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heat exchanger design in 4 steps.
d Examples
Heat exchanger design step by step
Shell and tube heat exchangers are widely used in various industries due to their versatility, efficiency, and robust design. They are capable of handling a wide range of temperature and pressure differentials, making them suitable for diverse applications, including power generation, oil refining, chemical processing, and HVAC systems. This article provides a step-by-step guide to designing a shell and tube heat exchanger, complete with formulas and an example to illustrate the process.
Step 1: Determine the Heat Duty:
The first step in designing a shell and tube heat exchanger is to calculate the heat duty, which represents the amount of heat that needs to be transferred between the two fluid streams. The formula for heat duty (Q) is as follows: heat exchanger design
Q = m_dot * Cp * ΔT
Where: Q = Heat duty (in kW or Btu/hr) m_dot = Mass flow rate of the fluid (in kg/s or lb/hr) Cp = Specific heat capacity of the fluid (in kW/kg·°C or Btu/lb·°F) ΔT = Temperature difference between the hot and cold fluids (in °C or °F)
Step 2: Determine the Overall Heat Transfer Coefficient (U):
The overall heat transfer coefficient (U) represents the overall efficiency of heat transfer in the exchanger. It depends on various factors, including the geometry, fluid properties, fouling, and construction materials. While estimating U can be complex, simplified correlations such as the Kern method can provide a good starting point. The formula for U is as follows:
1/U = 1/hi + Δx/k + Δr/K + Δy/ho
Where: hi = Inside heat transfer coefficient (in W/m²·°C or Btu/hr·ft²·°F) k = Thermal conductivity of the tube material (in W/m·°C or Btu/hr·ft·°F) Δx = Fouling resistance on the tube side (in m²·°C/W or ft²·°F·hr/Btu) Δr = Resistance of the tube wall (in m²·°C/W or ft²·°F·hr/Btu) Δy = Fouling resistance on the shell side (in m²·°C/W or ft²·°F·hr/Btu) ho = Outside heat transfer coefficient (in W/m²·°C or Btu/hr·ft²·°F)
Step 3: Determine the Required Surface Area (A):
The surface area of the heat exchanger determines the size and cost of the equipment. It can be calculated using the formula:
A = Q / (U * ΔTlm)
Where: A = Surface area (in m² or ft²) Q = Heat duty (in kW or Btu/hr) U = Overall heat transfer coefficient (in W/m²·°C or Btu/hr·ft²·°F) ΔTlm = Logarithmic Mean Temperature Difference (in °C or °F)
Step 4: Select Tube and Shell Specifications:
Based on the design requirements and available resources, select the tube and shell specifications such as diameter, length, material, and layout. Consider factors such as fluid properties, pressure drop limitations, maintenance accessibility, and corrosion resistance.
Step 5: Perform Iterative Calculations:
Using the selected tube and shell specifications, perform iterative calculations to determine the number of tubes required and their arrangement within the shell. Consider the tube-side and shell-side velocities, pressure drop, and flow distribution for optimum performance.
FNQs on heat exchanger design
Q1: What are the advantages of using a shell and tube heat exchanger?
A1: Shell and tube heat exchangers offer several advantages, including high thermal efficiency, robust design, ability to handle high pressure and temperature differentials, versatility for various applications, easy maintenance, and scalability.
Q2: How is the overall heat transfer coefficient (U) calculated?
A2: The overall heat transfer coefficient (U) is calculated using the formula: 1/U = 1/hi + Δx/k + Δr/K + Δy/ho, where hi is the inside heat transfer coefficient, k is the thermal conductivity of the tube material, Δx is the fouling resistance on the tube side, Δr is the resistance of the tube wall, and Δy is the fouling resistance on the shell side.
Q3: What is the logarithmic mean temperature difference (ΔTlm)?
A3: The logarithmic mean temperature difference (ΔTlm) is used to calculate the required surface area of the heat exchanger. It is calculated as (ΔT1 – ΔT2) / ln(ΔT1 / ΔT2), where ΔT1 is the temperature difference on the hot fluid side and ΔT2 is the temperature difference on the cold fluid side.
Q4: How does the tube material selection impact the heat exchanger design?
A4: The tube material selection is crucial as it affects the overall heat transfer rate, corrosion resistance, and mechanical strength of the heat exchanger. Factors such as fluid properties, operating conditions, and cost considerations influence the choice of tube material.
Q5: What is the significance of the tube-side and shell-side velocities in heat exchanger design?
A5: Tube-side and shell-side velocities are important parameters in heat exchanger design as they affect the heat transfer rate, pressure drop, and flow distribution within the exchanger. Proper velocity selection ensures efficient heat transfer while avoiding excessive pressure drops.
Q6: How can fouling on the tube and shell sides affect heat exchanger performance? A6: Fouling, the accumulation of deposits on heat transfer surfaces, reduces heat transfer efficiency and increases pressure drop. It is necessary to consider fouling resistances (Δx and Δy) in the overall heat transfer coefficient calculation and periodically clean or maintain the heat exchanger to mitigate fouling effects.
Q7: Can you explain the concept of temperature approach in heat exchangers? A7: Temperature approach refers to the temperature difference between the hot fluid and the cold fluid at the end of the heat transfer process. A smaller temperature approach indicates better heat transfer efficiency, but it may also result in higher pressure drops and increased exchanger size.
Q8: How does the size and cost of a shell and tube heat exchanger depend on the required surface area?
A8: The size and cost of a shell and tube heat exchanger are directly influenced by the required surface area. A larger surface area leads to a larger exchanger size and higher material costs. Therefore, optimizing the design to achieve the required heat transfer with the minimum surface area is desirable.
Q9: Are there any software tools available for shell and tube heat exchanger design?
A9: Yes, there are various software tools and computer-aided design (CAD) packages specifically designed for shell and tube heat exchanger design. These tools assist engineers in performing calculations, simulating heat transfer, optimizing designs, and evaluating performance under different operating conditions.
Q10: How do innovations in shell and tube heat exchanger design contribute to energy efficiency?
A10: Innovations in shell and tube heat exchanger design, such as compact and micro heat exchangers, enhanced surface geometries, and the incorporation of phase change materials, improve heat
Example of heat exchanger design
Let’s consider a case where we need to design a shell and tube heat exchanger to transfer heat from a hot fluid (stream 1) at 150°C to a cold fluid (stream 2) at 50°C. The hot fluid has a mass flow rate of 4 kg/s, a specific heat capacity of 4 kJ/kg·°C, and an inside heat transfer coefficient of 800 W/m²·°C. The cold fluid has a mass flow rate of 3 kg/s, a specific heat capacity of 3 kJ/kg·°C, and an outside heat transfer coefficient of 500 W/m²·°C. The tube material has a thermal conductivity of 50 W/m·°C.
- Calculate the heat duty (Q): Q = m_dot * Cp * ΔT Q = 4 * 4 * (150 – 50) = 1600 kW
- Calculate the overall heat transfer coefficient (U): 1/U = 1/hi + Δx/k + Δr/K + Δy/ho Assume fouling resistances Δx, Δr, and Δy to be negligible for simplicity. 1/U = 1/800 + 1/50 + 1/500 U = 77 W/m²·°C
- Calculate the required surface area (A): A = Q / (U * ΔTlm) Assume a temperature approach of 10°C. ΔTlm = (ΔT1 – ΔT2) / ln(ΔT1 / ΔT2) ΔTlm = (150 – 50) / ln(150 / 50) = 66.7°C A = 1600 / (77 * 66.7) = 2.04 m²
- Select tube and shell specifications: Based on the available options and constraints, choose appropriate tube and shell dimensions, materials, and layout.
- Perform iterative calculations: Determine the number of tubes and their arrangement within the shell, considering factors such as tube-side and shell-side velocities, pressure drop, and flow distribution.
Conclusion
Designing a shell and tube heat exchanger involves a systematic approach to ensure efficient heat transfer while meeting specific requirements. By following the step-by-step guide outlined above and utilizing the appropriate formulas, engineers can design heat exchangers that optimize energy efficiency, performance, and cost-effectiveness. Continuous advancements in heat exchanger design techniques, software tools, and materials are driving innovation in this field, contributing to the development of more efficient and sustainable thermal management solutions.
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