High and Dry

Drying systems are often one of the biggest power drains on industry. The removal of large amounts of liquid from product requires energy - and lots of it. And as environmental regulations force companies to replace volatile solvents - which evaporate at low temperatures - with water, it's becoming more and more important to install the most efficient drying systems available.

That's the message that the Technology Demonstration Centre at East Midlands Electricity's Nottingham headquarters is putting out. Established to inform companies about the best drying options available for their particular applications, the TDC hosted a recent seminar to explain the principles of electrical drying - and how apparently simple systems can transform processes.

The elements of drying are not complex, said Peter Mynott of Electronic Control Services. Mechanical methods are the most efficient at removing large volumes of water - such as wringing, squeezing, and the action of gravity. Exposure to dry air promotes evaporation from the surface of the material, while air movement stops the air surrounding the wet material from becoming saturated. Heat also promotes evaporation.

The question is, how can these be translated into electrically-powered processes? `Mechanical methods are a good bet, but implementation is a problem,' Mynott said. Options include sucking and, especially, blowing the liquid away - which is where a device called the air knife comes in.

The air knife is a curtain of air at low pressure, moving at near-sonic speed, which blasts the water off the surface of material moving past it.

The equipment needed is simple - an electrically-powered blower, a hose, and the air-knife itself, which is a box with an angled linear nozzle at one corner. Pressure builds up in the box, and is relieved by the air escaping through the nozzle, which creates the high-velocity curtain. `The engineering of the nozzle is the most important factor - it has to be a proper aerodynamic outlet, otherwise it won't be able to convert the pressure energy into the high velocities required,' said Mynott.

Drying the air is also relatively simple with electricity, particularly by heating - a 10 degrees C increase in air temperature halves its relative humidity. Convection ovens (which allow water vapour to escape through vents) are often used to provide a `quick fix' for both batch and continuous systems, said Mynott, but these have drawbacks - they are difficult to control and inefficient.

Much more efficient is a dehumidifer. This is basically a refrigerator, which traps the vapour as it escapes from the oven and cools it down, so the water condenses and escapes as a liquid. Heat recovered from the vapour is then returned to the process. As well as being many times as efficient as a convection oven, this allows both temperature and relative humidity to be controlled.

Direct heating of wet materials is often seen as the method of last resort for drying, but can also have its plus points, said Mynott. One advantage is that heat reduces the viscosity and surface tension of water, which makes it easier to remove it from materials.

The most common heating method today is via infra-red arrays, Mynott said. `Medium' wavelengths are the most commonly used, and fast-response emitter cartridges are now available which supply a large amount of power to a small area, making them particularly suited to drying.

Practical applications often involve combining several of these technologies, according to Ian Lewin of East Midlands Electricity. One good example of this was Wilson & Stafford, a hat manufacturer based Warwickshire. The company had two problems, Lewin said: the first common to all hat makers, the other concerned with a rather unusual raw material.

Many of Wilson & Stafford's hats are made out of a formless felt `hood'. This is formed into the final shape by steaming it and forcing it over a mould; it then has to be dried. Previously, the hats were dried in an elderly gas-fired and steam-heated convection oven, which was fuel hungry and difficult to control.

East Midlands recommended that the company replace this unit with a drying chamber, whose humidity was controlled by an electric dehumidifier. The chamber temperature is 35 degrees C and the relative humidity around 30 per cent - even so, the chamber can remove 350kg of water per day, and dry 3000 hats per load. Energy savings of 90 per cent were achieved immediately. Moreover, the hats emerging from the drying chamber are smooth - the old oven used to leave them rough, which necessitated a further conditioning stage of processing.

A further problem involved the use of a mesh spun out of banana skin fibres as a hat material. Banana produces one of the strongest natural fibres, explained Lewin, and the mesh produced makes very attractive hats. The problem is that it's floppy, and has to be stiffened using a water-based PVA glue before it can be formed into hats. Wilson & Stafford found that the glue tended to clog the holes in the mesh, and drying using hot-air blowers was taking far too long.

The answer proved to be an air-knife combined with a medium-wave IR array, Lewin said. The former blasts water and excess glue off of the surface of the banana fibre web; the latter dries off the residue in around 2sec. The system can dry a kilometre of web in a day.

Rolls-royce techniques

The problem of drying caused by replacement of VOC solvents with water-based compounds is illustrated by Rolls-Royce aero engines, said Lewin. The company manufactures thousands of turbine blades by casting them in moulds. These, of course, have to be completely crack-free and flawless. The company used to check this using a coloured alcohol-based dye solution - this could be poured into the mould and cracks would then be visible. The alcohol evaporated rapidly. Water-based dyes, which now have to be used, are not so keen to evaporate, and the company needed to speed up its drying times.

There were many difficulties with this particular process, said Lewin. The moulds had a complex shape, contained only very small amounts of water, and were temperature-sensitive - too much heat, and they'd crack.