Dual Coil Heat Flux

[spawn]

I need some advice from the experts out there please. I have been vaping for a long time now and recently got into RDAs (RTAs to be exact). I am currently using a Fogger v5 with dual coils, this is the build:

Dual twisted 30AWG, each coil is (using my own spreadsheet - similar to steam engine):

AWG	30	0.02847	Ω/mm	0.255	mm
Strands	2	0.51	mm
Ra	1.60	Ω	0.8	Ω
Θa	2.5	mm
Lb	5.0	mm
Pc	16.0	W
Resulting Coil Detail
La	107.40	mm	Wire Length
Hn	0.52	mm	Rise
Cn	11.058	mm	Circumference per wrap
Ln	11.071	mm	Arc Length / wrap
Nc	9.5		Number of wraps
Hc	4.9	mm	Coil Height
Vc	140.6079	mm3	Coil Volume
Sa	172.076	mm2	Wire Surface Area
PDa	92.98216	mW/mm2	Heat flux (Area)
PDVc	113.7916	mW/mm3	Heat flux (Volume)

The question I have is around the Heat Flux. Now as you can see it is quite low (in the cool range). However, if I try increase power to get a higher flux to around 150 I lose flavor and start getting dry hits. I have a few guesses around what's happening.

1. Lack of airflow (fogger is quite limited)? - less air means more heat leading to hotter coils and burnt liquid.
2. Wicking issues?
3. Chamber size? Having a small chamber volume means the heat is building up significantly causing the liquid to overheat? I.e. heat flux calculations per coil don't tell the whole story and is affected by chamber volume?

I have a Goblin RTA on it's way - that should take care of (1). I'm hoping I can take advantage of the large coil surface area to get into the 150mw/mm2 heat flux range.

I think the summarize the question is: why can't I go over 30W with the Fogger v5 (no matter how well I build the coil = huge surface area, decent wicks)?

Edit: If this is in the wrong section I apologize, please move if necessary. Thanks.

 

[State O' Flux]

I can help you with this, but... you've provided info I don't need, and omitted (or at least need a bit of clarity on).
Is this two strands per coil, or two stands total? You say two strands, but you don't give a value for twist pitch.

 

[spawn]

Each coil is two strands, twisted, (stats as per that copy/pasted spreadsheet) can't say what pitch though - probably around 1mm?

 

[State O' Flux]

Good, clear photos, spawn. Very helpful. I'd agree that the twist pitch is very near 1mm.

For two strands of 30 gauge, with a 1mm twist pitch per coil - at 1.6Ω per coil for a net resistance of 0.8Ω, at 30 watts, I come up with a HF of 334 mW/mm²... pretty warm, although not excessively so.
It's possible that your other condition issues below are more likely.

1. Lack of airflow (fogger is quite limited)? - less air means more heat leading to hotter coils and burnt liquid.
2. Wicking issues?
3. Chamber size? Having a small chamber volume means the heat is building up significantly causing the liquid to overheat? I.e. heat flux calculations per coil don't tell the whole story and is affected by chamber volume?

Regarding dry hits...

1. It's possible that the reverse is true, as regards air flow. If the greatest percentage of draw vacuum is pulling in fresh air, then the lesser percentage is inadequate to draw in/replenish the juice into the wick-in-coil / vaporization chamber. I see this frequently with clone KFL+ attys.

2. Wicking material that is overly compressed, and / or a tight fit in the coils (which does not appear to be the case in the photos) can restrict capillary action in general and more so to the wick-in-coil.
I take it this is cotton? You might consider rayon, as it's capillary action is usually superior to cotton.

3. Entirely possible, and to a greater degree as the tank liquid level drops... if you consider the liquid to function as a cooling medium, while a tank filled with air may function as insulation.

 

[TheJakeBailey]

Whoa. There's a lot of science going on in here. (slinks back out the door with the other idiots.)

 

[spawn]

Thank you. Just to clarify, each coil is 1.6 Ohm so that's two strands of 3.2 Ohm each twisted at 1mm pitch. What does that come out as in terms of HF?

I put it up on steam engine just so we can be on the same page:

Coil wrapping | Steam Engine | free vaping calculators

Now, the HF value I put in is 15W, as I am running two of these coils in parallel at 30W - so each is getting 15W, HF comes out to 83 which is too cool...

 

[spawn]

1. It's possible that the reverse is true, as regards air flow. If the greatest percentage of draw vacuum is pulling in fresh air, then the lesser percentage is inadequate to draw in/replenish the juice into the wick-in-coil / vaporization chamber. I see this frequently with clone KFL+ attys.

I think I may try a smaller diameter coil as it could be obstructing airflow as you can see in the pictures. I will try this build in the Goblin when I get it - that should answer the question around airflow?

2. Wicking material that is overly compressed, and / or a tight fit in the coils (which does not appear to be the case in the photos) can restrict capillary action in general and more so to the wick-in-coil.
I take it this is cotton? You might consider rayon, as it's capillary action is usually superior to cotton.

It's KGD, I may order some Rayon and try it, if I can improve wicking I should be able to increase the power.

3. Entirely possible, and to a greater degree as the tank liquid level drops... if you consider the liquid to function as a cooling medium, while a tank filled with air may function as insulation.

I suspect this is a limiting factor in RTAs.

Just wanted to state also that I am getting good flavor and vapor using this build but looking at the HF figures makes me suspect it's not entirely optimal.

 

[State O' Flux]

Ah ha!

So

• Wire gauge - 30
• Number of strands - 2
• Twist pitch - 1.3mm - adjusted slightly to fit the wire length of 107.4mm.
• Resistance - two 3.2Ω coils in parallel, for a net of 1.6Ω target resistance.
• Setup - dual parallel coils

HF at 15 watts - 83. At 30 watts - 167.

It appears we are in agreement now. ;-)

 

[cobaltxblue]

The issue could still be with compressed wicking since this is a Fogger (same as it could be with a Kayfun, Orchid, Billow or almost any other RTA). It might help to have a picture of the tails of the wicking or at least more info. Are you tucking the tails into the juice channels, if yes how thick, if no are the tails resting right at the top of the channels, are the flush cut or cut on an angle.

 

[State O' Flux]

Whoa. There's a lot of science going on in here. (slinks back out the door with the other idiots.)

You'll get into it sooner or later Jake. ;-)

Actually, this thread may tend to make it out as a lot more complicated than necessary, with spawn and I going back and forth... to get on the same page.

Although perhaps a lengthy read, especially as I recently broke it down into 2 parts... this article may improve your perception of using heat flux (as well as heat capacity and leg power loss) as a necessary element to your builds... give it a read when you can.

Edit: If reading isn't your thing, PM me a build sometime... and I'll tell you if all the adjustable elements are in happy harmony.

 

[spawn]

Here is a photo a how I wick it:



Think I'll try Rayon and see if it can keep up.

 

[spawn]

Ok, I got some rayon and after a few tries have got it wicking well. Getting the desired HF (35W). Problem solved. Thanks for the comments / input.

 

[Verb]

Nice thread. I learned some things. Happy to read, Rayon saved the build.

 

[catalinaflyer]

Whoa. There's a lot of science going on in here. (slinks back out the door with the other idiots.)

Phew, I thought I was the only one to back out of the classroom because I missed the last semester.

I thought I was getting a pretty good handle on vaping, regresses back to cartomizers in a tank for several months then stroll back into class with "yeah I got this" swagger only to find people talking about Heat Flux, tension wound coils etc. I was quickly picking my brain and thinking they were talking about something in a Delorean out of the movie Back to the Future but no, they are talking about vaping so I gave up, bought an Atlantis and Subtank and went back out in the parking lot to hang.