ME 492 MATERIALS IN ENGINEERING DESIGN
Materials and Process Selection for a Cryogenic Heat Exchanger
April 5, 2012 GROUP #4 Dawson James Jeffrey Powell James Stevenson Derek Visvanathan i
Executive Summary There is an increasing demand for natural gas as it is the cleanest burning of all of all fossil fuels. When pipelines cannot be used to transport the natural gas it is liquefied (liquefaction temperature ‐163°) as it has a greatly reduced volume, making truck and naval transportation more feasible. In order to achieve the low temperatures required to liquefy the natural gas a cryogenic heat exchanger must be used. This report outlines the selection procedure for the type, material and processes required to fabricate a cryogenic heat exchanger. A spiral in shell heat exchanger utilizes thin tubes filled with coolant wound on the inside of a of a cylindrical shell. This design maximizes the amount of surface of surface area between the tubing and the natural gas allowing for a high heat flow from the gas to the coolant. The tubes require less advanced processes to fabricate than fins and the spiral shell heat exchanger requires significantly less maintenance than a plate and fin exchanger. Heat exchangers used for this purpose are quite large with lengths up to 500m and diameters of 5m. of 5m. Wrought aluminum 2026 was found to be the best material for the tubing inside the heat exchanger. Aluminum alloys have a high thermal conductivity resulting in the desired heat flow in the coolant. Aluminum is also less expensive than nickel alloys and will not have adverse reactions with the natural gas which was a problem with copper and brass. Along with the thermal conductivity and cost, aluminum will be able to withstand the pressures inside the heat exchanger passing all objectives. To maximize heat transfer thin walled tubing will be used. There are many processes that are able to create the thin walled tubing but wire drawing was found to be the best option. Wire drawing can create small cross sectional areas required for the thin wall tubing and is compatible with the selected aluminum alloys. Wire drawing can also yield tight tolerances. Another process that is suitable is roll forming. With similar specifications as wire drawing it will also perform the required function. Wire drawing has lower tooling and capital costs than roll forming and is therefore the most ideal process. i
Table of Contents Executive Summary........................................................................................................................................
i
Reasons and Process of Liquefying Natural Gas ...........................................................................................
1
Introduction and Design Statement..............................................................................................................
1
Design Constraints and Selection Criteria.....................................................................................................
2
Function:...............................................................................................................................................
2
Constraints: ..........................................................................................................................................
2
Objectives:............................................................................................................................................
2
Free Variables:......................................................................................................................................
2
Material Indices:...................................................................................................................................
2
Conceptual designs .......................................................................................................................................
4
Material Selection for Heat Exchanger Tubes...............................................................................................
5
Limits: .......................................................................................................................................................
5
Material Selection Summary ....................................................................................................................
6
Process Selection for Heat Exchanger Tubes ................................................................................................
8
Justification for Materials and Processes .....................................................................................................
9
Cost Estimation ...........................................................................................................................................
11
Final Design and Materials..........................................................................................................................
11
Summary and Conclusion ...........................................................................................................................
12
Bibliography ................................................................................................................................................
13
Appendix .....................................................................................................................................................
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Maximize Heat Flow Per Unit Area:........................................................................................................
14
Maximize Heat Flow Per Unit Mass:.......................................................................................................
15
Maximize Heat Flow Per Unit Cost: ........................................................................................................
16
Cost estimation results ...........................................................................................................................
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Reasons and Process of Liquefying Natural Gas Natural gas consumption is increasing due to the demand for cleaner energy production. In many cases pipelines are not available to supply the natural gas directly from the extraction plant. In its natural gaseous state, natural gas takes up a large amount of volume which makes transportation very expensive. In order to decrease the shipping volume, natural gas can be liquefied which reduces its volume to approximately 1/600th of its size. The main difficulty in liquefying natural gas is its classification as a cryogenic fluid, meaning it condenses at below ‐150°C. Another reason for liquefying natural gas is that the process removes impurities which mean once it reaches its destination it only needs to be regasified prior to distribution. Liquid natural gas is also non‐ toxic and non‐corrosive however; it is explosive when put in contact with water. Once natural gas is extracted, it first goes through several cleaning processes. Any condensates are removed along with CO2 mercury & H2S. The natural gas also goes through a dehydration stage to remove any trace amounts of water. The gas then goes through several cooling stages with air fin heat exchangers and compressors until it reaches the final cryogenic heat exchanger. The cryogenic heat exchanger uses another liquefied gas, typically liquid nitrogen or oxygen in the liquefaction process. Finally, the liquid natural gas is put into cryogenic sea carriers or cryogenic road tankers and shipped to the final destination.
Introduction and Design Statement The materials and process selection will be determined for a cryogenic heat exchanger used in the liquefaction of natural gas. The process of liquefying natural gas requires a temperature below ‐163 degrees Celsius and in this process the other trace elements present in the natural gas are separated out leaving pure liquid methane. This process occurs at ambient pressure on the natural gas side but the refrigerant that travels through the heat exchanger may not operate at this pressure depending on the type of process utilized. Therefore the stress due to pressure difference must be considered along with the thermal conductivity of the tubing material. There are two types of heat exchangers used in this application, one is a coiled tube heat exchanger and the other is fin and plate heat exchanger they are commonly manufactured using aluminum and titanium alloys, this report will investigate the optimum material for this type of heat exchanger. The optimum process for manufacturing will also be determined.
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Design Constraints and Selection Criteria Function: Cryogenic heat exchanger used to cool natural gas to its saturation temperature (-163°C) at which point it will liquefy and become liquefied natural gas (LNG).
Constraints: •
Withstand the pressure difference between working fluids, ∆
•
Operate at temperature below to ‐163°
•
Moderate ductility so tubing can be bent
•
Does not corrode due to working fluids or by products (such as H2S)
•
Have excellent low temperature (cryogenic) durability
Objectives: •
Maximize the heat transfer/flow per area,
•
Minimize cost,
Free Variables: •
Wall thickness of tubing,
•
Material choice
Material Indices: The method of heat transfer through the tubing will be conduction which is given by:
Where:
∆
is the heat flux (⁄ ), is the thermal conductivity (⁄ ), ∆ is the temperature difference between the working fluids (, and is the tube wall thickness ().
Heat flow,, is given by:
Where:
is the heat flow (), and 2
is the surface area of the tubing ( ) 2. Substituting the heat flux from conduction yields the following:
∆
Wall thickness is a remaining free variable which relates the pressure differences between the working fluids.
Where:
∆
is the materials yield strength (), is the radius of the tube (), ∆ is the pressure difference between the working fluids (), and is the wall thickness.
Finally by substituting thickness a material index based on both yield strength and thermal conductivity can be attained.
∆ ∆
In order the minimize cost the heat flow per unit mass is used (/ ).
2 ∆ / 2 /
∆ 2 3
/
∆ ∆ 2
∆ / ∆ 2 Once this index is derived multiplying mass by cost will result in the index for cost.
Conceptual designs There are several types of heat exchangers that are effective for the liquefaction of natural gas. Each exchanger will use the coolant liquid methane in order to bring the natural gas to its saturation temperature through the use of a typical vapour compression refrigeration cycle. Figure 1 Shell and tube heat exchanger
One of the simplest heat exchanger is a shell and tube heat exchanger where many small tubes are ran through a reservoir of coolant. The small tubes allow for maximum surface area to be in contact with the coolant allowing for the highest heat flow from natural gas to the coolant. The majority of heat exchangers are counter flow in which the working fluids are pumped in opposite directions. So the inlet side of the natural gas will be the outlet side of the coolant and vice versa. Multiple tube passes can be used to increase the amount of surface area and heat transfer that occurs. A modification of the shell and tube heat exchanger is the
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Figure 2a Spiral wound heat exchanger
Figure 2b Plate and fin Heat exchanger
spiral wound heat exchanger. In this case the natural gas is in the shell area (A‐A shell stream) while coolant runs through the tube streams. This configuration has a very high tube surface area but is far more complex than a typical shell and tube heat exchanger. Plate and fin heat exchangers can have higher surface area for heat transfer. They are also able to withstand high pressures. Plates are sandwiched together with small fins in between to further increase the surface area. This type of heat exchanger is more difficult to manufacture than those that utilize tubing. In addition to manufacturing complications the increased surface area makes the fluid path ways very small. This can lead to an increase in clogging depending on the working fluids being used.
Material Selection for Heat Exchanger Tubes While determining the materials needed for the tubes in the LNG cryogentic heat exchanger three objectives were considered: • • •
Maximize Heat Flow per Unit Area Maximize Heat Flow per Unit Mass Maximize Heat Flow per Unit Cost
Limits: Elongation: >20% Strain Maximum Service Temperature: < ‐163 °C 5
Thermal Conductor or Insulator: Good Conductor Tolerance to Cryogenic Temperatures: Excellent
Material Selection Summary Using level 2 CES EduPack with the limits stated above, the following are the nine materials that pass the limit stage: •
Age‐hardening wrought Al‐alloys
•
Brass
•
Bronze
•
Commercially pure lead
•
Copper
•
Lead alloys
•
Nickel
•
Nickel‐based superalloys
•
Non age‐hardening wrought Al‐alloys
The top five materials using level 2 for each objective are: Maximize Heat Flow per Unit Area: •
Age‐hardening wrought Al‐alloys
•
Brass
•
Copper
•
Nickel
•
Non age‐hardening wrought Al‐alloys
Maximize Heat Flow per Unit Mass: •
Age‐hardening wrought Al‐alloys
•
Brass
•
Copper
•
Nickel
•
Non age‐hardening wrought Al‐alloys
Maximize Heat Flow per Unit Cost: •
Age‐hardening wrought Al‐alloys
•
Non age‐hardening wrought Al‐alloys
•
Brass
•
Bronze
6
•
Copper
Breaking down each alloy using level 3 CES EduPack allowed for a more accurate material selection for each of the stated objectives. Copper, brass, and bronze were rejected from the material selection process as they have an adverse reaction with H2S which is a by product of the liquefaction process. The following are the top ten materials in order of performance index for Aluminum and Nickel alloys: Maximize Heat Flow per Unit Area (see Figure 2): 1) Aluminum, 2026, wrought, T3511 2) Aluminum, 2024, wrought, T4 3) Aluminum, 6082, wrought, T4 4) Nickel, Duranickel Alloy 301, annealed & aged 5) Nickel‐Co‐Cr alloy, UDIMET 700, bar 6) Nickel, Permanickel Alloy 300, annealed 7) Nickel‐Fe‐Cr alloy, UDIMET 630, bar 8) Nickel‐Co‐Cr alloy, AEREX 350, cold worked, aged 9) 45Ni‐3Mo‐Fe soft magnetic alloy 10) Nickel, commercial purity, grade 200, soft (annealed)
Maximize Heat Flow per Unit Mass (see Figure 4): 1) Aluminum, 2026, wrought, T3511 2) Aluminum, 2024, wrought, T4 3) Nickel‐Fe‐Cr alloy, UDIMET 630, bar 4) Nickel‐Co‐Cr alloy, AEREX 350, cold worked, aged 5) Nickel, Duranickel Alloy 301, annealed & aged 6) Nickel‐Co‐Cr alloy, UDIMET 700, bar 7) Nickel‐Co‐Cr alloy, EP741NP 8) Aluminum, 6082, wrought, T4 9) Nickel‐Fe‐Cr alloy, D‐979, bar 10) 45Ni‐3Mo‐Fe soft magnetic alloy
Maximize Heat Flow per Unit Cost (see Figure 6): 1) Aluminum, 2026, wrought, T3511 2) Aluminum, 2024, wrought, T4 3) Aluminum, 6082, wrought, T4 4) Aluminum, S520.0: LM10‐TB, cast 5) Aluminum, 5154, wrought, O
7
6) Aluminum, 6060, wrought, T4 7) Aluminum, 2024, wrought, T0 8) Aluminum, 5251, wrought, O 9) Aluminum, commercial purity, 1080, wrought, O 10) 45Ni‐3Mo‐Fe soft magnetic alloy
Materials that meet each of the three design objectives are: 1) Aluminum, 2026, wrought, T3511 2) Aluminum, 2024, wrought, T4 3) Aluminum, 6082, wrought, T4 4) 45Ni‐3Mo‐Fe soft magnetic alloy
Process Selection for Heat Exchanger Tubes The field of available processes was first narrowed using the tree selection to use only processes compatible with aluminum. Using limit selection the list was narrowed to include only primary shaping processes, circular prismatic objects and continuous methods. Finally due to the thin wall thickness required, processes which couldn’t provide a thickness of less than 1 mm were removed. The final five processes were then ranked according to the range of thickness available below 1 mm. The top five processes for shaping the tubes are: 1. Wire drawing 2. Roll forming 3. Shape drawing 4. Cold shape rolling 5. Powder extrusion
Wire drawing consists of pulling a solid cylindrical blank through a hardened die in order to reduce its cross‐sectional area. Tube drawing is a subset of wire drawing where the cylindrical blank has a hollow center and a mandrel is placed inside to keep the interior diameter at the value that is required. This process allows for a wall thickness down to 0.1 mm and a tolerance of 0.01 – 0.04 mm. Roll forming consists of feeding a continuous sheet of metal through rollers in order to create the shape. For the heat exchanger tubes, an additional welding process 8
would have to be done in order to seal the seam between the sections. Shape drawing is similar to wire or tube drawing where a continuous blank is pulled through a die to create the final geometry. The difference is that a mandrel is not used to create the desired wall thickness which limits the minimum wall thickness. Cold shape rolling is similar to roll forming except the process is done at room temperature and produces better surface finish. The downside to cold shape rolling is the higher stresses involved which don’t allow for a section thickness less than 1 mm. finally, powder extrusion uses heated loose metal powder either formed into a billet or placed directly into a chamber. The material is then pressed through a die which forms it into the required shape. Powder extrusion has the same problem as cold shape rolling where the section thickness must be greater than 1 mm.
Justification for Materials and Processes In order to determine appropriate materials for the cryogenic heat exchanger tubes, objectives and constraints had to be defined. The main constraints associated with the heat exchanger tube material are: •
Minimum Elongation of 20% Strain
•
Maximum Service Temperature Less Than ‐163 °C
•
Good Thermal Conductor
•
Excellent Tolerance to Cryogenic Temperatures
Additionally, objectives were determined to incorporate performance indices with our material selections. Since the heat exchanger tubes main purpose is to transfer heat from one fluid to another, thermal conductivity is a major aspect with this material selection. Therefore considering mass, area, and cost with thermal conductivity; three objectives were considered: •
Maximize Heat Flow per Unit Area
•
Maximize Heat Flow per Unit Mass
•
Maximize Heat Flow per Unit Cost
Using level 2 CES EduPack 2011 with the limits and objectives stated above, the top five materials determined were found to be: 9
•
Age‐hardening wrought Al‐alloys
•
Brass
•
Copper
•
Nickel
•
Non age‐hardening wrought Al‐alloys
Due to high corrosion rates with hydrogen sulphide, which is a by‐product of natural gas, copper and brass were eliminated from our selection. With copper and copper alloys removed from our selection list, a level 3 CES EduPack analysis of the top materials was conducted incorporating the three performance indices stated earlier. This produced the top three materials which all performed well in each objective. Therefore, the top three overall materials for the heat exchanger tubes, in order of performance index, are: 5) Aluminum, 2026, wrought, T3511 6) Aluminum, 2024, wrought, T4 7) Aluminum, 6082, wrought, T4
Wire drawing was chosen for its wider range of available cross sectional areas. This process is also simpler than roll forming, which makes it less expensive. Wire drawing also consists of one process done in steps unlike roll forming which would require an extra welding step. Wire drawing allows a range of cross sectional areas of .01 to 10 mm with an excellent tolerance range of 0.01 to 0.04 mm. The process is also continuous which allows for the production of very long tubes, this is ideal due to a typical heat exchanger being 500 m in length.
Figure 3 Wire drawing process
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Cost Estimation The total amount of tubing was based on an average weight of 91000 kg. The comparison of potential materials can be seen in Table 1 of the appendix. From the table it can be seen that Aluminum 6082 is the cheapest, with a cost per unit weight of $1.258/kg which brings the total cost of the material to $243,780. This cost does not include the containment vessel and the welded mounting structure. For the chosen material more factors were considered such as thermal conductivity and aluminum, 2026, wrought, T3511 was chosen. Out of the two available processes, only the capital cost and the tooling cost could be considered. The cheapest process turned out to be wire drawing as seen in Table 2 with a combined cost of $104,720. The total cost of the heat exchanger tubing including material and process came to $360,430.
Final Design and Materials As there are significant by‐products created in the liquefaction of natural gas the plate and fin heat exchanger was rejected because the small path ways may become clogged during operation. It was decided that a spiral wound heat exchanger would better suited for this application. This heat exchanger has much higher surface area of tubing resulting in higher heat flow from the natural gas to the coolant when compared with a more conventional tube in shell exchanger. Also, thermal expansion and contraction can occur in a spiral tube heat exchanger without breaking. The tubing for this heat exchanger will be made out of the following aluminum alloys (in order from most to least desirable): 1) Aluminum, 2026, wrought, T3511 2) Aluminum, 2024, wrought, T4 3) Aluminum, 6082, wrought, T4
Aluminum has a high thermal conductivity allowing for maximum heat transfer throughout the heat exchanger. Aluminum alloys will not react with any of the coolants, natural gas or by products from the liquefaction so there is no risk of corrosion. Aluminum is also readily available and inexpensive material that is widely used in heat exchangers and other 11
mechanism. There is an added advantage because aluminum can be bent to create complex piping networks without breaking and is compatible with the process selected, wire drawing. Aluminum has lower yield strength than nickel alloys but the 2000 series aluminum alloys will be sufficient for this function. Typically the heat exchangers used to liquefy natural gas are 5m in diameter and range between 200m and 500m long. Using aluminum will keep weight and cost down while the spiral heat exchanger will reduce the maintenance required which, considering its size, is vital.
Summary and Conclusion The most desirable materials were all age hardened aluminum alloys which are low in cost, have high yield strength and high thermal conductivity. These materials were similar to those currently used in these types of natural gas heat exchangers which typically use aluminum. Other materials, such as copper, would be attractive had they not have had adverse reactions with by products of the reaction. Aluminum is also ductile so it can be bent into the spiral shapes once it has been manufactured into thin walled tubes. Perhaps the most important constraint that aluminum met was its low minimum operating temperature (below ‐200°C). After passing all constraints aluminum scored high in all three of the material indices: heat flow per unit area, heat flow per unit mass and heat flow per unit cost. Wire drawing is the process that best fits the requirements for making small thin walled tubes while still having low costs when compared to other methods. As an added benefit there will be no additional processes required other than the final assembly. After researching current materials and process we found that the methods and materials selected for the cryogenic heat exchanger are similar to what is currently used.
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Bibliography Air Products. (2008, February). Retrieved March 23, 2012, from http://www.airproducts.com/~/media/Files/PDF/industries/energy‐lng‐brochure‐0408.ashx Ashby, M. F. (2011). Materials Selection in Mechanical Design 4th Ed. Burlington: Butterworth‐ Heinemann. The Linde Group. (2008, December 09). Gryogenic Heat Exchangers for LNG Plants. Retrieved March 23, 2012, from http://www.hts.org.uk/downloads/Linde_LNG_HEX_09Dec2008_Extract.pdf
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Appendix
Maximize Heat Flow Per Unit Area: Material Indices:
Slope = ‐1
Figure 4 ‐ Level 2 CES EduPack 2011 Materials
Figure 5 ‐ Level 3 CES EduPack 2011 Al, Ni Alloys
14
Maximize Heat Flow Per Unit Mass: Material Indices:
Slope = 1
Figure 6 ‐ Level 2 CES EduPack Maximize Heat Flow per Unit Mass
Figure 7 ‐ Level 3 CES EduPack 2011 Al, Ni Alloys
15
Maximize Heat Flow Per Unit Cost: Material Indices:
Slope = 1
Figure 8 ‐ Level 2 CES EduPack Maximize Heat Flow per Unit Cost
Figure 9 ‐ Level 3 CES EduPack 2011 Al, Ni Alloys
16
Figure 10 ‐ Level 3 CES EduPack Process rank of section thicknes
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Cost estimation results
Table 1 Material cost
Material
Unit Cost ($/kg)
Avg. Tubing Weight
Total Cost
(kg)
($)
2.81
91,000
255,710
Aluminum, 2024, wrought, T4
2.85
91,000
259,350
Aluminum, 6082, wrought, T4
2.58
91,000
234,780
Aluminum, 2026, wrought, T3511
Table 2 Process cost
Process
Capital Cost
Tooling Cost
Total Cost ($)
($)
($)
Wire Drawing
95,200
9,520
104,720
Roll Forming
667,000
19,000
686,000
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