What is Double-Pipe (Hairpin) Heat Exchanger?
A Double-Pipe Heat Exchanger is the simplest type of heat exchanger in its construction. It consists of one pipe mounted concentrically inside another, larger pipe. The two fluids flow through the exchanger: one through the inner pipe and the other through the annular space (the gap) between the inner and outer pipes.
The name “Hairpin” comes from the common practice of connecting multiple double-pipe sections in a U-shaped return bend to form a longer, continuous flow path, resembling a hairpin.
Core Principle: Heat is transferred from the hotter fluid to the colder fluid through the wall of the inner pipe. The fluids can flow in the same direction (parallel flow) or in opposite directions (counter-flow), with counter-flow being the most common and efficient arrangement.
Key Components and Their Functions
- Inner Pipe (Tube): Carries one of the fluids. Its diameter is selected based on flow rate, pressure drop, and fouling characteristics.
- Outer Pipe (Shell): Houses the inner pipe and contains the second fluid in the annular space.
- Return Bend (Hairpin): A U-shaped connector at the end of a section that allows the fluid in the inner pipe to turn around and flow back in the opposite direction for the next section.
- Junction Box / End Closure: A chamber at the end of the outer pipe where the annular flow can be directed to the next section or to the outlet nozzle.
- Packing Box (Packed Joint): A critical component that allows for thermal expansion. The inner pipe expands and contracts with temperature changes relative to the outer pipe. The packing box provides a seal while allowing the inner pipe to slide freely, preventing stress and damage.
How It Works (Flow Patterns)
A single double-pipe section is relatively short. To achieve the required heat transfer area, multiple sections are connected in series.
- For the Inner Tube Fluid: The fluid enters one end of the inner tube, travels the length of the section, makes a 180° turn in the hairpin bend, and returns through the inner tube of the next section. This constitutes a two-pass arrangement per hairpin.
For the Annular (Shell-Side) Fluid: The fluid typically flows in a single, continuous pass through the annular space of all connected sections. This creates a
- pure counter-flow arrangement with the inner tube fluid, which is the most efficient configuration for heat transfer.
Advantages and Disadvantages
| Advantage | Disadvantage |
| Simple Construction: Easy to understand, manufacture, and maintain. | Low Compactness: Has a very high footprint and weight per unit of heat transfer area. It is the least compact design. |
| High Pressure & Temperature: Excellent for very high-pressure applications on the tube side (the inner pipe is essentially a pipe within a pipe). Also suitable for high temperatures. | Limited Capacity: Practical only for low flow rates (typically up to ~15 m³/h). The annular space becomes impractically large for high shell-side flow rates. |
| Easy to Clean & Maintain: The design allows for easy disassembly and mechanical cleaning of the inner pipe. | High Cost per Unit Area: Becomes very expensive for large heat duties due to high material and labor costs. |
| True Counter-Flow: The standard hairpin arrangement achieves true counter-flow, allowing for very close temperature approaches (where the outlet temperature of one fluid approaches the inlet temperature of the other). | High Pressure Drop (Annulus): The annular space can have a high pressure drop, especially if fins are added. |
| Modular & Flexible: Units can be easily stacked or arranged in series (for more duty) or parallel (for higher flow rates). | Inefficient Use of Space: The large amount of structural metal (outer pipe) for a relatively small heat transfer area makes it inefficient. |
| Fouling Resistance: Handles dirty or fouling fluids well on the tube side due to the large diameter and cleanability of the inner pipe. |
Common Types and Variations
- Standard Double-Pipe: A bare inner pipe inside a standard outer pipe.
- Finned Double-Pipe: The most common and important variation. The inner pipe is fitted with longitudinal fins on its outer surface. This is crucial because the heat transfer coefficient for air or gas is very poor.
- The fins greatly increase the effective surface area on the annulus side, dramatically improving heat transfer when one fluid is a gas or a viscous liquid.
- This is often called a “Finned Tube Hairpin” exchanger.
Common Applications
Double-pipe exchangers are used in applications where their specific advantages outweigh their disadvantages:
- Low Flow Rates: Small-capacity operations where the flow rate is too low for a shell and tube exchanger to be efficient or economical.
- High Pressures: Services where the operating pressure is very high (e.g., 1,000+ psi), making the simple pipe-within-a-pipe design robust and safe.
- High Temperatures: Heat transfer duties involving very high temperatures.
- Fouling Services: Handling viscous, sludgy, or fouling fluids that require regular mechanical cleaning.
- Duty in Series: As a supplementary exchanger, like a trim cooler or pre-heater, to adjust the final temperature of a fluid coming from a larger main exchanger.
the Double-Pipe (Hairpin) Heat Exchanger is a simple, robust, but space-inefficient design. It is not a high-capacity “workhorse” like the Shell and Tube, nor is it a compact “efficiency champion” like the Plate exchanger.
Instead, it is a “niche specialist” perfectly suited for specific jobs: low flow rates, very high pressures, and duties requiring true counter-flow in a simple, cleanable package. Its use of finned tubes also makes it the go-to choice for small-capacity gas cooling or heating applications.