Author: Paolo Rossotto

Use of original high quality components for RF Machines

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Why should we use expensive high quality components manufactured by well known companies?

In the fast developing part of the world, technology is acquired from Europe and the USA to manufacture items that are very good copies of the original parts developed and manufactured by the same company over many years.

It seems all very simple and logical but two vitally important parts of this process remain excluded:

  1. The historic knowledge of the product development (for example using a specific material because when it was tried with a different one 20 years ago, something else happened)
  2. The understanding of the importance of quality control and consistent manufacturing. (Just because something looks the part it does not mean it will work correctly).

What are the consequences of not using high quality original parts?

  1. Specifications which don’t match, drifting of parameter or tolerance
  2. Lack of manufacturing uniformity.

In a typical RF dielectric equipment, the generation of RF oscillations through a mechanical assembly and a triode valve (tube) does not require all individual parts to have precise values; variation of their specification, parameters or tolerance is rarely seen to affect the operation of the RF equipment.

Lack of manufacturing uniformity can pose a problem when replacing components in existing equipment, if the new units do not physically match the original ones.

Reliability is the big issue.  If RF dielectric equipment is part of a continuous production line, machine downtime carries a heavy financial cost.  Failure of components in critical sections of the circuit could lead to damage to other parts within the machine, resulting in further equipment breakdown even after the damaged items are replaced.  If a component of inferior quality is used which then fails prematurely, not only is there the cost of the unit itself but also that of locating the damaged element in the equipment, removing it, checking if the equivalent is in the stores and is available, before fitting the replacement.  The loss of production is likely to be a much greater cost.

An “equivalent” component may appear to be cheaper but is unlikely to cost less in the long run.

Based on our experience, we recommend using only high quality components.

Red8RF only supply high quality components sourced from leading manufacturers.  For further information contact info@red8rf.com

Rectifier Assembly Failure in RF Dryers

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Red8RF were recently asked to investigate the recurrent failure of the rectifier assembly in an RF dryer supplied by a leading manufacturer being used in the textile industry.

From experience our Engineers know that rectifier failures are often caused by power surges at start up; a characteristic of the design of the three phase power circuit which is employed by many of the leading manufacturers of RF dryers.

Upon further investigation it was found that although the rectifier diodes within the assembly were of a good quality, their ratings had insufficient safety margins for the application in which they were being used.  As a result, the surge from the power circuit had been causing prematurely failure of the rectifier assembly.

Red8RF Engineers designed, assembled, fitted and tested a revised control assembly for the power circuit which eliminates the frequent diode failures.

RF Dryer Soft Start System

Power circuit designed and commissioned by Red8RF

Once installed, the immediate reduction of down time meant that the solution provided by Red8RF paid for itself in the first six weeks of operation.

Red8RF are now in a position to offer this soft start solution to those customers who are looking to improve the reliability and performance of their Fastran, Elgotec, Stalam or RFS RF dryers.

For further information about improving the performance of your RF dryer and for other engineered solutions please contact Red8RF

Q: What impact would a defective rectifier assembly have on your manufacturing process?

A: The first indication of a possible problem is when product leaving the machine has a higher moisture content than desired, when all machine parameters have seemingly remained unchanged.

If as a result you have had to reduce the speed of the conveyor belt transporting product through the machine in order to obtain the desired moisture profile, then there is a possibility that the rectifier assembly is defective.

Typically a diode without a snubber RC circuit (Resistor in series to a Capacitor fitted in parallel to the diode) will fail open circuit. As a full wave three phase rectifier assembly consists of six rectifier legs, failure of a single diode will result in failure of one of the legs.  The open rectifier will result in a lower High Voltage DC being supplied to the valve, which will in turn translate in to less power being transmitted to the medium to be dried.

The obvious consequence is reduced throughput, but a defective rectifier assembly could lead to instability in the RF circuit, arcing in the oscillator, flipping of frequency, increased voltage spikes and ripple in the HT DC supply; all of which can result in possible failure of machine components.

If part of a rectifier assembly becomes defective it should be repaired as soon as possible to avoid consequential reduction of throughput and a deterioration of the machine’s condition.  Regular servicing should be conducted to prevent component failure and to ensure that your machine remains a productive asset in you manufacturing process.

Over recent years our Engineers have seen an increase in the number rectifier assembly failures as a result of diode failure.  As a result they have designed a rectifier assembly which addresses the weaknesses of existing designs.  Their work has resulted in a rectifier assembly which is of higher quality and superior performance when compared to the assemblies available from the leading manufacturers.

RF Dryer Rectifier Assembly

Rectifier Assembly designed and built by Red8RF

Our Engineers have experience of servicing and supplying spare parts for machines from manufacturers including Strayfield, RF Biocidics, Fastran, Elgotec, Stalam and RFS.  If you would like to discuss any aspect of your RF Dryer please contact Red8RF.

Are you compliant with the new EU EMF Legislation?

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A new European Directive 2013/35/EU has been ratified by the UK Government and will become a UK law in July 2016.

The Directive sets out the minimum health and safety requirements regarding the exposure of workers to the risk arising from the exposure to electromagnetic fields (EMF).

All employers have a duty to assess the risks arising from the work they undertake and to put in place protective or preventive measures to reduce the risks they identify. These duties are a requirement of the Framework Directive. The EMF Directive was introduced to help employers to comply with their general duties under the Framework Directive for the specific case of EMF in the workplace. As employers will already be complying with the requirements of the Framework Directive, most will find that they already fully comply with the EMF Directive and have nothing more to do.

If any RF dielectric heating equipment is being used it is necessary to regularly verify that the intensity of the electric field generated is within the admissible level. The level of these electric fields can only be measured with suitable instruments, regularly calibrated.

Red8RF Engineers have the experience and suitable calibrated instruments to carry out a survey of EMF emissions and to advise customers of any actions that may be required to comply with the Directive.

The legal requirement is to verify workers’ exposure to electromagnetic fields, however the survey carried out by Red8RF also verifies if any RF leakage is present near the RF seals. Whilst this is NOT a requirement of this specific directive, uncontrolled RF leakage can cause interference to surrounding instrumentation, electrical control devices and raise potential interference to communication channels, with the risk of alerting the communication agency of an unauthorised RF signal being generated and propagated.

For further information contact Red8RF

Further information on the European Directive 2013/35/EU can be found here

What is RF?

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Radio Frequency

Short for radio frequency, RF consists of waves of electromagnetic energy moving through space.  All types of electromagnetic energy are grouped together in the electromagnetic spectrum.  RF waves can be characterized by a wavelength and a frequency.  The number of waves generated in one second is known as frequency and it is measured in cycles per second or Hertz (Hz).  The wavelength is the distance covered by one complete cycle of the electromagnetic wave.  The higher the frequency, the shorter is the wavelength.  RF is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kHz to 300 GHz

Often the term RF is used to indicate the presence of electromagnetic field or RF energy.  RF energy is mainly used in telecommunications, emanated by transmitting antennas or to generate heat in many industrial applications.

These frequencies make up part of the electromagnetic radiation spectrum:

  • Ultra-low frequency (ULF) — 0-3 Hz
  • Extremely low frequency (ELF) — 3 Hz – 3 kHz
  • Very low frequency (VLF) — 3kHz – 30 kHz
  • Low frequency (LF) — 30 kHz – 300 kHz
  • Medium frequency (MF) — 300 kHz – 3 MHz
  • High frequency (HF) — 3MHz – 30 MHz
  • Very high frequency (VHF) — 30 MHz – 300 MHz
  • Ultra-high frequency (UHF)– 300MHz – 3 GHz
  • Super high frequency (SHF) — 3GHz – 30 GHz
  • Extremely high frequency (EHF) — 30GHz – 300 GHz

What is RF Dielectric Heating?

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Dielectric Heating

In conventional systems the transfer of heat to a body, whether by convection, radiation or conduction, singly or in combination, can only take place through its surface. Consequently the rate at which the internal body of the object heats depends primarily on its thermal conductivity. Heat transfer often has to be restricted in order to avoid damage due to overheating of the surface. This limit to the temperature gradient between the outside and the inside of a body leads to extended process times, poor equipment utilisation and in some cases batch operations instead of continuous flow.

An alternative approach which can eliminate some or all of the problems is dielectric heating, the generic name covering both radio frequency and microwave sectors of the electromagnetic spectrum. Both are widely used by industry because of their so called ‘volumetric’ heat transfer characteristics which avoid the need to heat the surface first.

The types of materials and products which can be processed using dielectric heating are basically non-metals which inevitably have poor thermal conductivity. These include food, textiles, paper, ceramics, plastics, pharmaceuticals, chemicals and timber.

The process operations with which it is associated include:

  • Curing of wood working adhesives
  • Drying of textiles and other bulk materials
  • Tempering and defrosting of frozen products
  • Curing of polymers & composites
  • Pre-vulcanisation heating of rubbers
  • Melting of fats and waxes
  • Granulation/drying of pharmaceuticals
  • Preheating in fibre board manufacture
  • Moisture profile correction in paper making
  • Post baking of biscuits
  • Baking in a steam atmosphere

The principal advantages of dielectric process heating are based on the fact that heat is generated within the substance itself. Some materials notably water, are much more susceptible to this form of heating than are the substrates in which they are held and consequently, preferential heating takes place. This can lead to significant process advantage; for example, a more uniform rate of drying which in turn results in a better quality finished product usually associated with faster production speeds. In non-aqueous systems, such as polymer curing, the internally generated heat reduces process times and improves plant utilisation.

Whether dielectric heating is suitable for a given product will depend not only on its loss factor but also to great extent on the product dimensions and shape as well as the way in which the electric field can be applied to it.

The Science of Dielectric Heating

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High Frequency Electric Field

Dielectric heating arises when a high frequency electric field is established within a material which is a poor electrical conductor. The ease with which electrical energy can be transferred to a body as well as the distribution of that energy, depends on the dielectric properties of the various constituent materials which make up the substance. The main dielectric property of interest is the Loss Factor, which principally determines the amount of energy absorbed by the material for a given electric field.

It is important to realise that the dielectric loss factor is not constant but varies with a number of parameters such as frequency, moisture content and temperature. It is possible to find information in the literature giving values of loss factors for certain common materials. However, these are normally measured at ambient temperature and at equilibrium moisture content and should be regarded only as a guide, for a material at a given temperature and moisture content, there is usually a frequency (or resonance) which gives a maximum value of loss factor.

For example, water has a maximum dipolar loss factor at about 20GHz. However the loss factor at other frequencies is usually sufficient for an alternative to be satisfactory.

The power transferred to a dielectric is given by:

P = 2 π f εo εr tanδ E²

Where:

    • P is the power of density in Watts/m³
    • f is the frequency in Hertz
    • εo  is the permittivity of free space = 8.85 x 10-12 Farad/metre
    • εr is the relative permittivity of the material to be heated
    • tanδ is the loss tangent
    • εr x tanδ = is the Loss Factor of the material being heated
    • E is the RF field strength or voltage gradient within the material, in Volts/metre

The frequencies used for dielectric heating are not chosen because of any relationship with the resonance of water or other molecules but are specified by international regulation in order to minimise the risk of interference with telecommunications.  The frequencies available include 13.56, 27.12 and 40.68 MHz in the radio frequency band and are usually referred to as “ISM” bands used exclusively for Industrial, Scientific and Medical applications.

Radio Frequency Dielectric Heating & Drying Equipment

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Radio Frequency

Radio frequency process heating and drying equipment can be conveniently described as a power supply, or generator, delivering power to an applicator or work circuit, which applies an electric field to the material to be heated.

Traditional standard RF generators consist of a single valve (usually a triode) configured as a self excited ‘class C’ oscillator typically a modified Colpitts, tuned anode/tuned grid circuit.

For valves to operate efficiently, a high DC voltage must be applied to the anode. This is produced from the mains three phase supply by a step-up transformer and full wave rectification. Power is drawn by the work circuit from the oscillator circuit through mutual inductance coupling, similar to the windings of a transformer, where the tuned oscillator circuit is the primary winding and the work circuit is the secondary.

RF Generators

Typical RF generators have a conversion efficiency of about 70% between mains supply and RF power output and single generators producing over 100kW are available. How much of this is transferred to the product depends on a number of things including the design of the applicator, the degree of optimisation of the coupling and by the product presentation in the applicator. Consequently it is not necessarily easy to use a radio frequency heater as a general purpose unit.

Triode valves are either air or water cooled and are still widely available.  Brand new valves will typically last in excess of 15000 hours in a well maintained RF generator.  Solid state generators are now also available for special applications in the semiconductor industry.

 

Applicators

fig1Radio frequency applicators are essentially capacitors, which contain the product requiring heating as the whole or a part of its dielectric. The simplest and most widely used is the through field or parallel plate electrode as shown in Figure 1.

 

 

Such a system would be used in a press, for example in plastics welding or wood gluing, in  which case there would be no air gap. However, when used for drying like in a textile dryer, an air space is required above the dielectric to allow for the movement of the product through the machine and for removal of the water vapour. This then means an increase in voltage between the plates in order to maintain an adequate field strength in the product. It is therefore important to consider the relative dimensions of the dielectric and air space capacitors to give the desired heating effect without an electrical discharge taking place. For very thin materials such as paper it may be necessary to use an alternative electrode configuration.

rf-fig2

 

Several other configurations are available for when the product dimensions or other factors make the use of the through-field system undesirable. These include the fringe field, also known as the stray field array shown in Figure 2 which is used in the processing of thin sheets and webs typically in the paper industry.

Another variant is the staggered through-field, also shown in Figure 2 in which the rods are arranged above and below the product. The best known application of this arrangement is in the post baking of biscuits.

 

These latter two arrays require that the product is transported through the applicator on a conveyor belt in order to achieve uniform heating.

The Benefits of Radio Frequency (RF) Dielectric Heating

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Selective Heating

Radio frequency dielectric heating will heat water in preference to most substrates with which it is associated. This means not only rapid heating but, in the case of drying, equalisation of moisture content in a product as the wetter areas may absorb more heat than the drier ones.

Volumetric Heating

Since energy is transferred by the interaction between the high frequency electric field and the responsive components in the substance, heat is generated throughout the volume of the body.

Efficient Use of Energy

In a RF system, it is possible to design the system so that power is drawn in the proportion to the amount of responsive material in the applicator, hence there is no energy used when the equipment is empty. When carrying out drying, baking or similar operations the energy output will rise and fall proportionally with the quantity of water present in the unit, resulting in high efficiencies.

Improved Product Quality

Because it can avoid case hardening and other surface damage because of the volumetric nature of the heating, the quality of a whole range of products, from textiles, to glass fibre, ceramics, polymers wood, paper and food, can be greatly enhanced.