Dear crawljg,

1)

Indeed, you have to summarize all the pressure drops from the material intake up to the blower intake.

The blower intake conditions (pressure and temperature) determine the air mass flow of a vacuum system.

And the air mass flow influences the pressure drop of the product carrying part of the system and the product non carrying part of the system.

Between the baghouse and the blower inlet, the pressure drop is only caused by air as there should be no product present there.

2)

If a particle changes in velocity, kinetic energy is involved.

A particle can be accelerated and decelerated by the airflow and gravity.

In a bend, the particle will be decelerated by friction. After the bend the gas flow accelerates the particle again.

Inter particle and wall collisions have a decelerating effect and result in subsequent acceleration. (This is causing the slip velocity of particles in relation to the gas velocity)

At a diameter increase, a particle can enter a bigger pipe at a higher velocity than the gas velocity and will be decelerated by the gas.

Then, some gas pressure is regained.

But the calculation method that is used, dictates the acceleration pressure drop that has to be calculated, as this is also related to the used product loss factor.

3)

The discharge piping of the blower increases the pressure over the blower and thereby the power consumption.

As the volumetric efficiency is related to the pressure drop over the blower, the mass low is also influenced.

As these influences are normally small, they are neglected.

(But they do exist)

Have a nice day. ■

dear crawlejg,

As you are in the proces of modifying an existing installation, you can calculate the K-factor yourself, using the performance data of that installation under the present conditions with the calculation spreadsheet you have.

After that, you can use the obtained K-factor to calculate the modifications.

success ■

Dear crawlejg,

Forgive me if I am wrong (as it is 3am now) but I think your statment "since the particles are speeding up only a small amount after the first section" is incorrect.

In fact if I have got the theory right, then with a vacumm system the velocity accelerates at a greater rate the further you go along the system.

Consider (for a perfect vacum system) that P inlet = 1 atmosphere while P blower = 0 atmospehere.

So (Pinlet-Pblower)/Pblower = 1/0 = infinity.

Via Bernoulli's eqn & conservation of energy, this leads to a theoretical end velocity of infinity or is it sqrt(infinity). Either way and even taking many inefficiencies into account you get increasing acceleration and large end velocity in vacuum systems. Thats why they are rather less efficient than positive pressure systems which have reasonable values for (Pout-Pin)/Pin and thus reasonable ratio between start and end velocity.

Anyway as I said its 3am here and im not sure I got the theory right (if not, then someone else can explain) but im pretty sure the basic concept is correct.

Cheers

Young&Silly ■

As far as particle acceleration at pick up point and after a bend is concerned it is mostly dependent on air velocities. There is published work looking into acceleration at pick up point and after bends in most cases it has been reported that acceleration lengths after bends are longer then after pick up points. This is probably due to less slip velocity which results in longer acceleration lengths.

Volumetric increase in air is double in vacuum then is positive pressure. This also leads to higher exit velocities in vacuum systems as compared to same pressure drops in pressure system, but this is normally corrected by stepping the conveying pipe. ■

dear Young&Silly

The air velocity in a pipe of a vacuum system can be represented by ;

vair = (Pumpvolume * (1 – vacuum)) / (p * A)

where:

Pumpvolume in m3

Vacuum = underpressure at pump intake in bar

p = absolute pressure at considered pipe line location in bar

A = pipe area in m2

For the end of the pipeline --- > p = ( 1 – vacuum)

resulting in an end velocity of:

vend = Pumpvolume / A = constant (Not infinite)

Acceleration above this velocity is impossible, due to the laws, you mentioned.

Mr Mantoo, the velocity increase is dictated by the pressure ratio of a pneumatic conveying system.

for a vacuum system, this is 1/(1-vacuum)

and for a pressure system this is (1+p)/1

Example;

vacuum = 0.75 bar (common for a cement unloading system) resulting in a velocity ratio of

vend / vbegin = 1 /0.25 = 4

pressure = 2.5 bar(o)

vend / vbegin = (1+2.5) / 1 = 3.5

best regards ■

Thank you Mr Teus for your explanation, I think it’s holding the stick form the other end. ■

Dear mr Teus Tuinenburg:

I also read mr agarwal's paper about dilute phase conveying and i'm also confused about a few things.

first of all, i'm working with several intakes in the system, more than 15, the material is barley powder, its an extremely fine powder. how does that method apply to a system with several intakes? anychance of a further explaination? also as its a cleaning system, there's no count about the amount of solids that are being pick in each point, so i see this as a huge problem to use this methond, any recomendations or ideas?

anyway if there's anyone how could help me with this I'd appreciate it...

P.D.: I could also work the calculations with air so I can at least get an estimate about the blower selection. ■