Emerging trends in cooling water systems
Although water technology(chemistry of water) in industrial water
treatment has made large advances over the past 60 years, the objectives
of successful cooling water systems treatment programme have remained
the same: extend equipment life and maintain efficient heat transfer.
This has been accomplished by minimizing corrosion of system metallurgy
as well as keeping metal surfaces free from deposition. From a historical
perspective, even the earliest Cooling Water Systems programmes have
addresses these concerns by the incorporation of concerns related
to corrosion and scale inhibitors and deposit control agents. Not
much of the changes or breakthroughs have taken place in atleast last
two decades in terms of speciality chemicals technology .
Emerging trends in cooling water systems programme are mainly in
- Use of environmentally safe chemicals
- Alkaline treatment
- Health related concerns
- Automatic dosing and monitoring
- Use of environmentally safe chemicals
Scanning of the recent work on cooling water treatment reveals that
the published literature has been mainly dedicated to the search
for environmentally safe corrosion and scale inhibitors. The present
research deals with types, structure, efficiency, biodegradability
and advantages of new compounds "mixtures". green water
chemistry and green chemicals(speciality chemicals) are defined
and the approach adapted in finding such compounds and the method
of evaluation are highlighted. A promising approach is represented
by the combination of biodegradable polymers and environmentally
safe amounts of phosphorous and chromium. New corrosion and scale
inhibitors other than the green ones, and the recent work on ozone
have been considered as well. Guidelines for the future work on
industrial water treatment chemicals are being redefined.
Cooling towers are used by office buildings and manufacturing plants
throughout the world to dissipate waste heat from air conditioning,
industrial and power generation processes. Recirculating water transfer
thermal energy from the building or industrial process to the atmosphere.
Atmosphere air blown through the cooling tower carries away the
heat. Environmental problems arise when water escapes from the system
in the form of droplets. Such water droplets carry with them various
chemicals that are used in the system. Some of these chemicals are
environmentally harmful. Hexavalent chromium is the one that is
of the most concern and warrants immediate attention.
Hexavalent chromium-based ("chrome") compounds are among
the most efficient and cheapest corrosion inhibitors available.
The trouble is, hexavalent chromium is a suspected carcinogen, and
is highly toxic. Chrome emissions from cooling towers in New Delhi
alone could cause as many as 700 cancer cases over a 70 year exposure
Various Governments have banned the use of hexavalent chromium
water treatment chemicals in open water circulating systems that
are potentially capable of emitting respirable hexavalent chrome.
This prohibition is expected to reduce the risk of cancer cases
due to cooling tower emissions to virtually zero. Additionally,
elimination of hexavalent chromium-based treatment chemicals will
eliminate the amount of hazardous and toxic wastes discarded through
blowdown. Non-chromate chemicals may also have some adverse impacts
on the environment. For example, while zinc based chemicals are
particularly dangerous to humans, they are highly toxic to marine
life. Similarly, phosphate discharges into lakes and ponds may cause
excessive algal growth leading to eutrophication problems. But in
comparison to the highly toxic chromate inhibitors, the substitute
chemicals are relatively innocuous and do not present the same environmental
problems that chromates do. Nevertheless, the impact of substitute
chemicals on the environment must be carefully analyzed before actually
- Alkaline treatment in cooling water systems
Conventional chromate/zinc and chromate/zinc/orthophosphate programmes
have traditionally been controlled at pH limit below 7 (and often
much lower) to avoid excessive precipitation of water salts. In
modern water treatment "chromate" has been discontinued
long back due to "environment considerations." On the
other hand now days almost all the good companies use "alkaline
cooling water technology" and take advantage of lower corrosivity
of high pH waters super saturated with respect to calcium carbonate.
Alkaline waters are generally less aggressive towards steel than
near neutral pH water because of their higher buffering capacity.
The deposit control position of the program utilizes a "polymer
technology" that allows the pH to be controlled at levels previously
thought to be too high.
It is encouraging to note that , 85% of the cooling towers operating
worldwide had already changed to non-chromate systems or alkaline
treatment.Most industrial cooling water systems now run at pH level
above 7 in order to take advantage of the alkalinity in the water
as an aid in corrosion control. Several different combination programmes
are available, offering corrosion and scale control in various ways
depending on system needs. The entire programme should include bio-fouling
control as an integral part of the operation.
Never let the cooling water system run without treatment since
alkaline system are generally well into the scaling range and will
precipitate calcium carbonate quickly if scale inhibitor are not
present. Recovery from such disasters may be difficult and expensive.
VARIOUS RULES FOR GOOD OPERATIONS OF ALKALINE COOLING WATER OPERATION
@ Be sure that operating personals are well trained and are able
to understand the
importance of good control of cooling water operations.
@ Install good feeding equipment, with backup parts if needed and
insist that it should
be well maintained.
@ Plan, through regular checking of water parameters, for unexpected
problems and develop a response plan. (Quick response to upset conditions
can mean the difference between a momentary problem and long term
loss of good performance or loss of costly equipment.)
@ Start up the system properly. Be sure the system is clean and
follow the supplier’s
recommendations for start up procedures and be sure that maximum
dosages, particularly with biocides are not exceeded.
- Health related concerns
In 1976, there was an outbreak of 221 pneumonia cases associated
with an American Legion Convention in Philadelphia, Pennsylvania.
It was determined that the bacterium legionella was the causative
agent of the outbreak. Since that time Legionellosis or Legionnaires
Disease as it is commonly referred to, has been a controversial
subject. Outbreaks of legionellosis continue to occur. Since cooling
tower systems have been implicated as sources of outbreaks, it is
important that water treatment professionals have an understanding
of what is known and what is not known about Legionnaires Disease.
Legionella is a fresh water bacterium. It has been isolated from
such sources as wet soil, lakes, rivers, and streams, cooling towers
and piping systems. The word most often used to describe legionella
is "ubiquitous", meaning omnipresent or being everywhere.
It is true that this organism is all around us and can be isolated from most fresh water sources.
Legionella is an aerobic bacterium. This means it requires the
presence of oxygen to survive and grow. In laboratory tests, the
bacterium has been shown to withstand a pH as low as 2.0. Temperatures
up to 55 Deg C (131 Deg F) can be tolerated and growth can occur
at temperatures up to 45 Deg C (113 Deg F). In general, the cells
are 0.5-1.0 micrometers in width and 1.0-3.0 micrometers in length.
The legionella bacterium grows at a slower rate than most other
fresh water bacteria.
Legionella depends upon the presence of appropriate nutrients and
minerals for growth. Two of the most important are an amino acid
called L-Cysteine and iron. Laboratory experiments have shown that
some species of legionella can multiply intracellularly within certain
free-living protozoa. In addition to protozoa, algae and non-legionella
bacterial have been shown to play important roles in the proliferation
of legionella. Algae provide growth stimulation of legionella through
substances produced by the algae. Non-legionella bacteria can be
used as sources for the amino acid L-Cysteine.
There are two forms of Legionnaires Disease; Pneumonia and Pontiac
Fever. Of the two forms the pneumonia is much more serious. The
pneumonia form of legionellosis is transmitted by the breathing
in the water vapors containing legionella. This allows the bacterium
to infect the lung tissue. The incubation period can be anywhere
from 2 to 10 days long. The attack rate is 1-4% of those exposed.
There is a 15- 20% mortality rate for persons actually coming down
with the pneumonia. The symptoms, which come on gradually, can include
fever (39 Deg C, 102 Deg F), headache, cough and upset stomach.
This form of legionellosis is generally thought to be seasonal with
the greatest number of outbreaks occurring in the summer and fall.
Persons most at risk to the pneumonia form of legionellosis include
adults, the elderly and those persons who have low immune tolerances.
Pontiac Fever is a mild, self-limiting form of legionellosis. It
has an incubation period of
only 36 hours. Pontiac Fever has an extremely high attack rate (95%),
yet there has never been a fatality attributed to this form of the
disease. Pontiac Fever is most likely to occur in working age adults.
This form of legionellosis has a rather sudden onset but recovery
generally occurs in 48
hours. The symptoms are a low-grade fever and headache.
There are currently 23 recognized species of legionella. Of these,
only 11 have been isolated from infected humans. One of these species,
Legionella pneumophila has been implicated as the causative agent
for approximately 85% of all known cases of legionellosis. The same
species of the bacterium can cause either the pneumonia or Pontiac
Fever. Still, it is not known what factors influence the bacterium
to cause one form of the disease instead of the other. Using a technique
called isoenzyme typing, each serogroup can be further subdivided.
The use of this technique has resulted in the discovery of 62 genetic
variations of L. pneumophila serogroup.
The identification of a legionella species in a given water source
does not prove any
association with disease. This is due to the genetic diversity of
Close surveillance of outbreaks is the primary indirect control
measure for legionellosis at this time.
So how are responsible water treatment professional supposed to
handle questions regarding Legionnaires Disease? The following common
sense recommendations are not designed to eliminate legionella,
but rather to minimize the potential of the bacteria to get out
of control and cause on outbreak of disease.
Keep the cooling water system operating within the established parameters.
The accumulation of suspended matter and organic matter can contribute
to the proliferation of legionella. Undertake an effective biocide
program that includes two, alternating biocides that function in
differing manners. (i.e. alternate a quaternary ammonium based biocide
with a carbamate biocide.) The control of other microflora within
a system can and does have a direct impact upon the growth of legionella.
Monitor levels of other bacteria within the system. Since legionella
grows at a rate slower
than most other bacteria, it is probable that the system would experience
other problems prior to legionella becoming a problem.
Inspect the site for air intake sites in the vicinity of the cooling
tower. By minimizing the
exposure of persons to water vapors from the cooling system, you
can minimize the potential for an outbreak of disease.
By following the above recommendations, best possible protection
against Legionnaires Disease can be achieved.
- Automatic dosing and monitoring
In most cases, too much emphasis is placed on the control and feed
of cooling water treatment chemicals and not nearly, enough emphasis
is placed on continuous monitoring of the system. While most end
users of cooling water specialty chemicals are aware of the monitoring
capabilities of the major specialty suppliers, few are aware that
having there on lab. For monitoring helps them in getting better
results. There are a number of good reasons for continuous performance
monitoring of cooling systems:
Changes in make-up water chemistry.
Changes in temperature due to seasonal variations.
Changes in system heat load due to production level changes.
Changes in biological control requirements due to seasonal variations.
Treatment Chemical quality control problems.
System feed and control problems.
System contamination from atmospheric or process leaks.
Most systems control and monitor pH and conductivity levels on
a continuous basis. While these two parameters are of primary importance,
two other parameters warrant attention:
1. Chlorine level if oxidizing biocides are utilized.
2. General corrosion rate and pitting tendency.
Chlorine Monitoring and Control
The importance of monitoring and control of chlorine levels in cooling
water systems, which utilize oxidizing biocides such as chlorine,
chlorine/bromine, or sodium hypochlorite, is very important. If
a true chlorine residual is not maintained, biological deposition
and/or under deposit corrosion will occur. If chlorine residuals
are too high, corrosion will occur.
Continuous monitoring of cooling systems for general corrosion and
pitting tendency is
critical to assuring performance of a cooling system treatment program.
Corrosion coupons can provide you data on the average corrosion
rate over a given period. Instantaneous readings of corrosion probes
provide a snapshot of system corrosion performance at any given
time. The cost of these instruments is reasonable; (approx. Rs.
6,000-8000) and the data provided is very valuable
Results of process or atmospheric contamination can be detected
The effects of the interaction of other additives on corrosion such
biocides can be evaluated. In some cases, we have found that some
non-oxidizing biocides also have a tendency to significantly increase
the pitting tendency of some waters.
Corrosion can be monitored on off shifts and weekends when close
attention may lack due to the
absence of supervisory personnel.
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