RADIO FREQUENCY [RF] DRYING

Radio Frequency Drying has evolved as an important step forward in the search for the perfect drying method. Radio Frequency [RF] Drying offers advantages over other conventional methods. It is globally accepted, adopted and used world-wide.

The RF generator creates an alternating electric field between two electrodes. The material to be dried is conveyed between the electrodes, where the alternating energy causes polar molecules in the water to continuously re-orient them selves to face opposite poles. The friction of this movement causes the water in the material to rapidly heat throughout the material's entire mass, thus drying the material.

RF drying generates thermal energy from within the product, there is no external source. This results in selective and endogenous drying, wherein the evaporation rate is directly proportional to the amount of electromagnetic energy supplied.

Increasingly, industries that require quick and uniform drying have turned to RF drying as the most effective method for drying and value-addition to their products. The textile industry, especially, relies on RF drying for its product enhancement; the steaming and bulking effect improves physical properties like elasticity, softness, hand and colour effect. Similarly the food/paper/wood etc. industries rely unquestioned on RF Dryers to obtain quality standards of global acceptance.

Some of the advantages of RF drying technology are:-

Energy Savings.

The efficiency of convection dryer drops significantly as the lower moisture level is reached and the dried product surface becomes a greater thermal insulator. Substantial saving in energy as at this point, but with more moisture to be removed, RF drying provides an energy-efficient means of achieving the desired moisture objectives, simply because the moisture alone is treated and energy is not wasted on having to heat the product, the environment as well as the water.

RF DRYING TEXTILES by Arun Monga Techical Director

This paper describes application of Radio Frequency techniques for removal of moisture from various products, particularly drying of Textile Yarn.

Radio Frequency drying principle is similar to that of the modern microwave oven used in household, where products are subjected to direct heating process in electromagnetic energy fields.

BASIC PRINCIPLE :

The temperature of any material may be increased either by direct or indirect heating. The traditional process whereby heat is generated remotely and then transferred to the material by conduction, convection and radiation, are all examples of Indirect Heating.

Direct methods of heating are those where the heat is generated within the material itself, with radio frequency and microwave methods being typical examples. This method of heating is also referred to as dielectric heating.

Most non-metallic substances (or dielectric materials) undergo a uniform rise in temperature when subjected to a high frequency electromagnetic field. Each time the direction of the applied RF field reverses, so the polarisation of the individual molecules within the substance changes. The work done by the polar molecules in realignment activity in accordance with the direction of electrostatic field generated by the high frequency electromagnetic energy causes friction, which manifests itself as heat, which in turn leads to rise in temperature. As may be expected, the higher the frequency of the electromagnetic field the faster the molecules move, thus creating more friction and consequently more heat is produced. Typical examples of Polar dielectric substances are Water, PVC, Nylon, Bakelite, Organic Oils, Rubber, Glass etc.

The frequency of the RF fields generated by devices for dielectric heating, are centered on 13.56 MHz and 27.12 MHz. These frequencies are effectively reserved for industrial, scientific and medical use in order to avoid possible interference with other legitimate users of the frequency spectrum. (All radio transmission is governed by the international codes of EMI and specific frequencies area allocated to users depending on their classification and requirements).

R.F. HEATING

Most substances differ in their reaction to direct heating, with water for example being highly receptive to the application of direct heating methods. In practice this means that any product containing moisture (be it food, paper, textiles etc.), can be rapidly dehydrated using Radio Frequency energy. Individual applications will require different techniques, of design and commissioning R.F.equipment in order to gain optimum performance.

During drying by RF heating process, the product passes through an electrode system and in doing so becomes the dielectric itself, which is then subjected to a powerful RF voltage at a frequency of 27.12 MHz. The molecules of a dipolar substance (such as water) readily realign themselves with this high frequency electromagnetic field and become heated very rapidly. Non polar substances on the other hand are not affected by this process.

The use of RF energy as a means of drying is highly efficient as only the water molecules become excited during the process, and energy is not dissipated throughout the mass of the product thereby avoiding unnecessary heating (or power consumption). In addition the heating is uniform, causing the water molecules to become excited simultaneously.

The use of RF energy as a drying medium also means that the inner working surfaces of the dryer (i.e. those coming into contact with the product) do not themselves require heating as a condition of the overall drying process. Furthermore the heating is self regulating. The temperature of the product increases and decreases in direct proportion to the amount of water contained in the product.

The working environment is improved. Conventional heaters give off "hot atmospheres" i.e. the external surfaces of the device itself become heated during the process. The external surfaces of RF generators however remain cool, thus providing a more comfortable working environment for the operators. The operation and maintenance of RF heaters is also a relatively simple task. The number of controls are minimised thereby enabling rapid training of operators.

To sum it all the interest of RF drying for textile materials is growing due to the following :

1. Substantial saving in energy as the moisture alone is treated and energy is not wasted on having to heat the product, the environment as well as the water.
2. Uniform drying throughout the mass of the product yielding good quality of homogeneously dry material.
3. Accurate drying within 1% moisture profile throughout the mass of the product.
4. No dye migration due to uniform heating.
5. Clean working environments and easy to operate equipment.
6. Easily lends itself to automation. Regulation of energy inputs based on actual work load conditions unlike the other steam drying equipment whose internal volume has to be heated regardless of the quantity of products.

R.F. CALCULATIONS

By international agreement the following radio frequencies are reserved for industrial use: 13.56 MHz ±0.05%; 27.12 MHz ±0.6%; 40.68 MHz ±0.05%. And for microwave heating the following frequencies are reserved: 2450 MHz; 896-915 MHz (industrial heating).

The heat generated within a given product is expressed by the formula :

Energy Input = Voltage X Frequency X Loss Factor(Polar Molecules).

This formula also explains why certain manufacturers use the 27.12 MHz frequency instead of 13.56 MHz, in spite of the design parameters being more stringent. There is a wider frequency tolerance and also a theoretical reduction of 41% in electrode voltage. This can be of importance as it avoids incidence of electrical breakdown within materials being processed and it also permits the drying of textile packages on steel springs. Where possible, it has been found desirable that heat input density and electrode voltage should be low for all materials.

As a "rule of thumb", 1kWh energy to remove 1 kg water is found to be a useful parameter. The precise formula is :

Temperature rise energy (kWh) = (kg X temp rise X specific heat)863

However, when calculating requirements, it is best to work from "worst case" data, i.e. - wintertime temperatures and maximum likely moisture content of input materials. Allowances of 10 - 20% should also be made for unavoidable radiated and convected heat losses which depend on the materials' ratio of surface area to volume.

APPLICATIONS

Packages and Muffs. Following the removal of free water from packages by hydroextraction or pre-drying with, for example, a rapid dryer, these are simply loaded onto a conveyor for passage through the RF unit. As much as one hour's production can be loaded.

Upon entering the RF zone, heat-energy is generated throughout each package in proportion to its moisture content with the result that very even heating and drying takes place. There is no mechanical damage to yarns and usually the dried materials have a much better feel and appearance. Whites are usually much whiter than from indirect drying.

Tops. After hydroextracting, tops are an ideal subject for RF drying. The small surface area to volume makes conventional drying techniques very inefficient by comparison. Moisture situated or held in the centers is instantly heated on arrival in the heating zone and is levelled until the required regain is reached. Meaning that the zone containing higher moisture gives out more vapour till such time that the moisture percentage in the entire mass becomes same.

Hanks. Traditionally, hanks have specific problems during the drying cycle. Suspension by mechanical means can result in a permanent wet spot with resultant discoloration. Metal band conveyorised dryers tend to produce wide moisture variations across the bandwidth. Neither system can give automatic moisture control, which is always available with RF ovens.

RF dryers dry hanks in a relaxed state usually on a polyester perforated band. This allows air to be directed through the band and across the surfaces of the hanks to assist steam removal.

Loose Stock. With suitable hopper and distribution equipment, loose fibres may be conveniently transferred from continuous or batch hydroextraction to the RF conveyor. Open weave polyester bands are not prone to colour contamination, and fibre retention is overcome by brush and air jet under the return pass of the band. Laborious cleaning down operations after each change of colour, associated with other types of dryer, are eliminated.

Hosiery and Made-up garments. An interesting application for radio frequency techniques is the drying of hosiery in bags and garments after hydroextracting. Particularly in the case of hosiery, which is normally made in synthetics, the amount of energy required is low and all parts of the articles are reduced to the same level of moisture, including the joints in tights. It is difficult to see how the conventional dryers can do this work efficiently.

Thread and warp drying. RF dryers are now being used extensively to dry sized warp threads. Heat is directly generated within the water based size so that almost instant drying can take place without in any way effecting the quality of the product. Yarn drying is also now being done by RF after the felting process for carpet and knitting yarn with much success.

Bales. In the case of wool and the ever increasing cost of transportation, it is usual to transport raw wool in bales using high compression. Sometimes this material is also stored at low temperatures. Opening up the compressed fibre has been a traditional cause of difficulties. RF heating of the bale before opening resolves this.

Now that very high compression bales are being used, RF can be used to eliminate another problem related to moisture content. With these bales it is of paramount importance that there are no wet spots prior to baling, since these can cause severe damage due to bacteriological effects. Drying with RF before baling ensures that there are no wet spots, since these are always preferentially heated with this technique.

Electronic dye fixation. Several RF units have been installed for dye fixation. The process has the advantage of being continuous and easily controllable. Basic research is now probably necessary on the related physical chemistry if this application is to become a widespread and major dye house process. There is no doubt that, from an electronics viewpoint, the process is simple and reliable.

SUITABILITY OF MATERIALS

The easiest of all materials to dry is polyester, whilst the most difficult appear to be the acrylics. The reasons are fairly straightforward and an attempt will now be made to explain some of the possible hazards. Polyester is totally non-polar and therefore totally ceases to heat when dry, and that is the reason why it is used for conveyor bands, from the RF point of view.

Some acrylics are highly polar and will continue to heat once dry and will eventually catch fire. It is, therefore, important to know how much water has to be evaporated and ensure that the equipment operator has a clear understanding of settings.

In fact, the situation is not quite as bad as it sounds since, when drying packages down to 2%, they actually leave the RF zone with moisture of around 4% with the remaining moisture being lost during cooling.

A further point where it is necessary to be careful is where certain types of whiteners are used on acrylics, since excess energy density can cause slight yellowing. The answer here is to ensure that power densities are always of a low order and this is one of the reasons why wide bands are preferable with high powered equipment.

Other materials rarely cause any problems, although some care has to be taken with polyamides since they are also polar.

ADVANTAGES OF RADIO FREQUENCY DRYING

Flexibility. The same machine can dry tops, packages, hanks, loose stock and bagged products. In operation the electrode assembly can be lowered or elevated by push-button control; the conveyor belt speed can be similarly varied and so too can the input voltage. These three variables enable so many different product configurations to be handled. The latest machines are now built with pre-set controls, allowing the operator to press one button and alter all settings simultaneously to cater for different loads. Change-over times accordingly are now matters of seconds only.

Yarn Improvement. RF does offer substantial (and quantifiable) advantages for textile manufacturers. Accuracy of moisture control to 1% is reliable and continuous. By relying on "direct" heating, it has been repeatedly found that dye migration is reduced thereby producing whites are "whiter" - particularly on cottons. Fibres have better bulk and handle. "Over-drying" and "wet spots" are eliminated.

Productivity. RF drying equipment offers great possibilities for increasing productivity. The units are immediately available after switching-on with no "warm-up" time being needed. Different packages with any number of different colours can be dried simultaneously. Large or small production batches can be similarly accepted. It is possible also accurately to monitor costs, in a way that is frequently difficult with traditional systems, because electrical energy is the only input and it is consumed fully by the product to be dried.

RF dryers can be moved easily to different locations and also lend themselves to automated loading and unloading. Maintenance can be performed adequately by normal plant electrical staff.

Operators generally learn quickly to like RF dryers. There is no environmental heat hazard, as no radiated heat is given off. The units are quiet in operation. The controls are easy to comprehend.