Ordinary pneumatic conveying calculations assume isothermal expansion of the conveying gas.
The question is then:” which temperature and why is the temperature assumed constant?”
The temperature of the conveying gas determines the gas velocity and thereby the resulting pressure drop and condensation.
The gas laws dictate that expanding gas cools down, but when the temperature is (by experience) approximately constant, where is the heat to keep the expanding gas temperature close to constant coming from?
The energy source that is used to convey the material through a pipeline is the temperature of the gas plus the temperatures of the material and the surroundings.
The temperature source of the gas and the material is always positive and the other sources can be positive or negative.
Pneumatic conveying is temperature driven.
The only conclusion is that there are heat exchanges between the conveying gas and the environment, friction generated heat, and the most important, the conveyed material.
As temperature is the stored energy in a gas and a material and there are many situations possible where the temperatures of the conveying gas (high after compression) and the surroundings (low in the arctics and high in the tropics) and the dissipated friction energy, the resulting mixture temperature along the pipeline is not easy to determine.
To calculate the temperature along a pipeline, starts at the outlet of the compressor, possibly followed by a cooler, and after that a pipeline (insulated or not insulated) to the feeder mixing point).
At the feeder mixing point, the gas encounters the conveyed material, which has a certain temperature, but certainly different from the gas temperature.
When 2 materials (gas and product) with different temperatures mix there is heat exchanged until a common temperature is reached.
The mixture temperature will normally be close to the material temperature, because of the relatively high specific heat content compared to the specific heat content of the gas.
At that location, the first chance of condensation occurs as the pressure is high and the temperature can be decreased to a RH of 100%
The condensation heat will increase the gas temperature and thereby the mixture temperature again
Conveying a very hot material will increase the mixture temperature and thereby the gas temperature even higher, resulting in a higher gas volume and higher velocities.
Along the pipeline, there will be friction energy dissipated in heat, causing an increase in mixture temperature and there will be heat exchanged through the pipe wall, whereby the mixture temperature can be increased or decreased.
As the mixture temperature has an influence on the gas- and material velocity, there is a chance that the conveying is entering a sedimentation region when the conveying temperature becomes too cold, due to too much external cooling (arctics).
All the above heat exchanges, occurring along the pipeline can be calculated and determine whether the conveying temperature increases or decreases along the pipeline and the influence on the gas velocity (and the material velocity).
A pneumatic conveying system starting with the mixture temperature and no heat interaction with the surroundings will always cool down, because the only heat sources are the gas and the material only a part of that heat source is returned into the mixture flow.
Heat exchanges with the surrounding will exist in reality always and influence the expansion coefficient not to be exactly 1 and therefore, pneumatic conveying is (almost) never isothermal.
By assuming an isothermal expansion in pneumatic conveying calculations, the accuracy of that calculation is unknown.
The above emphasizes the importance of understanding the gas laws and thermodynamics in relation to pneumatic conveying. ■
Isothermal pneumatic conveying.
Ordinary pneumatic conveying calculations assume isothermal expansion of the conveying gas.
The question is then:” which temperature and why is the temperature assumed constant?”
The temperature of the conveying gas determines the gas velocity and thereby the resulting pressure drop and condensation.
The gas laws dictate that expanding gas cools down, but when the temperature is (by experience) approximately constant, where is the heat to keep the expanding gas temperature close to constant coming from?
The energy source that is used to convey the material through a pipeline is the temperature of the gas plus the temperatures of the material and the surroundings.
The temperature source of the gas and the material is always positive and the other sources can be positive or negative.
Pneumatic conveying is temperature driven.
The only conclusion is that there are heat exchanges between the conveying gas and the environment, friction generated heat, and the most important, the conveyed material.
As temperature is the stored energy in a gas and a material and there are many situations possible where the temperatures of the conveying gas (high after compression) and the surroundings (low in the arctics and high in the tropics) and the dissipated friction energy, the resulting mixture temperature along the pipeline is not easy to determine.
To calculate the temperature along a pipeline, starts at the outlet of the compressor, possibly followed by a cooler, and after that a pipeline (insulated or not insulated) to the feeder mixing point).
At the feeder mixing point, the gas encounters the conveyed material, which has a certain temperature, but certainly different from the gas temperature.
When 2 materials (gas and product) with different temperatures mix there is heat exchanged until a common temperature is reached.
The mixture temperature will normally be close to the material temperature, because of the relatively high specific heat content compared to the specific heat content of the gas.
At that location, the first chance of condensation occurs as the pressure is high and the temperature can be decreased to a RH of 100%
The condensation heat will increase the gas temperature and thereby the mixture temperature again
Conveying a very hot material will increase the mixture temperature and thereby the gas temperature even higher, resulting in a higher gas volume and higher velocities.
Along the pipeline, there will be friction energy dissipated in heat, causing an increase in mixture temperature and there will be heat exchanged through the pipe wall, whereby the mixture temperature can be increased or decreased.
As the mixture temperature has an influence on the gas- and material velocity, there is a chance that the conveying is entering a sedimentation region when the conveying temperature becomes too cold, due to too much external cooling (arctics).
All the above heat exchanges, occurring along the pipeline can be calculated and determine whether the conveying temperature increases or decreases along the pipeline and the influence on the gas velocity (and the material velocity).
A pneumatic conveying system starting with the mixture temperature and no heat interaction with the surroundings will always cool down, because the only heat sources are the gas and the material only a part of that heat source is returned into the mixture flow.
Heat exchanges with the surrounding will exist in reality always and influence the expansion coefficient not to be exactly 1 and therefore, pneumatic conveying is (almost) never isothermal.
By assuming an isothermal expansion in pneumatic conveying calculations, the accuracy of that calculation is unknown.
The above emphasizes the importance of understanding the gas laws and thermodynamics in relation to pneumatic conveying. ■
Teus