Dear KChetan,

The gas temperature influences the gas density.

All pneumatic conveying parameters where gas density is involved are affected by the gas temperature.

SLR = mass flow of material / mass flow of gas

A higher compressor intake temperature results in a lower density and therefore in a lower mass flow of gas.

Hence, the SLR increases with a higher gas temperature.

If the intake temperature is constant, the mass flow of gas is constant and then a higher gas temperature in the conveying pipeline results in a lower gas density and therefore higher gas velocities.

The suspension velocity of the material decreases with a higher gas temperature.

Then material acceleration decreases and the material velocity will also be lower.

Whether a temperature increase of the conveying gas (mainly caused by the material temperature and depending on the pressure and compression principle) influences the pressure drop and capacity, depends largely on the compared installation. (Dilute or dense as defined by the Zenz diagram)

The inter action (or sometimes counter-inter action) of the many parameters in pneumatic conveying make calculations complex and the overall effects very difficult to predict.

Success

Teus ■

If the temperature gets high enough there might be a source of ignition.

That could affect the flow through the system.

(It's wind up for a Wednesday afternoon in Saudi) ■

Dear kChetan,

If the temperatures of conveying gas and material are not considered in a mathematical calculation, that must be seen as a shortcoming of that program.

The calculated pressure drop is depending on various partial pressure drops, which are influenced by the density of the air.

F.i.

Gas density decreases with increasing gas temperature.

The suspension velocity of the particles increase with lower gas density, hence with higher temperatures.

This results in a higher required gas velocity for keeping the particles in suspension and therefore requires a higher pressure drop for lifting the material.

This extra pressure drop would not have been calculated when the temperature was not considered.

Moreover, there are more heat flows to consider, which influence the gas temperature, s.a. heat exchange between conveying gas and the conveyed material, heat exchange with the surroundings, heat exchange from friction losses going back into the gas-material mixture, temperature drop due to the expansion of the conveying gas, etc.

A quite complex heat exchange system.

The mathematical model, I made takes into account all these factors.

If a mathematical calculation does not take into account the temperature influence will give probably acceptable results as long as the temperatures on which the mathematical calculation is based and the operating temperatures are the same.

Using this mathematical calculation for f.i. much higher temperatures will result in significant deviations between calculated performance and achieved performance.

Is your question related to a practical case?

Take care

Teus ■

Dear Chetan,

In my article on pneumatic conveying calculations it is obvious that gas density is a major parameter in successful conveying system design. And we also know that gas density varies with gas temperature. But in a typical conveying line the residence time is only a few seconds, therefore, there is not much change in gas temperature. In non-typical systems, gas temperature along the conveying line should be calculated or estimated and this change should be used to determine gas density along the conveying line.

For positive pressure systems, conveying air is generally cooled by using an air cooler at the blower discharge, to maintain a constant air temperature at the inlet of the conveying line.

Effect of temperature must also be used in the design of all of the conveying system components, especially if the gas temperature or the ambient temperature is too low or too high.

Regards,

Sincerely,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com ■

Dear Chetan, Mr Agarwal,

In addition to Mr. Agarwal’s reply, the following remarks:

The temperature change along the pipeline, due to heat exchange between particles and material is relatively small, because the material temperature determines to a great extend the mixture temperature.

This is due to the high heat content of the material, magnified by the Solid Loading Ratio.

Other heat exchanges do exist (as already explained in my earlier reply).

However the effect is dampened by the high heat capacity of the material compared to the heat capacity of the conveying gas.

In this respect, the cooling of the compressed conveying air after the compressor in a pressure conveying installation has a reduced effect, because the mixture temperature will always be close to the material temperature.

At low SLR, the pressure is lower and therefore also the compressed air temperature is lower.

Cooling the compressed air has therefore also a limited effect.

With cement conveying installations (which I designed, built and operated), the compressed air is very seldom cooled and yet, although the compressed air has a temperature of around 200 degrC and the cement temperature is approx 30 to 40 degrC, the outlet of the pressure tank is just hand warm.

This also proves that there is certainly a significant heat exchange, which could be expected, because of the small particles having a relatively large contact area.

The intake temperature of compressors determines the mass flow of the air and therefore are of greater influence.

When I was involved in big grain unloaders in the port of Rotterdam, it was common knowledge that the performance was better during cold and dry days in the winter.

Calculating the change in gas temperature along the conveying line assumes a significant heat exchange between the material particles, conveying gas and surroundings through the pipe wall.

This is indeed the case, because I experienced that, while the cement conveying pipe was hand warm at the beginning, the temperature at the end of the pipe was decreased to just a little above ambient temperature.

Ambient temperature and material temperature should not be neglected in pneumatic conveying.

Have a nice day

Teus ■

kchetan

A less academic (for which everyone is eternally grateful to Tues for his knowledge sharing) but more "real life" answer -- I have been involved in systems where the product(s) being conveyed are quite temperature sensitive and the change in temperature from a warm sunny day to the cooler evenings does make a difference.

The influence in the material to pipe wall friction portion of the calculations Teus and Amrit referred to come into play for sure.

Many polymer materials have this characteristics they can reach their softening point and start to have higher frictional losses as well as create more streamers

Granulated sugar has similar characteristics in that the product will begin to get sticky at elevated temperatures in the convey lines. ■