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Re: NEON- Ovens, another look (long)
At 06:13 AM 11/5/96 -0500, EclectiKat@aol.com wrote:
>Have just received an order that will require that I build an oven, which I
>intend to do now. Lots of tubes, all same length. Now my earlier questions
>are suddenly very important. Am tired of flaming long tubes although will
>still do it for shorter custom ones, looking forward to a more "hands off"
>approach.
>Still have a few nagging questions, tho, anyone want to help?
Well, for what it's worth, here are some notes on the direction
I'm taking. As I mentioned in my last post, I won't have the pictures
or a full update for a while 8^(= Warning: this is long.
- There are a few good books out on kiln building that have good chapters
(or more) on electric kilns. This is essentially what we're building.
I don't have any references handy right now, but check any big-city
library... that's where I looked at these books (I didn't buy any of
them, because I'm only planning to build one oven!). With some time,
I can track down a reference to the book I relied on most heavily -- if
you're interested, ask me and I'll go after it.
- Outside dimensions are 60" x 30" x 24". Inside dimensions are 6" less
in each direction: 54" x 24" x 18". From the sound of it this may be
around half of the heated volume that you need? My frame is 1" angle
iron, with expanded steel walls, floor and top (expanded steel is the
heavy steel 'mesh' sometimes welded to the surface of steel stairs
so people don't slip on the otherwise smooth surface).
- I have chosen to go with refractery blanket material instead of
refractery bricks. The cost is about the same, and the performance
is almost as good as bricks (certainly good enough for the temperatures
we're talking about). Construction is easier with the blanket. The
major downside is that once the blanket has been taken up to temperature
a couple times it gets brittle on the surface, so care must be taken not
to abrade it too roughly. The guy in town that has been building custom
ovens and furnaces for 30 years is sold on the blanket these days for
any application except extremely large furnaces or situations requiring
much higher temperatures than we're going to see. The blanket I'm
using is 6 lb per cubic foot, 1" thick. It's 3 layers thick on all interior
frame surfaces (required thickness calculated using temperature and
volume tables in the kiln building books). The brand name is KaoWool
Ceramic Fiber.
- I'm using a method of hanging the blanket that I learned from the oven
builder in town, and I purchased some of the parts from him. This'll be
hard without pictures: basically, I've got 14 guage stainless steel
wire bent around ceramic insulators used in the appliance industry
called 'PB125' insulators. Think of a tiny donut with the wire bent half
way around the circumfrence. The two ends of the wire sticking out away
from the donut are parallel, and they pass through a large (approx 2")
ceramic 'button'. The wires pass through the blanket, the button holds
the blanket in place on the walls, and the electric elements are suspended
by running them through the holes in the 'donuts'. On the outside wall,
the wires are bent back around the expanded steel mesh to hold everything
in place. Places where there aren't elements are handled the same way,
except there aren't any donuts. There is a layer of heavy duty aluminum
foil between the outer layer of blanket and the expanded steel mesh, to
keep fibers from the blanket inside (the stuff isn't nearly as bad as
asbestos, but it's still not good for the lungs -- wear a full face
respiratory filter when working with it).
- Again using tables from the kiln building books to calculate requirements
for the elements, I've got four horizontal elements running around the
inside circumfrence of the oven, equally spaced from top to bottom. The
elements are 16 guage Chromel-A wire, 20 ohms/ft, wound into 3/8" O.D.
coils of the appropriate length. These will be powered with 240VAC, and
will draw 12 amps each. My oven builder buddy wound these for me on a
special machine -- this may be the hardest thing to track down. My
guess is that there are companies that will do this mail-order, but I've
not tried to find any of them.
- Electrical connections to the elements are going to be made with fiberglass
insulated nickel wire rated to 200 degrees C. The connection will be
made with stainless heavy duty crimp butt connecters, and these will
be embedded in the refractery blanket when everything is together. Thus,
inside the oven you see the end of an element coil disappearing into the
blanket, and on the outside you see the nickel/fiberglass wire exiting.
- Each element has it's own infinite heat switch such as used on
commercial culinary ovens. This is so I can fine tune elements relative
to each other in case I need to overcome any temperature differences
top to bottom. Any appliance repair store can get these for you, they're
about $25 bucks apiece around here.
- I've mounted junction boxes and conduit on the outside of the oven for all
wiring. Even though this doesn't have to be inspected (because it won't
have a permanent connection to it's supply) I'm doing everything strictly
according to code. This thing will connected to 240VAC drawing around
48 amps when it's running -- I want it to be *safe*!
- The heart of the control system is a program controller I ordered from
Cole-Parmer. It's great for this application, and only cost $300.
Unfortunately, it's been discontinued (their vendor changed models) so
you can't get this exact one. They still carry several program
controllers, but the least expensive is around $500. In this price
range, these are all similar: 8-16 ramp/soak segments in a program, a
small number of programs that can be stored, usually open-collector
control outputs for driving solid-state relays. I know there are people
on the list that want to build their own, but after many years of doing
embedded computer engineering work, I found $300 to be a *real* cheap
way not to do yet another closed-loop adaptive feedback/control system
(do I sound burned out on that kind of work?).
- The controller will be driving a small solid-state relay from the open-
collector output. This relay drives a 60 amp mercury displacement
power relay that will actually switch the 4 elements. The load side
of the power relay will have a series 60 amp fuse installed in case
something should go wrong on that side of things. Oh, one other
thing: the solid-state relay is opto-isolated -- very important in my
mind, to keep from destroying the program controller if something goes
wrong on the load side. (Side note: it is possible to find solid-state
relays with load-side current ratings up to 75 amps, using a *big*
heat sink -- I have been told that reliability of these when driven
hard is not good, thus my choice of using a small solid-state relay
to trip the mercury displacement power relay).
That's all for now, I'm out to the shop to try to get another
couple hours of work in on building the oven before I have to start
tearing down my whole shop. If I get far enough to have useful pictures,
I'll get them up on the web somewhere -- if I don't, it'll be a few
months before that happens.
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