watson.fawkes
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Magnesium Oxide packing for cartridge heaters and thermocouple assemblies
Magnesium Oxide powder is a standard packing material for cartridge heaters and thermocouple assemblies. I've wanted a cartridge heater for a CuCl2
etch tank, and I've pretty much decided to make one out of glass, so I've been looking for the MgO. It's been a bit harder to find cheaply than I
might have guessed. Here are my preliminary results, and please add any sources you might know of.
Probably the most widespread source is your local vitamin and supplements store, where it's sold in powder form. I've seen USD 6 -10 per pound for 1/2
- 2 pound quantities.
Ceramics supply houses may carry it, where it's used in glaze formulation. Here's one vendor, that has 5 lbs for USD 13. It doesn't seem to be universally carried, though, since evidently many glaze formulators get their MgO in
the form of the carbonate as with dolomite.
I've found an assaying supplier that has 10 lbs for USD 23, and 55 lbs for USD 56.
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Swede
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I found the same thing as you have, WF. I was looking at creating an immersion heater for my Lead Dioxide plating experiments. MgO is not as common
as one might think. You've found cheaper sources than I did.
Have you considered buying one off the shelf? I did a bit of research into industrial cartridge heaters, and they are both plentiful and cheap on
eBay and from other surplus sources. $20 or so should set you up with a 500 to 1000 watt heater, encased in stainless or inconel. The higher the
wattage density, the more likely it is to be an inconel sheathe.
The cadillac of immersion heaters are those made by Process Technology, and are PTFE encased. The L-shaped heaters are the most useful, placing the
hot coil in a horizontal position optimized to heat smaller tanks. There's yet another eBay source for these, a guy usernamed SurplusPlating.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=38014...
I pulled the trigger on that one two days ago. Even if you don't need PTFE for a particular project, it's nice to have because it can be used in the
future for other, as yet unknown, projects.
Coupled with a $30 Chinese PID temperature controller, they will hold +/- 1 degree all day. You know I love DIY stuff, but the metallic-sheathed
cartridge heaters are really cheap, and something to consider.
What is your plan to create a cartridge heater? Are there any DIY resources on the web?
[Edited on 7-9-2009 by Swede]
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12AX7
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If you don't mind an extra step you can burn MgCO3 or Mg(OH)2 (easy to make from MgSO4) in a furnace.
Tim
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watson.fawkes
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Quote: Originally posted by Swede  | Have you considered buying one off the shelf? [...] $20 or so should set you up with a 500 to 1000 watt heater, encased in stainless or inconel.
[...] The cadillac of immersion heaters are those made by Process Technology, and are PTFE encased.
[...]
What is your plan to create a cartridge heater? Are there any DIY resources on the web? |
My immediate
application is heating a copper chloride etch tank for printed circuit boards. The tank body is mostly done, and the design uses a horizontal tube
heater lying parallel to the bottom between a pair of sparger pipes. I could put the heater elsewhere, but this design gets some heat circulation
without continuous pumping. Copper chloride is a pretty problematic corrosive. Both stainless steel and even inconel are not rated for this
application. See the Watlow Corrosion Guide, for example. Titanium is rated (you will be happy to know), as is PTFE, but I don't have any of either in my shop. I did
some looking around, and except for PTFE, there's not much out there in the used market. Add all this to the geometry I need, and I just decided to
learn how to make them myself.
As far as construction methods, the only material I've found has been indirect references in patent literature. The basic idea I've worked out is very
easy. You take a coil of wire, put in your tube, pack it with filler (MgO), and press it all together to compact the filler. The difficulty has to do
with keeping the coil away from the sides and the wire away from itself, both to prevent shorts. Solving this requires making some simple tooling;
I'll summarize here. Use a bit of all-thread as a core form around which to wrap the resistance wire and a jig to hold it concentric with the tube.
Wrap the wire and assemble the jig. Then pack the cavity between the core form and the tube with MgO. Now carefully unscrew the core form, leaving a
void; this step is why you want a jig. Fill this void with MgO. Now put on the tamping block, a cylinder with a couple of holes for the wire, and
press to compact. Take out the tamping block and seal the top with a casting resin, say, epoxy. For ordinary wire gauges, say 18-30 AWG, I'm pretty
sure this would work. For fine wires, I'm not quite as confident. The other kind of core form to consider is a hollow tube.
Getting adequate heat density isn't a problem. 1000W from 120 V mains takes about 20 in. of 30 ga. Kanthal A-1 (which I happen to have a spreadsheet
open about at the moment); nichrome will be about half again as much. If you wind around a 1/2-13 NC thread, that's a little less than an inch of
winding.
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garage chemist
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| Quote: Originally posted by watson.fawkes |
Getting adequate heat density isn't a problem. 1000W from 120 V mains takes about 20 in. of 30 ga. Kanthal A-1 (which I happen to have a spreadsheet
open about at the moment); nichrome will be about half again as much. If you wind around a 1/2-13 NC thread, that's a little less than an inch of
winding. |
This thing will burn out after 10 sec. maximum.
Have you ever worked with electric heat generation before? Do you know what "maximum surface load" of heating wire means?
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Swede
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I don't know enough of the theory to comment on garage chemist's statement, but as for preventing shorts on the wall, would it be possible to wrap a
thin sheet of glass cloth or similar between coil and tube wall? Another (possibly very stupid) thought - if the length and thickness of the
resistance wire is long and thick enough, would it be possible to thread it through a PTFE tube, say 1/8" OD", then coil the tubing around a form and
"set" the PTFE coil with heat? The void between wire and tube wall might be filled with a high temp silicone oil, and with the correct math, one
might possibly plan it so that the wire itself never exceeds the working temp of the teflon, but with a low enough wattage density, adequate heat
might be transferred to the solution.
Just thinking out loud a bit.
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dann2
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Hello,
If heating a tank to 90C, a coil of Nicrome in a ushaped glass tube (just air in the tube) is good enough so long as you are not in a big hurry to
heat up you tank. A variac will do as input.
As far as getting 1000W from an inch coil of Kantal. That might work if you are blowing a hurricane of air through the coil of wire to stop it burning
up. Not what you are needing to do.
Dann2
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watson.fawkes
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| Quote: Originally posted by garage chemist | This thing will burn out after 10 sec. maximum.
Have you ever worked with electric heat generation before? Do you know what "maximum surface load" of heating wire means? |
In open atmosphere, sure, it would burn out. Failure is from oxidation of the element. No oxygen, no oxidation. In a cartridge
heater you've got a sealed environment and small amounts of free oxygen. I've not seen specific engineering data, which is all proprietary, but it's
clear that manufacturers are running their elements at much higher surface loads than they'd be rated for in open atmosphere. For example, defects in
the MgO packing lead to failures; see the bragging at this manufacturer's site about recompacting the fill in order to avoid cracks in the packing.
And evidently there's something special about MgO as opposed to other fill materials that protects heating elements. My best guess is that Mg metal is
so much more reactive than the alloyed components of resistance wire that there's very little oxygen transfer from the fill MgO and the wire itself.
Unfortunately, I've not seen anything specific on it, just hints from reading. I'm not even sure that's what's happening. On the other hand,
manufacturers do brag about the purity of their filling material; see the above web page for some of that.
Above some temperature limit, though, you will get oxygen transfer and element failure. So perhaps I shouldn't have used 30 ga. wire as an example.
It's very possibly beyond that temperature limit (although it's not obvious to me that it is). Let me change the example to use 24 ga. wire, which is
about exactly twice the diameter of 30 ga. The corresponding wire length for identical heat output is about 4 times that for 30 ga., but surface load
goes down by a factor of eight. That's 6.6 feet of wire wound up in a coil 3.9 inches long, still plenty small enough. Given that I don't have data
for surface loading inside MgO fill, I'm going to have to proceed by trial and error. If you have such data, I'm all ears.
On the other hand, the total material volume of the wire goes up by a factor of eight. With heating wire being the most expensive per-unit cost in
constructing a heating cartridge it's no wonder that manufacturer run their elements at the highest surface load they can manage.
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watson.fawkes
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| Quote: Originally posted by Swede | | I don't know enough of the theory to comment on garage chemist's statement, but as for preventing shorts on the wall, would it be possible to wrap a
thin sheet of glass cloth or similar between coil and tube wall? |
Yes. Indeed, I've seen exactly such plans
in reverse for wrapping heating wire directly around a retort pipe. For insulation, they used asbestos paper (it's a 50 year old book); please
substitute one of the modern refractory papers. As a glue, they used a few coats of sodium silicate solution. There was a top wrap of a second paper
layer, as I recall. There's no reason why you couldn't do exactly the same on the interior wall of a metal pipe. Disadvantages are extra material cost
and a more-or-less difficult fabrication inside the pipe. If a heater were used for quick heat, the added expense might be worth it. For a heater used
primarily to maintain bath heat, I doubt the difference is worth it.
As for using PTFE, it might work, but I'm partial to refractory materials, which have more predictable failure properties.
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watson.fawkes
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Use of Mg metal as an oxygen getter inside a cartridge heater
Writing replies it's occurred to me that if the reactivity of Mg acts to hold oxygen, that you could use Mg metal as an oxygen getter, analogous to
the use of getters in vacuum tubes. The idea is to react away all the free oxygen inside the cartridge cavity with a slight excess of Mg metal
remaining.Mg powder is probably the best form, with its higher surface area. Oxygen diffusion is the rate-limiting factor for
equilibration, and it will happen faster with more surface area.
A burn-in period would be required, to avoid combustion of the Mg. All you really want is its oxidation, not a small bomb.
There will be a drop in pressure as the oxygen is gotten out. Since pressure is easier to seal than vacuum, the cartridge should be pressurized
before sealing, say at 1.3 atmospheres. In large volumes, you could fill with inert nitrogen and lower the amount of Mg powder.
You might have to worry about Mg bridging shorts, although they'll likely open up in the burn out period.
You don't need much Mg total, just a small molar excess of the amount of gas between the interstices of the fill material. For higher
performance, you could put more in and selectively reduce the metallic impurities in the MgO.
I'll let this note stand as a disclosure of prior art, since I have no interest in going into the cartridge heater business or developing
intellectual property for it.
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watson.fawkes
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| Quote: Originally posted by 12AX7 | | If you don't mind an extra step you can burn MgCO3 or Mg(OH)2 (easy to make from MgSO4) in a furnace. |
I
worked out a trial price for MgO from Mg(OH)2 from MgSO4 + 2NaOH. Prices I took were $1/lb for MgSO4 and $3/lb for NaOH. These are typical for
small-quantity, retail source packages. That comes out to $0.265 per mole for both reagents. That's $0.795 per mole of final MgO, or $8.94 per pound
before energy costs.
Blackboard chalk seems to be about $1.50 for a quarter-pound, or $6/lb. Cheaper is white powder chalk refills for a chalk line tool, at about $2/lb in
5 lb quantities. Calcining that comes out to $4.18/lb before energy costs.
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garage chemist
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Here, this is the document you need, the Kanthal Heating Alloys Handbook:
http://www.kanthal.com/C12570A7004E2D46/062CC3B124D69A8EC125...
Scroll down to the element types- they give maximum surface loadings of wire and element for every imaginable application.
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watson.fawkes
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OK, I'm officially embarrassed. I
already had a copy of that, but I hadn't read it all the way through. Here's the relevant bits: | Quote: | Cartridge Elements, Powder Filled, p. 16
Surface Load
On tube: 10-25 W/cm^2 (65-160 W/in^2) for elements with straight wire.
Other types: about 5 W/cm^2 (30 W/in^2) |
| Quote: | Metal Sheathed Tubular Elements, p. 14
Surface Load
Wire: Normally 2-4 times the element surface load.
Element: 2-25 W/cm^2 (13-161 W/in^2) |
The difference in descriptions is that the first has a ceramic core and
the second doesn't (see the document). Note that both these descriptions give a primary rating on the element. The first doesn't specify a
wire load directly, but it does indirectly, because the "straight wire" specification gives away something. Assuming that the outer jacket is, say, 6
times the diameter of the wire, that's an upper limit of 150 W/in^2. The upper limit in the second case is 50-100 W/in^2. Wires sitting in groove in
the side of a small kiln, on the other hand, are rated at 3-6 W/in^2. It's frustrating that they don't actually specify the wire load, but it seems
100 W/in^2 is a reasonably practical one, before derating for geometry.
The lower rating for "other types" of cartridge configuration must come from a geometry where the effective heat sink angle is reduced; it's a full
360° for a single wire, 180° for a hairpin turn element, etc. The geometrical derating seems to top out at about 1/π, which is
considering the wires to be close wound, to the limit of touching, so that the effective radiator looks like a cylindrical shell whose thickness is
the wire diameter. Thus πD surface area reduces to just D. This more-or-less corresponds with the factor-of-5 (or so) geometric difference
indicated.
Back to a canonical 1000 W cartridge. The element surface load for a cartridge 4" long by 3/4" diameter rod is 106 W/in^2 (actually a little lower,
since I'm disregarding the ends). That's on the high side, but perhaps feasible. Easier to justify is a 8" long by 1" diameter element, with a surface
load of 40 W/in^2. My original 30 ga. example was operating at a wire surface load of 1600 W/in^2, which is past the upper limit of feasibility. The
one at 24 ga. was at 200 W/in^2, which is at the boundary. At 21 ga., we're down to 70 W/in^2, which is OK, if a little high. The requisite length of
21 ga. wire winds around a 3/4-10 NC screw form in 6.8 inches, so that works for this element.
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not_important
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| Quote: Originally posted by watson.fawkes | I worked out a trial price for MgO from Mg(OH)2 from MgSO4 + 2NaOH. Prices I took were $1/lb for MgSO4 and $3/lb for NaOH. These are typical for
small-quantity, retail source packages. That comes out to $0.265 per mole for both reagents. That's $0.795 per mole of final MgO, or $8.94 per pound
before energy costs.
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Mg(OH)2, while not as bad as Al(OH)3, can be a bit difficult to filter. I'd suggest using Na2CO3 or NaHCO3, both sold at ceramics suppliers, to get
one of the basic carbonates. Calcining those to MgO is much easier than with CaCO3, decomposing around 550 C.
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Eclectic
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http://www.aquascience.net/filtration-media/index.cfm?id=121
particle size is about like fine kitty litter.
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watson.fawkes
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So that we have all the pricing information
on this page, this source is $17 (not including shipping) for a ten-pound box of Corosex, a >97% purity granular MgO. The manufacturer is Clack Corporation and here's their PDF product literature. (Incidentally, this is a good price; another vendor was twice as much.)
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watson.fawkes
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Daigger has "lab grade" magnesium oxide, $5.35 / 500 g. (That's $ 4.85 / lb, for comparison). It says "heavy powder", which I presume means "finely ground", with less
entrained air.
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densest
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MgO occurs as "light" and "heavy" powder. As initially prepared, it's usually the light form, but transforms pretty easily into the heavy form (but
not the reverse) The heavy form is indeed much more compact and importantly for your application, less reactive. MgO will absorb water and CO2 from
the air as CaO does but less avidly.
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Texium
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Thread Moved
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