Heat exchangers are devices that transfer heat between two or more fluids. They are classified based on different factors such as the type of fluids used, the flow pattern, and the mechanical design of the heat exchanger surface. Based on the mechanical design of the heat exchanger surface, heat exchangers are classified into three categories.

1. Shell and Tube Heat Exchangers
Shell and tube heat exchangers are the most common type of heat exchangers used in industrial applications. They consist of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid flows through the tubes while the other flows around the tubes in the shell. These types of heat exchangers are further classified based on the flow pattern of the fluids, such as co-current flow, counter-current flow, and cross-flow.

2. Plate Heat Exchangers
Plate heat exchangers are compact and have a high heat transfer rate compared to shell and tube heat exchangers. They consist of a series of thin plates with alternating channels for the two fluids. The fluids flow through alternate channels and are separated by the plates. These types of heat exchangers are easy to clean and maintain, making them ideal for applications where hygiene is critical.

3. Finned Tube Heat Exchangers
Finned tube heat exchangers are used when the fluids have a low heat transfer coefficient. They consist of a tube with fins attached to the outside. The fins increase the surface area of the tube, which enhances heat transfer. These types of heat exchangers are widely used in air conditioning and refrigeration systems.

In conclusion, heat exchangers are classified into three categories based on the mechanical design of the heat exchanger surface, which are shell and tube, plate, and finned tube heat exchangers.