A U-tube heat exchanger consists of a copper U-tube bundle mounted within a steel cylindrical shell, where the cold fluid stream circulates through the tubes and the hot fluid stream circulates through the shell. In contrast, a shell-and-tube heat exchanger has a tube bundle with straight tubes, rather than U-shaped tubes. While both types of heat exchangers are used in HVAC systems, U-tube heat exchangers are more commonly used due to their ease of maintenance and cleaning.