Steam Engine: Basic & advanced features: Pt. Two

Hello Mr. Flux, first a love letter: Nicely done!

Now an inquiry: You don't happen to know how Heat Capacities are calculated by steam-engine, do you? I was switching up builds today, going from 24ga to parallel 28ga, and looking over the numbers. As expected, Surface Area and Heat Flux are inversely proportional for a fixed wattage.

I'm curious if there's a nice relationship between Heat Capacity and certain values - I see it increases as length increases, and as I play around just now, it appears to be vary solely on the length of the wire - two coils weighing (approximately) 123mg will both have (approximately) a heat capacity of 56.6 units, no matter how the other variables change (wire gauge included).

readeuler;bt12758 said:

Hello Mr. Flux, first a love letter: Nicely done!

Now an inquiry: You don't happen to know how Heat Capacities are calculated by steam-engine, do you? I was switching up builds today, going from 24ga to parallel 28ga, and looking over the numbers. As expected, Surface Area and Heat Flux are inversely proportional for a fixed wattage.

I'm curious if there's a nice relationship between Heat Capacity and certain values - I see it increases as length increases, and as I play around just now, it appears to be vary solely on the length of the wire - two coils weighing (approximately) 123mg will both have (approximately) a heat capacity of 56.6 units, no matter how the other variables change (wire gauge included).

Click to expand...

Thanks very much boss.

I do not. I've looked high and low for the exact reference to the mJ K-1 value (I imagine the mJ is millijoule, but the second part, I don't know if it's Kelvin, or what the upper '-1' references) but I've not had any success.

Funny thing is, I've had a few conversations with Lars recently, and completely forgot about asking him to point me at the source material.

I see HC as something not quite as critical as heat flux, where a particular "temperature range" is (albeit somewhat subjectively) desirable for an individual... but instead, more a comparator value, in the sense that you are looking for the desirable HF first... with a universally desirable, low HC value second.

Regarding your last comment... I see HC (and HF) adjust with the variables of gauge, net resistance and parallel coil count... with wattage being the only variable that does not affect HC. Or am I missing your observation?

Next time I talk to Lars, I'll get the specs... 'cause it really bugs the hell out of me that I don't know.

Well, I'm more a math than physics guy, but Wikipedia tells me that HC is [change in Heat / change in Temperature]. I'm not 100% sure that we can measure heat in Joules (or, here, mJ), but that gets along with the K, for degrees Kelvin, with a subscript of -1 since we're dividing by a temperature (change); 1/K = K^(-1).

I guess looking at HC, I mean this: It seems to be that it is solely dependent on the mass of the wire. Anything affecting the mass (like resistance, wire gauge, # of wraps, etc) will affect the HC: more mass, more HC. If I look at any build on SE, I'm only able to change HC if I do something that affects the mass of the coil (wrap spacing won't, because we're keeping a fixed resistance there, for example). Upping the resistance will up the HC, since we're using more wire.

I know that water has a very high heat capacity; we've got to add a lot of energy in order to heat up water. My point being that I imagine Kanthal/Nichrome/Nickel all have some kind of normalized per-unit-mass HC value. I think Wikipedia is saying this:

Heat capacity is an extensive property of matter, meaning it is proportional to the size of the system. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass, or volume, so that the quantity is independent of the size or extent of the sample.

Click to expand...

It then says that Specific Heat is a name for that normalized per-unit-mass measure. So if you know the Specific Heat for Kanthal, and how massive your coil is, you've got the HC from multiplication.

I'm still amazed that Lars found some way of tying all this stuff together into a tidy little calculator. Finding values for the Specific Heat of Kanthal isn't hard, but it's dependent on temperature, and thus to calculate HC my way, you'd need to estimate the coil temp! Further, that he was able to use all the collected knowledge about Kanthal etc., to give us numbers that are physically significant to our builds. Of course more Kanthal takes longer to heat up, but now we can say exactly how so!

I agree about how much attention to pay to HC; it seems pretty safe to say that HC should ideally be low, and that it shouldn't be your number 1 priority in a build. But it probably plays a part in questions like "why does it take 3 seconds for my dual 24ga coils to heat up?" I'd say low HF is okay, as long as HC is correspondingly low as well.

I hope you don't mind me using your blog as an opportunity to learn physics through vaping

Oh, just for fun: Here's the link to my latest build that got me wondering about all of this. If we look up the Specific Heat Capacity of Kanthal at 20 Centigrade, it's 0.46 kJ/(kg K). My coils linked above weigh 77.057 mg. If we do the multiplication:

0.46*77.057 = 35.446, the HC value in steam-engine. You can verify for yourself why all the factors associated with SI-prefixes go away; it took me a good 5 minutes to convince myself! As a final fun footnote, the Specific Heat Capacity goes up as temperature goes up; at 200C it's about 20% more; this is physics' way of telling us that it gets harder to heat something up, the hotter it is!

readeuler said:

I hope you don't mind me using your blog as an opportunity to learn physics through vaping

Click to expand...

Not at all, 'P'... quite the contrary. We're on the same page for both a basic explanation of just what HC is, how it applies, as well as it's value when compared to other variables.

As this all started with the question: "so, what the hell does mJ K^-1 mean in the real world?"... being able to share that curiosity, along with the desire to translate that question into terms and examples that most can understand, well... it's always good to have a second (and bigger) enthusiastic brain working at a solution. ;-)

Your comments, conclusions and calculations, both here and in private communications over the past few days, have provided me with a rational, detailed secondary perspective as well as a valuable, "on the same page" validation... I appreciate you efforts very much.

Cheers!