Conduction and convection

FIG. 5-6 Temperature gradients for a steady flow of heat by conduction and convection from a warmer to a colder fluid separated by a solid wall. 

Radiation differs from conduction and convection not only in mathematical structure but in its much higher sensitivity to temperature. It is of dominating importance in furnaces because of their temperature, and in ciyogenic insulation because of the vacuum existing between particles. The temperature at which it accounts for roughly half of the total heat loss from a surface in air depends on such factors as surface emissivity and the convection coefficient. For pipes in free convection, this is room temperature for fine wires of low emissivity it is above red heat. Gases at combustion-chamber temperatures lose more than 90 percent of their energy by radiation from the carbon dioxide, water vapor, and particulate matter. 

Radiation is a process that is different from both conduction and convection, because the substances exchanging heat need not be touching and can even be separated by a vacuum. A law formulated by German physicist Max Planck in... 

Most heat transfer processes used in production facilities involve combinations of conduction and convection ti ansfer processes. For example, in heat exchangers the transfer of heat energy from the hot fluid to the coLl fluid involves tliree steps. First, the heat energy is transferred from the luH fluid to the exchanger tube, then through the exchanger tube wall, ctud finally from the tube wall to the cold fluid. The first and third steps are convection transfer processes, while the second step is conduction process. 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

Reducing heat transfer with gas fill. Conduction and convection cause heat transfer across the air spaces in multilayer windows. Although air is a relatively good insulator, other gases that have lower thermal conductivity can be sealed into the cavities... 

In order to perform effectively as an insulant a material must restrict heat flow by any (and preferably) all three methods of heat transfer. Most insulating materials adequately reduce conduction and convection elements by the cellular structure of the material. The radiation component is decreased by absorption into the body of the insulant and is further reduced by the application of bright foil outer facing to the product. 

Surfaces will absorb radiant heat and this factor is expressed also as the ratio to the absorptivity of a perfectly black body. Within the range of temperatures in refrigeration systems, i.e. - 70°C to + 50°C (203-323 K), the effect of radiation is small compared with the conductive and convective heat transfer, and the overall heat transfer factors in use include the radiation component. Within this temperature range, the emissivity and absorptivity factors are about equal. 

Any of various types of heat transfer equipment, whereby relatively cold water flowing over a surface will, by conduction and convection means, transfer heat away from a process. The most common types of heat exchangers are plate and frame and shell and tube designs. A boiler is also a type of heat exchanger. 

Because the mechanisms governing mass transfer are similar to those involved in both heat transfer by conduction and convection and in momentum transfer (fluid flow), quantitative relations exist between the three processes, and these are discussed in Chapter 12. There is generally more published information available on heat transfer than on mass transfer, and these relationships often therefore provide a useful means of estimating mass transfer coefficients. 

In the previous chapters, the stresses arising from relative motion within a fluid, the transfer of heat by conduction and convection, and the mechanism of mass transfer are all discussed. These three major processes of momentum, heat, and mass transfer have, however, been regarded as independent problems. 

For liquids boiling from a pool the boiling rate is limited by the heat transfer from the surroundings to the liquid in the pool. Heat is transferred (1) from the ground by conduction, (2) from the air by conduction and convection, and (3) by radiation from the sun and/or adjacent sources such as a fire. 

Fireproofing for the petroleum and related industries follow the same concept as other industries except that the possible fire exposures are more severe in nature. The primary destructive effects of fire in the petroleum industry is very high heat, very rapidly, in the form of radiation, conduction and convection. This causes the immediate collapse of structures made of exposed steel construction. Radiation and convection effects usually heavily outweigh the factor of heat conduction for the... 

Heat Flux - The rate of heat transfer per unit area normal to the direction of heat flow. It is the total heat transmitted by radiation, conduction, and convection. 

The char combustion is sustained by its own heat release. The heat release and heat transport is thereby coupled with the oxygen transport, which is usually the controlling factor. The heat evolved from reaction is transported by heat conduction and convection out of the particle. [73]... 

Radiation is the rate of heat transfer by electromagnetic waves emitted by matter. Unlike conduction and convection, radiation does not require an intervening medium to propagate. The basic rate of radiation heat-transfer equation between a high temperature (Th) black body and a low temperature Tf) black body is Stefan-Boltzmann s law ... 

Usually, the rate of heat transfer is a combination of conduction and convection in a heat exchanger system as illustrated in Fig. 7.1 and only the fluid temperature on either side of the solid surface is known. For steady state, the rate of conduction heat transfer and the rate of convection heat transfer are equal. The total resistance (R) of the combined rate of heat transfer is... 

The overall heat transfer coefficient is normally made up of several terms arising from the various resistances to the flow of heat. In the simple example mentioned above, there will be terms for heat transfer through the liquid by conduction and convection, for conduction through the metal wall of the tank and through any layers of insulating material and for heat loss from the outer skin to the surrounding air. 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

Conductive and convective heat transfer, thermal expln by 3 C496—C497... 

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