Question

Classify heat exchangers according to flow type, and explain the characteristics of each type.

Short answer

Answer: The three types of flow in heat exchangers are parallel flow, counterflow, and crossflow. Parallel flow heat exchangers have both fluids flowing in the same direction, simpler design, and less efficiency. Counterflow heat exchangers have fluids flowing in opposite directions, resulting in a more even temperature distribution and higher efficiency, but with a more complex design. Crossflow heat exchangers have fluids flowing perpendicular to each other, with efficiency lying between parallel and counterflow heat exchangers and design complexity between the two as well. Mixed crossflow heat exchangers offer better temperature control, while unmixed ones provide higher heat transfer rates.

Step-by-step solution

Types of Flow in Heat Exchangers

There are mainly three types of flow in heat exchangers: parallel flow, counterflow, and crossflow. Each type has its own advantages and disadvantages.

1. Parallel Flow Heat Exchangers

In parallel flow heat exchangers, both the hot and cold fluids flow in the same direction. The temperature of both fluids decreases or increases along the same path. Characteristics of parallel flow heat exchangers: - It has a simpler design compared to other types of heat exchangers. - The temperature difference between the hot and cold fluid is high at the entrance and low at the exit. This means that the driving force for heat transfer decreases along the heat exchanger. - It is less efficient than counterflow heat exchangers because the temperature difference decreases along the length, which reduces the overall heat transfer.

2. Counterflow Heat Exchangers

In counterflow heat exchangers, the hot and cold fluids flow in opposite directions. This allows a more even temperature distribution over the entire length of the heat exchanger. Characteristics of counterflow heat exchangers: - It is more efficient than parallel flow heat exchangers because the temperature difference between the hot and cold fluid is more or less constant along the length of the heat exchanger. This maintains a high driving force for heat transfer. - It can achieve a higher temperature change in the cold fluid compared to parallel flow heat exchangers. - The design is more complex than parallel flow heat exchangers.

3. Crossflow Heat Exchangers

In crossflow heat exchangers, the hot and cold fluids flow perpendicular to each other. There are two types of crossflow heat exchangers: unmixed and mixed. Characteristics of crossflow heat exchangers: - They have a lower heat transfer efficiency than counterflow heat exchangers but higher than parallel flow heat exchangers. - The design complexity lies between that of parallel flow and counterflow heat exchangers. - Mixed crossflow heat exchangers allow for better temperature control, while unmixed crossflow heat exchangers offer higher heat transfer rates. To summarize, heat exchangers can be classified based on their flow types: parallel flow, counterflow, and crossflow. Each type has its own advantages and disadvantages, as well as unique characteristics that influence their efficiency and design complexity.