materials selection 2 (mate sophomore series)

8/3/2019 Materials Selection 2 (Mate Sophomore Series)

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Team Mate

Dillon Lynch

Austin Schader

Chris Riley

5/22/10

Project #2

Materials Selection for a

Heat Exchanger



Summary:

During the materials selection, we sought a material for use in the interior tubes of a

heat exchanger. In order to optimize performance for this given application, materials of

low cost, high yield strength and high thermal conductivity were primarily considered.

However, other properties played a large role in our final choice of material. Materials

featuring a leak before break failure mechanism, corrosion resistance, a minimum

service temperature of 150˚ C and a ductility near 30% were desirable candidates.

In the selection process, we chose to maximize thermal conductivity and yield

strength while minimizing cost. After narrowing our results with the Cambridge

Engineering Selector (CES) and analyzing the top candidates with a Weighted

Performance Index, we determined 4037 low alloy steel to be the best material for the

interior tubes of the heat exchanger.

Needs Statement:

In the design of a heat exchanger, a material was needed for the interior tubes. The

chosen material must effectively transfer heat across the interface while maintaining its

structural integrity under applied stress. It should also be of low cost.

and does not fail due to the pressure difference across the interface and if it were to fail

it should leak before breaking. Additionally the material must have a minimum maximum

service temperature of 150˚, acceptable corrosion resistance to freshwater, minimum

ductility of 30%, and be formable into a tube shape.



Functional Statement:

The chosen material will serve as tubing. As such, it must be able to conduct heat from

hot steam to cold water within the heat exchanger. It must also be strong enough to

maintain its structural stability under intense pressure differences.

Objectives:

Our objectives for the material selection are to maximize thermal conductivity and yield

strength while minimizing cost. In order to account for this, our material analysis utilized

a property index that reflects these properties.

Constraints:

In addition to our several objectives, we have placed a number of appropriate constrains

upon the design. In order for the tubes to function properly, the material used should

have a minimum service temperature of 150˚ Celsius, offer an acceptable resistance to

freshwater corrosion, have a ductility of around 30% and be able to be formed into the

desired tube shape.

Discussion:

During our CES analysis, we chose to use multiple objectives in order to properly

evaluate each potential material for the heat exchanger tubing. First, we chose to limit

the shape of the design to a hollow asymmetrical cylinder by using the tree selection in



CES. This eliminated approximately 500 materials from our selection that could not be

formed into the desired shape of the tubing. We then chose to plot thermal conductivity

and cost on the Y-axis with leak before break on the X-axis. The resulting indices were

(Thermal conductivity * Yield Strength)/ Price on the Y-axis and (Fracture Toughness *

Fracture Toughness)/ Yield Strength on the X-axis. These two performance indices

served to show the materials with optimal heat transference across the device, minimal

cost and a maximum leak before break failure rating.

After constructing the plot of maximum thermal conductivity, minimum cost and

maximum leak before break rating, we then chose to limit our results to those that had

an acceptable rating for use with freshwater. This quality is relevant to the heat

exchanger tubing as water will flow within it. We then applied a limit to the minimum

service temperature (150˚ C) to our graph and a to ductility (30%). Together, these

criteria limited our results to 60 potential materials.

After applying the mentioned property limits, we constructed a Weighted

Performance Index (WPI). We chose to use the ratio of thermal conductivity and leak

before break values to create a line on our plot. This allowed us to chose the top six

materials for the application with respect to the graphed properties. The line was then

moved upward until only the top materials were represented on the graph. These

materials were then evaluated in the WPI.

Weighted Property Index:

In order to finalize the materials selection, weighting factors were assigned to each of

the properties considered. Thermal Conductivity * Yield strength)/Price was the first



weighting factor used and was imported from CES. This property was derived from the

function and objectives of the heat exchanger. The next weighting factor was leak

before break. This data was also created using CES while its weighting factor was

derived from the considered functions and objectives. The following two weighting

factors, minimum service temperature and corrosion resistance, were derived from the

constraints placed upon the heat exchanger tubing material. Minimum service

temperature was taken into consideration due to the necessity of the interior tubing to

operate at a high temperature. However, it would be beneficial to the heat exchanger's

performance for the tubing to have a minimum service temperature appreciably greater

than that of the limit. Corrosion resistance was an important factor to take into account

as water would be flowing through the tubing. If the chosen material was susceptible to

corrosion, it would not prove an effective chose for our application. Thankfully, all of our

top materials met our corrosion resistance constraint at acceptable levels. Because of

this, corrosion resistance was given a lowered weighting factor.

Results and Sensitivity Analysis:

According to the WPI, Low Alloy Steel AISI 4037, tempered at 540 °C & Oil quenched

(referred to as 4037), is the optimal material for the heat exchanger tubing. This is

mostly due to the fact that it had the highest value for Thermal Conductivity * Yield

strength)/Price of the six analyzed materials. Thermal Conductivity * Yield strength)/

Price was the highest rated factor in the WPI. Thus, it had much influence over our final

material selection. All six materials met our standards for corrosion. Because of this,

each material scored the same value in the WPI and had no overall effect in the making



of our decision. Service temperature did not carry a large amount of influence in the

WPI. Yet, it and the property of leak before break allowed for the optimal material to be

distinguished. Leak before break was the final weighting factor and had about the same

influence as service temperature. As stated before, Thermal Conductivity * Yield

strength/Price had the highest weight factor which ultimately led to our choice of 4037.

This is despite the fact that it held median values for leak before break and service

temperature.

If corrosion resistance was removed from our WPI, the same results would be

achieved. Altering the weighting factors of service temperature and leak before break

may impact the final result. However, 4037 steel had an appreciable lead over the other

material candidates in the application's most important factor. Therefore, its use in the

heat exchanger is more advantageous than that of the other five considered materials.

Composition:

4037 steel, like other low alloy steels contains constituents other that iron and carbon.

These added elements are used to alter the properties of the alloy for heightened

performance in a given application. Low alloy 4037 steel contains, in addition to iron

and carbon, manganese, molybdenum, phosphorus, sulfur and silicon. Manganese and

sulfur can act to prevent the steel from becoming too brittle. Molybdenum prevents the

growth of grains within the metal. This in turn increases the strength of the metal.

Silicon may also be included to increase the alloy's strength. This is achieved through

silicon's ability to form solid solutions within ferrite iron. In general, alloying carbon steel

with other elements increases a metal's strength, hardenability and resistance to



corrosion, properties that may prove desirable for the application of tubing in a heat

exchanger.

Conclusion:

After conducting our analysis, we have determined low alloy 4037 steel to be the

optimal material for the interior tubes of the heat exchanger. Of the materials we

considered, 4037 steel had desired qualities for an application involving great pressure

differences, high temperatures and a possibly corrosive environment. One aspect to

take into consideration is the ductility and manufacturability of the chosen material. In

our analysis we simply set a constraint on ductility. While a minimum ductility should

indeed be implemented in the investigation, this property may be considered more

deeply. The greater the metal's ductility, the easier it will be to manufacture into the

desired form. Thus, from a manufacturer's perspective, it may be beneficial to set a

maximum ductility value (one above which a material loses adequate pressure

sustaining ability) and try to maximize ductility up until this practical determined limit.

Should this ductility limit be implemented in the material selection, a minimum limit

should be placed upon yield strength to ensure that the material's heightened ductility

does not sacrifice its structural integrity when put under applied stress. The inclusion of

manganese, one of the constituents of 4037 steel, may increase the alloy's

machinability, and thus its manufacturability.

Works Cited:

"Alloy Steel." Wikipedia . 16 Apr. 2010. Web. 23 May 2010.

<http://en.wikipedia.org/wiki/Alloy_steel>.

materials selection 2 (mate sophomore series)

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