It is essential to increase or decrease the temperature of one or two flow streams in numerous engineering applications. A heat exchanger economically accomplishes this double action. A heat exchanger is a heat transfer equipment used to transfer internal thermal energy between two or more fluids available at various temperatures. In most heat exchangers, the fluids are separated by a heat transfer surface, and normally, they do not mix. Heat exchangers are utilized in various applications, such as power, the process, transportation, petroleum, cryogenic, air-conditioning, refrigeration, heat recovery, alternate fuels, and other industries. Power plants use heat exchangers to collect heat from hot waste gases to enhance power efficiency. Refrigerators employ heat exchangers to dump the heat from inside the fridge to the room that it is sitting in. Vehicles use heat exchangers to transmit waste heat to the atmosphere to prevent overheating.
Gas Turbines basically consist of three main sections:
In common, industrial heat exchangers have been categorized according to (1) construction, (2) pass arrangements, (3) phase of the process fluids, (4) transfer processes, (5) heat transfer mechanisms, and (6) Phase change mechanisms.
Based on the constructional details, heat exchangers are classified as follows
1. Tubular heat exchangers, which are double pipe, shell and tube, coiled tube.
2. Plate heat exchangers (PHEs) including gaskets, brazed, welded, spiral, panel coil, lamella.
3. Extended surface heat exchangers, which are tube-fin or plate-fin.
4. Regenerators which has fixed matrix or rotary matrix.
Based on the Pass Arrangements, they are either single pass or multipass. A liquid is considered to have made one pass if it flows through a section of the heat exchanger through its full length once. A fluid is reversed and flows through the flow length two or more times in a multipass arrangement.
According to the Phase of Fluids, Heat exchangers are classified as
1. Gas-liquid type, which is mostly tube-fin-type compact heat exchangers with the liquid on the tube side.
2. Liquid-liquid type, which is a shell and tube heat exchanger.
3. Gas–Gas, which is found in exhaust gas–air preheating, recuperators, rotary regenerators, intercoolers, and/or aftercoolers to cool supercharged engine intake air of some land-based diesel power packs and diesel locomotives, and cryogenic gas liquefaction systems.
According to Transfer Process, heat exchangers are classified as indirect contact type, including direct transfer type, storage type, fluidized bed, and direct contact type with cooling towers.
The primary heat transfer mechanisms employed for heat transfer from one fluid to the other one are:
1. Single-phase convection, forced or free,
2. Two-phase convection (condensation or evaporation) by forced or free convection, and
3. Combined convection and radiation.
Heat exchangers are also classified based on the phase change mechanisms as condensers and evaporators.
A heat exchanger comprises heat-exchanging components, such as a core or matrix containing the heat transfer surface including primary and secondary surface and fluid circulation elements such as headers or tanks, inlet and outlet nozzles, or pipes. Ordinarily, there are no moving parts in the heat exchanger; nevertheless, there are exemptions, such as a rotational regenerative exchanger or a scraped surface heat exchanger. The heat transfer surface is a plane of the exchanger core that is in direct contact with fluids and within which heat is transferred by conduction. That division of the surface that is in close contact with both the hot and cold fluids and conveys heat between them is attributed to the primary or direct surface. The excess of fins to the primary surface diminishes the thermal resistance on that side, increasing the whole of the heat transfer from the surface for the same temperature difference.
Various quantitative and qualitative design aspects and their interaction and interdependence arrive at an optimum heat exchanger design. Most of these considerations are dependent on each other and should be considered simultaneously to arrive iteratively at the optimum exchanger design based on an optimum system design approach. The process specification is one of the most critical steps in heat exchanger design. It includes problem specifications for operating conditions, exchanger type, flow arrangement, materials, and design/manufacturing/operation considerations.
The first and most important consideration is to select the design basis (i.e., design conditions). Next comes an analysis of the performance at the design point and off-design (turndown) conditions. The design basis would require the specification of operating conditions and the environment in which the heat exchanger will be operated. These include fluid mass flow rates (including fluid types and their thermophysical properties), inlet temperatures and pressures of both fluid streams, required heat duty and maximum allowed pressure drops on both fluid sides, fluctuations in inlet temperatures and pressures due to variations in the process or environment parameters, corrosiveness and fouling characteristics of the fluids, and the operating environment (from safety, corrosion/erosion, temperature level, and environmental impact points of view).
The choice of a particular flow arrangement is dependent on the required exchanger effectiveness, exchanger construction type, upstream and downstream ducting, packaging envelope/footprint, allowable thermal stresses, and other criteria and design constraints. The tube fluid is usually selected for shell and tube exchangers as the one having more fouling, high corrosiveness, high pressure, high temperature, increased hazard probability, high cost per unit mass, and/or low viscosity. The maximum allowable pressure drop will also dictate which fluid will be selected for the tube side (high-pressure fluid) and which for the shell side. Mechanical design is another essential aspect of ensuring the exchanger's mechanical integrity under steady-state, transient, startup, shutdown, upset, and part-load operating conditions during its design life.