Classification Of Heat Exchangers

Introduction
Heat: is energy in transit from one mass to another because of a temperature difference between the two. A form of energy associated with the motion of atoms or molecules and transferred from a body at a higher temperature to one at a lower temperature. Heat energy will move from a high energy state to that of a lower energy state. The process will continue until a state of equilibrium is reached. Equilibrium is the energy state where the material is at the same energy level as its surroundings.
A heat exchanger is defined as device used to transfer thermal energy (enthalpy) between two or more fluids, between the solid surface and a fluid. The fluids can be single compounds or mixtures. The typical applications of heat exchangers include cooling or heating of fluid stream of concern, evaporation or condensation of multi-component or single fluid stream. They are also used in heat rejection or heat recovery from a system.
The heat exchanger:
Is a piece of equipment built for efficient heat transfer from one medium to another? The media may be separated by a solid wall, so that they never mix, or they may be in direct contact.
Heat exchangers are found in most chemical or mechanical systems. They serve as the system's means of gaining or rejecting heat. Some of the more common applications are found in heating, ventilation and air conditioning (HVAC) systems, radiators on internal combustion engines, boilers, condensers, and as pre-heaters or coolers in fluid systems. This unit will review some specific heat exchanger applications
Heat exchangers consists of the heat exchanging elements such as matrix or core containing heat transfer surface and the fluid distribution elements such as manifolds, headers, tanks, seals, or pipes. Normally, there are no moving parts in the heat exchangers. The exceptions are in rotary regenerator (where the matrix is driven mechanically to rotate at some given speed)
Classification
Heat exchangers can be classified according to the transfer process, flow arrangement, construction, surface compactness, heat transfer mechanisms, and number of fluids. In this article, the heat exchangers are classified in heat transfer process. In addition, brief description is provided for each classification along with the selection criteria. The following are the main types:
Shell and Tube Heat Exchangers
These are tubular heat exchangers that are widely used in the industry for a number of reasons. They can be custom designed for virtually any operating conditions and capacity, such as from high vacuums to the ultra-high pressures (over 100 MPa). They are limited only by materials of construction. These exchangers can be designed for the special operating conditions such as heavy fouling, vibration, highly viscous fluids, corrosion, erosion, radioactivity and multi-component mixtures and so on. These are versatile exchangers that are made from broad range of materials (metal and non-metal) and in different sizes. They are widely used as process heat exchangers in chemical industries and petroleum-refining; as condensers, steam generators, oil coolers in the power plants, boiler feed water heaters, refrigeration applications.
The shell-and-tube exchangers are basically non-compact exchangers. The heat transfer surface area to volume ratio ranges from about 50 to 100 meter square. Therefore, they need considerable amount of support structure, space, and capital and installation costs. This explains why they are expensive compared to compact heat exchangers. However, they are more efficient and highly effective in heat transfer.
Compact Heat Exchangers
These heat exchangers are characterized by large heat transfer surface per unit volume of the exchanger, resulting in reduced space, weight, support structure, and footprint; reduced energy requirement and cost; improved process design, plant layout, and processing conditions; and low fluid inventory compared to conventional designs such as shell-and-tube exchangers. Extremely high heat transfer coefficients h are achievable with small-hydraulic- diameter flow passages with gases, liquids, and two-phase flows.
The unique characteristics of compact extended (plate-fin and tube-fin) surface exchangers, as compared with the conventional shell-and-tube exchangers, are: (1) there are many surfaces available with different orders of magnitude of surface area density; (2) there is flexibility in distributing surface area on the hot and cold sides as war- ranted by design considerations; and (3) there is generally substantial cost, weight, or volume savings.
Regenerators. The regenerator is a storage type exchanger. The heat transfer surface or elements are usually referred to as a matrix in the regenerator. In order to have continuous operation, either the matrix must be moved periodically into and out of the fixed streams of gases, as in a rotary regenerator.

References 1. Heat energy will move from a high energy state to that of a lower energy state. The process will continue until a state of equilibrium is reached. Equilibrium is the energy state where the material is at the same energy level as its surroundings. 2. Designing of heat exchangers. http://www.mie.uth.gr/ekp_yliko/CEP_Plate_and_Frame_HX.pdf