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Materials 4D03 Corrosion Report
12/04/14
Biomass Power Heat Exchanger
Process-Based Solution
Submitted by:
Jesse Carreau, 1219430
Carlos Orellana, 1213642
Eryk Taylor, 1220016
Patrick Fondevilla, 1213786
Submitted to:
Dr. Noël

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1.0 INTRODUCTION
1.1 Problem Statement
A biomass power heat exchanger has severely corroded after just six years of
operation and must be replaced. An investigation will be completed in efforts to forge a
process-based solution to understand why and how this happened and to significantly
reduce the amount of corrosion of a new heat exchanger.
1.2 Background
A heat exchanger is a device of varying size used frequently in industry as well as in
households. A heat exchanger can be found in anything from a fridge to a plane engine.
These practical machines are so widely used because it saves money by utilizing as much
heat as possible. They also transfer heat without interfering with a reaction or a process
because there is no transfer of fluid. Instead, there is a bypass of two fluids whose heat
will be changed by the other. In retrospect, a heat exchanger involves the transfer of heat
by introducing and running a contained fluid through some sort of network where another
fluid is present resulting in a desired change of temperature of both fluids. With respect to
this investigation, the heat exchanger involves the burning of biomass, which is just
recycled and waste wood [1].
Figure 1: Macroscopic inspection of the
corroded pipes.

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As it can be seen in Figure 1, air is driven through the inside of the above pipes,
under which the biomass is burned and the combustion gases are sent through a chimney
flue to contact the pipes. The material of the pipes and majority of the heat exchanger is
ASTM 513 MT-1010 carbon steel, which is a regular 1010 carbon steel mainly consisting
of iron, with the addition of manganese, phosphorus, sulphur and of course carbon. The
combustion gases within the chimney flue consist of CO2, H2O, O2, SOx, NOx and will be
considered throughout the investigation. As it can be seen in Figure 1, the corrosion at
the very beginning (within the first meter) is much worse than the rest and this will be the
focus point of the early investigation.
2.0 FORMS OF CORROSION
Many factors contribute to the corrosion of this carbon steel pipe. Since wood waste
materials are being burned to generate heat energy, several gases are released due to the
combustion reaction that is taking place. Specifically, CO2, H2O, O2, SOx, and NOx are
the main gases that are released. In general, the O2 can cause serious corrosion damage
by attaching to the walls of the pipe and forming oxides. Also, the H2O molecules can
attach to the pipes and cause further corrosion. Wear-related corrosion also contributes to
the depletion of the tube wall thickness. All metals contain some degree of surface
defects and can also contain pits. As the air that flows through the pipe in a laminar
fashion flows over these defects/pits, turbulent flow develops and causes further pitting
and corrosion. This is shown in Figure 2 below.
Figure 2: Development of turbulent flow,
causing further corrosion.

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These corrosion forms, however, would contribute to the corrosion along the entire 4
meters of the pipes. As one can see in Figure 3 above, the most significant corrosion
occurred within the first meter of the pipes and the final 3 meters were only depleted by
approximately 0.25 millimeters. Therefore, since the corrosion forms aforementioned
only contribute to the uniform corrosion of the pipes, a different form of corrosion must
have caused such significant damage in the first meter of the pipes. The surrounding
temperature is known to be 170°C and the inlet temperature of the combustion air is
known to be 37°C. As the flue gas rises over the pipes, the large temperature gradient
between the air in the pipes and the surrounding air produces condensation on the outside
of the pipe. The moisture is removed from the flue gas since it is cooling down when it
interacts with the surface of the pipes and this moisture is left on the surface of the pipes
in the form of condensation. As the air moves through the pipes and is heating up, a
smaller temperature gradient will exist between the air in the pipe and the flue gas and as
a result less condensation occurs in the rest of the pipe. As seen in Figure 3, the most
significant corrosion damage happens in the first meter of the pipes after which the air is
heated to the point where there is a smaller temperature gradient. This smaller
temperature gradient produces much condensation, leading to reduced corrosion damage
in the final 3 meters of the pipes.
Figure 3: Tube Wall Thickness vs. Distance from Tube Sheet

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In Figure 1 above, one can see how the corrosion took place on the bottom half of the
pipes, and not so much on the top half. This justifies why the likely mode of corrosion
was due to the condensation. The moisture would drip down the pipe due to gravity,
leading to corrosion on the bottom half of the pipe. The triangle surrounding the data
point in Figure 3 indicates the point at which the most significant corrosion damage
ends, which is precisely at the first meter mark of the pipes. After this point, more
uniform corrosion is present that is of much less significance. If the temperature at
precisely the first meter of the pipes can be calculated, then a plausible solution can be
implemented by pre-heating the air before entering the biomass power heat exchanger
pipes to that of the temperature at the first meter of the pipes. This will lead to much less
significant corrosion damage due to condensation and will extend the life of the pipes
greatly. The calculations and pre-heating method will be discussed in the following
section.
3.0 SOLUTION
In order to solve the corrosion in the carbon steel pipe, the condensation formed from
the contact of the hotter flue gas with the cooler air in the pipes must be prevented or
reduced. Completely preventing the formation of condensation is not feasible because hot
air must heat up the cold air for the heat exchange to work, therefore condensation will
always occur in the pipes. To account for this, the amount of condensation will be
reduced instead, and this process-based solution will reduce the amount of corrosion
forming on the outside of the pipe.
As discussed before, a significant amount of corrosion formed in the first meter of
pipe, so by calculating the temperature of the air at this point, the temperature that the
cooler air should be increased to in the solution will be known. A few assumptions were
made before the temperature was calculated. The velocity of air was assumed to be 20m/s
[2]. Using the range of diameters for ASTM 513 from ASTM International, the inner and
outer diameters were assumed to be 0.15m and 0.2m respectively [3]. Through
calculations shown after, it was determined that the length of the pipe in the heat

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exchanger should be approximately 6.48m. Finally, the properties for ASTM 513 MT-
1010 Carbon Steel were assumed to be similar to AISI 1010 Carbon Steel [4].
To reduce condensation, the temperature of the cooler air will be increased so that the
difference in temperature of the flue gas and pipe air will be decreased. A T6 aluminum
alloy concentric pipe will be installed at the inlet of the carbon steel pipe. T6 aluminum is
a cheap metal, priced at approximately $6/cm from a metal supplier, Metal Supermarkets.
It is also an effective metal because it has a high thermal conductivity (177 W/m*K) [2],
which measures how easily heat can travel through material.
In Figure 4, the lighter region represents the T6 aluminum alloy concentric pipe and
the darker area is the carbon steel pipe. Steam at 100ºC will flow through the concentric
pipe and heat up the cooler air flowing through the carbon steel pipe and heat it from
37ºC to approximately 90ºC. The steam flow will run parallel to the cooler air flow. By
decreasing the temperature gradient of the flue gas and the pipe, the amount of corrosion
formed in the pipe should decrease.
4.0 CONCLUSION
It was determined that in order to reduce as much oxidation as possible, attention
was focused on the corrosion being caused by the precipitation. In order to do this the air
must be pre-heated to decrease the temperature difference. Therefore the suggested
solution is to install an inexpensive heat exchanger to warm the air entering the biomass
heat exchanger and this will decrease the amount of corrosion present.
Figure 4: Concentric pipe, parallel flow [3]

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5.0 REFERENCES
[1] Woodford, C. (2009) Heat exchangers. Retrieved on Nov. 29 from
http://www.explainthatstuff.com/how-heat-exchangers-work.html.
[2] Fluid Velocities in Pipes. (n.d.). Retrieved November 30, 2014, from
http://www.engineeringtoolbox.com/fluid-velocities-pipes-d_1885.html
[3] Standard Specification for Electric Resistance Welded Carbon and Alloy Steel
Mechanical Tubing. (n.d.). Retrieved November 30, 2014, from
http://www.astm.org/Standards/A513.htm
[4] Incropera, F. (2007). Ch 11 Heat Exchangers, Appendix A. In Fundamentals of
Heat and Mass Transfer (6th ed.). Hoboken, NJ: John Wiley.

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6.0 Appendix

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