Chemical Engineering June 2011 - 40

Source: Ref. 1, with permission
from Dr. Muller-Steinhagen
Maintenance due
Feature Report
able for use in a chemical process due
to the temperatures or chemicals to
which the coating would be subjected.
In the reference just cited, PTFE coating
of the plates - while acceptable
for some chemical services - showed
higher fouling than on a standard
stainless-steel plate.
Work is being done by some researchers
to evaluate the use of thinner
(nano-composite) coatings on plate
surfaces to avoid the loss in heat conductivity
due to the coating thickness.
The author has seen the resistance to
fouling improved even further in spiral
heat exchangers by dipping the entire
heat exchanger in a phenolic bath
after the mechanical manufacturing
steps have been completed to improve
the fouling resistance of the surface.
Monitoring fouling in CHEs
The two most common indications that
a heat exchanger may be fouled are an
increase in pressure drop or a loss in
heat transfer. The author's experience
indicates that more heat exchangers
are taken out of service for cleaning or
maintenance due to an increase in pressure
drop than as a result of reduced
heat transfer. In CHEs, because of the
relatively smaller flow channels, this
increase in pressure drop can be caused
by the macro-fouling by large particles,
which are too big to pass through the
channels, as discussed earlier.
While this is less likely to happen
in the case of spiral heat exchangers,
which can have larger flow passages
(up to 25 mm), still the inlet distributor
or the discharge header of the spiral
could become constricted. Mitigation of
this area of possible pluggage in CHEs
was discussed earlier in this article,
pointing out the need for accessibility.
In the case of fouling brought on by
surface deposits due to micro-fouling,
this is less likely to happen in the high
wall-shear-stress channels of CHEs.
While high pressure drop can certainly
be an indication of fouling and
can cause disruption of plant operation
and additional costs, it must be
remembered that even if fouling were
to occur uniformly on the heat transfer
surface, the thermal conductivity
of the deposit as well as its thickness
determine the effect the deposit has
on heat transfer. So, if the increase in
pressure drop is determined by the
engineer to be tolerable, monitoring
the heat exchanger's thermal
performance at that point to determine
how severely it is fouled may
be in order before proceeding with
taking it offline for cleaning.
With respect to monitoring, the
Rf, Limit
x
x x
x x
x
x x
Time until
maintenance
x Based
on actual
conditions
Time
author regrets to say that he has
only seen it carried out in pilot
plant operations or carefully supervised
plant runs. There seems to be
little time, manpower or more often,
operating budget available in industry
today to carry out monitoring of fouling
in heat exchangers, although one
wonders if the time and manpower
were made available in some systematic
fashion, what savings to the plant
operating budget would result?
In addition, imagine having the
Figure 6. Plotting the fouling factor over
time allows estimation of when a heat
exchanger requires maintenance
is in operation. This " ageing " effect
would not occur in the bulk, moving
fluid stream as it travels through the
heat exchanger passages. Ageing is
well described in the 2009 paper by
Wilson [9], although the authors of
that paper caution that the ageing
model described in their work is based
on simple assumptions and requires
more research.
The intermediate approach beability
to plan maintenance work on
key heat exchangers in advance of
critical failures or underperformance.
While one could argue that this is the
aim of preventive maintenance (PM)
outages, how many times is PM carried
out on a folklore basis without the
engineer knowing the fouled condition
of the heat exchanger? In other words,
the PM may have been performed prematurely,
incurring unnecessary expense
and downtime.
The alternative approach to PM
is maintenance after failure (MAF),
which is, unfortunately, the path most
often traveled in operating plants.
With MAF, production is interrupted,
resulting in downtime, disruption of
shipping schedules, unhappy customers,
possible overtime for cleaning
crews and the emergency acquisition
of spare parts or completely new heat
exchangers. None of these is desirable
nor reflects the most economical scenario.
One other little-considered factor
also occurs if MAF is the default
philosophy: The author has seen many
times the effect of allowing a fouling
deposit to " age " on the surface of a
plate-and-frame heat exchanger. Ageing
seems to convert the deposit into a
nastier, more resilient (hence, tougher
to remove) form. Ageing occurs because
the fouling layer is a static one,
susceptible to constant high (or low)
wall temperatures as well as possible
chemical and physical reactions over
the entire time the heat exchanger
40 ChemiCal engineering www.Che.Com june 2011
tween PM and MAF is perhaps best
described by Christian Kuhlmann's
writings collected by Muller-Steinhagen
[1]. Kuhlmann's paper offers
the concept of status-oriented maintenance
(SOM). In SOM, the condition
of the heat exchanger at any
given time is determined as a result
of calculating the U value of the heat
exchanger, and thus, monitoring its
thermal performance using the wellknown
equation:
(4)
Or, rearranged,
(5)
Where LMTD is the log mean temperature
difference.
Knowing the end point temperatures
of each fluid and their flowrates
allows Q to be calculated using
(6)
(the engineer should compare Qhot
and Qcold for accuracy). At the same
time, LMTD can be easily calculated
as well, allowing U to be determined
for that set of conditions. Knowing the
U value at any given time and knowing
the Uclean, which was either measured
upon startup with a clean heat
exchanger or validated through other
means, the fouling factor at that point
in time can be calculated simply by:
Fouling factor (Rf)
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Chemical Engineering June 2011

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