Heat flow

Most important for the technical performance of the cooking system and thus for consmner satisfaction is the heat flow as it influences the boiling time, energy consumption, and temperature distribution in the cookware used. 

The remainder of the radiant energy from the heating coils reaches the insulation surface where it is mostly reflected (3). Less than 5% is absorbed (4). As the heating coils are in direct contact with the insulation, a small portion of energy is released through direct contact into the insulation (10). 

The heat flow between the panel and the cookware bottom is mainly influenced by the air gap separating them. When flat cookware is used with 

in the case of poor-quality cookware, a large air gap ( 1 mm) exists, a noteworthy share of energy will be transported by radiation. Depending on the cookware material, the radiant energy will be more or less absorbed from the cookware bottom. Enamel cookware absorbs more than 95%, stainless steel and aluminum cookware nearly 50% of the radiated energy. In any case, the heat flow will be hindered by the air gap, so that, for example, the short boiling time of good-quality cookware (narrow air gap) cannot be attained. 

When a good pot is used, characterized by its smooth contact to the glass ceramic surface, one hardly notices the influence from the size of the heating element. Using a large pot on a smaller heating element results in a boiling time of 9.5 min. 

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The heat flow through the system can be manipulated by changing the number of stages. Figure 3.13c shows the effect of an increase from three to six stages. 

Looking at the heat flows in Fig. 6.18a, some are negative, which is infeasible. Heat cannot be transferred up the temperature scale. To... 

More than 7.5 MW could be added from a hot utility to the first interval, but the objective is to find the minimum hot and cold utility. Thus from Fig. 6.186, QHmin = 7.5MW and Qcmm = 10MW. This corresponds with the values obtained from the composite curves in Fig. 6.5a. One further important piece of information can be deduced from the cascade in Fig. 6.186. The point where the heat flow goes to zero at T = 145°C corresponds to the pinch. Thus the actual hot and cold stream pinch temperatures are 150 and 140°C. Again, this agrees with the result from the composite curves in Fig. 6.5a. 

The initial setting for the heat cascade in Fig. 6.18a corresponds to the shifted composite curve setting in Fig. 6.15a where there is an overlap. The setting of the heat cascade for zero or positive heat flows in Fig. 6.186 corresponds to the shifted composite curve setting in Fig. 6.156. 

Finally, the heat cascade is shown in Fig. 6.21. Figure 6.21a shows the cascade with zero hot utility. This leads to negative heat flows, the largest of which is -1.84 MW. Adding 1.84 MW from a hot utility as shown in Fig. 6.216 gives = 1.84 MW, Qcmm = 1-84 MW, hot stream pinch temperature =... 

The grand composite curve is obtained by plotting the problem table cascade. A typical grand composite curve is shown in Fig. 6.24. It shows the heat flow through the process against temperature. It should be noted that the temperature plotted here is shifted temperature T and not actual temperature. Hot streams are represented ATn,in/2 colder and cold streams AT iJ2 hotter than they are in practice. Thus an allowance for ATj in is built into the construction. 

The point of zero heat flow in the grand composite curve in Fig. 6.24 is the pinch. The open jaws at the top and bottom represent Hmin and Qcmin, respectively. Thus the heat sink above the pinch and heat source below the pinch can be identified as shown in Fig. 

Heat flow required from the turbine exhaust = 21.9 MW From steam tables, inlet conditions at T, = 300°C and Pi = 41 bar are /ii = 2959kJkg ... 

As the amount of temperature cross increases, however, problems are encountered, as illustrated in Fig. 7i8c. Local reversal of heat flow may be encountered, which is wasteful in heat transfer area. The design may even become infeasible. 

Another design option that can be considered if a column will not fit is use of an intermediate reboiler or condenser. An intermediate condenser is illustrated in Fig. 14.5. The shape of the box is now altered because the intermediate condenser changes the heat flow through the column. The particular design shown in Fig. 14.5 would require that at least part of the heat rejected from the intermediate condenser be passed to the process. An analogous approach can be used to evaluate the possibilities for use of intermediate reboilers. Flower and Jackson," Kayihan, and Dhole and Linnhofl have presented procedures for the location of intermediate reboilers and condensers. 

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

The coefficients, L., are characteristic of the phenomenon of thermal diffusion, i.e. the flow of matter caused by a temperature gradient. In liquids, this is called the Soret effect [12]. A reciprocal effect associated with the coefficient L. is called the Dufour effect [12] and describes heat flow caused by concentration gradients. The... 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

Fig. 1. Sketch of heat flow through (a) a plane wall where the arrow indicates the direction of heat flow and (b) a series of composite walls, (c) The...

A second property, closely related to the first, is the abiHty of the heat pipe to effect heat-flux transformation. As long as the total heat flow is ia equiHbrium, the fluid streams connecting the evaporatiag and condensing regions essentially are unaffected by the local power densities ia these two... 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

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