Choosing the Right Type of Shell and Tube Heat Exchanger in Piping Systems

II JAY SHRI KRISHNA II

Shell and Tube Heat Exchangers are some of the most ever-present sections of equipment in Piping systems. They are silently confirming efficient heat transfer across various industries. But, there are different types of Shell and Tube Heat Exchangers, each with its unique design, strengths and applications, which we will discuss and understand here.

Choosing the Right Type of Shell and Tube Heat Exchanger in Piping Systems

Shell and Tube Heat Exchangers are used for transferring thermal energy between fluids. But with various types available, selecting the right one is important. Also, the selection is depending on key factors include temperature, pressure and fluid properties.

This post dives into the three main categories of Shell and Tube Heat Exchangers commonly found in Piping which are:

Fixed Tube, Floating Head & U-Tube Heat Exchanger

1. Fixed Tube Heat Exchanger: The tube bundle is welded to both sides of the tube sheet.

2. Floating Head Heat Exchanger: One tube sheet is fixed to the shell, while the other tube sheet has a loose-fitting connection to the shell, allowing for expansion and contraction.

3. U-Tube Heat Exchanger: The tubes are bent into a U-shape, eliminating the need for a second tube sheet.

Countercurrent Flow for Maximum Efficiency:

Shell and Tube Heat Exchanger is an arrangement of shell and tube. A key advantage of these type of heat exchangers is their ability to operate countercurrent flow.

In this pattern, the hot and cold fluids flow in opposite directions within the shell and tubes. This countercurrent arrangement, increases the temperature difference between the fluids throughout the exchanger which leading to more efficient heat transfer.

Increasing Residence Time with Multiple Passes:

While countercurrent flow helps efficiency. To increase resident time of the fluid and for greater heating or cooling effect, more number of passes has to be added in exchangers. By add in multiple paths for the fluid within the tubes, you can increase the residence time which allowing for more heat transfer to occur.

So, here we are discussing the different heat transfer paths within Shell and Tube Heat Exchangers, namely Fixed Tube sheet, Floating heads and U-tubes.

Here, the three main types of Shell and Tube Heat Exchangers:

1. Fixed Tube Sheet Heat Exchangers:

This is the most basic design, like a box with tubes welded on both ends. Perfect for situations where keeping costs low is important.

It works best when the fluids don't get super-hot or cold, as big temperature changes can damage the exchanger.

This design makes it hard to clean the outside of the tubes, so the fluids going through need to be pretty clean to avoid clogging.

Flow paths: The fluid on the outside (shell side) makes one pass through the box. The fluid inside the tubes (tube side) can take multiple paths back and forth using a special wall inside the box. Imagine a maze for the fluid!

Cleaning the inside: Even though the outside is tricky, the insides of the tubes can be cleaned by removing a cover and using a special brush.

2. Floating Head Heat Exchangers:

A regular heat exchanger like a metal box with tubes running through it. Generally, the tubes are fixed tightly in place. But a Floating Head Heat Exchanger is like that box with one loose end. That loose end lets the tubes slide back and forth a little bit. This sliding room is key because hot & cold fluids can make the tubes shrink or grow. The Floating Head Exchanger lets the tubes adjust without squeezing or pulling on the rest of the box, preventing damage.

Regular heat exchangers can crack if the tubes get hot and expand a lot. Floating head exchangers are like regular ones, but with one special side that can move a little. This lets the tubes grow or shrink freely without stressing the whole box. This makes them better for hot and cold fluids, easier to clean inside, and less likely to have problems with the tubes shaking. However, they are also, expensive to construct.

The fluid inside the box (shell side) flows lengthwise, which also helps prevent the tubes from vibrating too much.

3. U-Tube Heat Exchangers:

U-Tube Exchangers use long, continuous tubes bent like a U-shape.

This U-bend acts like a built-in spring, allowing the tubes to expand and contract as things heat up or cool down. No extra parts needed!

But there's a trade-off. The U-bends cause a decrease in flow efficiency and make maintaining cleanliness difficult.

U-tubes: These tubes bend back on themselves in a U-shape. This design allows for thermal expansion without a floating head, but the bends can create stress points during temperature changes. U-tubes are generally not recommended for high-pressure steam applications due to this stress concentration.

Steam application: Steam is a hot fluid that can cause significant thermal expansion. While U-tubes can handle some expansion, they are not ideal for extreme cases like steam because of the stress on the bends.

Floating Head Exchangers, on the other hand, are a better choice for steam due to their ability to accommodate large thermal expansion without stressing the tubes.

Balancing Needs with Design Choices:

Fixed tube sheets offer a simpler, more affordable design but limit thermal expansion and cleaning options.

Floating heads address these concerns but come at a higher cost.

U-tubes excel at handling temperature variations but may restrict flow path possibilities.

Considering these trade-offs confirms you select the most appropriate Shell and Tube Heat Exchanger for your particular application.

Material Selection, Codes and Standards, Advantages and Disadvantages:

These three designs for Shell and Tube Heat Exchangers offer different advantages, disadvantages, material selection, codes and standards making them suitable for different applications.

1. Fixed Tube Sheet Heat Exchangers:

Materials:

Common materials which include carbon steel, stainless steel, admiralty brass, and cupronickel.

These Selection is dependent on pressure, temperature & fluid compatibility.

Codes & Standards:

ASME Boiler and Pressure Vessel Code (BPVC) Section VIII-1 is the primary standard for pressure vessel design, often supplemented by TEMA (Tubular Exchanger Manufacturers Association) standards for specific heat exchanger requirements.

Advantages:

Simple and cost-effective design

Compact due to straight tubes

Right for low-pressure applications

Disadvantages:

Limited thermal expansion capability (can lead to stress and leaks)

Tough to clean shell side due to fixed tube sheet

Not ideal for high-temperature applications

2. Floating Head Heat Exchangers:

Materials:

Similar to fixed tube sheet Heat Exchangers, with additional options for high-temperature applications like Inconel or titanium.

Codes & Standards:

ASME BPVC Section VIII-1 and TEMA standards still, remain the guiding principles for design & manufacturing.

Advantages:

Put up thermal expansion through a movable tube sheet

This design facilitates cleaning on both the shell and tube sides for optimal heat exchanger maintenance.

Appropriate for wider pressure and temperature ranges

Disadvantages:

More complex & expensive design as compared to fixed tube sheet

Need proper alignment of floating head to avoid leakage

May have a little larger footprint due to additional space for head movement

3. U-Tube Heat Exchangers:

Materials:

Similar to fixed tube sheet and floating head Heat Exchangers, with U-bends requiring careful material selection to avoid stress concentration points.

Codes & Standards:

ASME BPVC Section VIII-1 and TEMA standards continue to rule or govern the design & manufacturing.

Advantages:

Exceptional thermal expansion capability due to the U-bend design

Good choice for high-temperature applications

Disadvantages:

Limited flow path configurations due to U-bend restriction

More complex and potentially costlier cleaning process on the U-bend side

May have a larger footprint due to the U-bend geometry

Let's compare Fixed Tube Sheet, Floating Head, and U-Tube Heat Exchangers:

Shell and Tube Heat Exchanger: Their Types and Comparison

1. Fixed Tube Sheet Heat Exchanger:

Simplest design: Least complex to manufacture, resulting in lower cost.

Limited thermal expansion: Can be susceptible to stress due to temperature differences between tubes and shell. In some cases, they might need a stretchy part called a "bellows" to handle big temperature changes.

Difficult cleaning: Tube cleaning on both sides is challenging, especially mechanically.

Tube replacement: Individual tube replacement is difficult.

2. Floating Head Heat Exchanger:

High thermal expansion: One head sheet is free to move, accommodating large temperature differences without stress.

Easy cleaning: Both tube bundle and shell side are accessible for mechanical cleaning.

Individual tube replacement: Easier than fixed tube sheet design.

Higher cost: More complex design with floating head mechanism leads to higher cost.

Leakage risk: Sealing around the floating head requires careful design and maintenance to prevent leaks.

3. U-Tube Heat Exchanger:

High thermal expansion: U-bends in the tubes absorb thermal expansion, reducing stress.

Limited cleaning: Inner tube surface cleaning is difficult due to the bends.

Limited tube layout: U-bends reduce the number of tubes that can fit in the shell, impacting efficiency.

Moderate cost: More expensive than fixed tube sheet due to bending process, but less than floating head.

Choosing the Right Design:

Consider these factors when selecting a Shell and Tube Heat Exchanger design:

Pressure and Temperature: Fixed tube sheet for low pressure/temperature, floating head for wider range, U-tube for high temperature.

Thermal Expansion: Floating head and U-tube handle large temperature differences better.

Cleaning Requirements: Floating head allows easier cleaning on both sides.

Cost: Fixed tube sheet is most economical, followed by floating head and U-tube.

Space Constraints: Fixed tube sheet may offer a more compact design.

Maintenance: Floating head is easiest to clean; then fixed tube sheet, and U-tube is most difficult.

Conclusion:

In conclusion, Fixed Tubes Sheet Heat Exchangers are cost-effective but limit expansion. Floating Heads Heat Exchangers allow for expansion & cleaning but are high-priced. U-Tubes Heat Exchangers handle temperature swings but limit flow patterns. Considering these aspects ensures an efficient & reliable heat exchanger for your specific requirements.

By understanding these different types & their applications, you can ensure optimum heat transfer performance within Piping system!

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