Access the full text.
Sign up today, get an introductory month for just $19.
References for this paper are not available at this time. We will be adding them shortly, thank you for your patience.
June, 1945 AIRCRAF T ENGINEERING Workshop and Production Sertion By Carl J. Madsen† IT H the popularity of high frequency heat load extremely difficult if not impossible. The Loss Factor ing growing by leaps and bounds, it is current s involved may also reach magnitudes Thi s is a term commonly used to express the desirable to stop for a moment now and then difficult to handle. degree to which materials will absorb energy by to take stock of the state of th e art. This can best Th e length of th e load may be such as t o produce dielectric heatin g technique. I t is easily measured by be done by breaking down the broad view and re standin g waves. This will b e discussed further under mean s of proper equipment. Referring to equation viewing only a narrow phase a t a time. This paper frequency limitations. (1), it is obvious tha t as the loss factor approaches is concerned only with dielectric heatin g (heating of Sometimes it is possible t o scan th e material, thus zero, it becomes increasingly difficult to transfer materials by a varying electric field), and reviews reducing th e electrode area to a value which permits power into the material. A practical lower limit the major limitations that prevent universal convenient adjustments and frequencies sufficiently in th e loss factor, of materials which can be heated application. high t o produce desired rate of heating. effectively, appears to be between 0·005 and 0·01. In considering thes e limitations, it mus t be under Pur e polystyrene, quartz and certain other loss Occasional applications are encountered where a stood that almost all of them are interlocked, to particula r area can be heated if the material is material s have loss factors considerably below this some degree, with each other. Hence, in discussing stacked, increasing the thickness dimension and value and are exceedingly difficult to hea t b y dielec each individually it is well t o remember these inter thereb y reducing th e capacity of the load. tri c methods. relations when considering an actual application. Th e upper limit probably blends into the range where th e material becomes sufficiently conductive Frequency a s to respond to inductive fields, although some Cost. I n general (since th e amoun t of energy introduced skin effect will probably be noted before this limit High-frequency energy is not a cheap source of into the material is proportional to frequency) the is reached. heat. In terms of dollars per kWh. it will cost desire is to use as high a frequency as possible. A approximately $0·025 to $0·04 including direct compromise is usually necessary however, due to Electrodes operating cost, maintenance and amortization. one or more limitations. A whole treatise could be written on electrode I t becomes economical by virtue of one or more Th e effect of large areas and low capacitive re design. One point I wish to cover in this discussion of these factors: actanc e was discussed previously. is th e fact that heat is not generated in the elec 1. I t produces a result not possible by other Th e length of th e material ma y be such as to pro trode s to any appreciable degree by the dielectric methods of heating. duce uneven heating due to standing wave effect. heatin g process. The electrodes may be heated by 2. I t increases the speed of a process and saves Th e higher the frequency used, the greater the hea t transfer from the dielectric material being pro labour and/or overall equipment cost. probabilit y of unequal voltage distribution over a cessed. Often this is undesirable du e t o th e tempera . 3 . I t produces or confines th e heating in th e part given length or area. If the longest dimensions of the tur e reduction of the adjacent material and may or point where needed, thus reducing overall power electrodes is a small fraction of a wave length, or if become a limiting factor in some processes. Various consumption. th e material is scanned, this effect is minimized. simple methods are in common use to minimize this 4. It is clean and compact. Ordinarily if the longest distance from the point of effect. 5. It reduces rejects, or improves the production connexion to th e edge or corner of th e high voltage quality of a product. electrode is less than 1/16 of a wave length, very Summary 6. I t is flexible in its application to a variety of little trouble will be experienced. Heating will be Dielectric heatin g is a very useful tool in industry, operations, processes or products. sufficiently uniform to produce a variation of less one which is finding new applications every day. These factors are often of sufficient importance to tha n 20 per cent over the area involved. In some However, as with any tool, ther e are certain limita make an application economically attractive. processes, this may be intolerable, restricting the tions which prevent its promiscuous application to maximu m frequency (or maximum dimensions of all types of problems. B y realizing these limitations, th e work) t o a still lower value. Applicability an d the series of compromises often necessary, a Th e maximum frequency which can be used Dielectric heating is applicable only to materials careful analysis will permit the average electrical withou t exceeding the above variation in heating normally considered poor conductors of electricity engineer to apply it intelligently to work which it can be determined from the formula: or insulators. When the electrical resistivity of a can do well. material drops below 1,000 ohm-cm., it is usually possible or necessary to use some technique other than dielectric heating . The stat e and characteristics of the material have considerable influence on the where f = frequency in megacycles division point, changing it as much as 10 t o 1. l =maximum distance from point of con British Standard Specifications A convenient formula for calculating th e heating nexion t o edge or corner of high voltage B.S. 971 .—Wrought Steels. in dielectric materials is given by: electrode in feet. Thi s new issue of B.S. 971 has been prepared in e' = dielectric constant of material (specific orde r to take account of the alterations that were inductiv e capacity). mad e in the revision of B.S. 970 and to include where W=rat e of heating in watts Anothe r limitation occasionally encountered is details of th e new steels tha t were added therein. A =are a of electrode in square inches tha t of generator design for high power at high Th e publication gives an explanation of all the d =thickness of material in inches frequencies. From a practical viewpoint at the steels included in B.S. 970; indicating the uses of f =frequency in cycles per second momen t it appears undesirable to use frequencies each steel, details of th e appropriate hea t treatment E =voltag e RMS. abov e 20 megacycles when powers above about an d information on the effect of the ruling section e" =los s factor of material=e ' tanδ 20 kW. are involved. on the properties. Any special features of th e steels ar e also pointed out. Th e following paragraphs discuss briefly each of these factors. I t has no t been possible in th e new issue t o include Voltage th e cross references to the various industrial speci Th e power input to the material is proportional fications which were given in th e first issue a s this Dimensions t o the voltage squared. Hence, if the voltage is could only be achieved by a widespread inquiry Th e dimensions of th e piece of dielectric material increased sufficiently, considerable power can be from industry of tb,c specifications that have been are important in tha t they may influence an appli supplied to the work at relatively low frequencies. introduce d during the four years since B.S. 970 cation as to frequency, rate of produuction or elec Two factors limit th e voltage which may be applied. wa s first prepared. I t is, of course, hoped tha t to a trode design. Th e total voltage between electrodes should be large extent the adoption of B.S. 970 has rendered I n general, dielectric heating becomes attractive kep t under 15,000 volts. I t is no t impossible t o use man y of th e individual specifications out of date. when the thickness of the material is sufficient to voltages above this value, but the additional pre Price: 7s. 6d. pos t free. make it difficult to hea t th e material throughout by caution s necessary to avoid corona and arc-overs conventional means. In terms of inches, this will are often more expensive than some other com B.S. 350, 1944—British Standard Conversion promise of th e engineering factors. vary with th e thermal conductivity of th e material. Factors and Tables. Th e thickness of th e material must be reasonably Th e voltage gradient, that is, the voltage per Par t One contains a list of basic British and uniform if uniform heating is desired. If the work inch, permissible across the material may be a Metric units. Par t Two, which is preceded b y a page limiting factor. The radio frequency voltage which is of such shape (for example wedge shaped), that of definitions affecting conversions, comprises a will punctur e a given dielectric material is generally two pieces of material can be placed in comple series of conversion tables—the linear measures muc h lower than the 60-cycle voltage which a mentar y positions and the resultant thickness be being based on a standard of 1 inch=25.4 milli materia l will stand . I t appears desirable to keep the uniform, the material will heat evenly. Special metres—t o six significant figures. Par t Three covers voltage gradient below 2,000 volts per inch in technique in electrode design and arrangement may conversion factors and multiples; while Parts porous materials and less tha n 5,000 volts per inch sometimes be used when dealing with irregular Fou r and Five contain selected conversion tables; thickness materials. The relation of thickness and on less porous. in the former from 1—100 and in the latter from voltage will be discussed under voltage. Anothe r typo of voltage limitation may be 1—1,000 units . Th e area of material to be heated will limit the reached in some applications where it is impossible Including , as it does, authoritative factors for t o have the electrodes in contact with th e material. frequency which ma y be used, in two ways. th e carrying out of every conceivable conversion This gives rise to a condition in which the space Th e capacitive reactance of a large area may operatio n with a large number of worked out betwee n th e electrodes is occupied by two materials become sufficiently low as t o make tuning of the examples , the booklet is invaluable and quite of widely different dielectric constants . Th e potential essential in th e drawing office. distributio n in this case may be such as to cause a †Electronics Engineer, Westinghouse Electric and Manufacturing Price , 3s. 6d . pos t free. breakdow n of th e surrounding medium (usually air). Company, East Pittsburgh, Pennsylvania.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jun 1, 1945
Access the full text.
Sign up today, get an introductory month for just $19.