1)Monitoring of Micrological
Micrological
monitoring in cooling systems is generally conducted either
by monitoring microorganism counts in the bulk water or by
directly measuring biofouling using a biological deposit monitor.
Monitoring bulk water organism counts is the traditional and
most widely used approach and is generally conduced using
the standard plate count (SPC) method. Other bio count assay
methods, including the popular dip stick method, are also
used with results of the analysis usually expressed as CFU
(colony forming units) per ml. Biocount results are used for
establishing microbiological trends for a system, for quantifying
and differentiating between the different types of troublesome
organisms, and for evaluating biocide effectiveness. Experience
with a particular cooling system will indicate what level
of bilk water organisms should be maintained and not exceeded
to ensure acceptable biocontrol for that system.
Legionella bacteria
in cooling water systems can also be monitored using plate
count methods by utilizing special culture media selective
for the growth of Legionella. An alternate approach also used
is a microscopic technique called the direct fluorescent antibody
(DFA) test. Either approach, when properly performed, permits
enumeration of Legionella in order to determine whether biocide
treatments are effectively keeping the numbers of Legionella
in check.
The second approach
to microbiological monitoring in cooling water systems is
to directly measure microbiological control using a biodeposit
or biofouling monitor. Several different types of biofouling
monitoring devices are used and are based on monitoring either
changes in pressure drop or heat transfer resistance or on
measuring biofilm directly using surface colonization coupons.
The biofilm monitor operates on a slip stream from the cooling
system and can provide real time assessment on system boifouling
conditions. Biofouling treatment program performance can then
be monitored and adjusted to maintain clean surface conditions
in the units. In combination with an appropriate bulk water
monitoring program complete cooling system microbiological
status can always be known and treatment program fined-tuned
for optimum performance.
Finally, special
methods have been developed to detect the presence of sulfate-reducing
bacteria (SRB), acid-producing bacteria (APC) and other anaerobic
species associated with microbiologically influenced corrosion
(MIC). These methods all require careful in-field sampling
techniques. The methods involve the use of either special
growth media or enzymatic reactions that generate a gas or
a color change that can be measured quantitatively.
2) Fouling Evaluations
Corrosion and
deposition are closely related in most cooling systems Dirty
systems, as a rule, show higher corrosion rates and more localized
corrosion than clean systems.
On-line, side
stream heat exchangers are often used to evaluate deposition
rates in cooling water systems. A plot of heat transfer rate
versus time can indicate the extent of deposition in an on-line
heat exchanger, but cannot indicate the nature of the deposits.
An on-line heat exchanger is a valuable tool in assessing
deposition tendencies on heat exchanger surface in cooling
systems. Both visual inspection and analysis of any deposits
formed can assist in identifying and controlling deposition
in the operating system.
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