Dear chavannilesh,

The discussion about dense- and dilute phase is coming back regularly.

The first problem is that different definitions exist, based on:

- solid loading ratio

- velocity

- Location of operating point in the Zenz diagram

- dense phase is the same as (controlled) plug conveying.

The, you ask for the optimum pneumatic conveying efficiency.

To have a meaningful discussion, we must agree on the definitions of:

- dense phase conveying

- dilute phase conveying

- pneumatic conveying efficiency

Since you put the question forward in this thread, what are your definitions? ■

Dear chavannilesh,

The first comment is that you are referring in the definitions to a variable “concentration” in %.

In pneumatic conveying, the term concentration is never used.

The variable Solid Loading Ratio, however, is commonly used to describe the ratio between material mass flow and gas mass flow.

(material mass flow)/(gas mass flow), which is a dimensionless variable.

This definition has a flaw, that a very long pipeline conveys per definition at a very low Solid Loading Ratio but can well be dense conveying when the velocity is “low”.

This definition has a flaw, that a very short pipeline conveys per definition at a very high Solid Loading Ratio but can well be dilute conveying when the velocity is “high”.

When the solids are not fully suspended, the system is operating close to the instable conveying zone in the Zenz diagram.

Therefore, the Zenz diagram is used to indicate the boundary between dense (left of the lowest point) and dilute (right of the lowest point) conveying.

Sometimes, dense conveying is mentioned as plug conveying.

This definition implicates the overall energy consumption per conveyed ton, including the compressor efficiency.

The pneumatic conveying efficiency can be defined as the expansion energy of the gas per conveyed ton, excluding the compressor efficiency.

Designing a system as close as possible to the sedimentation/choking boundary in the Zenz diagram for a defined capacity and pressure.

This design must be made for a range of pipe diameters until the required performance and/or efficiency is reached.

Maximum performance and maximum energy efficiency (lowest energy consumption) never coincide. ■

Dear chavannilesh,

Already explained:

This definition implicates the overall energy consumption per conveyed ton, including the compressor efficiency.

The pneumatic conveying efficiency can be defined as the expansion energy of the gas per conveyed ton, excluding the compressor efficiency.

The energetic efficiency of the fan/compressor does not affect the pneumatic conveying efficiency (exclusive the energetic efficiency of the fan/compressor)

The energetic efficiency of the overall pneumatic conveying efficiency (inclusive the energetic efficiency of the fan/compressor) is influenced by a fan/compressor.

The fan- and compressor characteristics (pressure/vacuum and air flow) determine the possible pneumatic conveying design.

A certain pneumatic conveying design, resulting in a required capacity, airflow and pressure/vacuum determines the compressor/fan selection.

The main difference is that a lobe blower or screw compressor with internal compression are Positive Displacement blower.

The displaced volume is independent of the intake conditions.

The displaced mass is dependent of the intake conditions.

A fan is an inertia pump, whereby the volume flow is depending on the pressure drop.

A higher pressure drop results in a lower air volume flow.

If a fan pressure drop increases too high, the pneumatic conveying chokes easily, due to the lower velocities because of the lower airflow.

At the same time, the pressure increases further, increasing the SLR, causing an even higher pressure drop.

Designing a pneumatic conveying system should not be underestimated in complexity. ■

Dear chavannilesh,

Pressure sensors, level sensors, PLC controls ■

Dear chavannilesh,

Vacuum or pressure system?

22 mesh = 797 microns = 0.797 mm

Vertical distance is missing

Material name is missing (required for solid loss factor)

Material density is missing ( required for suspension velocity

Altitude is missing

Method of feeding is missing (kettle system, rotary valve?)

Have you accounted for these variables in you calculation?

300 m3/hr: is that air compressor displacement? ■

Dear chavannilesh,

Obviously, you have designed for a solid loading ratio of approx. 2.9.

The full blower pressure is not used.

The velocities are OK.

Filter area = 10 m2 ■

Dear chavannilesh,

150 m flexible hose will increase the solid loss factor and thereby the pressure drop.

Are you planning soft hoses or reinforced hoses?

For conveying spices, a food product, I would expect stainless steel pipe.

You do not want a rubber taste in your meal.

I understood that the installation is a pressure system, and I calculated as such.

After the filter separator, a filter fan is not absolutely required, but will keep the silo pressure slightly below atmospheric. ■

Dear chavannilesh,

OK. A short distance.

Better to use a reinforced hose of the correct size.

As it concerns a food grade installation, the used conveying gas should be nitrogen.

A silo pressure above the atmospheric will, in case of a leak, blow spice into the environment.

If the silo pressure ais below atmospheric:

Possible leaks allow rain- or contaminated air ingress when the silo is placed outside, but prevents dust emission.

Self-cleaning food grade cartridge filter seem to be OK.

Consult filter manufacturers and authorities for approval ■

Dear chavannilesh,

OK

To prevent choking, the rotary valve must be stopped and the blower must keep running.

The rotary lock rpm can be used as a reference for keeping the capacity constant

The blower outlet must be switched to the atmosphere and when that mechanism fails, a spring loaded safety valve must open

You can increase the feeding until the conveying pressure becomes too high or the conveying pressure becomes unstable and fluctuating, ■

Dear chavannilesh,

Why a screw feeder?

Multiple rotary valves will do. ■

Dear chavannilesh,

OK

A screw feeder can leak material through the screw flights with air from the atmospheric to the vacuum inlet.

A rotary valve, experiencing the same pressure differential seals the material off better.

Screw feeders are available for the situation, where the pressure differential is reverse than compared in the case of a vacuum system feeder.

Possible air leakage through a vacuum system feeder has no significant influence on the pneumatic conveying performance.

The air inlet flow + the feeder leakage always equals the blower air flow.

Are you now referring to another system, or is this the same system of 150m? ■

Dear chavannilesh,

For a pressure system, the pressure transmitter is mounted in the clean air supply line.

For a vacuum system, the pressure transmitter is mounted in the clean air vacuum line.

When the pipeline starts to choke, the pressure (or vacuum) surpasses a maximum limit. That is the pressure (or vacuum) to stop the feeding.

Preferably not.

The measured pressure (or vacuum) is compared with a set pressure (or vacuum) to control the rpm of the feeder. ■

Have a look at

https://www.youtube.com/watch?v=1Da7iWH7F0 ■

Dear chavannilesh,

There is nothing to suggest, as you have all parameters listed as if determined.

Although there is some information missing.

- Type of soda ash

- particle density

- bulk density

- Particle size

- Ambient conditions

- Altitude a.s.l.

Considering the very short length and the very low SLR, the system will work, if the suspension velocity is not too high. ■