Hello all,
If I need an Al Block to heat up and I need the least possible dimensions but I need to increase its temperature from ambient temp(20 degree) to 60 degree. how many watts I need and what the dimensions should be? I do not know from which point I should start.
Thanks
You need to start with more specific information.
What parameters are fixed besides the material of the block? Or at least, what are their reasonable ranges?
Some things you haven't specified:
Is the block supposed to remain at 60 (C? F?) for as long as heat is being applied, or is there a temperature controller?
Check wirewound or power resistors. They come already sealed in an aluminium block. Problem solved.
Materials have a thermal resistance which is simply stated as watts per degree. For example if your block has a thermal resistance of 1 watt per degree, then 40 watts input will raise the temperature 40 degrees over ambient. As you can see there are watts, degrees and thermal resistance so any one of those quantities can be juggled to get the desired results. Thermal resistance will depend on several things such as the size and shape. It would be much easier to take a block of suitable dimmensions, dump power into it and measure the temperature rise.
Do you have a practical application you're trying to work out?
Start as simply as possible. Al has a specific heat capacity of 0.9 joules per gram. If your aluminum weighs 1 gram it will take 0.9 joules to raise the temp 1 degree C. To raise the temp 40 degrees then you need 40*0.9 = 36 Joules
A Watt is a Joule per second. So it will take 36 seconds at the rate of 1 watt to heat your one gram by 40 degrees using one watt of power.
If you want to heat 1 gram in 3.6 seconds then you will need 10 watts.
That's a good starting point. However, it assumes that there's no loss. In practice, there will be loss of heat from conduction, convection, radiation and imperfect transfer from the heat source. The extent of the loss depends on the environment. The temperature will stabilize when the rate of loss balances the rate of heating energy supplied. If heat loss is zero (impossible in practice but can be approximated), the temperature will continue to rise indefinitely as long as power is applied.
The numbers work out, pretty close, that you can use Spice to model thermal systems, using 1 gram of aluminum to be one farad, 1 K/W as one ohm, 1 K becoming 1 volt. You can approximate thermal diffusion with a string of R-Cs, sorta.
John
ext -
Yup, Is the block floating in space or attached to something? The smaller the block the faster you can rasie it's temperature. (As long as it's not attached to some other heat sink.) I always seem to end up with heaters in the 10 Watt range. Can you give us more details?
George H.
ons
60not
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1 1Yeah, and your heater is a current source 1 Watt of heat is an amp of current.
George H.
Thanks a lot for your comments. They gave me good starting point at least, but let me provide you with more detailed info to see if the suggestions you have written before is applicable to my case or not. In advance, your help is appreciated.
I am going to design a closed loop temperature controller .
The Al block is attached to a power transistor( from uppers face) and one power resistor on one of the lateral sides( may be two power resistors on two of the lateral sides)is/are placed. The Al block is placed in a room (consider 3*4m )and in ambient temperature of 25C. It will be heated up by a power resistor which is/are placed on the block. the temperature of the block is measured by a thermocouple and is reported to a temperature controller.The output of the temperature controller activate the heater circuit including voltage supply and power resistor.The Al block is used as a reference temperature so should be kept constant from the temperature setting until the end of the measurement.The measurement process is very fast and takes maybe less than 1 second even about 1 ms.As the power resistor is heated up by a voltage supply( by current through the resistor)then I need an estimation about the size of the Al block to see which power resistor and voltage supply I need.
Please, if you have any suggestion regarding my previous question or the set up that I described above, let me know. I appreciate your help.
Ela
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