Heating air in a wind tunnel.

It would also save a lot of energy by taking advantage of the considerable momentum of the moving air. You can add a heat-exchanger heat-cool section to regulate temperature.

One important issue, to what degree will the air flow be laminar vs turbulent? That makes a big difference in how effect the air flow is for heat-sink cooling. You can creatively modify the air path for one state or the other.

This from my COOL.TXT cheat-sheet file...

==========

If moving air removes heat from a volume, the exhaust air becomes hotter than the intake air.

The forced-air cooling equation is...

P = 169 * Qa * (T2/T1 -1)

P = power, watts Qa = air flow, CFM (cubic feet per minute) T2 = outlet temp, K (absolute temperature, Kelvins) T1 = inlet temp, K

For 1 deg C rise at 1 CFM, we get...

P = 169 * 1 * (301/300 - 1) = 0.5633 w

So, near room temp, an air flow of 1 CFM will be heated 1.775 deg c/watt.

Since 1 cubic foot = 28.31 liters, for Qm in LPM the equation becomes

P = 5.69 * Qm * (T2/T1 -1)

So one LPM has a heatsink capacity of 50.25 degC/watt

===============

So, 250 lpm, 20 to 70 degrees C rise, takes

P = 5.69 * 250 * ( (273+70)/(273+20) -1)

= 260 watts,

I think.

Check the CRC Handbook of Chemistry and Physics for the thermal properties of air.

Will the air recirculate and be reused, or will it be a pass through system?

But you need to design the tunnel to take advantage of the recirculation

-- badly designed recirculation is worse than open flow.

Google "wind tunnel turning vane" for help. It's really important for any efficiency at all.

John Perry

Good idea. The heat capacity will depend on moisture content as well, so the OP will have to figure what the ambient conditions are likely to be.

If the system recirculates the air, then the power required to heat the air will depend on its specific heat, the desired rate of temperature rise and the heat loss across the ductwork insulation.

Theoretically, with perfect insulation (and sufficient patience) 70°C can be attained with a very small power input.

I have a small project that requires an air flow of approximately

Any assistance would be gratefully received.

Hi John,

Thanks for that reference.

At present the system is still only on paper so we can do either. It is a one-off test rig for mobile phone base station modules and the customer has asked that the 'hot air' flow.

I have already considered recirculating the air as this would save a lot of heat. Obviously, all of the ducting would need to be lagged from a safety standpoint, but this would mean the air would not cool down. The units will generate a lot of heat too so once the air is at 70C in a recirculating system, it would need some cooling.

It would be simpler, but extremely eco-unfriendly, to have a straight through system. It just might need a lot of heat.

Thanks to John P, I found the following data (my figures are approximations):

Density of air at 70C 1kg/m3 Constant Pressure specific heat 1000J/kg.K

CPSH = 1000W.s/kg.K = 1W.s/l.K = 0.0167W.min/l.K

So for 250l/min and 50C rise in temperature

Pretty close.

Thanks to all for your help.

Speaking of airflow, I'm wondering how many watts it would take to heat a 1 L/min flow of air through a 1 cm diameter hole, to 155 C? Ambient can be anywhere from ~72F (~22C) to ~85F (~29C).

Thanks, Rich [ And in case you're wondering, yes, it's for a THC evaporator. %-}

. .

My previous calculations should apply. I don't think the hole diameter should matter except for the fact that you may get turbulence at the exit.

Density of air at 70C 1kg/m3 Constant Pressure specific heat 1000J/kg.K

CPSH = 1000W.s/kg.K = 1W.s/l.K = 0.0167W.min/l.K

Assume the worst case ambient of 22C the required rise in temperature is

The input power required depends on the efficiency of the heater. I'm planning on using one of these 'drainpipe' style heatsinks with high wattage resistors bolted to the outside.

Rich Grise, Plainclothes Hippie skrev:

Use the resistance of the heater to measure its temperature, long time ago when I was in school I build temperature controlled soldering iron that worked like that. If I dig really deep I might be able to find a schematic

-Lasse

Rich Grise, Plainclothes Hippie skrev:

Use the resistance of the heater wire to measure its temperature. long time ago in school I build a temperature controlled soldering iron that worked like that, if I dig really deep I might be able to find a schematic.

-Lasse

Hey, thanks for this! I have some nichrome wire, that I think would be perfect for this - and I can get some ceramic or maybe sculpt something out of clay, and see if there's such a thing as an adjustable aquarium pump, and I'll have an electrical THC evaporator. :-P

But if I want to temperature-control it, I have noticed that thermistors kind of peter out at 150C or so, so I guess I'd have to go thermocouple, right?

Thanks! Rich

Would prolly work better with nickel wire than with nichrome, wot with the tempco and all.

The basic (vice) principle is to drive the excitation on a Wheatstone bridge with the error so that the heater balances against a reference resistor (of course you'd want to make the side of the bridge with the reference resistor much higher in resistance; it's the ratio that counts). You also will want to keep the excitation on the correct side of naught, eh?

Best regards, Spehro Pefhany

Well, 12 bits and SPI is a little bit overkill - I was thinking in terms of a thermostat, like a comparator. I'll have to look up thermocouple circuits. In fact, I remember seeing something in some data book years ago - I'm sure I'll dredge up something! :-)

Thanks! Rich

When I was in the USAF one of the transmitters I worked on (I was a tech) had a "magnetron filament control loop" that did exactly that to control the cathode temperature. It's basically a bridge. I'd have to look up the tempco of nichrome, but that is another option. :-)

Thanks! Rich

For your purposes, an LM324 with a 1N4148 for compensation will work well enough. Get one of the guys at work to weld a ~AWG30 bead K T/C or buy one.

Best regards, Spehro Pefhany

Excellent idea! I'll have to scrounge up some thermocouple wire, or maybe bite the bullet. McMaster-Carr is just down the street. :-) I was looking at ceramic tube stock to make my little heater with, but they have alumina that you can only cut with a diamond wheel.

But hell, it's for a pot pipe - I could probably just snap a piece, and it'd come out better than what I could sculpt. ;-D

Thanks! Rich