Double Pipe (Tube-in-Tube) Heat Exchangers: How They Work & When to Use Them

A refinery sample cooler, a high-pressure gas trim cooler, or a small process stream with a tight temperature approach rarely needs a full multi-tube bundle. In these cases a tube in tube heat exchanger, also called a double pipe or hairpin exchanger, often does the job with less steel, simpler maintenance, and better thermal efficiency. This guide explains how double pipe heat exchangers work, where they outperform shell-and-tube designs, and which tubes and pipes go into them, so you can specify the right unit and the right material the first time.

A tube in tube heat exchanger, also called a double pipe or hairpin heat exchanger, is a heat exchanger in which one tube sits concentrically inside a larger pipe. One fluid flows through the inner tube while the second flows through the annular space between the two, and heat passes through the inner tube wall. Because the fluids can flow in true counter-current, double pipe exchangers handle close temperature approaches and temperature crosses well, and because their diameters are small, they suit high-pressure, small-duty service.

What Is a Tube-in-Tube (Double Pipe) Heat Exchanger?

Tube in tube heat exchangers go by several names depending on the vendor and the region, but the construction is the same. A tube-in-tube heat exchanger, a double pipe heat exchanger, and a tube and tube heat exchanger all describe an inner tube running concentrically inside an outer pipe. When the assembly is folded into a U-shape, so that two straight legs share a return bend, it is called a hairpin heat exchanger. A multitube hairpin carries a small bundle of tubes inside each outer leg instead of a single tube, which raises the surface area while keeping the same modular layout.

One point of confusion is worth clearing up early. In refrigeration and HVAC, “tube-in-tube” often refers to a coaxial coil, where a smaller tube is coiled inside a larger one to work as a condenser or evaporator in a chiller or heat pump. That is a related but different product. This guide covers the industrial process version used in refineries, petrochemical plants, and gas processing. For tubes used in refrigeration-style evaporators, see our introductory guide to evaporator tubes.

How Does a Double Pipe Heat Exchanger Work?

The principle is conduction and convection across a single metal wall. Heat flows from the hotter fluid, through the inner tube wall, into the colder fluid, and the two streams never mix. In a typical counter-current hairpin unit the sequence is:

1. One fluid enters the inner tube at one end of the exchanger. The second fluid enters the annulus, the gap between the inner tube and the outer pipe, at the opposite end. Entering from opposite ends creates counter-current flow.
2. As the fluids travel the length of the straight leg, heat moves through the inner tube wall from the hot stream to the cold stream.
3. At the end of the leg, the inner tube turns through a return bend and the annulus fluid crosses over to the next leg, so the two streams stay in counter-current.
4. Hairpin sections are then connected in series to add length, or in parallel to carry more flow, until the required duty is met.

Counter-current flow keeps the temperature difference between the two streams large along the whole length. This is why the cold stream can leave hotter than the hot stream leaves, a condition called a temperature cross. A single multi-pass shell-and-tube unit cannot achieve a large temperature cross efficiently and needs a correction factor applied to its mean temperature difference. A double pipe exchanger in true counter-current needs no such correction, so the same duty often takes less surface area. Our guide on why counter-flow heat exchanger tubes are more efficient covers the thermal principle in more depth.

Main Parts of a Double Pipe Heat Exchanger

• Inner tube: the seamless tube that carries one stream and forms the heat transfer wall. It can be bare or finned.
• Outer pipe: contains the annulus fluid. It is sized to leave a narrow annular gap around the inner tube and is commonly standard seamless pipe.
• Return bend and return head: connect adjacent legs so that straight sections can be folded into a hairpin.
• End closures and glands: in most designs these let the inner tube expand independently of the outer pipe and be withdrawn for inspection or cleaning.
• Nozzles and supports: connect the exchanger to process piping and carry the weight of the legs.

Where the annulus fluid has a poor heat transfer coefficient, such as a gas or a viscous oil, a finned inner tube (commonly with longitudinal fins) can raise the effective surface area substantially. Hairpin manufacturers quote anywhere from two to five times the surface of a bare tube. Our overview of fin tube types covers the fin geometries used across heat exchanger designs.

When Should You Use a Double Pipe Heat Exchanger?

Double pipe exchangers earn their place in a handful of well-defined situations:

• Small heat duty. Process heat transfer references generally put the economic ceiling at roughly 20 m² of surface per exchanger. Beyond that, the cost per square metre favours shell-and-tube.
• High pressure on one or both sides. Small diameters let the pressure be contained by pipe-sized components rather than large tubesheets and shells, so wall thickness stays manageable at pressures that would make a multi-tube bundle impractical.
• A close temperature approach or a temperature cross. True counter-current flow makes better use of the available temperature difference than multi-pass layouts.
• Fouling, viscous, or slurry streams. The straight legs can be opened and cleaned without dismantling a large bundle.
• Wide temperature differences. In most designs the inner tube is not fixed rigidly at both ends, so differential thermal expansion between the tube and the outer pipe causes little stress.
• Duties that may change. Sections can be added, removed, or re-piped when process conditions change.

This combination is why double pipe units appear as sample coolers, trim coolers, and small process duties in refineries and petrochemical plants.

Double Pipe vs Shell-and-Tube Heat Exchanger: Which Should You Specify?

The choice usually comes down to duty size first and pressure or temperature cross second. The table below summarizes the practical differences.

For the full range of shell-and-tube configurations, see our guide to tubular heat exchanger types and TEMA configurations. If a plate design is also on the table, our shell-and-tube vs plate heat exchanger comparison covers that decision.

Which Tubes and Pipes Go Into a Double Pipe Heat Exchanger?

For a tube supplier, this is where double pipe exchangers get specific. Performance and service life depend on the inner tube and outer pipe, not only on the thermal design.

Four specification points deserve attention before you place an order:

• Wall thickness. In high-pressure service the inner tube wall is set by the pressure difference and code calculations, not by a standard minimum. State the design pressure and corrosion allowance in your enquiry.
• Dimensional tolerance. The annulus is narrow, so outside diameter and wall consistency matter. Cold drawn seamless tubes hold tighter tolerances than hot finished tubes, which keeps the annular gap and the flow through it predictable.
• Bending. If the inner tube is formed as a U-bend, specify the bend radius, the minimum wall at the bend after forming, and whether post-bend heat treatment or stress relieving is required. Our guide to U-tube heat exchangers covers bend-related considerations.
• Certification. Ask for EN 10204 3.1 or 3.2 mill test reports, and IBR certification if the unit is part of a steam circuit. Our guide to seamless pipe grades helps you match the outer pipe grade to the service.

Anand Seamless manufactures seamless carbon steel, alloy steel, and stainless steel tubes and pipes, along with finished carbon steel and alloy steel U-bend tubes, for Indian heat exchanger fabricators. The company holds IBR Well Known Maker status, ISO 9001:2015 certification, and EIL vendor approval.

How to Specify a Double Pipe Heat Exchanger: A Practical Checklist

1. Define the duty and surface area. If the required area approaches 20 m² or more, compare the cost against a shell-and-tube unit before committing.
2. State pressure and temperature on both sides. Include the minimum approach temperature and any temperature cross, because these decide whether counter-current hairpin sections pay off.
3. Decide which stream goes in the inner tube. It is usually the higher-pressure, more corrosive, or more fouling stream, because the inner tube is the easiest side to clean and the cheapest to build in a better alloy.
4. Select materials for the inner tube and outer pipe separately. Match each to its own fluid, temperature, and corrosion allowance.
5. Choose bare or finned. Add fins when the annulus side has a low heat transfer coefficient, such as gas or viscous liquid.
6. Confirm code and inspection requirements. Agree the design code, hydrotest, NDT scope, and certification level with the exchanger designer before ordering tubes.

Conclusion

A tube in tube heat exchanger is not a smaller shell-and-tube unit. It is a different tool, best suited to small duties, high pressures, close temperature approaches, and streams that foul. When the duty grows past roughly 20 m², shell-and-tube usually wins on cost. When pressure is high or a temperature cross is needed, a double pipe or hairpin design often wins on both thermal efficiency and simplicity. Whichever you choose, the inner tube and outer pipe specification decides how long the exchanger runs without problems, so settle grade, wall thickness, tolerance, bending, and certification before the order goes out.

Frequently Asked Questions

Is a tube in tube heat exchanger the same as a double pipe heat exchanger?

Yes. Tube in tube, double pipe, and tube and tube heat exchanger are different names for the same construction: one tube running concentrically inside a larger pipe, with one fluid in the inner tube and the other in the annulus. In refrigeration, tube-in-tube can also refer to a coiled coaxial exchanger, which is a related but different product.

What is the difference between a double pipe and a hairpin heat exchanger?

A hairpin heat exchanger is a double pipe exchanger folded into a U-shape so that two straight legs share one return bend. This keeps the unit compact in length. Hairpins can carry a single inner tube or a small bundle of tubes in each leg, and several hairpin sections can be piped in series or parallel.

Why is counter-current flow an advantage in double pipe exchangers?

Counter-current flow maintains a larger temperature difference between the two streams along the whole length, and it allows a temperature cross, where the cold stream leaves hotter than the hot stream leaves. Multi-pass shell-and-tube units need a correction factor for this, while true counter-current double pipe units do not, so the same duty often needs less surface area.

Can a double pipe heat exchanger handle high pressure?

Yes. Because the diameters are small, the pressure is contained by pipe-sized components rather than large tubesheets and shells. The practical limit is set by wall thickness, material, and the applicable design code, so the design pressure must be stated clearly when specifying the inner tube and outer pipe.

What are the disadvantages of a double pipe heat exchanger?

The main drawbacks are limited surface area per exchanger, a bulky and heavy layout for the heat transferred, and a higher cost per square metre than shell-and-tube once the duty grows beyond roughly 20 m². Finned inner tubes and multiple hairpin sections can extend the capacity, but at some size shell-and-tube becomes the better choice.

Which tube materials are used in double pipe heat exchangers?

Common inner tube choices are SA-179 carbon steel for clean non-corrosive service at moderate temperature, SA-213 T11 or T22 alloy steel at higher temperature, and TP304L or TP316L stainless steel for corrosive fluids. The outer pipe is typically seamless carbon steel pipe such as ASTM A106 Grade B, upgraded where the annulus fluid is corrosive.

Need Tubes or Pipes for a Double Pipe or Hairpin Heat Exchanger?

Share your tube grade, outside diameter, wall thickness, quantity, bend requirements, and certification needs with the Anand Seamless team through the contact page or at inquiry@anandseamless.com, and they will help you confirm the right specification before you order.

Published by Anand Seamless Ltd., a seamless tube and pipe manufacturer based in Gujarat, India. This guide is general engineering information. Final design, material selection, and code compliance for any heat exchanger must be confirmed by the exchanger designer and the applicable project specification.