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Heat exchanger

Heat exchanger

In engineering a heat exchanger (or simply an exchanger) is an apparatus in which thermal energy is exchanged between two fluids that have different temperatures.

From the thermodynamic point of view, heat exchangers can be assimilated to open systems that work without exchanging work; in other words, they exchange matter and heat with the outside, but they do not exchange jobs.

In the solar thermal industry the heat exchanger is used to transfer the heat captured through the solar radiation that is found from fluid that circulates through the solar collectors to another fluid. This other fluid is the one that will circulate through the heating system or will be stored directly in an accumulator.

Although they are widely used in the industry to control the temperature of the process, many examples of heat exchangers are found in everyday life. Some examples of common heat exchangers are the car radiator and the heater for home heating.

Classification of heat exchangers

There are different classification criteria for heat exchangers based on different characteristics.

Classification by contact between streams

Depending on the contact mode between the two currents, the heat exchangers can be:

  • in direct contact: if the interfaces of the fluids that exchange heat are in direct contact with each other; The two currents exchange thermal energy and matter, that is, they are not separated by walls;
  • in indirect contact: if the fluids are not in direct contact with each other; this possibility occurs, for example, in the case of surface exchangers, in which the two fluids are separated by a surface that is crossed by the heat flow.

In the vast majority of cases, cold and hot bodies that circulate inside a heat exchanger are fluid (ie, in liquid, gas or vapor).

Classification of surface heat exchangers by construction model

Depending on the geometry of the exchanger, it can be defined (citing only the main cases):

Tubular heat exchangers

  • Double tube heat exchanger (or "concentric tubes"): the fluids flow in two coaxial tubes, one inside (tube or tube) and another outside (ring or shirt).
  • Shell and tube heat exchanger: one of the fluids goes through the inside of the tubes (generally with a circular section) and the other one through the outside of the tubes, in a specially designed chamber (housing).
  • Trombone heat exchanger (or drip heat exchanger): consists of a helical tube or serpentine inside which flows the process fluid that is cooled through a film of water dripping from above.
  • Scraped surface heat exchanger: the tubes of these exchangers have rotating blades inside that discard the internal surface of the tube;
  • Submerged surface heat exchanger. Coil for heating or cooling fluids.
  • Pipes inside the ovens: the tubes are heated by irradiation and exchange heat with the currents that pass through them.
  • Plate heat exchanger: the two fluids overlap the opposite sides of a sheet in alternate and insulated chambers. The geometry of these exchangers is similar to the filter press. A special case is the roll-bond exchanger, in which the channels on one side are internal to a monobloc sheet, while on the other side there is a fluid, generally stationary.
  • Spiral exchanger: the two fluids pass to the opposite sides of a sheet, usually smooth, in single long chambers, spirally wound.
  • Graphite or other material exchanger: the currents circulate in cylindrical holes, generally arranged orthogonally on both sides; Expanded surface heat exchangers.
  • Jacketed equipment.
  • Regenerative type: the currents are sent alternately inside an inert brick chamber (Cowper regenerator), or in particular rotating units in sheet metal (Ljungström exchanger).

Plate heat exchangers

Classification by compactness

The "compactness" of a heat exchanger is represented by the "surface area density" (expressed in m 2 / m 3 ), which is equal to the ratio between the area of ​​the exchange surface and the volume of the heat exchanger. hot.

In the event that the exchange of heat takes place between a gas and another fluid, depending on the density value of the surface area, the exchangers are divided into:

  • Compact heat exchangers: with a surface area density greater than 700 m 2 / m 3 ;
  • Non-compact heat exchangers: with a surface area density of less than 700 m 2 / m 3 .

In the event that the exchange takes place between two liquids or if it is associated with a phase change, we define:

  • Compact heat exchangers: with a surface area density greater than 400 m 2 / m 3 ;
  • Non-compact heat exchangers: with a surface area density of less than 400 m 2 / m 3 .

Classification of thermodynamic exchangers by process type

Depending on the process for which they are used, the exchangers can be of a sensitive type (only exchange sensible heat), cooler (a process fluid is cooled through a service fluid, heater (a process fluid is heated by a service fluid), superheater, boiler (a liquid is boiled), evaporator, condenser (an aeriform is condensed), collector.

Sometimes, the term "exchanger" is used with a narrower meaning, referring to the specific case in which the purpose of the equipment is the thermodynamic exchange between two process fluids, it is called "cooler" and "heater" when one of the Two streams consist of a service fluid, so the purpose is to cool or heat a process fluid.

Classification by thermal profile

Temperature profiles for countercurrent exchange and for exchange in the same current.

Most thermodynamic heat exchange processes are not isothermal, that is, they occur at a variable temperature. In other words, a body goes into a low temperature and heats up. The other enters at high temperature and cools. In the simplest cases, the thermal exchange between the two fluids can take place mainly in three ways:

  • Equitable current exchange: fluids move in parallel paths along the same line.
  • Countercurrent exchange: fluids move in parallel paths, but in opposite directions.
  • Cross current exchange: the fluids move along orthogonal paths.

Only in the case of counter-current exchange, the cold fluid outlet temperature can be higher than the hot fluid outlet temperature. In the case of equitable current exchange, the temperatures of the two fluids approach each other during the crossing of the heat exchanger and, theoretically, can reach the same value (equal to the equilibrium temperature of the two fluids) if the Exchange surface has an infinite area: this is obviously a limiting condition, unattainable in practice.

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Last review: February 13, 2019