Micro-heat exchangers

Because of the small thermal diffusion paths and the high specific surface area, micro heat exchangers are used for rapid heat transfer between hot and cold fluids. Compared to conventional heat exchangers, the size can be considerably 

The Influence of the wall material on the performance of a countercurrent micro heat exchanger was studied theoretically by Stief et al. [18]. The authors considered nitrogen gas flow through channels of 50 pm height, 500 pm width, and 10 mm length. The thickness of the separating walls was varied between 125 and 500 pm. 

If the thermal conductivity of the wall material is changed, different axial temperature profiles in the channels with hot and cold gases and within the channel walls are obtained. 

The performance of the heat exchanger is characterized by its efficiency, i hex which is the ratio of the actual heat transferred and the maximum. 

In the case of equal volume flows of two identical fluids, the efficiency is given by  

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Eurther research on convective transport under low Reynolds number, quasicontinuum conditions is needed before the optimal design of such a micro heat exchanger is possible. The cooling heat exchanger is usually thermally linked to a relatively massive substrate. The effects of this linkage need to be explored and accurate methods of predicting the heat-transfer and pressure-drop performance need to be developed. 

Chapter 4 is devoted to single-phase heat transfer. Data on heat transfer in circular micro-tubes and in rectangular, trapezoidal and triangular ducts are presented. Attention is drawn to the effect of energy dissipation, axial conduction and wall roughness on the thermal characteristics of flow. Specific problems connected with electro-osmotic heat transfer in micro-channels, three-dimensional heat transfer in micro-channel heat sinks and optimization of micro-heat exchangers are also discussed. 

Understanding the differences in two-phase flow characteristics between conventional size channels and micro-channels is also important for designing mini- or micro-heat exchangers, since the flow characteristics will affect the phase change heat transfer. 

First mention of microreactors uChem processing at KfK (now FZK) in Karlsruhe first micro heat exchangers manufactured 1939... 

Substantial heat-transfer intensification was also described for a special micro heat exchanger reactor [104]. By appropriate distribution of the gas-coolant stream, the axial temperature gradient can be decreased considerably, even under conditions corresponding to very large adiabatic temperature rises, e.g. of about 1400 °C. 

Nature as model for micro-reactor development general advantages of micro flow onset of industrial interest micro heat exchanger vision of mefhanol-fuel reforming costs stiU too prohibitive [231],... 

Off-the-shelf catalogue sales of micro reactors have just started [15]. With an increasing number of commercial products, quality control will become more important. Brandner et al. describe quality control for micro heat exchangers/reactors at the Forschungszentrum Karlsruhe [23]. All manufacturing steps are accompanied by quality control and documentation. Leak rates (down to 10 mbar 1 s for He) and overpressure resistance (up to 1000 bar at ambient temperature) are measured. Under standardized conditions, the mean hydraulic diameter is determined. Dynamic tests supplement this quality control. 

Manufacturing and testing of compact micro heat exchangers with high volumetric heat transfer coefficients,... 

I., Using micro heat exchangers as diagnostic tool for the process optimization of a gas phase reaction, in Proceedings of the VDE World Microtechnologies Congress, MICRO.tec 2000,... 

Schubert, K., Experimental determination of heat transfer coefficients in micro heat exchangers, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6, pp. 241-246 (11-14 March 2002),... 

Stief et al. [125] investigated the performance of coimter-current micro heat exchangers, especially with respect to the wall conduction effects and the choice of... 

Figure 2.30 Schematic drawing of a counter-current micro heat exchanger.

Figure 2.31 Characteristic temperature profiles in a counter-current micro heat exchanger for a very low (left), intermediate (middle) and very high (right) thermal conductivity of the wall material and equal volume flows inside the two channels, reproduced from [125],...

Stiee, T, Langer, O.-U., Schubert, K., Numerical investigations of opimal heat conductivity in micro heat exchangers,... 

Figure 3.9 Image of a completely mounted cross-flow micro heat exchanger with 1 cm active volume [46].

Optimized microfabrication and advanced assembly led to the use of thin platelets, in an original version 100 pm thick with a 80 pm micro channel depth, so that very thin walls (20 pm in the case sketched) remain for separating the fluids. Therefore, also the total inner reaction volume with respect to the total construction volume or the active internal surface area is very large. The latter surface amounts to 300 cm (for both the heat transfer and reaction sides) at a cubic volume of 1 cm. Indeed, the micro heat exchangers exhibited high heat transfer coefficients for gas [46] and liquid (Figure 3.10) [47, 48] flows. 

The whole set-up for partial oxidation comprises a micro mixer for safe handling of explosive mixtures downstream (flame-arrestor effect), a micro heat exchanger for pre-heating reactant gases, the pressure vessel with the monolith reactor, a double-pipe heat exchanger for product gas cooling and a pneumatic pressure control valve to allow operation at elevated pressure [3]. 

It comprises several wafers with different functional elements such as micro heat exchanger, mini fixed-bed catalyst chambers, manifolding structures and other components not disclosed. The flow partitioning was performed in a hierachic manner. A patent gives deeper insight into the generic construction architecture of the micro-reaction system. The wafers were bonded to a stack and equipped with fluid connections [137]. 

Micro heat exchanger material Stainless steel Micro hole in pgauze diameter depth 70 pm 250 pm... 

Micro heat exchangers and also any kind of micro channel devices, heated or cooled externally, offer considerably improved heat transfer owing to their large internal specific surface areas. Hence they offer unique possibilities to steer oxidations to increased selectivity of the partial-oxidation products. 

GS 11] ]R 5] At high mass flows and with a large heat release, an undesired, substantial temperature increase even for highly efficient micro heat exchanger-reactors is observed (Figure 3.53) [115]. 

D Microfab Micro Mixer-Micro Heat Exchangers... 

D Integrated Total Systems with Micro Mixing and Micro Heat Exchange Functions... 

Different from sole combinations of micro devices, this refers to a total system with many functional elements and flow-distribution and, recollecting zones, typically composed of 2-D plate-type architecture. Each of these plates usually has a separate fimction, comprising imit operations and reaction. Frequently, micro mixing and micro heat exchange fimctions and corresponding elements are employed. Often, the system can be composed of different elements resulting in different process flow combinations. Such an approach may be termed a construction kit. 

Micro heat exchangers based on microchannel architecture. Top left and bottom Reprinted with permission from W. Ehrfeld et al., Microreactors New Technology for Modern Chemistry. Wiley-VCH Weinheim, Germany, 2000. Top right Courtesy of the Institut fuer Mikrotechnik Mainz, GmbH. 

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