The conveying gas normally contains an amount of water vapor, which can condense while pressurized and/or cooled.
The amount of water vapor is determined by the RH of the intake air.
After the compressor:
-The air pressure has increased, increasing the RH
-The temperature has increased in case of polytropic or isochoric compression, decreasing the RH
-In case of isothermal compression, the temperature is not increased and the RH increases as only the pressure increases.
Depending on the compressor intake temperature and the ambient pressure and the ambient RH and the compression pressure and the compressed temperature, the RH of the compressed air is lower/equal or higher than the intake RH.
Above a RH=100% there will be water vapor condensation.
De pipeline between the compressor and the material mixing point is also a cooler, which can cause the first condensation in the clean air supply pipeline.
The next location where condensation can occur is the air/material mixing point.
Where the compressed air temperature can be up to 200 degrC, the material temperature can be close to the ambient temperature, resulting in a mixture temperature close to the material temperature. This is due to the much higher heat content of the material in relation to the heat content of the conveying air.
Along the pipeline the pressure reduces, causing the conveying air to become dryer.
However, if the conveying pipe (which is also a heat exchanger) runs through a very cold zone, the temperature along the pipeline reduces faster than the pressure, condensation can still occur.
The conveying gas is in reality not a dry gas but a gas mixture of air and water vapor.
And an air/water vapor mixture has a higher density, influencing the particle drag force and the partial pressure drops.
The sum of the partial pressure drops influences the gas velocity, which creates feedback on the total pressure drop and the gas volume.
As the gas expands along the pipeline, the gas is delivering work to the expansion while cooling down.
Some partial pressure drops are spent on friction, which return heat energy back to the gas and thereby increasing the gas temperature.
All these effects determine the local RH and whether there will be condensation or not.
If there is condensation, the condensation heat is spread over the gas and the material, increasing the mixture temperature, influencing the gas volume, density and thereby the velocities, and so on.
The occurrence of condensation in a pneumatic conveying system must be calculated as water condensation is important.
Example 1:
If there is condensation in a cement conveying installation, at least the condensed water will react with the cement.
This sounds worse than it is, because a proper design results in a low amount of condensed water, binding not more than 0.2% to 0.4% of the conveyed cement, evenly spread over the conveyed cement.
As the cement is later mixed with sand and gravel to make concrete the bonded cement during pneumatic conveying becomes totally irrelevant.
Condensation in fluidizing cloths can become a problem as the water condensates locally in the cloth and cause the cloth to choke.
A ordinary water cooler with a water separator is then always advised.
Example 2:
If Calcium Chloride is conveyed, a material that is used in regenerated adsorption dryers, the pneumatic conveying system suddenly becomes an air dryer and the CaCl2 is changed to a CaCl2- hydrate, which can form clumps.
And possible clumps will have a higher suspension velocity, which cannot be kept in suspension and choking is then possible.
Increasing the air velocity is then the wrong way to go, as the amount of water vapor is then also increased and the formation of clumps also.
The only solution to this problem is to dry the air to a dew point of -40 degrC at the conveying pressure, which requires a high-pressure compressor at f.i. 7.5 barg.
In vacuum conveying, the conveying temperature is the ambient temperature or the material temperature and because the pressure is decreasing along the pipeline, the RH is also decreasing.
An advantage, which make vacuum systems useful in food- and pharmaceutical applications.
The above emphasizes the importance of calculating the RH and the condensation of water along the pipeline as part of the pneumatic conveying algorithm. ■
Pneumatic conveying and condensation.
The conveying gas normally contains an amount of water vapor, which can condense while pressurized and/or cooled.
The amount of water vapor is determined by the RH of the intake air.
After the compressor:
-The air pressure has increased, increasing the RH
-The temperature has increased in case of polytropic or isochoric compression, decreasing the RH
-In case of isothermal compression, the temperature is not increased and the RH increases as only the pressure increases.
Depending on the compressor intake temperature and the ambient pressure and the ambient RH and the compression pressure and the compressed temperature, the RH of the compressed air is lower/equal or higher than the intake RH.
Above a RH=100% there will be water vapor condensation.
De pipeline between the compressor and the material mixing point is also a cooler, which can cause the first condensation in the clean air supply pipeline.
The next location where condensation can occur is the air/material mixing point.
Where the compressed air temperature can be up to 200 degrC, the material temperature can be close to the ambient temperature, resulting in a mixture temperature close to the material temperature. This is due to the much higher heat content of the material in relation to the heat content of the conveying air.
Along the pipeline the pressure reduces, causing the conveying air to become dryer.
However, if the conveying pipe (which is also a heat exchanger) runs through a very cold zone, the temperature along the pipeline reduces faster than the pressure, condensation can still occur.
The conveying gas is in reality not a dry gas but a gas mixture of air and water vapor.
And an air/water vapor mixture has a higher density, influencing the particle drag force and the partial pressure drops.
The sum of the partial pressure drops influences the gas velocity, which creates feedback on the total pressure drop and the gas volume.
As the gas expands along the pipeline, the gas is delivering work to the expansion while cooling down.
Some partial pressure drops are spent on friction, which return heat energy back to the gas and thereby increasing the gas temperature.
All these effects determine the local RH and whether there will be condensation or not.
If there is condensation, the condensation heat is spread over the gas and the material, increasing the mixture temperature, influencing the gas volume, density and thereby the velocities, and so on.
The occurrence of condensation in a pneumatic conveying system must be calculated as water condensation is important.
Example 1:
If there is condensation in a cement conveying installation, at least the condensed water will react with the cement.
This sounds worse than it is, because a proper design results in a low amount of condensed water, binding not more than 0.2% to 0.4% of the conveyed cement, evenly spread over the conveyed cement.
As the cement is later mixed with sand and gravel to make concrete the bonded cement during pneumatic conveying becomes totally irrelevant.
Condensation in fluidizing cloths can become a problem as the water condensates locally in the cloth and cause the cloth to choke.
A ordinary water cooler with a water separator is then always advised.
Example 2:
If Calcium Chloride is conveyed, a material that is used in regenerated adsorption dryers, the pneumatic conveying system suddenly becomes an air dryer and the CaCl2 is changed to a CaCl2- hydrate, which can form clumps.
And possible clumps will have a higher suspension velocity, which cannot be kept in suspension and choking is then possible.
Increasing the air velocity is then the wrong way to go, as the amount of water vapor is then also increased and the formation of clumps also.
The only solution to this problem is to dry the air to a dew point of -40 degrC at the conveying pressure, which requires a high-pressure compressor at f.i. 7.5 barg.
In vacuum conveying, the conveying temperature is the ambient temperature or the material temperature and because the pressure is decreasing along the pipeline, the RH is also decreasing.
An advantage, which make vacuum systems useful in food- and pharmaceutical applications.
The above emphasizes the importance of calculating the RH and the condensation of water along the pipeline as part of the pneumatic conveying algorithm. ■
Teus