WOOD TREATMENTS FOR COOLING TOWER STRUCTURES
Written by:
Darrell Smith,
Operations Manager, Conrad Forest Products
Dr. Jeff Morrell, Department of Forest Products, Oregon State University
for CTI - Cooling Tower Institute
The cooling tower industry subjects its wooden towers to the harshest
environments conceivable. The wet-dry cyclical conditions and structural
designs intensify the undisciplined seasoning of the wooden members,
resulting in warping, twisting and checking. Wood preservative treatment
is based on two separate -- yet interdependent-- requirements: penetration
and retention of the chemical preservative.
These requirements should be relevant to the environment to which
the wood will be subjected. Proper treatment of a specific wood specie
to acceptable standards defined and promulgated by the American Wood
Preservers Association (AWPA) will help address and prevent these problematic
conditions and increase the longevity of the cooling tower structure.
HEARTWOOD vs. SAPWOOD
There is a marked superiority in the decay resistance of the heartwood
in comparison with that of the sapwood in the wood species now used in
the cooling tower industry. The sapwood of all species is susceptible
to decay, whereas the heartwood is highly durable. The degree of durability
of the heartwood is directly related to the extractive contained and accumulated
in it. These changes in the heartwood by the extractive greatly increase
the resistance to the penetration of preservative treatments, which affects
the treatability of the wood and the operations the treating plant employs
in dealing with the treatment of those "hard-to-treat" species referred
to as "refractory". The most common misunderstanding is that heartwood
is materially stronger than sapwood. Results and comparisons of thousands
of tests performed by the Forest Products Laboratory fail to show any
significant differences in strength between sapwood and heartwood. The
greater permeability of the sapwood makes it superior from the standpoint
of the preservative treatment used and should be endorsed rather than
denounced.
WOOD SPECIES
Within the differing species of conifers (for example, redwood and Douglas
Fir), permeability varies, however, the sapwood of each remains easier
to treat than the heartwood. Generally, Douglas Fir's heartwood is harder
to penetrate with preservatives than redwood. Both, however, are required
to be incised by AWPA standards for uniform penetration. The heartwood
of "old-growth" redwood has long been valued for its excellent resistance
to decay, and for many years, redwood was the preferred cooling tower
material. However, with the depletion of the old growth stands, availability
and cost are making "old-growth" redwood prohibitive. The lumber industry
has converted to young-growth" redwood, which will increase the volume
of its sapwood. Evidence supports that the heartwood found in "young-growth"
redwood is not as decay resistant as that found in "old-growth", thereby
increasing the need for effective chemical preservation.
PROPER PREPARATION OF THE WOOD
The success of proper treatment is dependent upon the preparation of the
wood to receive the preservative. Air seasoning is the method most generally
employed to prepare the wood for treatment after incising. For various
reasons (e.g. lack of time, climatic conditions and lack of storage space),
other methods are used to prepare the wood. These artificial methods can
include steaming, vacuuming, and kiln drying. The sapwood of most species
seasons at a much faster rate than the heartwood. As a result, the sapwood
dries and begins to shrink, while the heartwood still retains its original
size. Checking of the sapwood is inevitable in such cases, and the checks
will later extend in all likelihood to the heartwood as it dries. The
size and severity of the checks can be limited to a considerable extent
by controlling the drying process through proper stacking, end coating
and kiln drying. End coating is not desirable in the treating process
because of the added cost, application, and contamination of the preservative,
and, most importantly, retardation of treatment. When the material reaches
19 % M.C. (moisture content), it may be considered "air seasoned". Artificial
methods used in preparing the wood can achieve good penetration and uniformity
of treatment at 28-37% M.C., especially with the use of proper incising.
DRYING DEFECTS
Drying defects (such as checking, warping and splitting) in Douglas Fir
and redwood frequently occur in the cooling towers because of the nature
in which the wood dries after being supersaturated with moisture by the
preservative treatments. Treating "to refusal" is ineffective in preventing
wood decay and insect attack because the drying defects exceed the envelope
of preservative treatment. However, if the wood is properly treated and
cared for after treatment and the relative humidity is taken into consideration
at the site of the cooling tower, the structural wood material will render
longer life with less maintenance.
INCISING
The impregnation of preservative chemical of the heartwood in Douglas
Fir and redwood poses challenges for the treater, but the treatment can
be markedly improved by incising. Incising is a mechanical process wherein
numerous slits are inserted in the wood parallel to the grain direction.
These slits increase the amount of cross section or end grain exposed
to the preservative during the treating process and aids in the retention
of the preservatives. There is no question that incising affects wood
strength by decreasing the effective cross section of the member. Decreases
typically range from 10 to 20% for dimensional lumber but decreases with
increasing wood size. As a matter of fact, 4x4 has no reduction. The loss
is more than offset by the improved treatment and performance of the finished
product. The deeper and more uniform the envelope of treatment, the less
likely it is to be compromised in service and the better its performance.
Ultimately, strength loss associated with incising is more than offset
by the performance improvements.
WATER TREATMENT PRACTICES
Another factor affecting the performance of both naturally durable woods
such as redwood and preservative treated wood has been the change in water
treatment practices as result of environmental regulations. Older practices
of including chemicals such as chromium in water for inhibiting corrosion
probably played a major role in both the fixation of preservative components
as well as the types of decay organisms present. Newer water treatment
systems contain more benign anti-corrosion chemicals which may have little
effect on fungi which degrade the wood. Thus, towers which use these newer
water treatment systems may be less forgiving of poor treatment. It is
generally difficult to overcome poor specifications and treatment with
remedial treatment. It would be far better and more cost effective to
develop specifications for initial preservative treatment to ensure that
treatment is delivered at the required level to a depth adequate for protection
and that the integrity of the preservative barrier remains intact. Ultimately,
the specifications the wood is treated to are the responsibility of the
purchaser, but if they are ill informed or have not asked the appropriate
questions, the quest for well-treated, long-lasting structural members
will be difficult.
CONCLUSION
While the successful treatment of wood used for cooling towers can pose
a challenge, a number of practices can increase the quality of treatment
and ensure more uniform performance. Adherence to the American Wood Preservers'
Association standards of treatment and penetration (C30-91) can confront
that challenge and maximize the value of wood as a material in the cooling
tower industry.
ACZA FLAME SPREAD CHARACTERISTICS
Dated: 1-22-97
NOTE FLAME SPREAD CLASSIFICATIONS:
- CLASS I 0-25 FLAME SPREAD
- CLASS II 26-75 FLAME SPREAD
- CLASS III 76-200 FLAME SPREAD
DOUGLAS FIR TREATED TO A RETENTION OF .40 LBS. PER CUBIC FOOT TESTED
AT THE UNDERWRITERS LABORATORIES PASSED CLASS II FLAME SPREAD REQUIREMENTS
THE PRELIMINARY TESTS INDICATE ABOUT 1/2 THE NORMAL FLAME SPREAD OF UNTREATED
DOUGLAS FIR WHICH IS 70 TO 100. DOUGLAS FIR WHEN TREATED WITH ACZA TO
A RETENTION OF 2.0 PER CUBIC FOOT WILL PASS CLASS I REQUIREMENTS.
REDWOOD TREATED TO .60 LBS. PER CUBIC FOOT AND TESTED AT THE UNDERWRITERS
LABORATORIES PASSED CLASS II FLAME SPREAD REQUIREMENTS. THE PRELIMINARY
TESTS INDICATE 1/3 THE NORMAL SPREAD OF UNTREATED REDWOOD. THE PRELIMINARY
TESTS ALSO INDICATE THAT REDWOOD TREATED WITH ACZA TO A RETENTION OF
2.0 LBS. PER CUBIC FOOT WILL PASS CLASS I.
THESE PRELIMINARY TESTS AT THE UNDERWRITERS LABORATORIES CONFIRM
AND DOCUMENT THAT ACZA PRESSURE TREATMENT OF DOUGLAS FIR OR REDWOOD
ENHANCES THE FIRE RESISTANT QUALITIES OF THE STRUCTURE WHEN SPECIFIED
AND USED.
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