化工原理英文教材传热设备heat-exchange equipment
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Special heat-transfer devices used to liquefy vapors by removing their latent heats are called condensers.
Condensers fall into two classes. In the first, called shelland-tube condenser, the condensing vapor and coolant are separated by a tube wall.
Flash p342
2-4 exchanger
The 1-2 exchanger has an important limitation. Because of the parallel-flow pass, the exchanger is unable to bring the exit temperature of one fluid very near to the entrance temperature of the other.
An even number of tube-side passes are used in multipass exchangers. The shell side may be either single-pass or multipass.
In multipass exchangers, floating heads are frequently used.
Flash p339
In an exchanger the shell-side and tube-side heattransfer coefficients are of comparable importance, and both must be large if a satisfactory overall coefficient is to be attained.
Including conduction-convection in exchangers, boilers, and condensers; radiation in furnaces and radiant heat dryer.
General design of heat-exchange equipment at first, material and energy balances are set up. From these results the required heat-transfer area is calculated. The quantities to be calculated are the overall heat-transfer coefficient, the average temperature difference.
A typical case is heating a fixed gas, such as air, by means of condensing steam. The individual coefficient for the steam is typically 100 to 200 times that for the air. The capacity of a unit area of heating surface will be low.
The velocity and turbulence of the shell-side liquid are as important as those of the tube-side fluid.
To promote crossflow and raise the average velocity of the shell-side fluid, baffles are installed in the shell.
The LMTD, as given by Eq(11-15), does not apply in this case, and it is customary to define a correction factor F. The correction factor is multiplied by the LMTD for countercurrent flow, the product is the true average temperature drop.
Correction of LMTD in multipass exchangers
In multipass exchangers which have more tube passes than shell passes, the flow is countercurrent in some sections and parallel in others.
Shell-and-tube heat exchangers
Single-pass1-1 exchanger
The simple double-pipe exchanger is inadequate for high heat-transfer rate. Shell-and-tube construction, such as that shown in figure, where one shell serves for many tubes, is more economical. This exchanger, because it has one shell -side pass and one tube-pass, is a 1-1 exchanger.
Δtm = F LMTD
Plate-type exchanger
For heat transfer between fluids at low or moderate pressure, below about 20 atm ,plate–type exchangers are competitive with shell-and-tube exchangers, especially where corrosion-resistant materials are required
Flash p343
The heat recovery of a 1-2 exchanger is inherently poor.
A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.
Figure 15.6 a and b shows factor f for 1-2 and 2-4 exchangers, respectively.
Each curved line in the figure corresponds to a constant value of the dimensionless ratio Z
Metal plate, usually with corrugated faces, are supported in a frame; hot fluid passes between alternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces.
Z Tha Thb Tcb Tca
The factor Z is the ratio of the fall temperature of the hot fluid to the rise in temperature of the cold fluid.
And the abscissas are values of the dimensionless
The plates are typically 5mm apart.
They can be readily separated for cleaning; additional area may be provided simply by adding more plates.
Condensers
Extended surfaces have been developed in which the outside area of tube is mul having the lower coefficient is brought into contact with the extended surface and flows outside the tubes , while the other fluid, having high coefficient, flows through the tubes.
化工原理 Principles of Chemical Industry
heat-exchange equipment
In industrial processes heat energy is transferred by a variety of methods. Heat exchangers are of importance for heat energy transport. There are lots of exchangers.
Multipass construction increases the fluid velocity, with a corresponding increase in the heat-transfer coefficient.
The disadvantages are that
(1) the exchanger is slightly more complicated ; (2) Some sections in the exchanger have parallel flow, which limits the temperature approach; (3) the friction loss through the equipment is increased because of the larger velocities and multiplication of exit and entrance losses.
In the second, called contact condensers, the coolant and vapor streams are physically mixed.
Extended-surface equipment
Difficult heat-exchange problems arise when one of two fluid streams has a much lower heat-transfer coefficient than the other.
multipass exchanger
The 1-1 exchanger has limitations, because when the tube-side flow is divided evenly among all the tubes, the velocity may be quite low, giving a low heat transfer coefficient.
ratio
H
H
Tcb Tca Tha Tca
The factor η is the heating effectiveness. From the numerical values of Z and η, factor F is read from Figure 15.6, and multiplied by the LMTD for countercurrent flow to give the true mean temperature.
Condensers fall into two classes. In the first, called shelland-tube condenser, the condensing vapor and coolant are separated by a tube wall.
Flash p342
2-4 exchanger
The 1-2 exchanger has an important limitation. Because of the parallel-flow pass, the exchanger is unable to bring the exit temperature of one fluid very near to the entrance temperature of the other.
An even number of tube-side passes are used in multipass exchangers. The shell side may be either single-pass or multipass.
In multipass exchangers, floating heads are frequently used.
Flash p339
In an exchanger the shell-side and tube-side heattransfer coefficients are of comparable importance, and both must be large if a satisfactory overall coefficient is to be attained.
Including conduction-convection in exchangers, boilers, and condensers; radiation in furnaces and radiant heat dryer.
General design of heat-exchange equipment at first, material and energy balances are set up. From these results the required heat-transfer area is calculated. The quantities to be calculated are the overall heat-transfer coefficient, the average temperature difference.
A typical case is heating a fixed gas, such as air, by means of condensing steam. The individual coefficient for the steam is typically 100 to 200 times that for the air. The capacity of a unit area of heating surface will be low.
The velocity and turbulence of the shell-side liquid are as important as those of the tube-side fluid.
To promote crossflow and raise the average velocity of the shell-side fluid, baffles are installed in the shell.
The LMTD, as given by Eq(11-15), does not apply in this case, and it is customary to define a correction factor F. The correction factor is multiplied by the LMTD for countercurrent flow, the product is the true average temperature drop.
Correction of LMTD in multipass exchangers
In multipass exchangers which have more tube passes than shell passes, the flow is countercurrent in some sections and parallel in others.
Shell-and-tube heat exchangers
Single-pass1-1 exchanger
The simple double-pipe exchanger is inadequate for high heat-transfer rate. Shell-and-tube construction, such as that shown in figure, where one shell serves for many tubes, is more economical. This exchanger, because it has one shell -side pass and one tube-pass, is a 1-1 exchanger.
Δtm = F LMTD
Plate-type exchanger
For heat transfer between fluids at low or moderate pressure, below about 20 atm ,plate–type exchangers are competitive with shell-and-tube exchangers, especially where corrosion-resistant materials are required
Flash p343
The heat recovery of a 1-2 exchanger is inherently poor.
A better recovery can be obtained by adding a longitudinal baffle to give two shell passes.
Figure 15.6 a and b shows factor f for 1-2 and 2-4 exchangers, respectively.
Each curved line in the figure corresponds to a constant value of the dimensionless ratio Z
Metal plate, usually with corrugated faces, are supported in a frame; hot fluid passes between alternate pairs of plates, exchanging heat with the cold fluid in the adjacent spaces.
Z Tha Thb Tcb Tca
The factor Z is the ratio of the fall temperature of the hot fluid to the rise in temperature of the cold fluid.
And the abscissas are values of the dimensionless
The plates are typically 5mm apart.
They can be readily separated for cleaning; additional area may be provided simply by adding more plates.
Condensers
Extended surfaces have been developed in which the outside area of tube is mul having the lower coefficient is brought into contact with the extended surface and flows outside the tubes , while the other fluid, having high coefficient, flows through the tubes.
化工原理 Principles of Chemical Industry
heat-exchange equipment
In industrial processes heat energy is transferred by a variety of methods. Heat exchangers are of importance for heat energy transport. There are lots of exchangers.
Multipass construction increases the fluid velocity, with a corresponding increase in the heat-transfer coefficient.
The disadvantages are that
(1) the exchanger is slightly more complicated ; (2) Some sections in the exchanger have parallel flow, which limits the temperature approach; (3) the friction loss through the equipment is increased because of the larger velocities and multiplication of exit and entrance losses.
In the second, called contact condensers, the coolant and vapor streams are physically mixed.
Extended-surface equipment
Difficult heat-exchange problems arise when one of two fluid streams has a much lower heat-transfer coefficient than the other.
multipass exchanger
The 1-1 exchanger has limitations, because when the tube-side flow is divided evenly among all the tubes, the velocity may be quite low, giving a low heat transfer coefficient.
ratio
H
H
Tcb Tca Tha Tca
The factor η is the heating effectiveness. From the numerical values of Z and η, factor F is read from Figure 15.6, and multiplied by the LMTD for countercurrent flow to give the true mean temperature.