Heat transfer is a complex phenomenon.In essence, as long as there is a temperature difference in one medium or between two mediums, heat transfer will occur.We put different types ofHeat transfer processIt is called heat transfer mode.The heat transfer process of objects can be divided into three basic heat transfer modes, namely: heat conductionThermal convectionandthermal radiationThe transfer of heat from high temperature to low temperature is called heat transfer process.[1]
Chinese name
Heat transfer process
Status
Important and basic process in chemical production
In chemical production, whetherchemical process Or physical process, almost all of them needquantity of heatThe import and export of. For example, in mostChemical reaction processAnd physical processes are carried out at a certain temperature. In order to make the system reach and maintain the specified temperature, it is necessary to heat or cool the materials in advance, and when many processes are carried out, it is also necessary to take away the heat released from the process or supplement the heat absorbed by the process in time
Heat transfer is an important and basic process in chemical production
Therefore, heat transfer operation is also an important unit operation in chemical production, and heat transfer equipment accounts for a large proportion of equipment investment in chemical plants, about 1/4 - 1/3
Heat transfer and thermodynamics are two subjects, which are related and different
Thermodynamics - the study of matterEquilibrium stateDetermine the total energy required for the system to change from one state to another equilibrium state
Heat transfer law of energy (heat) transfer
Heat transfer problem should be solved by combining energy conservation and heat transfer rate
objective
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1. Intensify heat transfer to speed up heat transfer rate
2. Reduce heat transfer and avoid heat transfer to reduce heat loss
Whatever the purpose, it is required to masterquantity of heatBasic principles of transmission
Reason and law
Delivery mode
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1. There are two types of heat transfer in chemical production:
Intermittent heat transfer Unstable heat transfer process with energy accumulation
Continuous heat transfer Stable heat transfer process, noenergyAccumulation, also
There is no relative displacement of particles in the system, only the vibration of microscopic particles such as particles (microparticles, molecules, atoms and free electrons),free electronA typical example issolidHeat transfer of
Convection heating
Relative displacement (particle motion) of particles in the systemHeat transferThere are two cases of convection:
Natural convection causes relative motion of particles due to different density caused by different internal temperature of the system(Natural convection)
Forced convection makes particles move relatively by external mechanical agitation or mechanical action(Forced convection)
Heat radiation The process of heat transfer in the form of field due to different temperatures
Contact mode
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1. Cold and heat sources (P210)
Heat source:
Electric heating has a wide range of applications,temperatureWide range
Saturated steam is most commonly used, not exceeding 180 ℃ (1MPa)
The flue gas is above 700 ℃, with slow heat transfer and difficult to control
High temperature heat carrier mineral oil, molten salt, etc., 180 ℃ - 540 ℃
Cold source:
Water, air, frozen brine, etc
2. Contact mode of three heat transfer media (P206)
In direct contact heat transfer, two mediums contact and mix
The two mediums do not contact and do not mix during the interwall heat transfer
(Most commonly used) gas gas, gas liquid, gas solid
Typical heat exchange equipment
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The basic equipment used in industrial heat transfer process is called heat exchanger. In chemical production, the most common is the heat exchange between two fluids
And most of them are wall to wall heat exchange, two fluids do not contact and do not mix
The cold and hot fluids are separated by the solid wall (heat transfer surface) in heat transfer, and the two fluids flow respectively on both sides of the mural
Typical heat exchangers areSleeve type heat exchangerAnd tubular heat exchangerThe simplest casing heat exchange consists of two concentric tubes with different diameters
During the heat transfer of two fluids, there may be phase change or no phase change. The heat transferred without phase change is mainly sensible heat, and when there is phase change, it is mainly latent heat, and there may also be sensible heat
according toThe second law of thermodynamicsWhere there is a temperature difference, the heat transfer process must occur. The driving force of the heat transfer process is the temperature difference, and the heat transfer and heat balance are the basis of the heat transfer calculation
