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Air heater with ni-chrome wire need input

Posted by terramir 
Air heater with ni-chrome wire need input
September 16, 2016 03:22PM
Want to build an air heater that will be powered by a pc power supply and will use both the 5V and 12V rails .
The air will need to be heated to no more than 55C actually I could probably get away with 49C (120F) I want to use an arduino chip 328p maybe.
The air will be circulated and returned to the heater via dryer duct. and then heated again. I can't have the heated Air exceed 55C because then it will start effecting plastic objects etc. I think I can do most of this myself, but I need three area's of suggestion.
I will print the "heaterbox component out of ABS" but I need suggestions on what material to use to hold the wires themselves.
Also what brand of 12V or 5V fans can stand temperatures of 55C.
I will use this to rid my bed/ my property of the old/new uprising pest (aka bedbugs).
My landlord refuses to do a whole house heat treatment but I saw neat beds, that can be heat treated (need to adapt this to my skill level and budget (budget virtually none) but I can reprap the box program an arduino and ni chrome wire is cheap ;-) and I can sew/velcro together 99 cent store cheap car sun shades ;-)
So and suggestions on a cheap way to hold the ni-chrome wire I dun want the wire really heated that harshly, but I do want it to be able to maintain an air temp of about 49C - 55C.
Also if someone can provide me with the resistance figures for different gauge ni-chrome that would be great so I can figure out length and watts of heat produced at both 5V and 12V
terramir
VDX
Re: Air heater with ni-chrome wire need input
September 16, 2016 04:39PM
... if you heat the wire below 300 degC, then cardboard impregnated with water glass (sodium silicate) can be used - otherwise you'll need something made from ceramic or mica sheets.


Viktor
Re: Air heater with ni-chrome wire need input
September 16, 2016 07:59PM
What is mica I have heard of the stuff ? And where would I get it?
terramir
Although I really dun want the wires that hot so when do I get sodium silicate

Edited 1 time(s). Last edit at 09/16/2016 08:01PM by terramir.
VDX
Re: Air heater with ni-chrome wire need input
September 17, 2016 10:18AM
... mica ("Glimmer" in German) is a crystalline mineral forming paperthin, transparent sheets and stacked/pressed together in blocks - was/is used as heat-conducting electrical isolation under transisors and MOSFET'S.

With impregnating paper/cardboard or cloth with sodium silicate makes it a littler harder/stiffer and fire-resistant.

You can even impregnate wood or plywood, what could be better for stability, than cardboard ...


Viktor
Re: Air heater with ni-chrome wire need input
September 17, 2016 12:06PM
How would I do this with wood/ply cut the pieces I need to hold the wires then soak it in a water glass mixture or should I just spray the stuff on the surfaces that will be holding the wires?
terramir
VDX
Re: Air heater with ni-chrome wire need input
September 17, 2016 01:29PM
... you can try with soaking and/or painting it with a brush on the edges/surfaces.

Comercially this is made with pressure, to get the fluid into the material ...


Viktor
Re: Air heater with ni-chrome wire need input
September 17, 2016 02:39PM
Now where do I get this stuff in California,
I looked on the web besides ebay not many options that
Don't require buying a ton of it very expensive for a common chemical honestly. Although I dunno how much I'll need either.
Well still need input on gauge vs voltage vs temperature
terramir

Edited 1 time(s). Last edit at 09/17/2016 02:40PM by terramir.
VDX
Re: Air heater with ni-chrome wire need input
September 17, 2016 04:28PM
... I've bought some sodium silicate in a 100ml bottle at my local pharmacy shop.

If you don't want to let it glow, then the ni-chrome wire normally used for styro-cutter will work.

I can buy some meters of this wire at local hardware stores.

Then simply try with a regulated PS at which voltage (and which current) it gets hot enough ...


Viktor
Re: Air heater with ni-chrome wire need input
September 18, 2016 10:24AM
I have some 32 gauge ni-chrome wire, but I really can't just be testing all willy-nilly, I want to design this right.
that means calculating what lengths and gauges to use
In parallel at 12 and 5V.
I know there should be formulae floating around here, and I also know that coiled lengths support about 1/2 the current due to induction choke kind of phenomenon.
Anyone here got some links for the formulae needed to make this work.
if I design this right because resistance increases with heat, I should be able without regulation maintain a wire temperature of 2xx degrees C without doing anything and then just use mosfets to bang on bang off based on exhaust temperature.
terramir
Re: Air heater with ni-chrome wire need input
September 25, 2016 12:42PM
For what it's worth I will add a few suggestions and cautions.

I've worked a bit with nichrome, having made a custom hot-end, and a plastic bender.

I strongly caution against using any flammable material next to it. You may get a hot spot, and it sags when it gets hot. There should be no possibility of fire.

What you might want to use is ceramic electric fence insulators.

However before you go on, you need to carefully consider the heat budget:
  1. How well are you going to be able to insulate the chamber with the mattress in it? You will need to know about how many watts will be lost at the required internal temp.
  2. You will need to heat the air somewhat above the required temperature, but below the decomposition temperature of the foam etc in the mattress. This relates to:
  3. How long will it take to get the mattress up to the temperature needed? The more watts the less time it will take. Not enough watts and it will never reach the desired temp.
  4. Given all of the above, how many watts are required?

I do not have formulas for any of the above.

For a fan, you may be able to use something like this Radiator Cooling Fan. My reasoning is that it is made for a fairly harsh environment (an engine bay) and may stand up for the time required. I have one like it in the heated chamber of my printer that can reach 60C. I don't run it all the time, but it's not dead yet.

You should also consider what would happen if the fan failed - would you have a meltdown/fire? Be very very careful, and do not run it when you are not there. You should have a built in mechanism to shut it down if the fan does fail.

You need to carefully calculate how many wires need to be used in parallel to achieve the desired resistance, different for each voltage of your power supply, and the amount of current each leg can supply (lower than the maximum possible).

Also the length of nichrome needs to be considered. A few short lengths will individually get to a much higher temp than more, longer lengths in parallel (given the same resistance), and air blowing across them. That would be the difference between a heater that glows and one that doesn't.

It sounds like you might, but if you do not fully understand ohms law at this point, please learn a lot more before continuing. I worry for your safety.

Instead of creating the heating coils yourself, it may be safer to modify a ceramic heater.

There are so many fatal ways this could go wrong that I haven't mentioned. Please be careful.


My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
Can you answer questions about Calibration, Printing issues, Mechanics? Write it up and improve the Wiki!
Re: Air heater with ni-chrome wire need input
September 25, 2016 12:55PM
Quote
terramir
I will use this to rid my bed/ my property of the old/new uprising pest (aka bedbugs).

One method pest control companies use: Get several sheets of foam board, build a box, put one or more space heaters inside with the item(s) and run them. Keep it above 45C for at least 90 minutes, use a remote style thermometer to monitor. For clothes use a dryer that can run the heat for 90 minutes.

Don't forget to put the mattress inside a mattress cover.

Edited 4 time(s). Last edit at 09/25/2016 12:58PM by stephenrc.
Re: Air heater with ni-chrome wire need input
September 27, 2016 07:14PM
ok, I understand the dangers involved so do I understand ohm's law.
ni-chrome wire has a tendency to increase in resistance as the temperature rises.
Been focusing on getting my prius back on the road, so I have not been able to dedicate much time to this project, but I know the following .
A. as long as the insulation losses in W is smaller than the power provided in W, as long as the temperature of the wires slightly exceed the temperature I wish to reach eventually I will reach that temperature.
B. Since ni-chrome wire will increase resistance as the temperature rises I could theoretically design a heating element that can never exceed a certain temperature, for example 150F @ 12V and 150F @ 5V as long as the voltage input is both 5V and 12V the wire will never exceed this temperature because the resistance of the wire rises as it reaches 150F the resistance increases to the point where it will not increase in temperature any longer. depending on the length of the wire for 12V and 5V as a coil it will also limit inrush currents because of the induction effect. this can be designed safely and securely.
C. I wanted to draw on the experience of the community to cut down on research hours, I can determine the max temp and current experimentally fairly easy, but without the numbers that I know someone around here has, this could take weeks instead of hours.
D most dryers I have access to do not reach the proper temperature to kill bedbugs and there eggs properly. commercially available dryers tend to put limits on the temperature to protect clothes and save energy, my own dryer is a 120V electric model and I really would have to dry my clothes for several cycles to be 100% sure.
E. ceramic heaters are designed for 120V I want this project to be 12V/ 5V to make it portable in a cinch. the 5V portion is just going to be enough for a load, so if I have to run it battery wise I could forgo the 5V coils and switch on additional 12V coils.
I just need the resistance numbers on ni-chrome wire at 20C and 50C and maybe 110C just so I can decide which gauge and lengths I am going for.
terramir
Re: Air heater with ni-chrome wire need input
October 18, 2016 03:53PM
[en.wikipedia.org]

could help.

Dave
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