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| A line of Strayfield RF textile Dryers |
OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Feeling the heat.
A new source of Q R Mexico ... and an Opportunity.
Electronics company C-Tech are proposing a new way to heat your TV dinner .... by RF and not by microwaves, as is now common. This is not a new idea, but it has now come of its time thanks to cheap(ish) semiconductor PA devices. RF heating in its various forms has been around for yonks. In industry, RF heating has a multitude of uses including drying, plastic-forming, diathermy etc.
In the 80's and 90's, GM1SXX worked for 'COATS The Threadmakers' who used RF heating in many of their threadmills to dry 'cheeses' of cotton and synthetic threads. We used Strayfield FASTRAN dryers.
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| A line of Strayfield RF textile Dryers |
The British company Strayfield are a major player in the RF drying field. Next time you eat a biscuit, you may just be eating something that has been post-bake dried in a Strayfield RF dryer. They make dryers for the timber, plastics, food, textile and other industries. In short, this sort of RF heating technology is old news...but for Joe Public, it's an exciting new way to heat food.
These Strayfield FASTRAN 'RF Driers' run around 30KW on 27.125Mhz and were very efficient at drying the soaking wet dyed material from our dyeing machines. Our dryers were of a carousel design. The operators loaded the cheeses (they look just like big round cheeses) onto vacuum 'hold-downs' that helped extract residual water as the RF did it's work at the heating chamber at the rear of the machine. We had the odd accident with these dryers... stuff like coolant lines becoming detached and the odd triode blowing up etc, but in general they were efficient devices. The downside is that they used 'free-running' oscillators based on a BIG ceramic 'lighthouse' type watercooled RF triode, and I mean BIG. Considering the last of any sort of obvious frequency control, they were quite stable when warmed up. The coolant used for the triode was demineralised water and the HT used was 10KV delivered from a three phase 415V AC supply. The ceramic triode used was directly heated, and I remember the heater current being 240 amps. The RF chokes in the heater lines were airspaced types formed from 15MM diameter copper, not really ideal for home use.
C-Tech of course, won't do things this way. Joe Public could never afford that. C-Tech have a prototype 2.5KW solid state prototype oven operating, hopefully using a crystal-controlled exciter! RF heating has several advantages over microwaves but the stumbling block, until recently, has been the cost of a multi-kilowatt solid-state PA. The proposed system will operate on 27.12Mhz or 13.58Mhz.
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| C-Tech's 2.4KW Fan Oven. |
The advantages of RF heating are more even penetration into food so not hot and cold spots like with microwaves and in addition, RF does a far better job of defrosting food. The C-Tech work while interesting raises the spectre of a new source of RF pollution should they be less than careful with their frequency control.
On the other hand, RF leakage requires far bigger openings than you'd ever find on a microwave type oven and is more likely to happen at the mains input side of things should the mains RF filter fail.
On a more positive note, I'm thinking that the PA devices used might just be able to do service in amateur applications, such as big PA's:-)
You can read more on the C-Tech Story at..
73 Al.
GM1SXX
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