1. Einleitung
How well an industrial heat exchanger transfers heat from one fluid stream to another depends on its design.
The concept seems straightforward enough.
Fluid 1 runs through tubes while fluid 2 flows around the tubes inside a shell.
Heat exchanger design in reality involves many interrelated decisions.
Parameters such as heat duty, temperature, flow rate, fluid properties, pressure drop, allowable fouling, materials of construction, and more all play a part. Additionally, designers must choose tube diameter and length, create a tube layout pattern, decide on baffle spacing, specify shell diameter, and establish flow configuration.
Often times, a change in one parameter will impact several others.
Increasing fluid velocity may improve heat transfer but it will likely increase pressure drop as well.
As a result, effective Shell and Tube Heat Exchanger Design balances thermal performance with mechanical requirements, maintenance concerns, and cost.
This article provides an overview of the major principles and factors involved.
2. What Is Shell and Tube Heat Exchanger Design?
Im Wesentlichen, Shell and Tube Heat Exchanger Design is the process by which we engineer an exchanger’s thermal and mechanical characteristics.
Heat exchanger design always starts with the process requirements.
Typical input data includes:
Hot fluid type
Cold fluid type
Hot fluid flow rate
Cold fluid flow rate
Inlet temperatures
Outlet temperatures
Operating pressures
Fluid properties
Allowable pressure drop
Fouling tendencies
Corrosion considerations
Available space for installation
Engineers use this information to calculate the heat transfer area needed to accomplish the required heat transfer.
Next, they work to develop the overall physical configuration of the heat exchanger.
This involves selecting items such as shell diameter, tube diameter, tube length, tube wall thickness, number of tubes, tube pitch, tube layout pattern, number of passes, baffle cut, baffle spacing, materials of construction, and nozzle sizes and dimensions.
As you can see, exchanger design involves both thermal considerations and mechanical design.
3. Basic Working Principle
A shell and tube heat exchanger exchanges thermal energy between two fluids that are separated by a solid barrier.
Heat transfer occurs across the tube wall.
Heat transfers from the hotter fluid to the tube wall by convection. Thermal energy then conducts through the tube wall and into the cooler fluid.
As a result, the overall heat transfer rate is dependent on many resistances to heat transfer.
These can include:
Convection resistance on the hot side
Conductive resistance through the tube wall
Convective resistance on the cold side
Fouling resistance
Note that heat exchanger area is only one factor affecting performance.
Fluid properties and flow details also have a significant impact.
4. Key Design Parameters
Prior to beginning detailed shell and tube heat exchanger design calculations, we must have a good understanding of our process data.
Wärmeabgabe
Heat duty is the amount of thermal energy that must be transferred per unit time.
Heat duty is the starting point for thermal sizing calculations.
Fluid Flow Rates
Fluid flow rates will affect heat capacity rates, fluid velocity, pressure drop, and convective heat transfer coefficients.
Inlet and Outlet Temperature
These values allow us to calculate the magnitude of the temperature change that must occur. Additionally, they help us determine the maximum temperature driving force available.
Fluid Properties
There are several fluid properties that we need to know including:
Density
Specific heat
Viscosity
Thermal conductivity
Keep in mind that fluid properties can change with temperature.
As a result, we should use properties that are representative of the actual operating conditions.
Operating Pressure
Operating pressure will have a direct effect on mechanical design.
Pressure can also affect the phase and properties of the process fluids.
Allowable Pressure Drop
All piping systems have some pressure limits.
As such, we must design the heat exchanger to maintain a pressure drop that is considered acceptable by plant standards.
5. Heat Duty Calculation
Determining heat duty is typically the first major step in shell and tube heat exchanger design.
When dealing with single phase fluids and no reactions or phase changes, we can relate heat duty to mass flow, heat capacity, and temperature change using the following relationship:
The basic relationship is:
Q = ṁ × Cp × ΔT
Where:
- Q= heat duty
- ṁ= mass flow rate
- Cp= specific heat capacity
- ΔT= temperature change
Basically, we can think of this relationship as the amount of heat a fluid can absorb or lose.
For instance, a larger fluid flow will require more thermal energy to achieve the same temperature change.
Similarly, the hotter we need to raise the process fluid, the greater the heat duty.
Ideally, we want the amount of heat lost by the hot process stream to equal the heat gained by the cold stream.
On actual heat exchanger design problems we may need to account for heat losses to the surroundings as well.
Heat exchanger applications involving phase changes require special consideration.
Latent heat must be accounted for when dealing with condensers, boilers, and evaporators. As a result, thermal calculations will look different than typical liquid heating applications.
6. Heat Transfer Area Design
Once we know our heat duty, we can proceed by estimating the heat transfer area required for our heat exchanger.
Assuming we know the overall heat transfer coefficient and can calculate an effective mean temperature difference, we can use the following relationship to estimate the required heat transfer area:
Q = U × A × ΔTₘ
Where:
- Q= heat transfer rate
- U= overall heat transfer coefficient
- A= heat transfer area
- ΔTₘ= effective mean temperature difference
Overall Heat Transfer Coefficient
The overall heat transfer coefficient U accounts for thermal resistance due to:
Convection on the tube side
– Conduction through the tube wall
– Convection on the shell side
– Fouling on the tube side
– Fouling on the shell side
A small U value will require a larger heat transfer area.
Temperature Difference
Notice that ΔTₘ is not just the subtraction of the two temperatures.
The temperature difference changes as we move along the length of the heat exchanger.
As a result, we usually take a logarithmic mean temperature difference for appropriate heat exchanger designs. Multipass exchangers may also require a correction factor to determine actual temperature difference.
7. Shell and Tube Sizing
Now that we have estimated the required heat transfer surface area, we can begin actual sizing of the heat exchanger.
Keep in mind that the total heat transfer surface area available is a function of several variables including:
Number of tubes
Tube outside diameter (OD)
Effective tube length
We can increase the available heat transfer area by adding more tubes.
Adding tubes will also increase the shell diameter.
Tubes can be made longer in order to decrease the total number of tubes.
Keep in mind that longer tubes will require more space for installation and cleaning.
Plant layout may be a large deciding factor between a taller or wider heat exchanger.
8. Tube Diameter, Length, and Thickness
Size and thickness of the heat exchanger tubes directly impacts heat transfer and mechanical design considerations.
Rohr-Durchmesser
Decreasing the tube diameter allows us to fit more tubes inside a given shell diameter.
Smaller tubes also affect fluid velocity.
One disadvantage of smaller tubes is the difficulty to clean them mechanically.
Rohr Länge
Increasing the length of each tube allows us to increase the surface area provided by each tube.
Physical limitations from the factory, shipping, or installation may restrict tube length.
Wanddicke des Rohres
Thickness of the tube wall will impact the mechanical strength of the tubes as well as thermal resistance.
Increasing the thickness of the tube wall will improve strength but it will also increase thermal resistance.
As such, we should choose tube thickness based on both mechanical and thermal needs.
9. Tube Layout and Tube Pitch
Tube layout impacts shellside flow, ability to clean, and shell diameter.
Triangular
If we set the tubes up in a triangular layout we can fit more tubes within the shell diameter.
Additionally, this layout can increase turbulence on the shellside of the heat exchanger.
One drawback is that it may be more difficult to clean between the tubes.
Square
Using a square tube layout allows us straighter lines to clean between the tubes.
As a result, it can become advantageous to use this layout when shellside mechanical cleaning is a concern.
Rotated Square Pattern
Rotating the square pattern is another way to strike a balance between tube density and flow characteristics.
Tube Pitch
Tube pitch is defined as the center to center distance between two adjacent tubes.
Tube pitch will affect many things such as:
Tube density
Shellside flow area
Pressure drop
Ease of cleaning
Mechanical layout
Therefore we must choose our tube pitch wisely.
10. Tube Pass Design
Tube passes refer to how many times the fluid on the tubeside must travel the length of the exchanger.
Typical designs include:
Increasing tube passes will increase fluid velocity on the tubeside.
This will improve our tubeside heat transfer coefficient.
However, pressure drop will also increase as a result.
As such, more passes does not always mean a better heat exchanger design.
We must balance heat transfer enhancement with pumping power.
11. Baffle Design
There are two main purposes of baffles.
Tube Support
ShellSide Flow Guidance
Without proper baffles, shellside fluid can flow through lessefficient paths.
ShellSide Velocity
Smaller baffle spacing increases shellside velocity.
This increases heat transfer but pressure drop as well.
Larger spacing allows lower flow resistance, but shellside heat transfer may suffer.
Baffle Cut
The baffle cut determines shellside flow window and crossflow area.
As such, baffle cut impacts heat transfer and pressure drop.
Tube Support
Another role of baffles is limiting the unsupported length of tubes.
This minimizes vibration which could damage tubes.
As such, thermal and mechanical design considerations must go handinhand.
12. ShellSide and TubeSide Pressure Drop
Allowable pressure drop is always a limiting factor of Shell and Tube Heat Exchanger Design.
Increasing fluid velocity improves heat transfer, but results in greater friction losses.
TubeSide Pressure Drop
Pressure drop on the tubeside depends on:
– Fluid Velocity
– Tube Diameter
– Tube Length
– Pass Count
– Fluid Viscosity
– Entrance/Exit Losses
ShellSide Pressure Drop
Pressure drop on the shellside depends on:
– Baffle Spacing
– Baffle Cut
– Shell Diameter
– Tube Configuration
– Fluid Velocity
– Fluid Properties
– Leakage/Bypass streams
Ideally, pressure drop should fall within allowable limits for the process system. Exceeding this may increase pumping or compression costs.
13. Flow Arrangement
Flow direction impacts temperature driving force through the heat exchanger.
Parallel Flow
Fluids enter on the same general end and move in the same general direction.
Gegenstrom
Fluids run through the exchanger in opposite directions.
Counter flow usually allows a more favorable temperature distribution for heat transfer.
Heat exchangers often use more complicated flows than these two examples.
Pass arrangements on the tubeside or shellside can drastically alter true temperature profile.
Engineers should evaluate entire flow arrangement when designing heat exchangers.
14. Material Selection
Materials of Construction should be compatible with process fluids.
Some common tube/shell materials include:
Kohlenstoffstahl
Carbon steel has decent mechanical properties and low cost when used with acceptable fluids.
Edelstahl
Stainless steel allows for greater corrosion resistance. Grades 304L and 316L are common in process equipment.
Copper Alloys
Copper alloys allow for greater thermal conductivity.
Nickellegierungen
Nickel and other highperformance materials can handle certain chemicals and high temperatures.
Titan
Titanium works wonders in certain applications but comes at a cost.
Equipment cost should be weighed against performance requirements.
15. Fouling and Cleaning Considerations
Fouling presents another form of thermal resistance.
Examples of fouling include:
- Scale
- Sediment
- Biological growth
- Corrosion
- Process buildup
Heat transfer decreases as fouling materials buildup on surfaces.
Pressure drop can increase as well. As such, engineers should anticipate fouling during the design phase.
Typically fouling is defined by adding a fouling resistance to thermal calculations.
One should be careful not to add too large of a design margin. Otherwise, equipment may be oversized.
Can’t see through your process stream? Consider how your fluid may foul over time.
Include cleaning provisions if your fluid has severe fouling tendencies.
Simple access to tube bundles allows for easier cleaning.
Removable tube bundle heads allow cleaning on the shellside.
16. Common Shell and Tube Configurations
Fixed Tube Sheet
Both shell and tube sheets are fixed relative to each other.
Simple design. Beware of thermal expansion and shellside cleaning.
U Tube
The tubes form a U shape to allow for expansion without a second fixed tube sheet.
Floating Head
Floating head allows for expansion and sometimes removable tube bundles.
17. Shell and Tube Heat Exchanger Design Process Step by Step
Step 1: Define Process Conditions
- Temperature
- Pressure
- Flow rate
- Physical properties
- Fluid type
Step 2: Calculate Heat Duty
Step 3: Determine Temperature Driving Force
Step 4: Estimate Overall Heat Transfer Coefficient U
Use estimated values or thermal resistance charts to arrive at U
Step 5: Estimate Heat Transfer Area
Calculate rough heat transfer area needed
Step 6: Select Tube Dimensions
Tube ID, Wall thickness, and Length
Step 7: Determine Number of Tubes
Approximate number of tubes needed
Step 8: Select Tube Layout
Triangular? Square? Something else?
Step 9: Estimate Shell Diameter
Find shell diameter from geometry of tube bundle.
Step 10: Select Baffles
Type, Cut, and Spacing of baffles
Step 11: Calculate Heat Transfer Coefficients
Evaluate Ai (tubeside), Ui(J), and hi (shellside)
Step 12: Check Pressure Drop
Ensure tubeside and shellside pressure drop are within limits.
Step 13: Iterate
Revise any numbers, velocities, passes, or baffles as needed.
Step 14: Mechanical Design Details
Shell thickness, tube sheets, nozzles, supports, bolting, and more.
18. Common Design Mistakes
Fouling
Maybe your heat exchanger works well enough when it’s nice and clean.
What about after several months of your process fluid sitting inside?
Pressure Drop
Velocity increases heat transfer but too much velocity causes excessive pressure drop.
Material Selection
Just because a material can handle the temperature doesn’t mean your process wants to react with it.
Flow Distribution
Maldistribution lowers true heat transfer area and creates hot spots.
Cleaning
Can you easily clean the heat exchanger? If your process fluid fouls, you should be able to clean it.
Thermal Expansion
Did you know the shell expands a different amount than the tubes? What about bowing of tubes?
Heat Transfer Area
Sure you can always add heat transfer area, but what about everything else mentioned above?
19. Custom Heat Exchanger Design
Many industrial heat exchanger projects require custom design.
Adjustable parameters include but are not limited to:
- Wärmeübertragungsfläche
- Shell diameter
- Rohrdurchmesser
- Länge des Rohrs
- Tube thickness
- Tube material
- Tube pitch
- Tube arrangement
- Number of passes
- Baffle cut
- Baffle spacing
- Nozzle locations
- Connection style
- Equipment orientation
Heat exchanger designs can also use finned tubes.
External fin surface can improve heat transfer when your application has an issue with heat transfer on one side.
Fin types can include low fin, welded fin, extruded fin, embedded fin, and more.
Low Fin Tubes for Heat Exchanger
Hochfrequenzgeschweißtes Rippenrohr
Extruded Fin Tubes for Heat Exchanger
20. Why Choose Bangwin Thermal?
Bangwin Thermal specializes in industrial heat exchanger equipment and fin tubes.
Different processes require different thermal solutions.
That’s why Bangwin Thermal can work with you to understand:
- Fluid composition
- Flow Rate
- Heat duty
- Inlet temperature
- Desired Outlet temperature
- Operating Pressure
- Material needs
- Allowable pressure drop
- Installation dimensions
With this information, Bangwin Thermal can help you determine what heat exchanger will work best for your process.
Need more external surface area? Bangwin Thermal specializes fin tube technology.
This includes standard tubes, low fin tubes, G fin tubes, extruded fin tubes, laser welded fin tubes, and high frequency welded fin tubes.
Contact Bangwin Thermal today to see how we can help with your custom heat exchanger needs.
21. FAQ
l What information should I have before Shell and Tube Heat Exchanger Design?
You should know: Fluid Types, Flow Rate, Temperatures, Pressures, Fluid Properties, Allowable Pressure drop, Material Requirements, Fouling Details, Installation Requirements.
l What are the steps to size a shell and tube heat exchanger?
Determine heat duty, determine temperature driving force, estimate overall heat transfer coefficient U, and Estimate heat transfer area.
l How do you size a heat exchanger?
Calculate required heat transfer area then choose tube dimensions and length to produce enough surface area.
l What layout should I use for my tubes?
Triangular has more tube density. Square allows better access to tubes for cleaning. Consider process specifics when choosing layout.
l Why are baffles needed?
Without baffles, shellside fluid can bypass tubes. Baffles force fluid to flow across tubes. Baffles also support the tubes and control shellside velocity.
l How does tube size matter?
Changing tube diameter impacts velocity, pressure drop, heat transfer area, fouling tendencies, and cleaning access.
l How many passes do you put in a heat exchanger?
Depends on the exchanger. Depends on your velocity and pressure drop requirements.
l What is the best material for a shell and tube heat exchanger?
Depends on your process.
l What metals are commonly used for heat exchangers?
Carbon steel, stainless steel, copper, nickel, and titanium are common materials for heat exchangers.
l Do finned tubes increase heat transfer?
Yes, they increase external heat transfer area.
l Can Bangwin Thermal design custom heat exchangers?
Yes! Give us a call or send us an email. Send us your process information and we’ll help determine your heat exchanger needs.
22. Conclusion
Shell and Tube Heat Exchanger Design is much more complicated than just attaching tubes to a shell.
An engineer should understand process conditions and heat duty.
Temperature driving force and required heat transfer area should be determined.
Tube diameter, length, quantity, pitch, and layout determine the tube bundle configuration.
Baffle configuration dictates shellside flow details.
Do not forget about pressure drop limitations.
Fouling tendencies, thermal expansion, cleaning provisions, and maintenance are all important to longterm success.
When designing shell and tube heat exchangers, engineers must balance thermal performance, equipment costs, and maintenance requirements.
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