Author Topic: Newbie understanding joule heating  (Read 757 times)

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Offline rthorntn

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Newbie understanding joule heating
« on: July 20, 2018, 02:27:35 pm »
Hi,

Sorry for the newbie questions.

Say I have a hard (fixed) requirement of 48V power, up to 20A, it must be DC.

Similar to a hairdryer I want to create hot air, assume the fan speed is independently controlled.

There would be a microcontroller and a PID.

Questions:

How do you design a resistive heater to use 960W?
Does equal wattage of AC or DC in to a resistive heater result in the same temperature?
How do you best switch the heater on/off (960W at 100% duty cycle, that would be an issue for MOSFET because it would have to dissipate that)?
Is there a way you can work out the maximum temperature of the air without building it, I guess it's a function of heater and fan duty cycle (lowest fan speed plus the maximum heater power)?

Thanks.

Richard
 

Offline vk6zgo

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Re: Newbie understanding joule heating
« Reply #1 on: July 20, 2018, 04:48:40 pm »
 :box:
Hi,

Sorry for the newbie questions.

Say I have a hard (fixed) requirement of 48V power, up to 20A, it must be DC.

Similar to a hairdryer I want to create hot air, assume the fan speed is independently controlled.

There would be a microcontroller and a PID.

Questions:

How do you design a resistive heater to use 960W?

As you quoted 960W with the given voltage & current, you already know
P=VI.

Knowing the needed voltage & current, you can derive the resistance.
R=V/I
In this case, 48/20 = 4.8 ohms.

960 watt is a serious amount of power, so that 4.8 ohms needs to be pretty rugged, or it will "have a short life but a happy one."

There are almost certainly ways of deriving the specs for such an element, but I would suggest looking at the elements in devices that already have ratings of around 1kw.
Perhaps elements in parallel rather than series, or something like that.

Quote
Does equal wattage of AC or DC in to a resistive heater result in the same temperature?
Yes, this is why we use RMS values for AC, rather than the more intuitive Average.
48v RMS AC will produce the same power dissipation in the element as 48v DC.
Quote

How do you best switch the heater on/off (960W at 100% duty cycle, that would be an issue for MOSFET because it would have to dissipate that)?

The switching device doesn't dissipate the 960W, its dissipation is entirely due to its own resistance.
If you had to dissipate the entire devices operating power, if you switched off a bar heater, you would vaporise the switch on the GPO it is plugged into.

It isn't a matter of power, it is really the current handling ability of the switching device.
Electric kettles used to (maybe still do ) use bi-metal switches, but,of course, they are operating at full Mains voltage, so the current is a lot lower.

If the 48v is ultimately derived from the Mains, I suggest doing the switching there, or perhaps as a function of a SMPS.
Quote
Is there a way you can work out the maximum temperature of the air without building it, I guess it's a function of heater and fan duty cycle (lowest fan speed plus the maximum heater power)?

Thanks.

Richard
You could measure the air temp of a similarly rated Mains operated heater, but, depending on how you organise your 4.8ohms, the elements may not be all exposed to the air in the same way as the comparison device.
 
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Offline Nusa

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Re: Newbie understanding joule heating
« Reply #2 on: July 20, 2018, 06:07:32 pm »
You might find this useful on the subject of heat transfer:
https://www.engineersedge.com/heat_transfer/heat_transfer_table_content.htm

Even if you've managed the math for your situation, confirming your answers empirically with a prototype is still a really good idea.

Don't forget safety considerations. What happens if the fan fails? What happens if the air flow is blocked? Unless the heating element will also fail before something *really bad* happens, you likely need a thermal shutoff. Depending on the type of device, there may be other legal requirements. Tilt switches for space heaters, for example.
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #3 on: July 21, 2018, 09:22:04 am »
Thanks!

The 48V DC is not derived from the mains.

Is there a way to safely switch the 1KW heater on/off with a microcontroller to control the temperature output accurately?
 

Offline vk6zgo

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Re: Newbie understanding joule heating
« Reply #4 on: July 21, 2018, 12:44:13 pm »
Thanks!

The 48V DC is not derived from the mains.
Bummer! :(
Quote
Is there a way to safely switch the 1KW heater on/off with a microcontroller to control the temperature output accurately?
You can control almost anything from a micro controller, with an appropriate interface.
Devices you could use are:- heavy duty automotive (truck) relays, suitably rated power MOSFETs, or
complete modules specifically designed for the purpose.

I wouldn't expect any solution to be very compact, as they are power devices, which tends to make them a bit "clunky".
« Last Edit: July 21, 2018, 12:46:34 pm by vk6zgo »
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #5 on: July 23, 2018, 07:16:33 am »
Thanks.

I would be grateful if someone could post an example switching device or even some tips on how to find one (for example on Mouser, under MOSFET, what are the important attributes)?

By "clunky", roughly what are we talking (LWH, I would like to use the most elegant and compact solution available)?

Could such a device be driven directly from an MCU pin and would it need active cooling?
 

Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #6 on: August 02, 2018, 12:28:53 pm »
[newbie alert] Ok so say I have a 48v DC power source and a 4.8 ohm cartridge heater that can handle 10A, if I connect it all up will the cartridge heater just draw 10A, even if the power supply is 12.5A, I think it will just draw 10A because we are controlling voltage and resistance which fixes our amps but it's a sanity check?
 

Offline rstofer

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Re: Newbie understanding joule heating
« Reply #7 on: August 02, 2018, 12:51:16 pm »
Current is a result, not a cause.  You have 48V and 4.8 Ohms so P = E2 / R or 480 Watts.

There are MANY MOSFETS that can handle 10A.  Most can handle MUCH more current.  The problem is getting them to turn on with only logic level voltages.  Vgs for full current has to be a logic level, like 5V in the case of the Arduino.  Otherwise, you need a MOSFET Driver.arch for 'logic level mosfet 100A 60V' or something like that.

 
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Offline hamster_nz

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Re: Newbie understanding joule heating
« Reply #8 on: August 02, 2018, 01:07:32 pm »
So, say you have your nominal 4 ohm heating element.

Directly connected across your 48V DC source will give you = V^2/R = 48*48/4 = 576 Watts

And it will draw I = V/R = 48/4 = 12A

So for something that can switch 48VDC/12A at a reasonable price, I would suggest you investigate something like the 25A version of this:

https://www.aliexpress.com/store/product/DC-DC-Single-Phase-Solid-State-Relay-DC-DC-SSR-Module-10A-25A-40A-DD-Input/108414_32860686417.html

I'm not saying you should use THAT one, just that you should investigate using something like it, but one where you are happy it meets your needs.

These can be driven by a micro-controller with ease.

« Last Edit: August 02, 2018, 01:09:25 pm by hamster_nz »
 
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Online IanB

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Re: Newbie understanding joule heating
« Reply #9 on: August 02, 2018, 02:01:51 pm »
Is there a way you can work out the maximum temperature of the air without building it?

Yes. Temperature rise equals power input divided by mass flow of air divided by specific heat capacity of air.

The specific heat capacity of air you can look up.

The mass flow of air through the fan you have to estimate. Use the fan datasheet to find the volumetric flow and multiply that by the density of air (which you can look up).

The air flow will vary with the flow resistance through the heating coil. So your estimate of the temperature rise will only be approximate, unless you measure the pressure drop with a manometer and look up the flow on the fan performance curve.
I'm not an EE--what am I doing here?
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #10 on: August 02, 2018, 04:21:02 pm »
OK, so I actually have some "Dernord" 12v 100w "17-11" cartridge heaters here, similar to these (the amazon ones say 17-03 not sure what my 17-11 means, anyone?):

https://www.amazon.com/Dernord-Cartridge-Electric-Stainless-Replacement/dp/B0741664G5

So with what I'm learning are they 1.44 ohms, I'm just asking OK, so if I connected one of these to a big bench supply at 24v supply it would draw 16.66 amps and put out 400W, say I used a bench power supply and gradually increased the amps from 1 amp, would it be safe to do?  Essentially at 26v it would output pretty near to 480W but I just want to know if it would be OK at a shade under 20 amps?

Update: the "Dernord" is 100mm x 8mm do we have any idea is it would have a density of over 50W/cm², if it did would that impact anything, is there a way to check if a resistor can handle a specific amount of watts without a datasheet?
« Last Edit: August 02, 2018, 04:46:37 pm by rthorntn »
 

Online IanB

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Re: Newbie understanding joule heating
« Reply #11 on: August 02, 2018, 05:11:31 pm »
If a cartridge heater is rated for 12 V 100 W then it is not safe to try and operate it at 24 V 400 W. If you try it will likely destroy itself in short order.

If you wanted 400 W from these, you would use 4 of them giving 4 x 100 W = 400 W.

Also bear in mind these are in no way suitable for heating air. They are not meant to run hot, and in any case they do not have a suitable surface area for contact with the air stream to transfer the heat. One could in principle fit some kind of finned heat sink to it, but it would still not be a very convenient arrangement.

The usual way to make an air heater is to obtain nichrome resistance wire (or similar) of the right thickness, form it into coils, and place the coils into an air duct. You can figure out the right thickness and length of the wire by considering the resistance per unit length.
I'm not an EE--what am I doing here?
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #12 on: August 02, 2018, 05:38:32 pm »
Thanks IanB!

I was thinking of using a finned heatsink encasing the cartridge heater and that placed in the air duct, more for safety (and ease of serviceability) than anything else.

I need to find more out about nichrome resistance wire, a calculator I found seems to suggest that at 48v I need 18.7m of 2.3mm wire to do 462w, clearly that's not happening.
 

Offline larsdenmark

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Re: Newbie understanding joule heating
« Reply #13 on: August 02, 2018, 05:49:16 pm »
A few points:
Resistance changes with temperature so you have to take the operating temperature of the wire into account. Since you don’t know the temperature you have to perform a few experiments.

If the fan fails the wire could get extremely hot and start a fire. Monitor the fan and make you don’t have any power on the heating wire when the fan doesn’t run.

 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #14 on: August 02, 2018, 05:55:53 pm »
My maximum temperature I want to create is 250 degrees C in 12 minutes.

I'm not liking this wire idea, coiling up lengths of wire by hand, it seems to be more for AC mains.
 

Online Hero999

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Re: Newbie understanding joule heating
« Reply #15 on: August 02, 2018, 05:57:27 pm »
A few points:
Resistance changes with temperature so you have to take the operating temperature of the wire into account. Since you don’t know the temperature you have to perform a few experiments.

If the fan fails the wire could get extremely hot and start a fire. Monitor the fan and make you don’t have any power on the heating wire when the fan doesn’t run.
And add a thermal fuse to protect against fire, if the controller develops a fault or the MOSFET switching the element fails short circuit.
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #16 on: August 02, 2018, 06:04:29 pm »
I'm learning a lot, thanks everyone!
 

Offline hamster_nz

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Re: Newbie understanding joule heating
« Reply #17 on: August 02, 2018, 06:45:52 pm »
My local hobby shop has 4m rolls of thinner nichrome wire that is about 14 ohms per meter - a meter of that over 48V would be around 100W, and would most likely glow red hot. I used to use about 900mm and a 12V supply to make a hot wire foam cutter.

But it makes me think... What are you trying to do?

Watts is a unit of energy, not heat, Even a few Watts can heat a well-insulated thing very hot.

And 250 degrees is as hot as an oven. No hairdryer blows 250 degree air...

And why 48V?

Mabe just run a mains heater (of whatever sort you need) off of a 1000W DC/AC inverter?
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #18 on: August 03, 2018, 11:25:51 am »
Thanks hamster_nz.

It's more like a portable air cooker with up to a 15 minute run time.

I want it to run directly off of DC, solar, maybe with something like an e-bike battery pack, and I don't care that it makes it more complicated and costly.

It will be efficient and insulated but I want to be able to adjust the temperature ramp fairly accurately.

 

Offline Eka

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Re: Newbie understanding joule heating
« Reply #19 on: August 05, 2018, 01:30:15 pm »
Current is a result, not a cause.  You have 48V and 4.8 Ohms so P = E2 / R or 480 Watts.

There are MANY MOSFETS that can handle 10A.  Most can handle MUCH more current.  The problem is getting them to turn on with only logic level voltages.  Vgs for full current has to be a logic level, like 5V in the case of the Arduino.  Otherwise, you need a MOSFET Driver.arch for 'logic level mosfet 100A 60V' or something like that.
For this much current, use a MOSFET driver chip. That way the FET turns on fully and fast so little heating happens in it.
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #20 on: August 10, 2018, 01:36:19 pm »
Thanks all.

OK so I just had a thought.

I have 4 x "Dernord" 12v 100w "17-11" cartridge heaters and a 48V 15A PSU.

Could I wire the 4 cartridge heaters in series to draw 8.33A at 48V from the PSU putting out 400W?

 
 

Offline Eka

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Re: Newbie understanding joule heating
« Reply #21 on: August 13, 2018, 11:35:44 am »
Thanks all.

OK so I just had a thought.

I have 4 x "Dernord" 12v 100w "17-11" cartridge heaters and a 48V 15A PSU.

Could I wire the 4 cartridge heaters in series to draw 8.33A at 48V from the PSU putting out 400W?
Yes, it's basically the equivalent of 4 resistors in series wired to a power supply with 4X the voltage. So the watts just add up, and the amp draw will be the same.
 
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Offline CatalinaWOW

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Re: Newbie understanding joule heating
« Reply #22 on: August 14, 2018, 12:16:05 pm »
Don't give up on the nichrome.  There is a reason that almost all  commercial products doing this sort of thing use it or one of its cousins.

Winding it isn't onerous.  You need a form.  They used to be made of mica, then asbestos.  One has gotten rare and pricey.  The other is unpopular because it kills people.  But there are several ceramics and other materials that are insulators and can stand the heat.  Notches on the edge keep the nichrome where it belongs and it is amazing how rapidly the length builds up as you add turns around a form of the appropriate size for your project.

Another approach is to cannibalize an old toaster or room heater.  It will have both the forms and the wire.  Just adjust the length to compensate for the difference between mains and your voltage.
 
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Offline rthorntn

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Re: Newbie understanding joule heating
« Reply #23 on: August 14, 2018, 03:44:00 pm »
Thanks, I might end up with nichrome but I'm working with cartridge heaters now because it's easy and I'm pretty sure it will do what I want.

 


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