Enhanced heating

D. P. Colvin, J. C. Mulligan, and Y. G. Bryant, Enhanced heat transfer in environmental systems using microencapsulatedphase change materials, 22nd International Conference on Environmental Systems, 1992. U.S. Pat. 4,807,696, D. P. Colvin and J. C. Mulligan (1989). 

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. 

In a batch reactor, the reaclants are loaded at once the concentration then varies with time, but at any one time it is uniform throughout. Agitation seiwes to mix separate feeds initially and to enhance heat transfer. In a semibatch operation, some of the reactants are charged at once and the others are then charged gradually. 

Motionless mixers continuously interchange fluid elements between the walls and the center of the conduit, thereby providing enhanced heat transfer and relatively uniform residence times. Distributive mixing is usually excellent however, dispersive mixing may be poor, especially when viscosity ratios are high,... 

Mention may also be made here of a number of polyfunctional compounds such as allyl methacrylate and glycol dimethacrylates which have been used to produce a cross-linked sheet of enhanced heat resistance compared with conventional poly(methyl methacrylate). Some manufacturers supply the sheet in an incompletely cross-linked state which allows a limited amount of forming after which the sheet may be further heated to complete the cure. 

There has also been a demand in recent years for an ABS-type material with an enhanced heat distortion temperature. In ABS polymers this is largely controlled by the Tg of the resin or glassy component. Consequently three approaches to raising the distortion temperature have been developed. They... 

The use of ABS has in recent years met considerable competition on two fronts, particularly in automotive applications. For lower cost applications, where demands of finish and heat resistance are not too severe, blends of polypropylene and ethylene-propylene rubbers have found application (see Chapters 11 and 31). On the other hand, where enhanced heat resistance and surface hardness are required in conjunction with excellent impact properties, polycarbonate-ABS alloys (see Section 20.8) have found many applications. These materials have also replaced ABS in a number of electrical fittings and housings for business and domestic applications. Where improved heat distortion temperature and good electrical insulation properties (including tracking resistance) are important, then ABS may be replaced by poly(butylene terephthalate). 

In some countries the extensive use of asbestos as a filler is somewhat discouraged because of the hazards associated with its use. In other parts of the world moulding compositions of enhanced heat resistance have been developed by the use of especially heat-resisting polymers used in conjunction with asbestos and other mineral fillers. 

One way to obtain enhanced heat and solvent resistance in SBR-based PSA is sulfur addition. It is difficult to obtain sufficient cure with sulfur during normal drying cycles (2-5 min at 150-180°C). Furthermore, cure via sulfur may continue after leaving the oven, thus degrading adhesion. When used in contact with copper, the sulfur may promote corrosion. The use of phenolic overcomes all of these problems despite the fact that its natural cure speed is also insufficient for the time frame available. This is overcome through addition of one of the accelerators... 

Little or no maintenance requirements Small space requirements Available m many construction materials No power requirements other than pumping Mixing achieved m short conduit lengths Minimal chance of material hangup or plugging Short residence times NaiTow residence time distribution Enhanced heat transfer Cost effective... 

Some processes have large heat transfer requirements. This may result in large inventories of material within the heat transfer equipment. If the material is thermally unstable it would be inherently safer to reduce the residence time in the heat exchanger. Options to minimize heat exchanger inventory include the use of different types of heat exchangers. Inventories in shell and tube heat exchangers can be reduced by the use of turbulators in the tubes to enhance heat transfer coefficients, and by placing the more hazardous material on the tube side. 

Figure 10-1OM. Finned tube with internal ribs enhances heat transfer inside as well as outside the tubes. (Used by permission High Performance Tube, Inc., Finned Tube Data Book. )...

Peng XF, Wang BX (1994a) Cooling characteristics with micro-channeled structures. J Enhanced Heat Transfer 1 315-326... 

A possible way to enhance heat transfer in systems with high power density is to change the phase in the micro-channels embedded in the device. This has motivated a number of theoretical and experimental investigations covering various aspects of heat transfer in micro-channel heat sinks with phase change. 

To enhance heat transfer between the particle and the environment... 

Values for the various parameters in these equations can be estimated from published correlations. See Suggestions for Further Reading. It turns out, however, that bubbling fluidized beds do not perform particularly well as chemical reactors. At or near incipient fluidization, the reactor approximates piston flow. The small catalyst particles give effectiveness factors near 1, and the pressure drop—equal to the weight of the catalyst—is moderate. However, the catalyst particles are essentially quiescent so that heat transfer to the vessel walls is poor. At higher flow rates, the bubbles promote mixing in the emulsion phase and enhance heat transfer, but at the cost of increased axial dispersion. 

One of the most often cited advantages of micro reactors is their prevention of hot spots by strongly enhanced heat dissipation. Hence higher dosing of reactants, i.e. higher reactant concentration and/or higher catalyst loading, may be possible... 

Peng, X. F., Wang, B. X., Cooling characteristics with microchanneled structures, J. Enhanced Heat Transfer 1 (1994) 315-326. 

Compared with laboratory fixed-bed reactors or conventional extruded monoliths, such a microstructured monolith is smaller in characteristic dimensions, lower in pressure loss by optimized fluid guiding and constructed from the catalytic material solely [3]. The latter aspect also leads to enhanced heat distribution within the micro channels, giving more uniform temperature profiles. 

The studies mentioned above referred to the general advantages of micro flow processing in terms of enhanced heat and mass transfer [85] (see a more detailed description in [42]). 

Strategies for enhanced heat control resulting in a new micro reactor design are briefly mentioned in [37]. 

In contrast, flat-blade stirrers and turbines mainly cause radial movement (see Figs. 5.4-5 and 5.4-6). An anchor is used to enhance heat transfer between the reaction mixture and the cooling medium. 

For ice on coil type ice storage tank, water inside the tank is sometimes agitated to enhance heat transfer on coil surface. Small water or air pumps are used for this purpose. This agitation is effective until ice is formed on the coil, because temperature during ice forming is uniformly freezing point. Degree... 

If the same duty is to be serviced by enhanced heat transfer without additional area ... 

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