In Anhydrous Caustic Soda (NaOH) plants, various types of heat exchangers may be used depending on the specific requirements of the process and design standards such as ASME Section VIII Div 1 & Div 2, TEMA, API 660, HTRI, and PVElite. Here are some common types of heat exchangers that might be employed:

1. Shell and Tube Heat Exchangers: These are the most common type of heat exchangers used in industrial applications, including chemical plants. They consist of a series of tubes through which one fluid flows, surrounded by another fluid that flows through a larger shell surrounding the tubes. They can handle high pressures and temperatures, making them suitable for many chemical processes.

2. Plate Heat Exchangers: These exchangers consist of multiple thin, slightly separated plates that have very large surface areas and fluid flow passages for heat transfer. They are compact, efficient, and suitable for applications where close temperature approaches are required.

3. Air Cooled Heat Exchangers (ACHE): In cases where using water or other liquids for cooling is not practical, ACHEs use air as the cooling medium. They consist of finned tubes exposed to air flow, facilitating heat exchange.

4. Double Pipe Heat Exchangers: These are simple, compact heat exchangers consisting of one pipe inside another larger pipe. They are often used for smaller scale applications or where simplicity and ease of maintenance are important.

5. Plate and Frame Heat Exchangers: Similar to plate heat exchangers but with a different configuration, plate and frame heat exchangers use multiple layers of plates and gaskets to separate the fluids and enhance heat transfer efficiency.

6. Spiral Heat Exchangers: These exchangers have two concentric spiral channels, allowing for counter-current flow of the two fluids. They are compact and can handle high fouling fluids.

7. Finned Tube Heat Exchangers: Typically used for air heating or cooling, finned tube heat exchangers increase the heat transfer surface area by adding fins to the tubes, which improves heat transfer efficiency.

The selection of the type of heat exchanger depends on factors such as the specific requirements of the NaOH production process (including temperature, pressure, flow rates, and fluid properties), space constraints, maintenance considerations, and regulatory standards outlined in ASME Section VIII Div 1 & Div 2, TEMA, API 660, HTRI, and PVElite. Each of these standards provides guidelines and specifications that influence the design, fabrication, inspection, and testing of heat exchangers used in industrial plants.