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RyanS

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1. Heat transfer theory

Interesting. So the heater I linked must be relying on a really low air flow rate? http://www.hmc-products.com/fotos/image/VARIA/Varia/0021-7000a/MANUAL%20USER/0021-8001a-0102a.pdf being a 40mm fan at low speed. I actually have it running at the moment and about 8cm away you can barely feel the flow. it does generate a bit of heat though. Accross the face of the fan are about 6 power resistors. Could it be possible that low flow combined with power resistors (rather than nichrome wire), allowed them to keep the power consumption as low as 34 watts? although I can't see whether the source is resistors or resistance wire is going to make a difference. Is one more efficient?
2. Heat transfer theory

Thanks. I'm not sure if I'm applying this formula correctly but using this fan as an example: http://www.gelidsolutions.com/products/index.php?lid=2&cid=5&id=28&tab=2 Flow rate is 27 CFM == 12,740 cm3/s minimum room temperature is usually 20C I want to raise by 35C to get 55C So 12,740 cm3/s X 0.00121J/cm3 = 15.4W per 1C X 35C = 540 watts!! so I need 0.27 ohm heating element for 12V. that's not going to work. The only sensible result I can get is drive the fan at a very low speed for 1000cm3/s to get 42 watts, but I don't even know if you can get the fan that slow. (900 RPM is the specified minimum, but flow rate at that speed, who knows). I was basically looking only to heat a small area around my computer mouse, at the moment I use a 25 watt halogen lamp which is a pain because bulbs keep blowing. Ideally I was hoping to design for 40 watts, maximum. similar to the 40mm fan heater I linked in my OP, just 80mm for a wider area.
3. Heat transfer theory

Hi, I'm new to the forum, and I'm a beginner into electronics. I'm trying to design a small fan heater using an 80mm PC fan and a nichrome wire heating element. It will be used as a hand warmer for bad circulation. I have a similar device which uses a 40mm fan, a commercial product that is very expensive (read: overpriced). but I want to build a second one that is a little larger. http://www.hmc-products.com/fotos/image/VARIA/Varia/0021-7000a/MANUAL%20USER/0021-8001a-0102a.pdf its 24V and 33 watts So far I'm thinking of this design: - Arduino Duemillnove or Nano (AT Mega328) to generate PWM, and make it PC controllable via USB - power transistors or MOSFETS to handle the heating element and fan currents - the 40mm fan uses resistors as the heat source, but I think using nichrome wire would allow more even coverage in front of the face or the 80mm fan. - 12V as the power source (Arduino is separate and USB powered), because the fan is 12V then I don't need to convert from 24-12V -it should be able to produce 55 degrees Celcius air temperature, 20mm away from the element Heres a PDF of the hardware layout: http://clients.rasvisual.com/temp/80mm_heater.pdf QUESTIONS: - is the distance of the nichrome element of 15mm away from the fan at the temperature I require likely to cause any problems for the fan? although I imagine all heat is directed away from the fan. - I will have an enclosure around the whole device, with air inlet holes around the top and side of the PCB mounting area. Is this sufficient to draw cool air over the heatsunk transistors to keep them from overheating? - FINALLY; and in relation to my topic of Heat transfer theory, I'm sure there is a formula to work out the required wattage of heating element to produce the desired temperature at 20mm (55degC), but I'm having trouble locating it or what to search for. Best I can do is that a fan heater is technically a forced convection heat transfer device, I'm looking at laminar flow across the element and forced air has a heat transfer coefficient or btw 10 - 100(W/m2K I just have no idea how to derive the required wattage. If anyone is familiar with these formulas I would greatly appreciate their advice. Then I can move on and choose the appropriate transistors, etc.
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