Which of the following is NOT a main type of heat exchanger?

The choice of "thermal jar" is correct as it is not recognized as a main type of heat exchanger. Heat exchangers are designed to efficiently transfer heat between two or more fluids and are crucial in various industrial processes for heating, cooling, and energy recovery.

Shell and tube heat exchangers consist of a series of tubes, one set carrying the hot fluid and the other the cold fluid, allowing for effective heat transfer while providing a robust design suitable for high-pressure applications. Plate heat exchangers use thin plates stacked together to form multiple channels for fluids to flow through, allowing for a compact and efficient heat transfer surface. Finned tube heat exchangers are designed with extended surfaces that increase the effective area for heat transfer, making them particularly useful in applications where heat transfer is required between air and fluid.

In contrast, a "thermal jar" does not serve as a recognized or standard configuration for transferring heat in industrial applications, thus making it the correct answer to identify as not being a main type of heat exchanger.