Types of heat transfer

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. 

The transfer of heat from one molecule to an adjaeent molecule while the particles remain in fixed positions relative to each other is conduction. For example, if a piece of pipe has a hot fluid on the inside and a eold fluid on the outside, heat is transferred through the wall of the pipe by conduc tion. This is illustrated in Figure 2-1. The molecules stay intact, relative to each other, but the heat is transferred from molecule to molecule by the process of conduetion. This type of heat transfer occurs in solids or, to a much lesser extent, within fluids that are relatively stagnant. 

This type of heat transfer may be described by an equation that is simi lar to the conduction equation. The rate of flow of heat is proportional to the tcmpcraiure difference betw een the hot and cold liquid, and the heat transtei area. It is expressed ... 

The transfer of heat to and from process fluids is an essential part of most chemical processes. The most commonly used type of heat transfer equipment is the shell and tube heat exchanger. The chemical process industries use four principal types of heat exchanger. 

The range of chemacal, petrochemical, and refining processes requires some type of heat transfer involving... 

Heat transfer in these systems conforms to the general laws of other types of heat transfer, because heat is still... 

Often, a reasonable and convenient way to understand the heat transfer process in a heat exchanger unit is to break down the types of heat transfer that must occur such as, vapor subcooling to dew point, condensation, and liquid subcooling. Each of these demands heat transfer of a different type, using different AT values, film coefficients, and fouling factors. This is illustrated in Figure 10-36. It is possible to properly determine a weighted overall temperature... 

When a liquid is vaporized in horizontal tubes, the inidal overall coefficient is several dmes the value for forced convecdon single-phase heat transfer. As the amount of vapor increases up to 100%, the coefficient falls off, down to a gas convecdon coefficient. The work of McAdams is some of the limited literature in this type of heat transfer. 

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. 

The shell and tube exchanger is by far the most commonly used type of heat-transfer equipment used in the chemical and allied industries. The advantages of this type are ... 

The factors that can affect the rate of heat transfer within a reactor are the speed and type of agitation, the type of heat transfer surface (coil or jacket), the nature of the reaction fluids (Newtonian or non-Newtonian), and the geometry of the vessel. Baffles are essential in agitated batch or semi-batch reactors to increase turbulence which affects the heat transfer rate as well as the reaction rates. For Reynolds numbers less than 1000, the presence of baffles may increase the heat transfer rate up to 35% [180]. 

Figure 5.1 shows, conceptually, four commonly used types of heat transfer equipment, although many more refined designs of such equipment exist. On a smaller scale, a double-tube type is used, whereas on an industrial scale a shell-and-tube type heat exchanger is frequently used. 

Conductive and Convective Heat Transfer, Thermo Explosion by. There are three fundamental types of heat transfer conduction, convection radiation. All three types may occur at the same time, but it is advisable to consider the heat thransfer by each type in any particular case. Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of either body. Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temp differences in forced convection, the motion is produced by mechanical means. Radiation is the transfer of heat from one body to another, not in contact with it, by means of wave motion thru space (Ref 5)... 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

Any of various types of heat-transfer equipment, whereby relatively cold water flowing over a surface will, by conduction and convection... 

HEAT TRANSFER AREA NEEDED TO COOL REACTOR Assume water inlet of 80°F with a 10°F temperature rise. A reasonable overall heat transfer coefficient for this type of heat transfer may be calculated as 45 Btu/(hXft2X°F). 

Determination of appropriate coefficients of heat transfer is required for design calculations on heat-transfer operations. These coefficients can sometimes be estimated on the basis of past experience, or they can be calculated from empirical or theoretical equations developed by other workers in the field. Many semiempirical equations for the evaluation of heat-transfer coefficients have been published. Each of these equations has its limitations, and the engineer must recognize the fact that these limitations exist. A summary of useful and reliable design equations for estimating heat-transfer coefficients under various conditions is presented in this chapter. Additional relations and discussion of special types of heat-transfer equipment and calculation methods are presented in the numerous books and articles that have been published on the general subject of heat transfer. 

The convection beat transfer coefficient is a strong function of velocity the higher the velocity, the higher die convection heat transfer coefficient. The fluid velocities associated with natural convection are low, typically less than 1 nVs. Therefore, the heat transfer coefficients encountered iti natural convection are usually much lower than those encountered in forced convection. Yet several types of heat transfer equipment are designed to operate under natural convection conditions instead of forced convection, because natural convection does not require the use of a fluid mover. 

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