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Fouling of an air-cooled heat exchanger, and alternative design approach.
Conference Paper · December 2009
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Fouling of an air cooled heat exchanger; an alternative design approach
HJ Nel a, IF Lombaard b, L Liebenberg a, JP Meyer a*
a
Department of Mechanical and Aeronautical Engineering, University of Pretoria, b Sasol Technology, South Africa
*
Author for correspondence (jmeyer@up.ac.za)
Keywords: Air cooled heat exchangers, fouling, design procedures, modelling
ABSTRACT: Air cooled heat exchangers (ACHEs) are an important part of industrial heat transfer systems,
especially where clean cooling water is scarce. Unfortunately, fouling of ACHEs is a serious problem since
the quality of air cannot practically be controlled. To account for this at present it is standard practice to design an ACHE with fouling factors, which increase the required unit size. These fouling factors are essentially
additional conductive resistances meant to model the insulating effects that the fouling layer introduces.
ACHEs designed in this manner will initially over perform, but the performance will decrease over time often
to levels far below design conditions. The aim of this paper, however, is to demonstrate that air-side fouling
will affect performance primarily by decreasing the air-side mass flow rate due to an increased air-side pressure drop, and not principally by increasing the conductive resistances as implied by the constant fouling factor method. This will be shown by assuming a uniform fouling layer throughout an ACHE which over time
increases in thickness, and by using existing pressure drop and heat transfer coefficient correlations as well as
a fan curve to simulate the change in the unit’s performance. It will then be shown that performance is affected most significantly by the increase in air-side pressure drop and related decreased air-side mass flow
rate. Finally, a new design procedure will be outlined that should help ACHEs to perform more consistently
throughout their lifetime.
1
INTRODUCTION
In the current climate of resource management
and energy efficiency it is becoming increasingly
important to manage available resources, especially
when these resources are in short supply, such as the
supply of clean water in South Africa. This makes
dry cooling air-cooled heat exchangers (ACHE’s) a
very attractive option since they use freely available
atmospheric air as the cooling medium. Using atmospheric air has its price, however, namely that the
quality and cleanliness of the air cannot easily be
guaranteed. This causes the ACHE’s to foul over
time and to lose performance. Unfortunately, the
performance levels often drop below the fouled design conditions despite being designed according to
widely acceptable industry standards (e.g. TEMA).
At present the standard practice for fouled heat
exchanger design is to add two additional and constant fouling factors which are based on the process
fluids used. This technique is over 50 years old and
is described in any classic heat transfer text, such as
Çengel (2006). The main source of values for the
fouling factors is the Standards of the Tubular Ex-
changer Manufacturers Association (TEMA),
(2007).
This method effectively increases the required
size of the unit in order to transfer the design thermal load under fouled conditions. Unfortunately
fouling is not present at the beginning of the unit’s
lifetime and consequently the unit will overperform.
The unit’s performance will then also decrease over
time, and, if left uncleaned, will eventually perform
substantially below design conditions.
One of the main assumptions of the constant fouling factor method is that it assumes conductive resistances are the only change to the unit, and that no
changes in flow rates or heat transfer coefficients are
expected. This is contrary to experimental measurements. Shah (2005) reports experimental measurements by Bemrose and Bott on fouling of ACHE’s
using calcium carbonate particles. The Colburn-j
factor for these tests decreased slightly, while the
friction factor increased by 40-150%. Shah (2005)
also gives Marner’s results of the fouling of a plate
fin ACHE using the exhaust gasses from a turbine
combusting No. 2 diesel fuel. The tests were run for
eight hours and showed fouling layer thicknesses between 50 and 200 micrometers. The heat exchanger
effectiveness decreased only 3% while mass flow
reductions between 6-11% were measured as a result
of the increased air-side pressure drop caused by the
fouling layer. This clearly shows that the assumption
of constant flow rates is flawed.
Probably the main reason that the constant fouling factor approach has remained so widely used,
despite its flaws, is that the actual process of fouling
is exceedingly difficult to model accurately. Bott
(1995) and Müller-Steinhagen (2000) list some of
the models for the various fouling mechanisms, such
as crystallisation, deposition, etc. Unfortunately,
these models cannot account for the interactions between the various fouling mechanisms and they require a great deal of empirical constants and
contaminant concentration information that are not
available a priori to the designer. This then also
means that accurate modelling of the fouling process
in the design phase is not practical, and has led to
the relatively simple constant fouling factor approach remaining the preferred method.
Nevertheless, even though detailed modelling of
the fouling process is not feasible, it is proposed
here that the constant fouling factor assumption (particularly with respect to regarding no change in the
flow rate), is fundamentally flawed. Below it will also be shown graphically to what degree the decrease
in air-side flow rate will eventually cause performance losses greater than would be expected from the
use of the TEMA (2007) fouling factors. Therefore,
the aim of this paper is to present an improved design approach that can account for ACHE air side
fouling. This method will then more accurately
model the primary causes of reduced performance
than the constant fouling factor method, and result in
more consistent heat exchanger performance over
any given period.
2
THEORETICAL ANALYSIS
The first step used here in analysing the performance of a fouled ACHE is the identification of an
appropriate fouling model. Modelling the depositional process itself is an incredibly complex task
and relies on so many parameters that it cannot be
readily performed. A simpler and more usable
scheme will be to assume that the fouling layer
thickens uniformly throughout the bundle. This is
modelled by uniformly increasing the outer diameter
of the tubes and the thickness of the fins with a layer
of fouling. This scheme is simple and crude but has
the advantage that, for thin layer thicknesses, the
pressure drop and heat transfer coefficient correlations that were originally used to design the bundle
can still be used to infer what operational changes
the fouling layer will bring about.
Thus, this scheme was implemented by first taking the design parameters of an ACHE test section
(c.f. Table 1) and feeding those parameters into the
Briggs and Robertson (1966) pressure drop correlation, as well as the Briggs and Young (1963) and
Ganguli (1998) heat transfer coefficient correlations.
The next step was to model the fouling layer by incrementally increasing the outer tube diameter and
fin thickness. The new parameters were passed to
the correlations to determine what performance
trends could be expected. The first, and most important trend, was that the air-side pressure drop increased much faster than was initially anticipated.
Figure 1 shows pressure drop curves for the unit
with several different fouling layer thicknesses as
well as the fan curve of a typical axial fan that would
be used with this unit.
It is apparent that the fouling layer has a negative
effect on the pressure drop characteristics of the unit,
which is in line with other measurements reported
by Bott (1995). However, the most important effect
is only seen once the fan curve is superimposed onto
the pressure drop curves (the intersection of a pressure drop curve and the fan curve denotes the operating point of the unit). The effect of the fouling layer
is to move the operating point of the unit to lower
face velocities, and consequently lower mass flow
rates and higher pressure drops.
Table 1. Geometrical and Thermal factors for test section
Factor
Value
Fin Material
Aluminium, k = 204 W/m K
Outer Fin Diameter
58 mm
Fin Root Diameter
25.4 mm
Fin Shape
G-Fin,
Mean Fin Thickness
0.46 mm
Fin Pitch
2.54 mm
Tube Material
Steel, k = 50 W/m K
Tube Outer Diameter
25.4 mm
Tube Inner Diameter
19.86 mm
Tube Arrangement
Staggered
Number of Tube Rows
6
Number of Tubes per row
Actual
13
Effective
12.5
Transverse Tube Pitch
60 mm
Longitudinal Tube Pitch
52 mm
Length of Finned Tube
0.75 m
Air Inlet Temperature
18°C
Air Inlet Pressure
101 kPa
Water Inlet Temperature
60°C
Water Flow Rate
3.9 kg/s
Next, once the various operating points were
identified the performance of the unit at each point
was modelled by using the Briggs and Young and
Ganguli correlations to estimate the heat transfer coefficients for the modified geometry at the appropriate flow rate. The thermal insulating effect of the
fouling was taken into account by adding a cylindrical thermal resistance to the bare tube surfaces. A
thermal conductivity of the fouling layer was as-
sumed at 1 W/m K, which is an acceptable average
value, based upon data presented by MüllerSteinhagen (2000). The fin efficiency was also modified appropriately based upon work presented by
Kröger (1998). The in-tube conditions were held
constant for all operating conditions. All this information was used to simulate the unit’s performance
using a simple effectiveness–Number of Transfer
Units model, Çengel (2006), and this was compared
to the decrease in performance predicted by a
TEMA (2007) type air-side fouling resistance of
0.00018 m2K/W. The results in Figure 2 then illustrate the problem with simply using a constant fouling factor. The constant fouling factor method
predicts a performance loss of less than 5%, while
the uniform fouling thickness approach shows that a
layer of only 50 micrometers would have the same
effect, and that any further increase would cause
significant additional performance losses.
This again demonstrates the problem of using a
constant thermal resistance to account for fouling.
Initially the unit will overperform, but this performance will decrease with time continually to values
far below design conditions. Furthermore, since
most industrial processes require stable performance, both over- and underperformance are undesirable. The only way to overcome this while using the
constant fouling factor approach is to adjust the fanmotor system to give the appropriate flow rate. The
constant fouling factor approach will oversize the
unit initially, but this does nothing to counteract the
actual fouling mechanism over time.
Figure 3. Decrease in maximum heat transfer
There is one more parameter that needs to be investigated if a more complete understanding of the
implications of air side fouling is required. This parameter is the maximum heat transfer of a unit. The
maximum heat transfer of a unit is the amount of
heat an infinitely large unit can transfer, subject to
the fluids flow rates, specific heats and inlet temperatures. It is defined by Çengel (2006) as:
(1)
In the case of ACHE’s the air side should almost
always have the lowest product of mass flow rate
and specific heat due to air’s low density and specific heat. It has already been shown that the air side
fouling will decrease the air side mass flow rate and
the effect of this on the unit’s maximum heat transfer is shown in Figure 3. This shows that the effect
of fouling is to reduce the maximum heat transfer
the unit is capable of. The fouling factor approach,
or any other approach that modifies the unit’s area,
such as the cleanliness factor approach, attempts to
add area to combat the loss in performance but Figure 3 shows that fouling decreases a parameter that
is independent of area. While the addition of area
can increase the heat transfer, it does nothing to address the underlying problem.
3
EXPERIMENTAL RESULTS
Figure 1. Fouling-affected operating points
Figure 2. Fouling-affected heat transfer
With regards to suggested methodology the biggest problem is that it is well known that ACHE’s do
not foul uniformly. The unit usually experiences
heavy fouling on the first row and less fouling
downstream through the bundle, although in some
cases the amount of fouling does start to increase as
the exit side of the bundle is reached. This means
that the uniform fouling model cannot be used to
give accurate performance predictions.
In an attempt to test the validity of the trends predicted by the uniform fouling theory several test sections of 0.75 m by 0.75 m were prepared from
severely fouled units. Unfortunately these test sec-
tions lost virtually all of the fouling due to sensitivity during handling and transport. This made a controlled laboratory investigation impossible.
The alternative was to performance test a fouled
unit in the field, which was done on a Sasol ACHE
in December 2008. The unit was heavily fouled with
several areas on the front face being completely
blocked. The approach taken was that since the increase in pressure drop could not accurately be
measured, the face velocity at the exit would be
measured at 140 points using a calibrated anemometer. The unit was then professionally cleaned by
GEA Nilenca, a heat exchanger servicing company,
and the measurements were taken again at the same
points.
The unit has a design face velocity of 3.42 m/s;
prior to cleaning the unit the average face velocity
was 1.66 m/s, 49% of the rated value. Furthermore
the unit removed 13.5 MW of heat (with six of the
20 fans inoperative for servicing) before cleaning.
After cleaning seven fans were inoperative, but the
face velocity had increased to 4.2 m/s, 122% of the
rated value, and the heat removal had increased to
22.4 MW, an increase of 66%. The reason the face
velocity was higher than rated is due to the fact that
the plant engineers had increased the fan’s blade angles over time to counteract the declining performance of the unit.
This shows that the fouling had caused a significant increase in the air-side pressure drop which led
to the significant decrease in air-side face velocities,
as predicted by the uniform fouling approach.
4
ALTERNATIVE DESIGN PROCEDURE
It is proposed that the constant fouling factor approach is fundamentally flawed, regardless of the
fact that designers do not currently have access to
acceptable alternative methods. The authors would
like to propose a new design procedure as well as
several suggestions regarding the design of ACHE’s.
The first step toward designing a fouling resistant
ACHE is to consider the environment in which the
unit will operate and trying to eliminate or manage
the fouling. Kröger (1998) gives some examples of
typical fouling sources and some methods to control
this. This approach can be extremely effective if a
particular source is responsible for the majority of
the fouling. However, it is not possible to foresee all
possible fouling sources, and if the sources are identified it is not always possible to eliminate them. As
a general rule the authors would advise designers to
avoid very fine tube and fin pitches as these would
cause higher baseline pressure drops that would
make it easier for the unit to collect fouling. Bott
(1995) and Müller-Steinhagen (2000) suggest that,
for heat exchangers in general, fluid velocities be
kept as high as possible as this will decrease the
amount of deposition that occurs.
Next an alternative design approach to the constant fouling factor method can be implemented as
described here. The fouling factor method leads to
the oversizing and the subsequent initial overperformance of the unit. One way to deal with this situation is to adjust the fan-motor system so that the
flow rate is correctly modified. Once the unit’s performance has decreased below the design conditions,
the operators will slowly increase the blade angle
over time to combat the pressure drop increase.
This procedure works until the operators have
used the entire pressure margin available, after
which the fan will stall and the unit would not function effectively. The authors would like to suggest
that the air-side fouling factor be neglected entirely
and that the fan be designed for an increased pressure drop,
, which would be the baseline
, plus an additional percentage
pressure drop,
of the baseline pressure drop, (Equation 2). This
would lead to the scenario where at clean conditions
the fan would supply more flow than required. This
would require that, as with the fouling factor method, the blade angles be turned down during clean
service and slowly increased as the unit fouls. An alternative to constantly adjusting the blade angles is
to use a variable speed drive to adjust the fan speed
to ensure the correct flow rate. Either of these two
methods allows the operators to adjust the flow and
it is up to the designer to decide which method is
more appropriate for each situation.
(2)
The advantage of this method is that the fan
would have a larger available pressure margin to
deal with the fouling-induced pressure drop increase,
and that the unit will be able to operate at design
conditions for longer than a unit designed using the
constant fouling factor approach. Hence, this results
in a more efficient design. This method does require
a larger fan and motor system but does not oversize
the physical heat exchanger unit. The designer therefore needs to perform a full economic investigation
which evaluates the cost implications of both approaches, the production losses that would result if
the unit underperforms (primarily in the fouling factor method), required cleaning intervals and cost as
well all other cost implications for the plant and unit
design before a decision is made on which method to
use.
At this stage the authors can make no concrete
recommendations regarding values of , however, a
tentative suggestion is that values between 0.1 and
0.2 should be used depending on the level of fouling
expected and the time between cleaning. Many more
measurements are required to make this method accurate and efficient.
The most important aspect that designers need to
be made aware of is that no matter what method is
used to account for fouling, the unit will still foul.
This means that eventually the unit’s performance
will decrease past acceptable limits. This further implies that regular cleaning of the unit will be required no matter what fouling approach is used and
that cleaning of the unit should be built into the lifecycle costing of the unit. The authors therefore recommend that the designers evaluate the importance
of the unit, the environment in which the unit will
operate, and the service intervals for the unit. Based
upon that information the appropriate design procedure should be used that will allow the unit to perform at the correct levels in the service interval.
Prior to servicing the unit should be inspected to determine the level of cleaning required and the cleaning can then be performed during the service interval
to minimise its effect on the process. Designers
should also be aware that even though cleaning can
be expensive, regular preventative cleaning will ensure minimal production losses and increase the lifetime of the unit, which should outweigh the cleaning
costs.
5
CONCLUSION
This paper presents an alternative to the current
practice of adding a TEMA type fouling factor to the
design of an ACHE. It has been shown through the
uniform fouling assumption that air-side fouling will
cause a dramatic reduction in the air-side mass flow
rate due to the increased air-side pressure drop. This
is in contrast with the fouling factor method which
assumes that the flow rate remains constant. Proof of
the reduction in flow rate was presented in the form
of pre- and post cleaning air-side face velocity measurements. Finally, a new design procedure has been
outlined that should help ACHE’s to perform more
consistently throughout their lifetime, as long as
they are subjected to regular cleaning.
ACKNOWLEDGEMENTS
This project was a final year capstone project at
the University of Pretoria in the Department of Mechanical and Aeronautical Engineering and was executed by the first author. This project was made
possible through the assistance of several individuals
and companies:
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 Prof. D.G. Kröger of the University of Stellenbosch for his invaluable assistance and
experience with ACHE’s and their design
 Mr Danie Nell of GEA Nilenca for the access
to the cleaning process and measurements
and as well as all the other invaluable support which made this research possible
 Mr Cobus Zietsman and the rest of the laboratory staff at the University of Stellenbosch
 Mr Warwick Hayes of Sasol for initiating the
cleaning project and assisting with the measurements
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Tubular Exchanger Manufacturers Association,
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Shah R.K. and Sekulic D.P, 2003, Fundamentals
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Bott T.R, 1995, Fouling of Heat Exchangers, Elsevier.
Müller-Steinhagen H, 2000, Heat Exchanger
Fouling: Mitigation and Cleaning Technologies,
IChemE.
Briggs D.E and Robinson K.K, 1966, Pressure
Drop of Air Flowing Across Triangular Pitch
Banks of Finned Tubes, Chemical Engineering
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and Cooling Towers, University of Stellenbosch.