Author Topic: heating elements, a better way?  (Read 2999 times)

Offline shipto

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heating elements, a better way?
« on: April 13, 2016, 03:50:43 PM »
If anyones noticed I am currently trying (with limited success at the moment) to build a electric furnace based on the gingery lil Bertha design.
I was thinking today about the way the element is mounted in grooves in the side walls which is pretty much standard as far as i can tell even with commercial units, Is this the best way however? I was thinking that the element mounted at the bottom would be better then I thought it would be much better if the element had a gap underneath and around the sides, this should create a recycling convection effect and the temps should rise much quicker and more evenly throughout the furness.
Sadly I am far behind the self imposed timetable I have set myself and dont have the time to look more into this so I am throwing it out there in case anyone else wants to have a go.
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
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Offline hermetic

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Re: heating elements, a better way?
« Reply #1 on: April 13, 2016, 06:00:39 PM »
Hi Shipto, have you any pics? are you using solid cooker type elements or wire wound fire type. I have seen small kilns and alloy melters with both. With the cooker ring type element, they are underneath the crucible set into a fireclak moulding so that the crucible sits on the moulding, not the element. With the wire coils, they are fitted in slots in the side walls of the furnace, set back into the refractory and sometimes have refractory pencil made from porcelain or similar inside the coil to support the elements, and prevent them sagging out of the slots when they are hot, any help?

Offline shipto

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Re: heating elements, a better way?
« Reply #2 on: April 13, 2016, 07:14:30 PM »
Hi hermetic the pictures of my efforts are in the "metal stuff" section, this is just something I was mulling over today while at work and I thought I would post the idea I know its not a new idea convecter heaters use this principle on a much bigger scale after all.
I was thinking some kind of ceramic housing for the element with feet to hold it off the ground of the furnace chamber.
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
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Offline PK

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Re: heating elements, a better way?
« Reply #3 on: April 13, 2016, 07:28:39 PM »
In my little electric furnace, I suspect that a lot of heating takes place by radiation. 

Offline BillTodd

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Re: heating elements, a better way?
« Reply #4 on: April 14, 2016, 05:14:39 AM »
As suggested by PK above, the vast majority of heat is transferred by radiation, not by convection.  The vacuum fernaces at work  (obviously) have no convecting gas in them when they're on yet heat up just fine.

The heat transfer is then a function of surface area, so heating coils on all faces is better.

If you want faster heating, you'll need to reduce the mass being heated (for a given input power). One way is by induction ,where the heat is primarily generated in the part directly .

Bill

Offline JHovel

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Re: heating elements, a better way?
« Reply #5 on: April 14, 2016, 06:19:54 AM »
I suspect the reason for not haing an element in the bottom is for mechanical protection.
if something melts or falls over all hell could break loose.... :zap:
Cheers,
Joe

Offline PK

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Re: heating elements, a better way?
« Reply #6 on: April 14, 2016, 06:49:16 AM »
I suspect the reason for not haing an element in the bottom is for mechanical protection.
if something melts or falls over all hell could break loose.... :zap:

:-) I think the phrase "What could possibly go wrong?" was coined in a foundry.
You really have to just accept that it's a bit risky and press on..... 

Worst case would be that you trip a breaker......

Another reason not to put the element in the bottom is because of the thing I forget the name of.....bugger... But it's basically a property of the element wire that limits the power density (ie Watts per unit area) you can have before the wire overheats and cracks. 
I ended up having to fit a higher resistance element, so I could space the coils more. This dropped the unit from 250V 15A to 250V 9A with a corresponding increase in time to melt.
It's more of a problem with small furnaces (like mine)... I really should have sprung for carbide bar elements... Or just built a bigger furnace...
I digress... On a small furnace you want as much surface area as you can get to keep the watts/m^2 low...


Offline shipto

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Re: heating elements, a better way?
« Reply #7 on: April 14, 2016, 03:14:30 PM »
ok I will consign that idea to the "not worth trying" section of my brain then.
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
https://myshedblog.wordpress.com/