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Unit 4 Dirt Factor

This document describes the different types of fouling that can occur in heat exchangers, including chemical, biological, deposition, and corrosion fouling. It also discusses how fouling factors affect the design and performance of heat exchangers, as well as design and cleaning considerations to minimize fouling.

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12 views4 pages

Unit 4 Dirt Factor

This document describes the different types of fouling that can occur in heat exchangers, including chemical, biological, deposition, and corrosion fouling. It also discusses how fouling factors affect the design and performance of heat exchangers, as well as design and cleaning considerations to minimize fouling.

Uploaded by

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© © All Rights Reserved
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Unit4

Factors of fouling in exchangers


heat

These are normally provided by the client based on their


experience with the operation of your plant in production or process,
but if these are not defined or delimited appropriately within
certain levels can make the engineer's work completely useless due to
very skilled in the design of the exchanger. They represent the
theoretical resistance to the flow of heat due to the accumulation of a layer
of dirt or any other substance on one or both sides of the
surface of the tube, but they are often 'thickened' by the end user in
an attempt to minimize the frequency of cleaning stops for the
exchanger. In fact, this practice can, if not selected
correctly, lead to a higher frequency of stops for
cleaning.

The mechanisms by which soiling occurs vary with


the application but can be broadly classified into four types
clearly identifiable.

Types of pollution

. Chemical contamination, in which chemical changes occur in the fluid


causing a layer of soiling to be deposited on the
surface (internal or external) of the tubes. A common example of
this phenomenon is the expansion in a pot or boiler caused by the
calcium salt deposition in the heating elements
as the solubility of salts decreases with the increase in
temperature. This type is out of the designer's control.
heat exchangers but it can be minimized by controlling
Carefully the temperature of the tube in contact with the fluid.
When this type of soiling occurs, it is usually
removed by chemical treatment or mechanical processes
steel brushes, drills or even high-pressure water guns
in some cases).
. Biological fouling, caused by the growth of
organisms in the fluid that settle on the surface. This type
it is also beyond the control of the designer of the exchanger but
it can be influenced by the choice of materials since
some, notably non-ferrous brass, are poisonous to
some organisms. When this type of fouling occurs
it is usually removed through chemical treatment or processes
abrasive mechanics.
. Deposition-induced fouling, where particles in the fluid
they accumulate on the surface when the velocity falls below
certain critical level. This is largely under the control of
designer since the critical speed of any combination
fluid/particle can be calculated to allow for a design in the
that the minimum speed should always be greater than the critical one. Mount the
vertically mounted heat exchanger can also minimize the
effects since gravity tends to pull the particles out of the
exchanger outside the heat exchange surface.
When this type of soiling occurs, it is usually
removed through mechanical brushing processes.
. Contamination due to corrosion, in which a layer resulting from the
corrosion accumulates on the surface of the tube, forming a layer
extra, usually made of a material with a high level of resistance
thermal. Through the appropriate selection of materials
construction the effects can be minimized since there is a
manufacturer's arrangement of exchangers a wide range of
corrosion-resistant materials based on stainless steel.

Corrugated tubes

The use of corrugated tubes has proven beneficial for


minimize the effects of at least two of these mechanisms:
emptiness due to deposit, because a higher level of turbulence is
generated at a lower speed and chemical fouling, because the
improvement in the thermal exchange coefficients when there is
corrugation in the tube results in tube wall temperatures
closer to the temperature of the bulk of the fluid. More details of
you areadvantagesare available for more information.

Identifcationofthefoulingfactor

The fouling factors provide a numerical value, which


reflects the resistance to heat transfer between fluid media. For example,
an increase in the resistance of a particular fluid reflects a value
of the greater fouling factor, such as 0.001 ft2-°F-hr/BTU, or
resistance to soiling. Normally, designers of
heat exchangers compensate for the fouling factors
higher increasing the surface area of the same. Although still
it will cause contamination, there is more surface area for exchange
functional of heat, continuing with the appropriate operations.

Causesofthefoulingfactor

Many causes contribute to the dirtiness within a


heat exchanger. The chemical reactions within the fluid that
runs produce deposits that adhere to the surface of the
exchanger. Internal corrosion can occur if the exchanger
of heat is not made of a material resistant to it, such
like stainless steel. The deposit of dirt materializes
when the fluid flowing does not reach the correct speed inside the
heat exchanger. The material settles outside the liquid on the
interior surfaces, forming a layer of sediment. The organisms
that can grow within the fluid create biological fouling,
that eventually binds to the surface. In addition, crystallization
produced when the salts dissolve in the fluid reservoir along
of the surface of the heat exchanger.

Influencesofthefoulingfactor

Other physical elements influence the values of the factor of


emptiness. The temperature of the wall of the exchanger, as well as
the fluid, contribute to the soiling. Excessive temperatures,
whether from the wall or the fluid, they create more resistance to the exchange of
heat, which hinders the process of heat transfer in general.
The fluid velocity must remain within the specification of the
manufacturer to prevent sediment deposition along the
surface of the exchanger.
Considerationsforcleaning

Heat exchangers that use tubular bundles are more


more difficult to clean than those that are shaped like plates. The uses
industries of heat exchangers, such as refineries,
They benefit from the use of plate heat exchangers. It is
it is possible to clean this type of exchangers daily,
providing the highest production efficiency. The industries that
they use tubular heat exchangers can only clean the
sets every few months, or even once a year, possibly
they malfunction between cleanings due to excessive dirt
internal.

Designproblems

Although increasing the surface area helps the heat exchanger


heat works for a longer time with a bit of dirt, the
excessively large surfaces of heat exchangers
the fluid speed decreases. As a result, the deposits dissolve the
fluid and contribute to a greater contamination than that of a
smaller surface heat exchanger.

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