A fixed tube sheet heat exchanger is undoubtedly the most economical option available, and this can be attributed to the simplicity of its fabrication process. The straightforward design of this heat exchanger, where the tubes are fixed to a stationary tube sheet, allows for easy and cost-effective manufacturing. However, this advantage is counterbalanced by the maintenance challenges that arise during its operational lifetime. Periodic cleaning and replacement of tubes are essential to ensure optimal performance, and while the inside of the tubes can be easily cleaned using mechanical means, such as forcing a wire brush or worm through the tubes, cleaning the outside of the tubes poses a significant difficulty.
The outside of the tubes can only be cleaned by removing the entire tube bundle from the heat exchanger, which is a cumbersome and time-consuming process. This not only leads to increased maintenance costs but also results in extended downtime, ultimately affecting the overall efficiency of the heat exchanger. In recognition of these challenges, many heat exchanger manufacturers have started providing removable tube bundles, which can be easily taken out for cleaning and maintenance. This design modification has significantly improved the maintainability of fixed tube sheet heat exchangers, making them a more viable option for industries where frequent cleaning and maintenance are crucial. Nevertheless, the inherent design limitations of fixed tube sheet heat exchangers mean that they may not be the best choice for applications where fouling and corrosion are significant concerns.
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Floating head-type shell and tube heat exchanger |
Design Formulas
The design of a Floating Head Heat Exchanger involves calculating the size of the heat exchanger based on the heat transfer requirements. The following formulas are commonly used:
1. Heat Transfer Area: A = Q / U * ΔT2. Shell Diameter: D = √(4 * A / π)3. Tube Length: L = A / (π * D_t * N)
where:
- A = heat transfer area- Q = heat transfer rate- U = overall heat transfer coefficient- ΔT = temperature difference- D = shell diameter- D_t = tube diameter- N = number of tubes
The ingenious design of the floating head exchanger provides a solution to the challenges posed by fixed tube sheet heat exchangers. The floating head exchanger's configuration allows for considerable expansion of the tubes, ensuring optimal performance and minimizing the risk of damage.
At the heart of this design lies the clever arrangement of the tube sheets. The exchanger tubes are securely fixed at both ends, with one end attached to the stationary tube sheet and the other to the floating tube sheet. The stationary tube sheet is firmly clamped between the shells, providing a stable anchor point. Conversely, the floating tube sheet is clamped between the floating head and a specially designed clamp ring. This ring, split in half to permit dismantling, is strategically positioned at the back of the tube sheet, allowing for effortless removal of the tube bundle.
To facilitate the withdrawal of the tube bundle from the channel end, the floating tube sheet is carefully crafted to be slightly smaller in diameter than the inside diameter of the shell. This thoughtful design element ensures a smooth and hassle-free removal process. Furthermore, the channel is equipped with inlet and outlet connections for the tube-side fluid, streamlining the flow of fluids through the exchanger.
The shell, which encases the tube bundle, is sealed by a shell cover or bonnet on the floating head side. Notably, the shell cover at the floating head end is larger than the other end, allowing the tubes to be positioned as close as possible to the edge of the fixed tube sheet. This deliberate design choice enhances the overall efficiency of the heat exchanger.
The crowning feature of the floating head exchanger is its ability to accommodate differential thermal expansion between the shell and the tube bundle. By allowing the tube sheet, along with the floating head, to move freely.
It is widely used in the chemical industry and is suitable for rigorous duties associated with high pressure and temperature and also with dirty fluids.
Classification and Applications
Floating head heat exchangers can be categorized into two primary types:
1. Internal Floating Head: This configuration features a floating head located within the shell. It can be further subdivided into two designs:
- With Clamp Ring: This design incorporates a clamp ring to secure the floating head in place.
- Without Clamp Ring: This design omits the clamp ring, relying on alternative means to secure the floating head.
2. External Floating Head: In this configuration, the floating head is situated outside the shell.
Applications
Floating head heat exchangers find extensive use in various industrial applications, including:
- Steam Superheaters: These heat exchangers are employed to superheat steam, increasing its temperature beyond the saturation point.
- Phase Change Units (Reboilers): Utilized in reboilers to facilitate phase change operations, such as vaporization or condensation.
Advantages
- Removable Tubes: The tubes of the exchanger can be easily removed for inspection, mechanical cleaning, and maintenance, reducing downtime and increasing efficiency.
- Elimination of Differential Expansion Problems: The floating head design accommodates differential thermal expansion between the shell and tubes, minimizing the risk of damage and ensuring reliable operation.
The Log Mean Temperature Difference (LMTD) calculation for a Floating Head Heat Exchanger is a crucial step in determining its thermal performance. Here's a step-by-step guide to calculate LMTD:
Assumptions
- Counter-current flow (hot fluid flows in one direction, and cold fluid flows in the opposite direction)
- Constant heat transfer coefficients
- Negligible heat losses
LMTD Calculation
1. Determine the temperature differences:
- ΔT1 = Th_i - Tc_o (temperature difference between hot fluid inlet and cold fluid outlet)
- ΔT2 = Th_o - Tc_i (temperature difference between hot fluid outlet and cold fluid inlet)
2. Calculate the LMTD:
LMTD = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2)
where ln is the natural logarithm
Correction Factors
For Floating Head Heat Exchangers, a correction factor (F) is applied to the LMTD to account for the non-counter-current flow in the shell. The correction factor depends on the heat exchanger's configuration and operating conditions.
Calculation Example
Suppose we have the following operating conditions:
- Hot fluid inlet temperature (Th_i) = 150°C
- Hot fluid outlet temperature (Th_o) = 80°C
- Cold fluid inlet temperature (Tc_i) = 20°C
- Cold fluid outlet temperature (Tc_o) = 60°C
1. Calculate the temperature differences:
- ΔT1 = 150°C - 60°C = 90°C
- ΔT2 = 80°C - 20°C = 60°C
2. Calculate the LMTD:
- LMTD = (90°C - 60°C) / ln(90°C / 60°C) ≈ 73.4°C
Note: This is a simplified example and does not take into account the correction factor (F) or other complexities that may arise in real-world applications.
Software Tools
For more accurate and complex LMTD calculations, consider using specialized software tools like:
- HTRI (Heat Transfer Research, Inc.)- ASPEN Plus- EES (Engineering Equation Solver)
These tools can help you account for various factors that affect the LMTD, such as non-counter-current flow, heat transfer coefficients, and fluid properties.