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Originally Posted by billbartuska
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I know of martins spreadsheet and in fact rewrote it myself to include a split loop calculations. I then rewrote that one to the version I have now to prevent errors. The reason of the question is I am currently adding the thermal data at the moment and wanted to know “the enthutiast” as billA would have said way of doing things rather than the text book.
With regards fan vs flow rate the industry standard thing is to say air thermal capacity should be a little higher than water thermal capacity. i.e.
Flow rate water * 4200 J/kg/C = air flow rate * 1000 J/kg/C
This will tell you if you need to add more fans (ie a bigger rad) or if your rad is too small. It also suggests that the performance of a two fan rad with screaming deltas should be equal to a quiet rad with 3 fans. This is useful as you can easily test if your rad needs more thermal capacity by upping the flow rate with cheap high cfm fans.
The other thing with rads is its abit hard to estimate what exactly detla T is. Water temperature varies across the radiator surface and air temperature varies through the device. A pass rad has two delta Ts of whatever you specify as air goes through the colder side then through the warmer side. It then matters which way the air is flowing and you have to compute the effect of heat dump on the second pass.
Standard procedure is to use the LMTD (for counter current flow) as the temperature difference with a correction factor (either constant like 0.7 or based on thermal capacities0 cos the result you get out of that is wrong.
See wikipedia for the LMTD as writing it out is hard
http://en.wikipedia.org/wiki/Log_mea...ure_difference
There is another method called eff-NTU which is based on the how close the performance of the rad is too an ideal one of same size then using empirical correction factors.
The two above approaches have the advantage that rad performance is just one C/W number. Similarly it doesn’t matter what fan you use only the air flow rate. So you can type away CFM numbers to your hearts content or use fan curves to estimate how things perform on a rad. Both these approaches have the disadvantage that they are some what uninteligiable to an average reader involving a lot of voodoo magic and they involve long, boring, hard to check formulas that I wanted to avoid.