Question

For a specified fluid pair, inlet temperatures, and mass flow rates, what kind of heat exchanger will have the highest effectiveness: double-pipe parallel- flow, double-pipe counterflow, cross-flow, or multipass shell-and-tube heat exchanger?

Short answer

Answer: The multipass shell-and-tube heat exchanger has the potential to have the highest effectiveness, given the specific fluid pair, inlet temperatures, and mass flow rates. However, the effectiveness of any heat exchanger depends on its specific design, materials, and operating conditions.

Step-by-step solution

Understand heat exchanger effectiveness

Heat exchanger effectiveness is the ratio of the actual heat transfer to the maximum possible heat transfer between the fluids. The higher the effectiveness, the better the heat exchanger is at transferring heat. Different heat exchanger configurations have different effectiveness values, depending on their design.

Investigate double-pipe parallel-flow heat exchanger

In a double-pipe parallel-flow heat exchanger, fluids flow in the same direction. The temperature difference between the fluids decreases along the length of the exchanger, reducing the heat transfer rate. Thus, the effectiveness of this type of heat exchanger is lower than other types.

Investigate double-pipe counterflow heat exchanger

In a double-pipe counterflow heat exchanger, fluids flow in opposite directions. This arrangement maintains a more constant temperature difference between the fluids, which leads to a higher heat transfer rate. The effectiveness of this type of exchanger is higher than the parallel-flow heat exchanger.

Investigate cross-flow heat exchanger

In a cross-flow heat exchanger, fluids flow perpendicularly to each other. This arrangement provides a high rate of heat transfer, resulting in higher effectiveness than both parallel-flow and counterflow heat exchangers. However, the effectiveness of a cross-flow heat exchanger depends on the specific flow arrangement (i.e., whether the flows are mixed or unmixed).

Investigate multipass shell-and-tube heat exchanger

In a multipass shell-and-tube heat exchanger, fluids flow through multiple passes within the tubes and shell. This configuration increases the number of heat transfer surfaces, resulting in a higher heat transfer rate and effectiveness. The effectiveness of a multipass shell-and-tube exchanger depends on the number of passes and the specific design.

Determine the heat exchanger with the highest effectiveness

From the information provided in steps 2-5, and knowing the specific fluid pair, inlet temperatures, and mass flow rates, one can conclude that the multipass shell-and-tube heat exchanger has the potential to have the highest effectiveness. However, it is important to note that the effectiveness of any heat exchanger depends on its specific design, materials, and operating conditions.

Key concepts

Double-Pipe Parallel-Flow

In a double-pipe parallel-flow heat exchanger, both fluids enter and move in the same direction. This setup is simple and straightforward.
Because both fluids travel side-by-side from start to finish, the temperature difference between them slowly decreases along the length of the heat exchanger.

• The diminishing temperature difference means less heat transfer occurs over time.
• As a result, this type of exchanger has comparatively lower effectiveness.
• It’s easy to understand and implement, making it suitable for systems where simplicity is needed over efficiency.

In general, while the double-pipe parallel-flow system is easy to construct and use, the gradually reducing heat transfer potential causes its effectiveness to be on the lower side compared to other configurations.

Double-Pipe Counterflow

A double-pipe counterflow heat exchanger features fluids flowing in opposite directions. This design maximizes the temperature gradient, as one fluid becomes cooler while the other becomes hotter, maintaining a consistent temperature difference along the entire length.

• This consistent temperature difference allows for more effective heat transfer.
• Therefore, counterflow arrangements usually exhibit higher effectiveness than parallel-flow
• They’re ideal in applications where efficiency is a crucial factor.

The double-pipe counterflow heat exchanger's design leverages the opposing movement of fluids to enhance its ability to transfer heat, making it superior in effectiveness to the parallel-flow configuration.

Cross-Flow Heat Exchanger

In the cross-flow heat exchanger, one fluid flows perpendicular to another, leading to a more varied temperature profile.
This setup benefits from a large surface area for heat transfer, boosting its effectiveness.

• This exchanger can be configured with either mixed or unmixed flow.
• Unmixed flow on both sides achieves higher effectiveness.
• However, the effectiveness can vary depending on how the flows are mixed or unmixed.

Cross-flow heat exchanges are often used in applications needing good heat transfer rates without the complexity of shell and tube designs. While their effectiveness is generally higher than parallel and counterflow systems, the actual values can depend on specific configurations and design intricacies.

Multipass Shell-and-Tube

The multipass shell-and-tube heat exchanger is intricate but highly efficient.
Its design involves fluids passing through tubes with several turns or loops within the shell, maximizing the contact surface for heat exchange.

• More complex and multiple passes provide more surfaces for heat transfer.
• This results in a higher heat transfer rate than single-pass designs.
• Consequently, this exchanger often boasts the highest effectiveness of those considered here.

Because of its potential complexity and flexibility, the multipass shell-and-tube design can be tailored to specific operational needs, making it exceptionally versatile and effective in diverse industrial applications. It's a popular choice where maximum heat exchange is needed within compact space.