looking for TEC wisdom

Latent heat of fusion

George H.

If it were freezing water, you'd know--it isn't subtle when that starts happening, and you don't need any 1 cc worth to see it.

You being an old cryogenics guy, you know all the following, but just in case:

How are you getting the heat out of the baseplate?

Are the TEC wires on the hot side?

What's the thermal interface material?

Have you calculated the heat leak through the sleeve washers?

Is that black boa constrictor the temperature sensor cable? Seems pretty large.

Those are some seriously giant screws for a cold plate, even if they're SS. For screws that large, I'd definitely be using nylon or polyimide. (Nylon dies when it gets too dry.)

If none of those things is the problem, the TEC was probably already damaged.

Cheers

Phil Hobbs

How hot did the hot side get when you hit 0C on the cold side? You are always fighting against resistive heating to pump heat from the cold side to the hot side so you need to shift a fair amount in the heatsink.

I'd try again with thermal grease and a decent sized finned aluminium heat sink on your hot side with a small fan for good measure. You should be able to vary the temperature of the cold side by improving the heatsinking of the hot side. TEC's will quite happily freeze water out of the air if you have high humidity and ice forms on the cold side. I recall one muppet who left a cooled CCD camera on for 24 hours and came back the next night to find it useless with ice in all the wrong places. Took the technicians a while to put it right.

It's a 1/2" x 2'x2' Al optical bread board.. to room air.

Yup

(?) Aluminum with some thermal grease

Nope.

Grin, an in stock thing used elsewhere.

Giant? 2-56? OK this is my big screw up. I didn't calculate the resistance of my 2-56 SS screws. (area ~4mm^2, length ~12 mm) SS conductivity ~20 W/K-m gives ~150 thermal 'ohms' (K/W) per screw.

I'll try again, Thanks Phil.

Right ~8 watts. I only stuck a sensor on the hot plate after the TEC was ~1/2 broken... closer to 10 Watts, at which point the hot plate was 45 C.

OK I was hoping not to have to use a fan. Seems like I'll also need some insulation that keeps the water out too.

Thanks, George H.

I have an app where I want two containers of water, one at ~108*F and the other about ~ 38*F. I was thinking about two aluminum plates as the sides of the containers. with the TEC's sandwiched between the aluminum sheets. My concern, how well will the TEC's work when the hot side sees 108* and the cold side sees 38*F? Each container will have water circulated out (at individual times) Hot side will loose heat, cold side will gain heat. I realize I will have to add TEC's to one of the tanks additional to the ones that transfer heat from one to the other. (I don't know which one, possibly both) to keep the temps were I want them. Also some stirring within each tank. And insulation. Your thoughts? Mikek

Not doable with a single-stage TEC--their maximum delta-T is about 65 degre es C, and that's where the cooling power goes to zero.

Cheers

Phil Hobbs

Hmm. I specified *F which computes to 42.22*C and 3.33*C for a difference of 39*C. That's getting close to maximum delta-T. I need to figure out some capacities and heat loss/gain for each section and a flow rate. Thanks, Mikek

imes)

egrees C, and that's where the cooling power goes to zero.

A lot depends on the ambient temperature, and how well insulated the two wa ter tanks are.

Peltier junctions are much better at pumping heat into their hot side than they are at cooling their cold side.

If the main job of the junction is to keep the 42.3C tank warmer than ambie nt, you might be in with a chance.

My 1996 paper has a an expression for heat transfer (in Watts as a function of the amps flowing through the junction) as a function of temperature dif ference that you might find useful. The hot side dumps more watts than the cold side absorbs, for obvious reasons, since the hot side gets all the res istive heating going on in the junction, as well as the heat shifted by the Peltier effect.

Sloman A.W., Buggs P., Molloy J., and Stewart D. ?A microcontroller

-based driver to stabilise the temperature of an optical stage to 1mK in th e range 4C to 38C, using a Peltier heat pump and a thermistor sensor? ? Measurement Science and Technology, 7 1653-64 (1996)

E-mail me - snipped-for-privacy@ieee.org - and I'll send you a copy (for private st udy only, of course).

Ahh, that's a usable tidbit, that tells me I would need additional TECs on the cold side and addition heatsinking with fan to eliminate the heat removed. Thanks Mikek

Never mind, didn't read carefully. That 108 C water would have been fun. ;)

Cheers

Phil Hobbs

dual times)

;)

Under a bit of pressure.

says that you only have to push up the pressure to about 40% above atmosphe ric to get the boiling point over 108C.

l times)

degrees C, and that's where the cooling power goes to zero.

o water tanks are.

han they are at cooling their cold side.

mbient, you might be in with a chance.

tion of the amps flowing through the junction) as a function of temperature difference that you might find useful. The hot side dumps more watts than the cold side absorbs, for obvious reasons, since the hot side gets all the resistive heating going on in the junction, as well as the heat shifted by the Peltier effect.

Actually, your original approach to the problem was wrong.

You need to think about two temperature controllers, one holding some water at 3.3 degrees Celcius, and another holding another lot of water at 42.2 d egrees Celcius.

Depending on the ambient temperature, you probably are going to have to pum p heat out of the cooler pot of water, and pump heat into the hotter pot.

If you take water out of either, and replace that water with water that sta rts off at ambient temperature, that's a further complication (and calls fo r a more heat-shifting capacity if you want to move much water).

If the two pots of water are physically close, you will have a deal with a fixed and constant heat flow from one to the other, but that won't be the i nteresting bit of the job.