Question

When is a heat exchanger classified as being compact? Do you think a double- pipe heat exchanger can be classified as a compact heat exchanger?

Short answer

Why or why not? Answer: No, a double-pipe heat exchanger generally does not meet the criteria for being classified as a compact heat exchanger. This is because compact heat exchangers have a large heat transfer surface area per unit volume (typically greater than 700 m^2/m^3), while double-pipe heat exchangers have a low surface area per unit volume due to their relatively simple concentric tube geometry. Compact heat exchangers usually have more complex geometries and deliver higher heat transfer rates within a small volume, whereas double-pipe heat exchangers are more suitable for smaller applications with relatively low heat transfer surface area requirements.

Step-by-step solution

Understand the concept of a compact heat exchanger

A compact heat exchanger is one in which the heat transfer surface area per unit volume is large, typically greater than 700 m^2/m^3. A heat exchanger is considered compact to improve its effectiveness and efficiency by accommodating a large heat transfer surface within a small volume, resulting in higher heat transfer rates.

Learn about double-pipe heat exchangers

A double-pipe heat exchanger, also known as a hairpin or concentric tube heat exchanger, consists of two concentric tubes or pipes. One fluid flows through the inner tube while the other fluid flows through the annular space between the inner and outer tube. The fluid can flow in the same direction (parallel flow) or in opposite directions (counter flow). These heat exchangers are generally used for smaller applications where the required heat transfer surface area is relatively low.

Compare the surface area per unit volume of a double-pipe heat exchanger to the criteria for a compact heat exchanger

As mentioned earlier, a compact heat exchanger typically has a surface area per unit volume greater than 700 m^2/m^3. In a double-pipe heat exchanger, the heat transfer surface area is limited by the geometry of the concentric tubes, which primarily consists of the outer surface of the inner tube and the inner surface of the outer tube. This geometry does not provide as high a surface area per unit volume as other compact heat exchangers, such as plate, fin, or folded-tube heat exchangers, which have more complex geometries designed to increase the heat transfer surface area.

Conclusion

Based on the comparison of the heat transfer surface area per unit volume, a double-pipe heat exchanger generally does not meet the criteria for being classified as a compact heat exchanger. Compact heat exchangers typically have more complex geometries and deliver higher heat transfer rates within a small volume, while double-pipe heat exchangers are more suitable for smaller applications with relatively low heat transfer surface area requirements.

Key concepts

Compact Heat Exchanger

A compact heat exchanger is designed to efficiently manage heat transfer within a very small space. These heat exchangers maximize the amount of heat transfer surface area in relation to the volume they occupy. For a heat exchanger to be classified as compact, its heat transfer surface area must exceed 700 square meters per cubic meter \( (m^2/m^3) \).
This is achieved by employing intricate designs such as plate or fin geometries. The benefits of compact heat exchangers include:

• Improved heat transfer rates - due to the high surface area
• Greater efficiency in heat exchange
• Better space utilization - making them ideal for use where space is limited

Although achieving a high surface area-to-volume ratio can be complex and expensive, the improved performance often justifies the design effort.

Double-Pipe Heat Exchanger

The double-pipe heat exchanger, also recognized as a hairpin or concentric tube heat exchanger, is one of the simplest designs for exchanging heat between two fluids. It consists of two pipes, one inside the other. The inner pipe carries one fluid, while the other fluid flows in the space between the two pipes.
These heat exchangers can operate in:

• Parallel flow – both fluids move in the same direction
• Counter flow – fluids move in opposite directions, which is often more efficient

Double-pipe heat exchangers are favored for their straightforward design and ease of maintenance, used primarily for smaller scale applications. However, they typically feature a limited heat transfer surface area. This makes them less suitable for applications requiring high heat transfer rates.

Heat Transfer Surface Area

The concept of heat transfer surface area is central to understanding the efficiency of heat exchangers. It refers to the surface through which heat is exchanged between fluids. The larger the surface area, the more opportunity there is for heat to be transferred, which boosts the exchanger's effectiveness.
Compact heat exchangers are designed to maximize this surface area within a given volume. A higher surface area is often achieved with innovative designs such as folded or finned tubes, effectively facilitating greater heat exchange without occupying extra space. In contrast, double-pipe heat exchangers have a limited geometry. Their surface area consists primarily of the outside surface of the inner tube and the inside surface of the outer tube, resulting in a lower surface area per unit volume compared to more complex compact designs.
Increasing the heat transfer surface area is key for improving efficiency in heat exchanger technology, but it also requires balancing cost, space, and design complexity.