Direct-contact heat exchangers

Published: 2020-01-02 09:15:29
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There are various advantages of the indirect-contact heat exchangers and one is that there is no mixing of the fluids which mean that the properties of the fluids are not affected and are therefore the fluids remain in their original properties. Secondly, the cooling process in the indirect heat exchangers is faster as the fluids in the component move in opposite direction which makes the cooling process to be efficient. Thirdly, the design of the indirect-contact heat exchangers is made in a flexible manner to accommodate wider varieties of temperatures and pressures (Oechsle & Spindler, 2016). One disadvantage of the indirect-contact heat exchangers is that they require more materials to construct and this makes their design and production to be complicated. This requires more resources and technical skills to construct the indirect-contact heat exchangers hence making them more expensive as compared to the direct-contact heat exchangers.

Direct-contact heat exchangers

There are various advantages of the direct-contact heat exchangers and one of them is that they are easier to construct as compared to the indirect-contact heat exchanges, this is because they do not require special mechanism of heat exchange in the system. The second advantage is that they have very high rates of heat transfers that can be achieved hence they are more efficient in cooling, third, these types of heat exchange are comparatively inexpensive because of their relatively simple construction (Horst, Rottengruber, Seifert & Ringler, 2013). The type of heat exchanger does not have the fouling problem because there is no heat transfer wall between the two fluids that engage in the exchange of heat. There are however various disadvantages of them is that their construction is limited to the conditions when the direct contact between fluids is permissible.

Materials used in the Construction

Tubes

Tubes are of great importance in the construction of the heat exchangers, tubes used are of diameters 10 inches to more than 100 inches. The tubes are manufactured using the industry standards, in common instances, the 0.625 to 1.5-inch diameter tubes that are used in the manufacture of heat exchangers are made from low carbon steel, copper, stainless steel, copper-nickel, titanium, Inconel and hastelloy (Wang, et al., 2015). The tubes can be made through the drawing or through welding and the high-quality electro resistance welded tubes have good grain structure at the points that are welded. In some special cases of heat transfer applications, the tubes are often made with extruded and with fins and the interior are rifled.

Tube sheets

The second component of the heat exchangers is the tube sheets and these are usually made of round and flattened metal sheet. There is teen drilling of the holes on the tube ends in order to fit for the teen ends in patterns that are relative to one another. The tube sheets are mostly made of the same material as the tubes and then fitted with the hydraulic or pneumatic pressure roller to the tube sheet, this makes the holes to be drilled easily and then reamed (Hwang, Kim, Min & Jeong, 2012). They can otherwise be machined grooved.

Shell Assembly

The shell is made using either a pipe or a rolled plate metal, the shell assembly mostly uses steel because the shell assembly is mostly exposed to extreme temperatures and area with high probabilities of getting corrosion. Moreover, there are other metals and alloys that can be employed in the construction of the shell assembly and they need to specified to ensure that they have the right properties (Balistreri et al., 2013).

Bonnets and end channels

The bonnets and ends channels help in the regulation of the flow of fluids in the tube-side circuit and they are either cast or fabricated. They are mounted against the sheet of the tube using a bolt and a gasket assembly, however, many designers incorporate the use of machined groove channel in the tube sheet that seals the joint (Hwang, Yamamoto & Torii, 2014).

Baffles

These are the final components in the construction of the heat exchangers and they function in two ways where first they function as tube guides to prevent the vibration from the flow of induced eddies and secondly, they help in directing the shell-side fluids across the bundle to increase the velocity and the turbulence to increase the rate of transfer (Oechsle & Spindler, 2016).

 

References

Hwang, S. W., Kim, D. H., Min, J. K., & Jeong, J. H. (2012). CFD analysis of fin tube heat exchanger with a pair of delta winglet vortex generators. Journal of mechanical science and technology, 26(9), 2949-2958.

Wang, N., Gao, W., Liu, T., Zhang, Y., Niu, W., & Zhang, D. (2015, December). Supply Power Capacity Design Method of Active Heat Exchanger for Precision Machine Tool Structure. In COMPUTER SCIENCE and ENGINEERING TECHNOLOGY (CSET2015), MEDICAL SCIENCE and BIOLOGICAL ENGINEERING (MSBE2015)-PROCEEDINGS of the 2015 INTERNATIONAL CONFERENCE on CSET and MSBE (p. 25). World Scientific.

Balistreri, S. F., Steele, J. W., Caron, M. E., Laliberte, Y. J., & Shaw, L. A. (2013). International Space Station Common Cabin Air Assembly Condensing Heat Exchanger Hydrophilic Coating Operation, Recovery, and Lessons Learned.

Hwang, I. J., Yamamoto, T., & Torii, S. (2014). NUMERICAL ANALYSIS ON PLATE HEAT EXCHANGER FOR SEPARABLE HERRINGBONE TYPE. International Journal of Numerical Methods and Applications, 11(1), 1.

Horst, T. A., Rottengruber, H. S., Seifert, M., & Ringler, J. (2013). Dynamic heat exchanger model for performance prediction and control system design of automotive waste heat recovery systems. Applied Energy, 105, 293-303.

Oechsle, U., & Spindler, K. (2016, September). Investigation of micro-and nanostructured coatings for heat exchanger surfaces in an ice store. In Journal of Physics: Conference Series (Vol. 745, No. 3, p. 032135). IOP Publishing.

 

 

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