If you look at the fan characteristic curves there is a very sharp drop in the flow rate with the increase in pressure. This causes problems during pneumatic conveying. Fan conveying is only suitable for low density solids at low rates and short distances. ■

Dear Mr Giraldo,

Generally two types of air pumps are used.

1) positive displacement pumps :

- Roots-type blowers

- Screw-compressors with internal compression

- Screw-compressors with internal compression and pre-inlet

- piston compressors

2) flow-pumps :

- turbine compressors

Operation of a positive displacement-pump in a

pneumatic conveying system in vacuum

The mass-flow displaced by a positive displacement-pump is

determined by the pressure and temperature of the air at the

intake.

Because of the positive displacement character of the pump ,

the displaced volume is practically constant.

Therefore the mass flow decreases when the vacuum on the

intake increases.

As a result of this ,that at low loading-ratios ,the

air-velocities in the first part ,and with that also the

product-velocities ,will be higher.

An advantage of this phenomena is ,that the starting of the

pneumatic vacuum-transport is guaranteed by the high

air-velocities in the suction-nozzle at low system-vacuum.

At low loading-ratio ,the positive displacement-pump with

an isochoric compression ,consumes almost proportional to the

lower induced vacuum ,less energy.

Adjusting the airflow for different products can be done by

controlling the rpm of the pump (no-loss method) or by

letting false air into the system (method with energy losses)

Operation of a turbine compressor in a pneumatic conveying system

The basic principle of a turbine-compressor is by transferring

kinetic energy to the air through a rotating impeller-wheel.

In a (adjustable) diffuser ,the kinetic energy is converted

into pressure.

The mass-flow displaced by a turbine compressor is constant

at a defined position of the diffuser and when maintained at

a constant pressure-ratio.

In a pneumatic convey system, the pressure-ratio will vary

with the loading-ratio.

In order to maintain a constant pressure-ratio, the difference

in pressure between the pump and the system has to be

throttled by a pressure-controlled valve in series with

pump-intake or -outlet.

This results in the pump always operating in a defined

working point on its curves.

An adjustable diffuser allows the flow to be controlled

without loss of energy.

Once adjusted, the turbine-compressor consumes a constant

energy, regardless the system pressure drop.

The throttle valve, which induces energy losses, creates

in partial load (low loading ratios) energy losses, which

cannot be neglected.

Because of the air-mass flow being constant, the air-velocity

at the end of a pipeline of a vacuum system will decrease with

decreasing system vacuum (lower loading ratios).

This also results in lower product velocities.

The constant mass flow also results in a constant air-velocity

at the suction-nozzle.

This is a disadvantage regarding the starting of a vacuum transport because an increased airflow should have created an increased pressure drop at the nozzle.

This is the case when a positive displacement-pump is applied.

Operation of a fan compressor in a pneumatic conveying system

A ordinary fan works as a turbine without the regulation throttle valve and diffuser.

A fan therefore will deliver more air at lower pressure ratios and lss air at higher pressure ratios.

The air flow is dependant on the pressure ratio in such a way that, when the pressure increases because of a momentarily higher loading ratio, the air flow decreases.

That causes a even higher loading ratio, resulting in more pressure drop, leading to a blockage of the pipeline.

At low loading ratios the air flow becomes more than proportionally high, with possible damaging the material.

All for now

teus ■

Daniel,

I have used centrifugal fans for pneumatic conveying systems in which the system pressure drop is less than 1 psi. I use fans whose performance curve is not very steep and also use a pressure control valve to keep the fan's discharge flow constant.

For your 2000 lbs/hr system, you may consider using fans if the system pressure drop is less than about 1 psi.

Regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■