costs of FRP structures
due to the high durability in corrosive environments, this
increased construction cost of 10% appears to be
nothing, making FRP a suitable material for cooling tower
structures.
CONCLUSION
An evaluation of applicable materials for an industrial
cooling tower located was presented in this study.
Advantages and disadvantages of different sets of
materials including reinforced concrete and FRP for
cooling tower structure were discussed. After evaluating
each material characteristic, FRP was selected as the
best structural material for cooling tower construction
mainly due to its superior performance in sea water
corrosive environment. From the economical point of
view, though the construction cost FRP structure is a little
higher, this can be easily balanced by less maintenance
costs of FRP structure considering its high durability in
hostile environments.
REFERENCES
CTI STD 137 (). Fiberglass Pultruded Structural Products for Use
in Cooling Towers, pp. 4-8.
ACI 318 (). Building code requirements for Reinforced Concrete,
pp. 69-169.
ASCE 7 (). Minimum Design Loads for buildings a
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