Peltier Guidance

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mbaker33

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Hi all,

I've been mulling over a fermentation chamber (basically the one on Wort-O-Matic) for a while now and the ice situation always hangs me up. I don't have the space fore a chest freezer to freeze milk jugs or anything like that and my actual fridge freezer is generally pretty packed already with general food items. So, the typical fermentation chamber doesn't really fit.

Enter the peltier, which I've been reading mixed reviews about all over the place. It seems where most people fail is when trying to attach it directly two something other than a heat-sink with a fan blowing the cool air into the chamber. I'd like to hear from anyone who has had success with these devices on sizing and design. I'd like to be able to heat and cool, and I have an Arduino with thermisters and all that. So, really, I just need the design/peltier specific plans to get my wheels off the ground.

Thanks in advance.

Mark
 
hi, i make precision temperature controlled enclosures for a living. we use TEC modules all the time. the thing that you have to keep in mind, is that you have to dump a lot of heat. basically you have to size your heat sink to be large enough to dump the heat you want to loose from your wort, and the heat from the electricity that you are pumping into your modules. if you don't do that, your module will quickly saturate (reach the maximum temperature differential between the 2 sides), now you have a heater instead of a chiller.

hope that helps. feel fee to get detailed.
 
Thanks Warthog, I am aware of the heatsink concerns for sure. And I understand that's 90% of the failures. Where I struggle is making sure I don't add myself to that percentage. :)

Do you have any recomendations for a TEC controller/Heatsinks that will drop a chamber of roughly 24"x24"x15" to 20* below the ambient temperature? I won't need to get that low all the time, but when I want to, I'd like to be able to.

I'd also like to be able to heat using the peltier, which I understand is possible just by flipping the polarity on the voltage.

Thanks again for your input. It sounds like you know WAY more about these devices than I do for sure. I just know that the *should* work for what I want. :)

Mark:rockin:
 
at this point there's some real math involved to do it right. but here's enough to select some heatsinks (your next step).
first know your heat load: that's the sum of everything that makes heat. your fermenting beverage, heat gain through the walls of your enclosure, and heat from your circulating fan (to blow all that cool you are making around). from this discussion here, your wort is giving off 20w, assuming your box is made of 1" thick blue poly foam (R6/inch), with a 20 f dT, then that's about 20w too. by coincidence, a good size of circulating fan is about 20w also. look for a heat sink that has the lowest thermal resistance you can find. it will be given in deg c/w (remember to add resistance for the interface between your TEC module and the back of the heatsink) once you have this, you will know how cold your cold side heatsink needs to be (it will be quite cold). from there, use the equation: (watts*1.76)/dT. this gives you the number of cubic feet per minute of air you need to move inside the box (find a fan that will do that, re-do the math, using the power of the real fan you will use. then look here and find a TEC module with enough umpf to do the job.
next: multiply the volts and amps of your power supply to give you watts (do not exceed vmax or imax), then add this to the cooling watts (60) to get how many watts you need to ditch. select the heat sink for your hot side (remember, that the hot side cannot get warmer than the dT number - your cold side temperature). now all you need to do is figure out how much air you need to move over the hot side heatsink, to keep the cold side cold enough to do the job (use the same formula from above). easy!


good luck. i can crunch some numbers for you if you need it
 
Would you need a fan in a box that small or could the TEC be mounted at the top and rely on the fact that cold air will sink?

I was looking into a TEC a while ago and the big sell for me is that I could switch from cooling to heating by just flipping the leads. The biggest downside that I saw was the inefficiencies of a TEC would waste too much power for me since I was planning on using a 7cf chest freezer...a small single bucket ferm chamber should be manageable.
 
it might work, but i wouldn't count on it. the heatsink will loose a lot of efficiency when air is not really moving over it. the more air the better your efficiency.
 
Hey warthog, some great info here, thanks. I have a few simple questions, but I figure I'll research it 1st to not be that annoying guy. But from your experience, has the peltier tech/cost made it a better solution for small chilling loads we might encounter as homebrewers?
 
you know, i actually haven't looked into it. single modules are pretty cheap, but add a power supply, heatsinks and fans, it all adds up. when i build these things they are used in semiconductor fabs, and nanotech labs, where conventional refrigerants are a big no no, i typically make then very big too, the last one i built had a capacity of 24,000w, and controlled to +/- 0.005c, it also cost almost $100k (whole thing, enclosure, chiller, control system etc). but i would think for small loads, less than say a few hundred watts, its probably still cheaper than buying an lg air conditioner at home depot.
 
at this point there's some real math involved to do it right. but here's enough to select some heatsinks (your next step).
first know your heat load: that's the sum of everything that makes heat. your fermenting beverage, heat gain through the walls of your enclosure, and heat from your circulating fan (to blow all that cool you are making around). from this discussion here, your wort is giving off 20w, assuming your box is made of 1" thick blue poly foam (R6/inch), with a 20 f dT, then that's about 20w too. by coincidence, a good size of circulating fan is about 20w also. look for a heat sink that has the lowest thermal resistance you can find. it will be given in deg c/w (remember to add resistance for the interface between your TEC module and the back of the heatsink) once you have this, you will know how cold your cold side heatsink needs to be (it will be quite cold). from there, use the equation: (watts*1.76)/dT. this gives you the number of cubic feet per minute of air you need to move inside the box (find a fan that will do that, re-do the math, using the power of the real fan you will use. then look here and find a TEC module with enough umpf to do the job.
next: multiply the volts and amps of your power supply to give you watts (do not exceed vmax or imax), then add this to the cooling watts (60) to get how many watts you need to ditch. select the heat sink for your hot side (remember, that the hot side cannot get warmer than the dT number - your cold side temperature). now all you need to do is figure out how much air you need to move over the hot side heatsink, to keep the cold side cold enough to do the job (use the same formula from above). easy!


good luck. i can crunch some numbers for you if you need it

Holy crap, that is some amazing information. I understand the principles you discuss, but really don't know how to apply them. If you could give me a rough idea of what I'm looking for, that would make my day.

I'm actually using 2" foamboard and was going to use either a large PC CPU fan or desk fan to force the air around. I'm open to whatever is recomended, but I can't break the bank on it.

Thanks again for all your help!

Mark
 
your 2" foam board will cut the external influence by about 1/2. so you will have 30w plus your fan. somewhere on the fan you plan on using will be something giving its watts (it may be in the form of volts and amps, just multiply them together to get watts). the other thing you will need to know is how much air the fan moves around. sometimes that's written on the fan, sometimes, you have to do an internet search. if you can get me that information, i can point you to a good heatsink. i don't really have time to design it for you, but i don't mind giving you a hand.
 
Thanks WartHog, that's a good starting point. I'll do some digging over the next couple of days and see what I can find. I don't expect you to design it for me, I'm sorry if that was implied.
 
WartHog, I'm sorry to be a pest here, but I did a quick search and found a CPU fan with a fairly high CFM that I would like to use. If it won't work, let me know, if I'm headed down the wrong path, let me know.

the CFM is 78.41 CFM and the Input Wattage is 3.96.

http://www.newegg.com/Product/Product.aspx?Item=N82E16835119127

Would this work for both sides of the peltier? Or is it typical to use a smaller fan on the inside and a larger one to vent the heat?

Thanks,

Mark
 
that fan will definitely work inside (if you can get at least 50% of that air to flow over the heatsink. you will need to get your heatsink fins 2c (3.6f) below the temperature you want. so if you want 13c interior (~55f) then you need to cool your heatsink fins to 11c (51.4f). this seems pretty do-able to me.

now find yourself a heatsink. modules are typically 40mmx40mm, (1.54" sq) so find something that will fit on that. lots of long thin fins are best. remember thermal grease will add resistance too, use a silver filled kind ($$$, but you won't need a lot - as a matter of fact, you want as thin a coat as possible).

oh, forgot about the outside. too early to tell, but typically you need to move a boatload more air, or use a massively larger heatsink for the hotside.
 
Alright WartHog, I'm thinking something like this on the inside:
https://www.sparkfun.com/products/10686

I'm still looking for something that will fit that 120mm fan and still be 40mm square for the hot side. It's harder than I thought it would be, mostly because most heat sinks I find in that size range don't have a measurement of the base because they are meant for CPU's.
 
the selecting of the right coldside/tec module/hotside/fan combination is an iterative process. here's a first pass.

the hs/fan combo is a pretty little thing. there is some information but not the thermal resistance. its not particularly easy to figure out the thermal resistance of a heatsink. you might find something similar here, or maybe you can call sparkfun, and ask if they have a curve for that assembly. just for the sake of making progress, i'll make a guess. let's say that heatsink has a thermal resistance of 0.3 c/w at the air speed from your fan (it'll be close to that i think)

- a quick note here: you may notice that the watts/cfm calculation involves the volume of air, and the heatsink curves show linear airspeed. you need to take the volume of air from your fan that crosses the fins, and divide that by the cross section of the open area, and that will give you the airspeed ((ft^3/min)/ft^2=ft/min).

so you want to dump 30w. we got from before a theoretical best case of 2c drop. 0.3c/w is not great (you may be stuck with it though). anyway, that means you will need to get the base of the heatsink 10c lower than the desired temperature to dump your 30 watts, but we know from earlier, that the fan supplied will be up to the task.

time to look at the module, and the hotside.

something to know about peltier modules. their rated power is for when the cold side and the hot side are the same temperature. they get worse from there. it is close to impossible to make this happen, you want to keep dT as small as possible (TECs all have curves so that you can find one with the right watts at the dT you will have).

time to put some numbers to this: if you want 55f (13c) in your box, then your module needs to get down to 37f (3c). ideally you would want the hotside to also be 3c, but that ain't gonna happen. but we can assume that we can cool the hotside with ambient air, to 33c. that's a dT across the tec of 30c. spending a little bit of time on the custom thermoelectric website i found this little guy ($46.50 ea). if you look at the curve for dT 30c, you can see it reaches 30w at about 3.25a. and looking over to the other curve, 12v at 3.25a (39w) gives you 30c dT, so that all works out.

cool

now we want our hotside to be at 33c or 9c above ambient (i'm assuming a 75f - 24c -ambient temperature). so we add the load 30w and the electrical power 39w and get 69w. our handy dandy airflow calculation tells us we need at least 13.5 cfm to do this. so far its sounding pretty good.

let's see if that heatsink will work on the hotside too, that would be pretty convenient. so just like on the coldside, the greater the dT the more watts it can transfer to the air. at 0.3c/w that means the heatsink should be 44.7c to extract the full 69w. so that's higher than our 33c needed for our tec. that makes for a 41.7 dT across the tec. so no, that sink won't work (the selected tec barely has 30w at 40c dT, but it draws a lot more power, upping the amount of heat you need to dump on the hotside now you have to dump 174w). now your temperature rise on the hs will be 52.2c (hs temp of 76.2c - giving a 73.2 dT on the tec, which will saturate the module, and you now have a heater.) your only option here is to pick a different heatsink for the hotside, or both the hot and coldsides.

a better heatsink on the coldside will mean warmer temperatures on the coldside of the tec, lowering dT across the tec, lowering the watts needed to cool to the desired temperature, thus lowering the amount of waste heat etc, etc.

i think what happens to most people who try to do this, is they end up just winging it, and as you see, all this stuff is inter-related, and winging it doesn't cut the hops.

there you go, have fun.
 
WartHog, thanks again for your brilliance in that post.

I found a giant heatsink that I think I could get 2x 60mm fans installed on for the hot side. Finding the numbers you are referring to has proven to be almost impossible short of going the custom route. It really seems like you know what you are doing, so I'm sure you know where to look for them, but I just can't find them.

Thanks again for your guidance on this. I feel like I'm at least not stabbing in the dark anymore, it's more like stabbing at dusk, just before the sun is finally set. :)

Cheers :mug:
 
i've been doing this for the last 6 years, so i've had a lot of practice. at work, we always go with custom heatsinks, look at aavid, they've got all the engineering data there. i think custom thermoelectric also sells sinks, ferrotec, also sells these things, as well as melcor/laird. i don't think you will find serious engineering specs on any of the hobby distributor's sites
 
Thinking of experimenting w/ tec peltiers attached to a corney keg for laggering beer powered by a pc power supply. It sounds like the hot side heat sink requirements are significant and a passive CPU heatsink might not be adequate.

Good info in this thread.
 
the biggest challenge with using TECs is getting the waste heat away. if you are just trying to make it cooler, its not too hard, if you want to remove a specific amount of heat, then you have to do some thinking.
 
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