In most heat exchangers, fluids are separated by a heat transfer surface that does not allow them to mix. This is called a direct transfer type, or recuperators. Heat exchangers that allow for intermittent transfer between fluids are called indirect transfer types or storage types.
Shell and tube heat exchangers are made with round tubes attached to cylindrical shells with their axes coaxial with the shell axis. According to Heat Exchangers by Dr. Osama Abdel-Bary, the specific differences between these types of exchangers are found in the construction features. These types are found in petroleum-refining and chemical industries as well as in waste heat recovery applications.
Double pipe heat exchangers consist of one pipe placed inside another one with a larger diameter. The sections are connected in a series or parallel arrangements, depending on need. These types of devices are small and are used for small amounts of heat transfer (up to 50 m), according to Dr. Abdel-Bary.
Compact heat exchangers are classified according to the kind of compact elements found within them. There are five different types, including circular and flattened circular tubes, tubular surfaces, surfaces with flow normal, plate fin surfaces and finned tube surfaces. All five types have one important element, which is the core (a pair of parallel plates with metal connections).
Plate and frame heat exchangers are built on thin plates that do not accommodate very high pressures and temperatures. They are lightweight and small and are often used in weight-sensitive locations such as ships and oil production platforms. Plate and frame heat exchangers may also be classified as plate, spiral plate, lamella and plate coil exchangers, according to Dr. Abdel-Bary.
Spiral exchangers consist of two long, parallel plates arranged into a spiral. The open sides are sealed off and held in place by clamps. The top and bottom covers are removable, thus allowing for the spiral exchanger to be easily cleaned.
Regenerative exchangers store heat and remove it from a surface. These types of exchangers are designed with a matrix that can move easily in to and out of fixed hot and cold fluid streams. Unfortunately, regenerators are known to suffer from leakage and fluid problems, according to Dr. Abdel-Bary. Nevertheless, they are used extensively in electrical power generating stations for air preheating.
Scrapped surface exchangers are used in situations where the fluid is likely to crystallize on cooling. They have a wide range of applications including food, chemical, pharmaceutical and petrochemical industries.