Heat exchanger

The heat exchangers (including the heat exchanger or heat exchanger) is an apparatus, the thermal energy transfers from one material flow to another.

4.1 Both media gaseous
4.2 A medium gaseous, a liquid
4.3 A gaseous medium, a phase transition in the gas / liquid
4.4 A medium liquid phase transition in a gaseous / liquid
4.5 Both liquid media
4.6 Other

Classification

Heat exchangers are arranged in a three structured classification of the thermal processes in terms of heat transfer:

Direct heat transfer is based on the operation of combined heat and mass transfer in separable material flows. Representative application is the wet cooling tower.
Indirect heat transfer is characterized in that material flows are separated by a heat transmissive wall space. Heat exchanger of this class are also called recuperator. This class includes, for example, radiators and heat exchangers.
Half Indirect heat transfer uses the characteristics of a heat accumulator. In this case, both substances are brought even schedule the heat storage in contact. The heat accumulator is alternately heated by the hotter medium, and then cooled by the colder medium, so as to transfer thermal energy from the hotter to the colder medium. Heat exchanger of this class are called regenerator. For example, here the heat wheel is classified.

The amount of heat transfer to each other are heavily dependent on the geometric leadership of both streams. The management of material flows must be distinguished in three basic forms.

Counter-current leads the substances so that they flow past each accommodating. Ideally, the temperature of the material flows are exchanged, that is, that the original cold medium reaches the temperature of the initially hot fluid and vice versa. Required for this ideal case, the same heat capacity flow on both sides of the heat exchanger. In addition, the heat exchanger would have an efficiency of 100 percent have. For these reasons, an exchange of temperatures is only approximately possible in practice. A use case represents the heat recovery
DC performs the materials so that they pass alongside each other in the same direction. Ideally, both material temperatures are equalized and are always between the initial temperatures. The direct current is used, for example, if a fast, reliable cooling is necessary. A disadvantage may be the stress of the material due to the temperature differences.
Crossflow leads the streams so that cross their directions. This material guide located in outcome between counter and DC. The cross-flow is used when one wants to bring the current to a specific temperature, for example by means of a heat exchanger.

Combinations of the basic shapes are common, as this supplement their benefits.

Countercurrent flow leaves the substances in total accommodating flow past each other, even though they often cross on their way. Ideally, the temperature of the material flows are exchanged as in countercurrent.

Performance of a heat exchanger

The efficiency in terms of the first law of thermodynamics for a heat exchanger is the ratio of the recorded thermal energy to the cold side to outgoing energy on the warm side. Since insulation reduces the heat loss to the environment, but does not prevent, a portion of the useful heat lost. Depending on how large the temperature difference between the media and the environment, this loss can be more or less large.

The performance of a heat exchanger is then great, if he is able to warm up as much as possible to be heated material flow as much as possible and cool the other stream. A natural limit for this is described by the second law of thermodynamics, according to which heat always flows from the hot to the cold stream.

An example:

The temperature change rate, called in air handling technology and heat recovery, compare the reached from the real heat exchanger apparatus temperature change with the theoretically possible. For example, assume that the reheating water ( inlet: 10 ° C ) is heated to 48 ° C at the outlet of the heat exchanger, so it is getting warmer by 38 K. Then the temperature efficiency is 38/40 = 0.95 or 95%. We now increase the amount of water that flows every second through the heat exchanger, then the achievable temperature will change. This means that the temperature efficiency is dependent on the operating conditions. This is a statement like " The heat exchanger has a (temperature) efficiency of 95 %. " Without further information is incomplete and is not in itself actionable statement dar.

In the automotive industry, the term of the Q100 has coined to characterize the performance of a heat exchanger.

In general, a counter-flow heat exchanger under otherwise identical conditions, more heat transfers as a direct -current heat exchanger. The reason is the higher mean temperature difference of the counterflow heat exchanger along the surface, which is crucial for the higher heat flow.

The heat transfer performance determined to be:

Here, the heat transfer coefficient [ W / ( m² K) ], the transmission area [m²] and the mean temperature difference [ K]. The average temperature difference (also: logarithmic mean temperature difference ) is calculated from the temperature difference of the media on the one hand, (), and on the other side () of the heat exchanger. In this direct current, the temperature difference on the entry side and on the exit side and the counter-flow, however, the temperature differences between each inlet of the one medium and the outlet of the other medium.

In the air - cooling technology is acc. SI base units DIN 8941 instead of using.

Execution

General

For a good efficiency of the material that separates the media must have good thermal conductivity and large surface area. Next, the heat transfer between the surface and flowing media must be as well. For a turbulent flow is low. This takes place primarily at a high Reynolds number. This means that the flow velocity is high and the viscosity of the medium should be low. However, increased speed, and a large wetted surface area also means an increased amount of energy for pumping the media through the heat exchanger.

In heat exchangers in which a medium is a liquid, the other medium is a gas (usually air), the heat capacity per volume of media is very different. Therefore, the gas must flow much more as a liquid, and it is necessary to increase the area for heat transfer to the gas. This is often done by fins or plates, such as high temperature heating elements, the cooling coils at the rear of a refrigerator or air conditioner and the radiator of the car.

Materials

Heat exchangers consist in most cases made of metal, but also of enamel, plastic, glass or silicon carbide. In the climate area comes mainly copper and aluminum because of the good thermal conductivity are used. In the industry, especially steel and stainless steel used here especially since the resistance of the materials is required. Radiator, however, are now mostly used to be made of sheet steel, gray cast iron. Plastic, enamel, glass or silicon carbide can be used for heat exchangers in the chemical industry, where the aggressiveness of the fluids do not allow the use of metallic materials. Silicon carbide can be used because of its extreme thermal stability (decomposition temperature above 2,200 ° C ) even when the heat exchangers, the material temperatures are above the limit of application of the metals. However, such high-temperature ceramic heat exchangers are still in development.

Designs

Here only the types of heat exchangers for liquid and gaseous media treated:

Heat exchangers for direct heat transfer

Wet cooling towers are used for re-cooling tasks in power plants. In this case, hot water is cooled in direct contact with the ambient air.

Recuperators

Recuperators have the two media for each a separate room.

Plate heat exchanger: Numerous parallel plates, the spaces are taken alternately from one and other media. A special form of the plate is the
A spiral heat exchanger, where instead of flat plates, a spirally wound sheet metal is used.
Tube heat exchangers or shell and tube heat exchangers: Due to the tubes ( " tube room ", usually a plurality of parallel tubes ) are being pumped or otherwise promoted. The pipes are located in the so-called shell space, a boiler, by the another media flows. Above all, shell and tube heat exchanger with a large number of parallel tubes are specialized in producing relatively expensive (many welds ).
U-tube heat exchanger in which the tubes are bent into a U -shape. Advantage is that the tube bundle can be easily inserted into the vessel and removed, because it is attached only on one side (for example, welded into the lid of the vessel ).
Jacket tube heat exchanger consist of two concentric tubes; the medium in the inner tube is heated or cooled by the medium in the outer tube ( usually water). This design is used with highly viscous or solids laden fluids ( for example, suspensions, slurries ), but has a low heat transfer surface area and thus a low efficiency. It is particularly suitable for high pressure in the inner tube.
Heating coil and cooling coil are a combination of pipes ( for the liquid medium ) and fixed thereto blades ( for the gaseous medium).
Countercurrent bed heat exchanger is a recuperative heat exchanger, which is composed of a plurality of lamella Wärmeübertragerschichten.

Regenerators

Regenerators are alternately flows through the hot and the cold medium and function because of their heat capacity.

Regenerators are used primarily for gases; the heat energy is stored in a solid state and discharged subsequently from the same surface to the other air stream. A distinction mobile storage materials as for the rotary heat exchangers, air preheaters and Stirling engine.
Stationary storage materials as for the hot blast stoves

In rotary heat registers, for example, refractory bricks, aluminum sheets, for regenerators in Stirling engines and copper braids for hot blast stoves used.

Applications

Both media gaseous

Exhaust heat to preheat the combustion air sucked in industrial plants, such as rotary heat exchangers and hot blast stoves.
Exhaust air heat recovery heat recovery, so heating the supply air in the ventilation or air-conditioned buildings for passive houses, the so-called air -to-air heat exchanger.
Air / air heat exchanger for cabinet cooling.

A gaseous medium, a liquid

Air heating or cooling for direct thermal treatment of the supply air in air conditioning systems
Space heating via radiators, convector as: Characteristic is the ribbed design, creating large surfaces are achieved.
Regenerative heat recovery for heating or cooling of the supply air in air-conditioned buildings.
Feed water of steam generators ( " economizer ").
Air / water heat exchanger for cabinet cooling.
Heat transfer to the hot water in gas heaters.
Charge air cooler for internal combustion engines

A medium gas, a phase transition in the gas / liquid

Condensation in steam turbines.
Evaporation in steam boilers from coal and nuclear power plants.
Evaporation and condensation of the refrigerant in air conditioning systems.
Air heat transfer of heat pumps.
Heat dissipation through the cooling coil in and on the back wall of refrigerators.
Heat pipe ( heat pipe )
Heat transfer in condensation dryers ( without exhaust port).

A liquid medium, a phase transition in the gas / liquid

Condenser of the heat pump in heating systems

Both liquid media

Groundwater heat transfer for heat pumps.
Heat transfer in ocean geothermic Gradientkraftwerken ( " ocean thermal power plant ").
Heat transfer in storage tanks of solar thermal systems.
Heat transfer between sea water and cooling water of marine diesel engines.
Heat transfer in district heating networks
From Biology: In order to minimize heat loss upon contact with the cold ground, run the wires into penguin feet as a counter-flow heat exchanger.

Other

The following applications do not belong to the heat exchangers, since the heat is not transferred between two moving fluids: geothermal heat exchanger, half-pipe coil, heat sink, radiator

Aluminium Solar plant Radiator (engine cooling)

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