Heat exchangers fluid physical properties

Hewitt, G. E, ed. 2008. HEDH Heat Exchanger Design Handbook, rev. ed. New York Begell House. This five-volume set is divided into broad topical areas theory, fluid mechanics, and heat transfer, thermal and hydraulic design of heat exchangers, mechanical design of heat exchangers, and physical properties. This last section is particularly data-focused. 

The fluid physical properties required for heat-exchanger design are density, viscosity, thermal conductivity and temperature-enthalpy correlations (specific and latent heats). Sources of physical property data are given in Chapter 8. The thermal conductivities of commonly used tube materials are given in Table 12.6. 

Note the similarity between Equations (c) and (a), where x = ht and y = U. From a standard heat transfer coefficient correlation (Gebhart, 1971), you can find that ht also varies according to Ktwt°, where Kt is a coefficient that depends on the fluid physical properties and the exchanger geometry. If we lump 1 /hs and 1 /hf together into one constant l/hsf, the semiempirical model becomes... 

This U value is called as clean U value because fouling resistances (/ j, / o) are not taken into account in equation (6.2). The film coefficients, hi and h, can be calculated based on the fluids physical properties and the geometry of the heat exchanger. For example, for U-tube exchangers with streams all liquid or aU vapor (no boiling and condensing), the correlation (Dittus and Boelter, 1930) is used to estimate the tube side Nusselt... 

The heat exchanger is operated in countercurrent configuration in order to improve the properties of the heat transfer between fluids. As a consequence, the mean temperature differences can be modeled, within the domain of the physically realizable temperature x G [52], by... 

From a practical viewpoint it is noteworthy that the tube sizes used approximated industrial heat-exchanger tubes. Accordingly, it would appear to be a sound procedure to use the correlations proposed for fluids of approximately the same physical properties and near the ranges of conditions studied, which may be summarized as follows ... 

After the model fluid has been found, the model temperature, T0 M, and the physical properties of the model system assigned to it are fixed. With the given dimension, dM, of the model apparatus, lid also is fixed. The condition lid = idem (8.49 b) also stipulates the size of the full-scale heat exchanger, dT, before the model experiments have been performed ... 

All types of equipment, exposed to fluids in motion, are subject to the erosion-corrosion phenomena. This can include pipeline networks (particularly curves, elbows and T-squares), floodgates, pumps, centrifugal fans, helixes, wheels of turbine, tubes of intersections of heat exchangers and measuring devices. In many cases, failures due to erosion-corrosion occur in a relatively short time.16 Most metals are susceptible to erosion-corrosion in the liquid phase under specific conditions. Resistance of metals depends on the physical and chemical properties of the corrosion product and/or the... 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

The magnitude of the individual coefficients will depend on the nature of the heat transfer process (conduction, convection, condensation, boiling, or radiation), on the physical properties of the fluids, on the fluid flow rates, and on the physical arrangement of the heat transfer surface. As the physical layout of the exchanger cannot be determined until the area is known, the design of an exchanger is of necessity a trial-and-error procedure. The steps in a typical design procedure are as follows ... 

Petukhov, B.J. Popov, N.V. Theoretical calculation of heat exchange and frictional resistance in turbulent flow in tubes of an incompressible fluid with variable physical properties. High Temperature 1 (1963) 69-83... 

B. S. Petukhov, and V. V. Kirillov, The Problem of Heat Exchange in the Turbulent Flow of Liquids in Tubes, (in Russian) Teploenergetika, (4/4) 63-68,1958 see also B. S. Petukhov and V. N. Popov, Theoretical Calculation of Heat Exchange in Turbulent Flow in Tubes of an Incompressible Fluid with Variable Physical Properties, High Temp., (1/1) 69-83,1963. 

Damping Characteristics. Damping causes vibrations to decay in an elastic structure and depends on the vibration frequency, the material of the elastic structure, the geometry, and the physical properties of the surrounding fluid (in the case of a shell-and-tube heat exchanger, the surrounding fluid is the shell fluid). 

Not only will the transfer of a foulant to a heat exchanger surface depend upon the physical properties of the constituents of the system, it will also depend upon the concentration gradient between the bulk fluid and the fluid/surface interface. 

---