Filling the vacuum

Compared with alternative means of getting solids material from A to B, such as mechanical conveyors and bulk containers, moving granules and powders in a pipe has obvious attractions.

The transport mechanism can be continuous; the material being transported is totally contained; and the infrastructure required seems reasonably straightforward - a feed vessel, a pump/blower, a pipe, and a receiver vessel. As with most good ideas, however, it is rarely that simple.

First, you have to decide between pneumatic (that is, positive pressure) or vacuum conveying. The former offers the advantage of allowing higher pressures to be used than vacuum systems, which by definition are limited to a maximum 'pulling' power of 1bar.

This is why traditionally high volume, long distance conveying of materials such as coal, mineral ores, cement and so on has been done in pneumatic systems. These are often fitted with booster stations at strategic points along the pipe length to make up for pressure losses along the line. But this can be quite an energy-intensive operation compared with the far lower power requirements of a vacuum system.

For in-process conveying, the properties of the material to be conveyed also have to be considered when taking the next decision. This is selecting between dense phase conveying and dilute, or lean, phase conveying. In lean phase, whether pneumatic or vacuum, the volume and velocity of the transporting gas (usually air, though inert gases like nitrogen can be used in some applications) has to be high enough to carry the solids along with it - and the density and concentration of the solids have to be low enough for them to stay in suspension in the fast moving gas stream.

Technically, in lean phase the gas velocity has to be above what is known as the 'saltation velocity', which is the point at which the solids concentration becomes too high for the solids to remain in suspension and they start to drop out and deposit on the bottom of the pipe.

This is where we move into the realms of dense phase conveying. The main air stream may no longer be strong enough to carry the solid particles along with it, but as long as the solids material is fluidisable it can still be transported through the pipe. Just like a liquid, the fluidised layer of solids flows through the pipe either in waves or plugs of material, pulled along by the gas flow - which no longer has to be at high velocity since the solids are not entrained in the main airflow.

The obvious advantage dense phase has over lean phase is that a smaller volume of air can convey more solids. But there are other, less immediately obvious benefits that become clear when considering specific applications, such as some of those tackled by Stockport-based Pneumatic Conveying Systems (PCS).

In addition to its eponymous business, PCS has recently been developing and refining the concept of dense phase vacuum conveying (DPVC). As sales and technical director Roger Burgess explains: 'everyone in the powder handling business is aware of positive pressure, dense phase pneumatic conveying. It's most often associated with large throughputs over long distances... however, DPVC systems are certainly effective when moving high tonnages of product over short distance of up to 20m. This satisfies a significant number of industrial applications and offers some important operational advantages.'

He points out that the main advantage of high velocity, lean phase vacuum systems is the total containment of the products being conveyed. It's not simply that the products are conveyed in a pipe, but rather that vacuum systems guarantee that there are no emissions into the environment. If there is any leakage in the system, air will flow in, not out.

Burgess says that with DPVC, PCS has now taken vacuum systems to the next level. 'DPVC systems offer the same environmental protection [as lean phase vacuum] but move products at low velocity, either in slugs or as a continuous stream of dense but aerated product.' Either way, he says the densely packed product moves as one - eliminating the segregation of product that can be a problem in high velocity, lean phase systems.

For producers of many food and pharmaceutical products, this can be of enormous benefit. Having produced carefully blended foodstuffs or pharmaceutical formulations, the last thing these companies want is for the material's composition to change during conveying from process to packaging.

Low velocity also means less attrition and less abrasion as product moves through the pipeline - and much less impact damage when the product is brought to a halt in the receiving hopper. 'The gentle nature of DPVC systems,' says Burgess, 'can be confirmed by a PCS system conveying spray-dried milk powder at 8000kg/h over 20m using a 7.5kW exhauster (vacuum pump). There is little or no damage to this most fragile of products.' Other fragile products successfully handled include freeze-dried coffee and baby foods.

Another possible benefit of DPVC is again explained by Burgess. 'As powder moves through the pipe in such a densely fluidised condition,' he says, 'there can be a syphonic effect created as it discharges into the receiving hopper. Throughputs of up to ten times conventional conveying rates have been achieved for some products - for virtually the same power consumption.'

<b>Meeting a stiff challenge</b>

A good example of the benefits of dense phase vacuum conveying is Pneumatic Conveying Systems' recent installation for National Starch at its factory in Goole, Yorkshire.

The factory produces a wide variety of standard and specialist liquid starches for use in the food and papermaking industries. Just over a year ago, chief engineer Sean Wright was looking to reduce the manual loading of the starch slurry mixer, along with improving the working environment and reducing packaging waste. This was at a time when the mixer had to be charged with up to 7tonnes at a time, loaded by operatives emptying 25kg bags of starch into a vented sack tip hopper.

The ideal solution was to switch to having the starch delivered in FIBCs. Only six or seven of the containers need to be unloaded per batch, leaving a small amount to be added manually for viscosity adjustment. What was needed, though, was an efficient transfer system to rapidly deliver the starch from the FIBC discharge station to the mixer without any dust escaping to atmosphere.

National Starch's specification was for the mixer to be loaded with powdered starch at a continuous and uniform rate of 12 000kg/h. After deciding to place the order with PCS, both Wright and plant co-ordinator Peter Holgate visited PCS's test centre in Stockport to witness trials on their starch products.

Relying on empirical data, and a wealth of experience from previous installations, PCS had sized a dense phase vacuum system with a 4kW exhauster ready for the trials at full scale production rates. The trials firmly demonstrated the efficiency of the fully fluidised feed bin and the vacuum receiving hopper with its rotary valve outlet. PCS believes that, as the scale-up theory is not totally reliable, there is no substitute for carrying out full scale trials on the actual product and conveying it over the actual distances to be encountered on site.