Transfer performance

Convection Heat Transfer. Convective heat transfer occurs when heat is transferred from a soHd surface to a moving fluid owing to the temperature difference between the soHd and fluid. Convective heat transfer depends on several factors, such as temperature difference between soHd and fluid, fluid velocity, fluid thermal conductivity, turbulence level of the moving fluid, surface roughness of the soHd surface, etc. Owing to the complex nature of convective heat transfer, experimental tests are often needed to determine the convective heat-transfer performance of a given system. Such experimental data are often presented in the form of dimensionless correlations. 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

Between 1 s and 1 min specific contact time, conduction heat-transfer performance decreases theoretically as the 0.29 power of contact time. This is consistent with empirical data from several forms of indirect-heat dryers which show performance variation as the 0.4 power of rotational speed (21). In agitator-stirred and rotating indirect-heat dryers, specific contact time can be related to rotational speed provided that speed does not affect the physical properties of the material. To describe the mixing efficiency of various devices, the concept of a mixing parameter is employed. An ideal mixer has a parameter of 1. 

The heat-transfer performance capacity of cylinder diyers is not easy to estimate without a knowledge of the sheet tenmerature, which, in turn, is difficult to predict. According to published data, steam temperature is the largest single factor affecting capacity. Overall evaporation rates based on the total surface area of the diyers cover a range of 3.4 to 23 kg water/(h m ) [0.7 to 4.8 lb water/(h fF)]. 

A novel variation is a cyhndrical model equipped with a tube bundle to resemble a sheU-and-tube heat exchanger with a bloated shell [Chem. Proce.s.s., 20 (Nov. 15, 1968)]. Conical ends provide for redistribution of burden between passes. The improved heat-transfer performance is shown by Fig. 11-61. 

Seawater Evaporators The production of potable water from saline waters represents a large and growing field of application for evaporators. Extensive work done in this field to 1972 was summarized in the annual Saline Water Conversion Repoi ts of the Office of Sahne Water, U.S. Department of the Interior. Steam economies on the order of 10 kg evaporation/kg steam are usually justified because (1) unit production capacities are high, (2) fixed charges are low on capital used for pubhc works (i.e., they use long amortization periods and have low interest rates, with no other return on investment considered), (3) heat-transfer performance is comparable with that of pure water, and (4) properly treated seawater causes httle deterioration due to scahng or fouhng. 

Final vacuum versus water temperature, water cost, heat-transfer performance, and product quality. 

The heat exchanger effectiveness shows how close the heat exchanger is operating to the maximum heat transfer performance. Equation (9.5) is valid for any type of heat exchanger. 

The gas dispersion ring or sparger can be a special design with holes or a single pipe entering the underside of the impeller, and there will be very little differences in mass transfer performance. References [25] and [29] provide valuable detail for considering design for gas dispersion/ mass transfer. 

Thus, for mass transfer performance design a specific design HETP value should be established, which in effect represents the range from point B through E for Cj values above point F, the HETP values will be greater (and thus less efficient contact). 

Structured packings maintain mass-transfer performance with minimum penalty for pressure drop [108]. Two models are presented for calculating pressure drop (1) Bravo-Rocha-Fair [111] and (2) Stichlmair-Bravo-Fair [112]. Each method is qmte involved with rather complex equations to calculate the factor to ultimately calculate a pressure drop. The authors [108] recommend for design using... 

If the tube-side pressure drop exceeds a critical allowable value for the process system, then recheck by either lowering the flow rate and changing the temperature levels or reassume a unit with fewer passes on tube side or more tubes per pass. The unit must then be rechecked for the effect of changes on heat transfer performance. 

For heat transfer performance, horizontal baffles to isolate tube-side passes in horizontal bundles are preferred over vertical baffles that isolate groups of tubes in vertical columns. The expansion of capacity by adding more tube bundles or sections in parallel is easier, and the MTD is better with the horizontal pass plates. The fan drive may be by any of the available means, including ... 

Camavas, T. C. Heat Transfer Performance of Internally Pinned Tubes in Turbulent Plow, Heat Transfier Eng., . 4, April une... 

Lin Q, Jiang F, Wang X-Q, Han Z, Tai Y-C, Lew J, Ho C-M (2000) MEMS Thermal Shear-Stress Sensors Experiments, Theory and Modehng, Technical Digest, Solid State Sensors and Actuators Workshop, Hilton Head, SC, 4—8 June 2000, pp 304-307 Lin TY, Yang CY (2007) An experimental investigation of forced convection heat transfer performance in micro-tubes by the method of hquid crystal thermography. Int. J. Heat Mass Transfer 50 4736-4742... 

Boger et al. [50] analyzed the performance of MLRs with internal density-driven circulation (IMLR). They found the gas-hquid mass transfer superior and the overall mass transfer performance comparable with those for slurry reactors. 

A comparison of the heat transfer performance of different reactors presented in Table 12.1 shows the main advantages of HEX reactors [10]. 

Table 12.4 Comparison of different HEX reactors according to the heat transfer performances.

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