One of the most common
operational challenges encountered with heat exchangers is fouling.
Fouling is the buildup of sediments and debris on the surface area of a
heat exchanger that inhibits heat transfer. Fouling will reduce heat
transfer, impede fluid flow, and increase the pressure drop across the
heat exchanger. As with many operational concerns, proper planning at
the design stage can minimize the effects of fouling down the road.
Designers use fouling factors
to maximize the lifespan, runtime and efficiency of a heat exchanger by
accounting for the amount of fouling an exchanger will sustain over a
period of time. This often results in many
applications, such as increasing the surface area of a heat exchanger,
so that fouling will not have as much of an effect. Inefineries, heat
exchangers will have to perform for several years without a cleaning.
This means that the heat exchanger must be able to function efficiently
for long periods of time. Compensating for fouling by enlarging surface
area allows heat exchangers to function with years of fouling.
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Types of Fouling
There are several types of fouling, each
forming depending on the type of fluid and conditions. The following are
some of the more common fouling mechanisms;
Crystallization
is one of the most common type of fouling.
Certain salts commonly present in natural waters have a lower solubility
in warm water than cold. Therefore, when cooling water is heated during
the cooling process (particularly at the tube wall) these dissolved
salts will crystallize on the surface in the form of scale. [Common
Solution: reducing the temperature of the heat transfer surface often
softens the deposits]
Sedimentation, the depositing of dirt,
sand, rust, and other small matter is also common when fresh water is
used. This can be controlled to a degree by the heat exchanger design.
[Common Solution: velocity control]
Biological Organic Growth material
occurs from chemical reactions, and can cause considerable damage
when built up. [Common Solution: material selection]
Chemical Reaction Coking
appears where hydrocarbon deposits in a high
temperature application. [Common Solution: reducing the temperature
between the fluid and the heat transfer surface]
Corrosion
can destroy surface areas of the heat exchangers, creating costly damage. Fouling will slow
down heat transfer and damage equipment unless it is dealt with accordingly. [Common Solution: material selection]
Freezing Fouling
results from overcooling at the heat transfer surface causing solidification of some of the fluid stream components. [Common Solution: reducing the temperature gradient between the fluid and the heat transfer surface.]
Fouling factor
The most common way to account for the effects of fouling in a tubular heat exchanger is the application of a fouling factor.
The fouling factor is a predetermined number that represents the amount
of fouling a particular heat exchanger transferring a particular fluid
will sustain. In the heat transfer equation the fouling factor is added
to the other thermal resistances to calculate the Total Thermal Resistance which is the reciprocal of U clean.
There is no direct calculation to determine the appropriate fouling
factor to use for a given fluid in a particular application, however
guidelines do exist to help determine an appropriate fouling factor. The
most common compilation of fouling factors, to be used for a variety of
fluid in various applications, is supplied by Tubular Exchanger
Manufacturers Association (TEMA). The below table is a list of general
fouling factors used for shell and tube heat exchangers and common
fluids and applications.
Fluid
|
Fouling Resistance (ft2-
°
F-hr/BTU)
|
Transformer Oil
|
0.001
|
Steam
|
0.0005
|
Compressed Air
|
0.001
|
Hydraulic Fluid
|
0.001
|
Glycol Solutions
|
0.002
|
Refined Lube Oil
|
0.001
|
Sea Water
|
0.0005 (up to 125
°
F) 0.001 (over 125
°
F)
|
Cooling Tower Water
|
0.001 (up to 125
°
F) 0.002 (over 125
°
F)
|
River Water (minimum) (tube velocity
#
3 fps)
|
0.002 (up to 125
°
F) 0.003 (over 125
°
F)
|
River Water (minimum) (tube velocity > 3 fps)
|
0.001 (up to 125
°
F) 0.002 (over 125
°
F)
|
River Water (average) (tube velocity
#
3 fps)
|
0.003 (up to 125
°
F) 0.004 (over 125
°
F)
|
River Water (average) (tube velocity > 3 fps)
|
0.002 (up to 125
°
F) 0.003 (over 125
°
F)
|
River Water (muddy or silty) (tube velocity
#
3 fps)
|
0.003 (up to 125
°
F) 0.004 (over 125
°
F)
|
River Water (muddy or silty) (tube velocity > 3 fps)
|
0.002 (up to 125
°
F) 0.003 (over 125
°
F)
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Fouling and Plate Heat Exchangers
The manner in which fouling and fouling factors
apply to plate exchangers is different from tubular heat exchangers.
There is a high degree of turbulence in a plate heat exchanger which
increases the rate of foulant removal and, in effect, make the plate
heat exchanger less prone to fouling. In addition, there is a more
uniform velocity profile in a plate heat exchanger than in most shell
and tube heat exchanger designs eliminating zones of low velocity which
are particularly prone to fouling.
Plate heat exchangers typically have a higher
U factor than shell and tube heat exchangers and often significantly
higher. It was previously described that in the heat transfer equation
the Total Thermal Resistance is the reciprocal of U clean,
therefore the Total Thermal Resistance in a plate heat exchanger is
often significantly less than the same application of a shell and tube
heat exchanger. Applying a typical fouling factor developed for a shell
and tube heat exchanger to a plate and frame design therefore will have a
greater proportional effect on the U factor resulting in a greater
overdesign of the exchanger. On the downside, due to the higher U factor
and lower surface area, the development of fouling affects plate heat
exchangers more significantly so controlling the development of fouling
is very important in plate heat exchangers.
In most plate heat exchanger applications
specifying a percent of excess surface is more practical than specifying
the TEMA fouling factors described above.
Minimizing fouling
Fouling depends on the type of heat exchanger,
and the kind of fluids being transferred. Due to different designs,
composition, and transfer fluid, each type of heat exchanger will suffer
fouling in unique ways. The tube side of a shell and tube heat
exchanger is usually easy to clean but the shell side can be more
difficult to access. Plate heat
exchangers can be taken apart for cleaning on both sides. Some heat
exchangers can be cleaned every night when the equipment is not in use,
while others can only be cleaned every few months or years. In order to
reduce the amount of fouling in a heat exchanger, equipment should be
cleaned as often as possible.
If a plate heat exchanger were to suffer from
the effects of fouling, extra plates can be added to re-gain
performance if the space permits in the frame.
Design Considerations to Decrease Effects of Fouling
There are a number of accommodations a
designer may use once they have figured out how much fouling to expect
in a particular unit.
-
A high level of turbulence keeps sediments from
settling on the surface of the heat exchanger, and also helps clean off
any fouling so it is important to ensure that the design velocities are
high enough to mitigate fouling but not too high to promote erosion.
-
Try to keep a uniformly high velocity throughout
the entire exchanger, so that sediments are not able to settle. Keep the
amount of low velocity turns and ‘dead’ spots to a minimum, so that
fouling will not accumulate.
-
Consider how often the unit needs to be cleaned, and provide easy access to make this process easier.
-
For a plate heat exchanger, select a frame size
that will accommodate additional plates in the case that more surface is
needed because of a loss of performance due to the effects of fouling.
Excessive Fouling Factors
It seems that proper planning for the future
and good design practice would result in the specification of a higher
than required fouling factor for safety sake. Anticipating fouling is
good practice, over-designing significantly however can actually
facilitate fouling. Specifying
too large a fouling factor will often result in more flow area that can
result in lower velocities in the exchanger and actually promote
fouling. Another risk is that an oversized exchanger operating clean
will overperform and a possible reaction would be to reduce the cooling
water flow which would reduce the velocity - promoting fouling. It is
important with all heat exchangers to operate as close to the design
flow conditions as possible.
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