化工原理英文教材传热原理Principles of heat flow in fluids
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化工原理 Principles of Chemical Industry
Principles of heat flow in fluids
Typical heat-exchange equipment
Single-pass shell-and-tube condenser
Expansion joint
It is clear from Fig.11-4 that Δt can vary considerably from point to point along the tube, and, therefore, the flux also varies with tube length.
The local flux dq/dA is related to the local value of Δt by the equation
because, as inspection of Figs11-4a and b will show, it is
not possible with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow.
The temperatures plotted Fig11-4 are average stream temperatures.
The temperature so defined is called the average or mixing-cup stream temperature.
Overall heat-transfer coefficient
Double-tube heat exchanger It is assembled of standard metal pipe and standarized return bends and return heads. shown in Fig.11-3.
One fluid flows through the inside pipe and second fluid through the annular space between the outside and inside pipes.
Where it is important to change the temperature of at least one fluid rapidly.
Energy balances
Enthalpy balances in heat exchangers
Heat transfer to or from the ambient is not desired in practice, and it is usually reduced to a small magnitude by suitable insulation.
Single-pass shell-and-tube condenser
For the vapor flowing in a condenser If the vapor entering the condenser (shell) is not superheated and the condensate leaving the condenser is not subcooled, the temperature throughout the shell-side of the condenser is constant.
Heat flux and heat transfer coefficient Heat flux
The rate of heat transfer per unit area is called the heat flux.
In many types of heat-transfer equipment the transfer surfaces are constructed from tubes.
Double-pipe exchanger are useful when not more than 9 to 14 m2 of surface is required.
For larger capacities , more elaborate shell-and-tube exchangers, containing up to thousand of square meter of area, are used.
Temperature ºC
Temp of condensing vapor T
Δt
Δt2
Δt1
Length of tube m
The flow type with the counterflow is commonly used. Parallel flow is rarely used in a single-pass exchanger such as that shown in Fig11-3
q=mh(Hh1-Hh2)= mc(Hc2 - Hc1)
If constant specific heats are assumed, the overall enthalpy balance for a heat exchanger becomes
q=mhCph (Th1-Th2)= mcCpc (tc2 - tc1) (11-6)
In the multipass exchanger ollowing situation:
In special situation where it is necessary to limit the maximum temperature of the cooler fluid; or limit the min temperature of hot fluid.
For the cold fluid, it can gain heat q=mc(Hc2 - Hc1)
Neglecting the heat transfer with the ambient. The heat lost by the warm fluid is gained by the cold fluid, therefore
dq dA
U
(T11-9t )
The quantity U is called the local overall heattransfer coefficient.
It is necessary to specify the area.
If A is taken as the outside tube area Ao, U becomes a coefficient based on that area and is written Uo.
It is customary to neglect it in comparison with the heat transfer through the wall of the tubes from the warm fluid to the cold fluid.
For the warm fluid, it can loss heat. q=mh(Hh1-Hh2)
If the two fluids enter at the same end of the exchanger and flow in the same direction to the other end, the flow is called parallel.
The temperature -length curves for parallel flow are shown in Figure
Steam and other vapor is introduced through nozzle F into the shell-side space surrounding the tubes, condensate is withdrawn through connection G, and any noncondensable gas that might enter with the inlet vapor is removed through vent K.
It is reasonable to expect the heat flux to be proportional to a driving force. The driving force is taken as Δt=T-t, which is the overall local temperature difference.
For the fluid flowing in an exchanger The temperature of the fluid in the tubes increases continuously as the fluid flows through the tubes.
The temperatures of the condensing vapor and of the liquid are plotted against the tube length in Figure
Heat flux may be based either on the inside area or the outside area of the tubes.
Average temperature of fluid stream
Because the temperature varies throughout the cross section of the stream , it is necessary to state what is meant by the temperature of the stream.
Countercurrent and parallel-current flow
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case is shown in Fig.11-4a
connection G leads to a trap, which is a device that allows flow of liquid but holds back vapor.
The fluid to be heated is pumped through connection H into channel D2.
It consists essentially of a bundle of parallel tubes A, the ends of which are expanded into tube sheets B1 and B2.
The tube is inside a cylindrical shell C and is provided with two channels D1 and D2, one at each end, and two channel covers E1 and E2.
Principles of heat flow in fluids
Typical heat-exchange equipment
Single-pass shell-and-tube condenser
Expansion joint
It is clear from Fig.11-4 that Δt can vary considerably from point to point along the tube, and, therefore, the flux also varies with tube length.
The local flux dq/dA is related to the local value of Δt by the equation
because, as inspection of Figs11-4a and b will show, it is
not possible with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat that can be transferred is less than that possible in countercurrent flow.
The temperatures plotted Fig11-4 are average stream temperatures.
The temperature so defined is called the average or mixing-cup stream temperature.
Overall heat-transfer coefficient
Double-tube heat exchanger It is assembled of standard metal pipe and standarized return bends and return heads. shown in Fig.11-3.
One fluid flows through the inside pipe and second fluid through the annular space between the outside and inside pipes.
Where it is important to change the temperature of at least one fluid rapidly.
Energy balances
Enthalpy balances in heat exchangers
Heat transfer to or from the ambient is not desired in practice, and it is usually reduced to a small magnitude by suitable insulation.
Single-pass shell-and-tube condenser
For the vapor flowing in a condenser If the vapor entering the condenser (shell) is not superheated and the condensate leaving the condenser is not subcooled, the temperature throughout the shell-side of the condenser is constant.
Heat flux and heat transfer coefficient Heat flux
The rate of heat transfer per unit area is called the heat flux.
In many types of heat-transfer equipment the transfer surfaces are constructed from tubes.
Double-pipe exchanger are useful when not more than 9 to 14 m2 of surface is required.
For larger capacities , more elaborate shell-and-tube exchangers, containing up to thousand of square meter of area, are used.
Temperature ºC
Temp of condensing vapor T
Δt
Δt2
Δt1
Length of tube m
The flow type with the counterflow is commonly used. Parallel flow is rarely used in a single-pass exchanger such as that shown in Fig11-3
q=mh(Hh1-Hh2)= mc(Hc2 - Hc1)
If constant specific heats are assumed, the overall enthalpy balance for a heat exchanger becomes
q=mhCph (Th1-Th2)= mcCpc (tc2 - tc1) (11-6)
In the multipass exchanger ollowing situation:
In special situation where it is necessary to limit the maximum temperature of the cooler fluid; or limit the min temperature of hot fluid.
For the cold fluid, it can gain heat q=mc(Hc2 - Hc1)
Neglecting the heat transfer with the ambient. The heat lost by the warm fluid is gained by the cold fluid, therefore
dq dA
U
(T11-9t )
The quantity U is called the local overall heattransfer coefficient.
It is necessary to specify the area.
If A is taken as the outside tube area Ao, U becomes a coefficient based on that area and is written Uo.
It is customary to neglect it in comparison with the heat transfer through the wall of the tubes from the warm fluid to the cold fluid.
For the warm fluid, it can loss heat. q=mh(Hh1-Hh2)
If the two fluids enter at the same end of the exchanger and flow in the same direction to the other end, the flow is called parallel.
The temperature -length curves for parallel flow are shown in Figure
Steam and other vapor is introduced through nozzle F into the shell-side space surrounding the tubes, condensate is withdrawn through connection G, and any noncondensable gas that might enter with the inlet vapor is removed through vent K.
It is reasonable to expect the heat flux to be proportional to a driving force. The driving force is taken as Δt=T-t, which is the overall local temperature difference.
For the fluid flowing in an exchanger The temperature of the fluid in the tubes increases continuously as the fluid flows through the tubes.
The temperatures of the condensing vapor and of the liquid are plotted against the tube length in Figure
Heat flux may be based either on the inside area or the outside area of the tubes.
Average temperature of fluid stream
Because the temperature varies throughout the cross section of the stream , it is necessary to state what is meant by the temperature of the stream.
Countercurrent and parallel-current flow
The two fluids enter at different ends of the exchanger and pass in opposite directions through the unit.
It is called counterflow or countercurrent flow. The temperature-length curves for this case is shown in Fig.11-4a
connection G leads to a trap, which is a device that allows flow of liquid but holds back vapor.
The fluid to be heated is pumped through connection H into channel D2.
It consists essentially of a bundle of parallel tubes A, the ends of which are expanded into tube sheets B1 and B2.
The tube is inside a cylindrical shell C and is provided with two channels D1 and D2, one at each end, and two channel covers E1 and E2.