Temperature control heat transfer

A perfect temperature-controlled heat-transfer surface is difficult to achieve, but it is closely simulated in practice by using a control fluid on one side of, for example, a metal tube. The tube wall should be thin and, ideally, the heat-transfer resistance comparatively large for the other fluid on the working side of the tube the latter surface is then effectively temperature-controlled and responds only to changes in the control fluid. 

Temperature control is accomplished in one of three general ways. One method is by controlling the temperature of the platen, usually by means of an integral channel in the platen through which temperature-controlled heat transfer fluid flows. Second, the temperature of the slurry itself can be regulated prior to being dispensed onto the platen. Finally, a means of heating the backside of the wafer can be built into the carrier [42,43]. 

A jacketed vessel, as shown in Figure 3.23, is often used for maintaining temperature however, it has limited surface area and low heat transfer coefficients. Sometimes internal reactor cooling coils are also used to provide additional heat transfer area. In order to manipulate the heat transfer, maximum flow is maintained in a circulation loop, while the jacket temperature is adjusted by bringing in and letting out coolant. The reactor temperature controller provides a setpoint to the jacket temperature controller. Heat transfer is linear and proportional to the temperature difference. 

A fluidi2ed-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems ia temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accompHshed by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

An expression for the net rate of phase change can also be derived by assuming that the phase-change process is controlled solely by the rate at which heat can be transferred between the bulk liquid and the liquid surface. In this penetration theory approach, the liquid surface temperature is assumed to equal the gas-phase temperature. The heat transfer within a liquid element is assumed to occur by pure conduction, and therefore... 

Vinyl chloride polymerization occurs via an exothermic radical reaction. In fact, the reaction is approximately 25% more exothermic than polyethylene polymerization. The highly exothermic nature of the reaction and the strong molecular weight dependence on temperature make heat transfer, and its control, critical to the manufacture of polyvinyl chloride. 

The initial stage of boiling is usually controlled by the heat transfer from the ground. This is especially true for a spill of liquid with a normal boiling point below ambient temperature or ground temperature. The heat transfer from the ground is modeled with a simple onedimensional heat conduction equation, given by... 

Thermal conductivity may be defined as the quantity of heat passing per unit time normally through unit area of a material of unit thickness for unit temperature difference between the faces. In the steady state, i.e. when the temperature at any point in the material is constant with time, conductivity is the parameter which controls heat transfer. It is then related to the heat flow and temperature gradient by ... 

Most of the reactions that proceed in FFB are strongly exothermic or endothermic, and heat must be removed from or supplied into the bed in order to control the bed temperature. So heat transfer in FFB is of great importance for design and operation. 

Other factors also contribute to improved furnace efficiency. Higher oxy/fuel flame temperatures improve heat transfer to the glass melt by radiation. Tighter control of stoichiometry in the flame region allows the furnace to run with a lower excess oxygen ratio. For full oxy/fuel, radiation heat losses through ports are reduced since fewer ports are needed. 

In addition, reactor operations are also classified by the way their temperature (or heat transfer) is controlled. Three operational conditions are commonly used (i) isothermal operation—the same temperatures exist throughout the reactor, (ii) adiabatic operation—no heat is transferred into or out of the reactor, and (iii) non-isothermal operation— the operation is neither isothermal nor adiabatic. 

It consisted of a single-pass, five-plate, water-cooled condenser capable of independent flow and temperature control. Heat was supplied to the evaporator reservoir, so that it could be operated at selected temperature levels. The rotating assembly, consisting of a shaft with spacers capable of accepting six disks 12 inches in diameter, of varying thickness and composition, was equipped with a variable speed drive. With the arrangement shown, approximately 2 sq. feet of disk-condenser surface area was available for mass transfer. Tests were performed on this unit at atmospheric pressure, using city water (125 p.p.m. of dissolved solids) as feed, to determine the effect on distillation rate of (1) reservoir tem-... 

In the steady state, when the temperature at any point is constant with time, this parameter controls heat transfer as follows ... 

For gas-liquid-liquid reactions equipment similar to that used for liquid-liquid reactions are employed. The hydrodynamics in these reactors is extremely complex because of the three phases and their convoluted interactions. An example is the grazing behavior of small solid particles enhancing mass transfer at gas-liquid interfaces. The scale-up from laboratory to the production site thus poses numerous problems with respect to the reactant s mixing, temperature control (heat removal), catalyst selectivity, and its deactivation [1]. The performance of such processes can be predicted analytically only to a limited extent for reactors with well-defined flow patterns. 

---