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Re: inductive heaters
Dear All Hooton,
Reading the list about induction heaters I will try tonight to give some answers
to some FAQ.
As one of the major neonshops in Holland is doing to oven pumping technique I
know (and see weekly) that they can convert a 200 mA electrode in about 10 till
12 seconds. The mainly used 80 mA electrode is converted properly (over cherry
red) in about 4 seconds.
One can heat up with a hand-held heat applicator a complete oven (I mean the
electrode's) of over 40 tubues in about halve an hour. The installed electrical
power of the HF generator is about 1 kVA and the device should be on castors nto
permit moving around the oven. It is air-cooled nowadays (iso the old
water-cooled types) and the hand held workhead have 2 to 2,5 metres of cable and
a relay controle button on the handheld. We use here in Holland the lead glass
fluride or a fluoride peroxide formula activated electrode. carbonates can be
used but since the decomposition to oxides is rather slow and needs a shell
temperature of over 900 C or more, they are rarely chosen for the oven pumping
method as the glass is heated up in temperature more then enough. When sealing
on the tubes to the by-pass pipe outside (round about) the oven the mercury
reservoir should be just outside the oven and any constrictions made in the
pumping stem should be uniform and not less than about 2 mm. This is not for the
pumping speed but for a uniform back filling pressure in the tubes. As there is
a too grat difference between pumping tems the back filling pressure will change
from tube to tube and well so that the tube with the most narrow stem will have
the less amount of rare gass in it. This is especially important for the large
ovens, als is proper aenneling of bends and joins, as a sudden crack and or
inrush (of air at atmospherical pressure) will spoil several tubes of the batch
and causes unwanted delays.
Switch on the heat and let the temperature rise to 450 C in about one hour. The
vacuummmust be 5.10 E -1 mabar so as you all say 5 micron. Switch off the heat
and lift the lid of the oven. Activate with the HF the electrodes, but make sure
that one is heating the not tubulated electrode first. Duiring this procedure
the HV pump is continuously on at full speed in it's operating area. We only use
dry pumps for this purpose like the turbo-molecular, the molecular-drag or the
drag-hybride pump. Make sure that you are not schwitching on the HF coil before
the work coil is completely over the shall and have past the pinch of the
electrode, as the pinch will break when you are heating up the lead through
dumet wires. When all electrodes are converted from inactive to an active one
and outgassed you can heat up the tubulated one again for a short period of time
as that one can absorp free coming gases out of the tubes binder, wall,
fluorescent layer and the other electrode at the other side of the tube. The
after restoring a good vacuum, like 1 micron or better, close the oven lid again
and heat, bake and anneal for halve an hour with a temperature of about 450 C.
The heaters then should be switch of and we backfill the tubes when the have a
temperature of 40 till 50 C at the desired pressure. When there are different
tube diameters in the oven, bacckfill first the widest ones, melt them off from
the round about the oven glass manifold and backfill then the smaaler diameters
by incresing the rare gas pressure.
As one of my students built his own HF Induction Heater. He promised me to
publish the blue prints together with an explanation how to build a HF device by
your own. The HF model we use here in Holland have a output power of 1 kW and a
frequency range from 50 to 200 kHz and is air cooled. The air cooled device is
much more handy than the water cooled one's as the water cooled one's like to a
box of Pandora with all these hose's through the unit and your hand-held.
As I have here comprehensive information of HF units I will produce hereunder
some information for these guys at the other side of the Atlantic who are
interested in this kind of information and especially for them who asked for
more information.
In brief induction heating is a non-contact method of heating electrically
conducting materials. It involves a source of alternating current (the induction
heater), an induction coil (called the work coil) and the part to be heated
(called the work piece). ((for sure I don't know if this translation is correct
for your language, but I hope that you all know what I mean,. If not, reply to
me))
When an electrical current is made to alternate in a work coil, this produces an
alternating magnetic field, a current will be developed in that part. The work
coil can de compared with the primary coil of a transformer and the work pice as
a short circuit secondary coil (winding?) The power that the current develops
depends on several factors, like:
The kilowatt rating of the induction heater.
The electrical resistivity of the work piece.
The configuration of the work coil and its relationship to the work piece.
For the last remark I must explain that you need a work coil for every different
type of electrode, so when you use the 25, 50, 45, 80, 90 and 100 mA
electrodeshells you need 6 working coils that can be fitted onto your handheld.
The intensity of the heating effect is local to the work coil and is greatest at
the surface. The effect reduces as the distance from the surface increases. In
order to heat small shells and to be able to use compact work coils, it is
necessary to develop an alternating current of high frequency.
As the power developed by the induction heater is coupled to the work piece by a
magnetic field, the ube may contain a controlled atmosphere or a vacuum. With
the induction heating method it is possible to achieve temperature far in excess
of 2000 C.
I asked my local supplier if they have an agent in the USA or a collegue
manufacurere. If so, I will put the name and adress on the list next week.
Hope that you enjoyd my explanation is meanwhile signing with his
best regards
dirk a. boonstra