Convection and Radiation

In combustion operations, it has been estimated that 20% of the reaction heat is released directly as radiant energy. The remaining heat energy resides in the combustion products, from which about 30% of the energy is then released as radiation. The presence of water vapor in the combustion gases itself may have some appreciable effect upon the gas emissivity and radiation. 

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There are three heat-transfer modes, ie, conduction, convection, and radiation, each of which may play a role in the selection of a heat exchanger for a particular appHcation. The basic design principles of heat exchangers are also important, as are the analysis methods employed to determine the right size heat exchanger. 

Heat Transfer in Rotary Kilns. Heat transfer in rotary kilns occurs by conduction, convection, and radiation. In a highly simplified model, the treatment of radiation can be explained by applying a one-dimensional furnace approximation (19). The gas is assumed to be in plug flow the absorptivity, a, and emissivity, S, of the gas are assumed equal (a = e ) and the presence of water in the soHds is taken into account. Energy balances are performed on both the gas and soHd streams. Parallel or countercurrent kilns can be specified. 

An estimate of the relative importance of convection and radiation can be obtained from the ratio of the radiation-to-convection transfer rates. This dimensionless number reduces to... 

Because the evaporation of the solvent is an endothermic process, heat must be suppHed to the system, either through conduction, convection, radiation, or a combination of these methods. The total energy flux into a unit area of coating, is the sum of the fluxes resulting from conduction, convection, and radiation (see Heat exchange technology, HEAT thansfer). 

The maximum velocity at the axis is twice the average, whereas the velocity at the wall is zero. The effect of the burner wall is to cool the flame locally and decrease the burning velocity of the mixture. This results in flame stabilization. However, if the heat-transfer processes (conduction, convection, and radiation) involved in cooling the flame are somehow impeded, the rate of heat loss is decreased and the local reduction in burning velocity may no longer take place. This could result in upstream propagation of the flame. 

There are three fundamental types of heat transfer conduction, convection, and radiation. All three types may occur at the same time, and it is advisable to consider the heat transfer by each type in any particular case. 

Simultaneous Loss by Radiation The heat transferred by radiation is often of significant magnitude in the loss of heat from surfaces to the surroundings because of the diathermanous nature of atmospheric gases (air). It is convenient to represent radiant-heat transfer, for this case, as a radiation film coefficient which is added to the film coefficient for convection, giving the combined coefficient for convection and radiation (h + hf In Fig. 5-7 values of the film coefficient for radiation are plotted against the two surface temperatures for emissivity = 1.0. 

In any operation in which a material undergoes a change of phase, provision must be made for the addition or removal of heat to provide For the latent heat of the change of phase plus any other sensible heating or cooling that occurs in the process. Heat may be transferred by any one or a combination of the three modes—conduction, convection, and radiation. The process involving change of phase involves mass transfer simultaneous with heat transfer. 

Foam Insulation Since foams are not homogeneous materials, their apparent thermal conductivity is dependent upon the bulk density of tne insulation, the gas used to foam the insulation, and the mean temperature of the insulation. Heat conduction through a foam is determined by convection and radiation within the cells and by conduction in the solid structure. Evacuation of a foam is effective in reducing its thermal conductivity, indicating a partially open cellular structure, but the resulting values are stiU considerably higher than either multilayer or evacuated powder insulations. 

I Overheating shortens the life of a capacitor. Adequate ventilation and cooling facilities, through convection and radiation, must be made available at the place of installation. When housed inside a cubicle, as in a capacitor control panel and in a tier fonnation. sufficient space must be provided between each unit. Adequate... 

By forced convection The factors that can influence the temperature of the enclosure, installed outdoors are wind and snow, other than forced cooling. But their effect on actual cooling may be small. Sometimes this happens and sometimes not. It is better to ignore this effect when estimating various thermal effects. Natural convection and radiation will take account of this. 

For conduction the heat resistance is the distance divided by the heat conductivity, R = 8/X.A, and the heat conductance is heat conductivity divided by distance, U = X.A/8. For convection and radiation the heat resistance is 1 divided by the heat transfer factor, 1/aA, and the heat conductance is the same as the heat transfer factor, U aA. A coefficient of heat flow is also used, the K value, which is the total conductance ... 

The sum includes concentric cylinder layers, such as the layer between the outer and inner diameters of the pipe or a possible thermal insulation layer. For each layer the corresponding heat conductivity Aj is used. The outer heat transfer fac-ror is the sum of the proportions of convection and radiation. Note Very thin pipes or wires should not be insulated. Because the outer diameter of the insulation is smaller than A/a , the resistance is less than that without the insulation.)... 

The total heat load is introduced by each source by convection and radiation ... 

In the second approach, the energy release is split by a predefined (mostly constant) factor between convection and radiation. The convective part is directly transferred as energy gain to the room air, while the radiative part is distributed to the surrounding walls by the area-weighted method or the view-factor method. 

ThcrL uie three distinet ways in which heat may pass from a source to a receiver, although most engineering applications are combinations of two or three. These are conduetion, convection, and radiation... 

Heat transfer is the energy flow that occurs between bodies as a result of a temperature difference. There are three commonly accepted modes of heat transfer conduction, convection, and radiation. Although it is common to have two or even all three modes ot heat transfer present in a given process, we will initiate the discussion as though each mode of heat transfer is distinct. 

Many everyday heat flows, such as those through windows and walls, involve all three heat transfer mechanisms—conduction, convection, and radiation. In these situations, engineers often approximate the calculation of these heat flows using the concept of R values, or resistance to heat flow. The R value combines the effects of all three mechanisms into a single coefficient. 

The resistances of the Sheetrock, siding, and insulation may be viewed as conductive resistances, while the resistance at the two surfaces combine the effects of convection and radiation between the surface and its surroundings. Typical values for these resistances in units of (W/m — W/m"-°C)" ((Btu/h-ft -°F) ) are as follows ... 

To measure the efficiency of a whole window, special testing takes into account all heat transfer from conduction, convection, and radiation. Certain values are used to represent the thermal and solar efficiency of high-performance windows by measuring reduced thermal heat loss (measured by the U-... 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

Btu/(hr) (ft2)(°F/ft) i = inside wall pipe o = outside wall surface of pipe Heat loss from fluid inside pipe through exterior insulation to outside air. Combined convection and radiation ... 

Heat transfer from the surface of an insulated or uninsulated pipe in air involves convection and radiation. In still air more heat is lost by radiation than convection. The heat loss from an insulated or bare pipe is, in Btu/hr ... 

This is the rate of heat transfer from a surface to the surrounding air (or fluid) due to conduction convection and radiation. It is generally used only in still-air conditions and when the temperature difference between surface and ambient is of the order of 30 K. It is obtained by dividing the thermal transmission per unit area in watts per square meter by the temperature difference between the surface and the surrounding air. It is expressed as W/nf K. 

Table 26.1 Heat gain by convection and radiation from singie common window giass for 22 March and 22 September (W/m of masonry opening) (The Trane Company, 1977, used by permission)...

Now that the overall coefficient U has been broken down into its component parts, each of the individual coefficients /q, hi, and hi must be evaluated. This can be done from a knowledge of the nature of the heat transfer process in each of the media. A study will therefore be made of how these individual coefficients can be calculated for conduction, convection, and radiation. 

Dry wall region Ultimately, if a large fraction of the feed is vaporised, the wall dries out and any remaining liquid is present as a mist. Heat transfer in this region is by convection and radiation to the vapour. This condition is unlikely to occur in commercial reboilers and vaporisers. 

A more complete energy balance will be used that includes transport by conduction, convection, and radiation. The new energy balance equation over a small volume element takes the form... 

The heat transfer into the boundary surface of a compartment occurs by convection and radiation from the enclosure, and then conduction through the walls. For illustration, a solid boundary element will be represented as a uniform material having thickness, 6, thermal conductivity, k, specific heat, c, and density, p. Its back surface will be considered at a fixed temperature, T0. 

For a fully developed fire, conduction commonly overshadows convection and radiation therefore, a limiting approximation is that h hk, which implies Tw T. This result applies to structural and boundary elements that are insulated, or even to concrete structural elements. This boundary condition is conservative in that it gives the maximum possible compartment temperature. 

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