Bugs and Calculations

[Grench]

IFF you're going to try something like that, may I recommend having it mounted high in the non-wired hinge side void (where Pandora has light pipes) and blowing -in- such that the SD card slots become the exit vent?

If the fan pushes -in- 1 mole of 21*C air and the air picks up enough energy to move to 100*C, a considerably larger volume of air will exit than enter.

1 mole of air weighs ~28.8 grams.
1 mole of air at 1 atmosphere and 21*C is ~24 liters (volume per mole in).
1 mole of air at SLC is ~24.5 liters.
1 mole of air at 1 atmosphere and 100*C is ~30.6 liters (volume per mole out).

Air Specific heat = 1.01J/gC

Changing air from 21*C to 100*C for 1g of air = (1*79)/1.01 = 78.2 Joules transferred per gram of air in.

Assuming the system needs to shed 3 Watts per hour, 3 Watt-hours ~ 10800 Joules.

10800/78.2 = 138.1 g of air per hour.
138.1 g ~ 4.79 moles ~ 115 liters per hour ~ 2 liters per minute in.

It wouldn't have to blow all that much air in to be effective. If you're going to consider this, be sure to have someone else run the numbers. What I did above is very back of the envelope and doesn't take into account any of the other methods for the heat to get out.

Boiling point of water won't melt the solder on the chips. I considered the above a 'worst case' scenario.

Yes, I am using * as a degree marker since the US QWERTY keyboard I'm on does not have the little degree circle symbol and I'm not interested in looking it up to type in by code a dozen times.

OK, it took me a bit to find it, but it looks like the max temp on the OMAP 5 is 85*C. Assuming good mixing and relatively slow air, the air exiting then could be around 75*C. Yes, that is still hot. No, I'm not overly concerned - and I wasn't concerned at 100*C. This is simply a math exercise.

Changing air from 21*C to 75*C for 1g of air = (1*64)/1.01 = 63.36 Joules transferred per gram of air in.
10800/63.36 = 170.44 g of air per hour.
170.44 g ~ 5.9 moles ~ 142 liters per hour ~ 2.37 liters per minute in.

For perspective, humans at rest inhale about 6-8 liters per minute, roughly 400-500ml per breath, 15 breaths per minute (then exhale a bit more as it warms and adds CO2).

All better?

 

[rygD]

I didn't know most humans were such shallow breathers. I probably am too when I am not paying attention to it.

I figured the 100 degrees was just an illustrative round number...

 

[Djoga'Ro]

Don't know. What CPU temperatures are people fine with and why?
I don't know what working under hightened temperatures does to a CPU - meaning when, how or why it gets bad.

I guess to get the air's temp to just 10 °K below the CPU you need some channeling through a fine ripped heat exchanger. Or just pump more and have the air coming out be somewhat cooler.

 

[Grench]

I didn't know most humans were such shallow breathers. I probably am too when I am not paying attention to it.

I figured the 100 degrees was just an illustrative round number...

That's pretty much it. I figured, "What number would be easy to work with and pretty bad if it got there?" No worries - fixed it.

Don't know. What CPU temperatures are people fine with and why?
I don't know what working under hightened temperatures does to a CPU - meaning when, how or why it gets bad.

I guess to get the air's temp to just 10 °K below the CPU you need some channeling through a fine ripped heat exchanger. Or just pump more and have the air coming out be somewhat cooler.

Considering that the original, and as far as I know, ongoing, plan is to have the unit dissipate heat naturally through the case with no provisions for a fan or external radiator, the above may not even be necessary. BUT - the question was implied by ED's statement that the fan would be noisy. So, I started thinking, which is usually a mistake, that the air flow and thus fan power needed to dissipate 3W of energy wouldn't really be that much - but how much would it really be? So I worked it out.

Someone who knows thermodynamics better than I do please check the numbers if anyone is going to take these results seriously.

But, as far as I can tell, the Pyra could be adequately cooled with a one way valve and a straw connected to the user's mouth. Step one to creation of the Borg has been found.

 

[Djoga'Ro]

I'm not bashing, just adding.

But, as far as I can tell, the Pyra could be adequately cooled with a one way valve and a straw connected to the user's mouth. Step one to creation of the Borg has been found.

That user should have quite dry lungs then to avoid short circuits.
EDIT: The whole device being wamer than the users breathe should do the trick to, of course.

 

[Klumpen]

I'm not bashing, just adding.

That user should have quite dry lungs then to avoid short circuits.

Water traps:
https://kiltsandmore.com/bagpipes-accessories/water-traps-moisture-control--:c222:len:1.html

 

[Djoga'Ro]

Water traps:
https://kiltsandmore.com/bagpipes-accessories/water-traps-moisture-control--:c222:len:1.html

Yeah, add a condenser before that and you're golden.

 

[Grench]

Or simply have the user inhale through the Pyra so the Pyra gets cold air. If a smoker's pipe can cool the burning embers enough to not burn the lungs, then the tube length from the Pyra to the user should be adequate to... time for a few disclaimers.

Note - the above is in jest and should not be attempted by anyone. The 'cooling through human inhalation' is noted through the coincidence that human respiration air exchange rate is in a similar range of magnitude and should be adequate to cool the Pyra.

No implication that the Pyra itself needs any additional cooling beyond whatever ED has in mind should be implied. This was a math exercise only in purpose to figure out how much air flow really was required to move 3Wh of heat.

It turns out that 3Wh isn't all that much heat and would take a very long time to heat up a cup of noodles. Someone else's turn - how long would it take for a 3Wh power source to heat a 250ml serving of Ramen type noodles to boiling? Assume an enclosed styrofoam container and 21*C ambient temperature with water starting at ambient with a lossless heater.

 

[Djoga'Ro]

Or simply have the user inhale through the Pyra so the Pyra gets cold air. If a smoker's pipe can cool the burning embers enough to not burn the lungs, then the tube length from the Pyra to the user should be adequate to...

You should advise the user to not suck too hard.

It turns out that 3Wh isn't all that much heat and would take a very long time to heat up a cup of noodles. Someone else's turn - how long would it take for a 3Wh power source to heat a 250ml serving of Ramen type noodles to boiling? Assume an enclosed styrofoam container and 21*C ambient temperature with water starting at ambient with a lossless heater.

What's the specific heat capacity of such a serving again? Nevermind, I'm too lazy anyway.

 

[Pocak]

It turns out that 3Wh isn't all that much heat and would take a very long time to heat up a cup of noodles. Someone else's turn - how long would it take for a 3Wh power source to heat a 250ml serving of Ramen type noodles to boiling? Assume an enclosed styrofoam container and 21*C ambient temperature with water starting at ambient with a lossless heater.

Let me try. Assuming 250ml of water plus 75 grams of noodles of 1700 J / Kg°C specific heat, and ignoring spices and the cup (those are only a few grams anyway), I get 25.6Wh of energy needed to heat it from 21°C to 100°C. Interestingly, that's in the same ballpark as the Pyra's battery capacity.
If the cup is thermally isolated, it would take 8.5 hours to bring to boil using 3W. (You mean watts, right? As in power. Not watt-hours, which is energy.) But in practice, with air taking heat away at the same time, it would never reach boiling temperature, and you would just end up with warm, soggy, raw noodles, and bacteria competing with molds to digest it.

 

[FBnil]

I have succesfully, in the past, put a wet tissue just above the Pandora left Nub (where the heat concentrates)...water evaporation powered cooling.

http://www.timeanddate.com/countdown/generic?iso=20160501T08&p0=37&msg=Pyra+Preorders&font=cursive#

 

[Grench]

Let me try. Assuming 250ml of water plus 75 grams of noodles of 1700 J / Kg°C specific heat, and ignoring spices and the cup (those are only a few grams anyway), I get 25.6Wh of energy needed to heat it from 21°C to 100°C. Interestingly, that's in the same ballpark as the Pyra's battery capacity.
If the cup is thermally isolated, it would take 8.5 hours to bring to boil using 3W. (You mean watts, right? As in power. Not watt-hours, which is energy.) But in practice, with air taking heat away at the same time, it would never reach boiling temperature, and you would just end up with warm, soggy, raw noodles, and bacteria competing with molds to digest it.

That fits the mental math of it. 3W isn't much.