Spring 2005

Obstructed fan curve data

What's the most important thing? Knowing the fan performance curve in your system. In the collaborative project, lots of different obstructions showed small effects on the predicted operating point. Adding a cosmetic bezel to the vent openings produced an operating point 30-50% lower than what you'd predict if you used the catalog fan curve. Ouch! Do your homework!

Coming up: CoolingZone Summit, August 2005

Look for a focus on high heat flux electronics cooling, and my short course on liquid cooling. More info at http://www.coolingzone.com/summit2005

See previous newsletters and CoolingZone articles list.

Spreadsheet Tricks for Thermal Engineers : Calculations

At the Semi-Therm conference, I did a tutorial on my favorite tricks in Excel -- which, according to Tony Kordyban, is "the toolbox you already own" for thermal calculations. The tutorial covered tricks for calculations and modeling, and others for data analysis. There weren't any handouts, but here is a quick summary of the highlights that I use for calculations:

Use labels when writing formulas to help you debug them. I've described this feature before, in a previous newsletter, but it's so little used that I thought I'd explain it again. You have to turn this option on using Tools/Options/Calculations; check the box next to Accept Labels in Formulas. You also have to be a bit choosy on the label that will be the variable name. To start, I usually write the label/variable name in column A. Choose names that the software won't confuse with a cell reference -- so at least three letters. Spaces are usually okay; if the software gets confused, you can always enclose the name in single quotes when you write the formula. Once you have a few labels, write the formulas in column B. As an example, you can write the formula as "=thickness/(k_layer*area)". Note that these labels work only within the same worksheet. Download example.

Another way to use sensible variable names is to use the Defined Names capability. This lets you designate values for the entire workbook, and is great for passing constants between worksheets. There are several ways to define the names; you can start with the labels next to the values, select the label and the value cells, and Insert/Name/Create; or you can Insert/Name/Define and put in your own name and reference. With the reference, you can give a name to an array (for example, a table you'll be using for lookup), a fixed range or cell (Sheet1!$B$16), or a relative range or cell (Sheet1!B$16 will look at cell 16 in whichever column is currently asking for the value). Better still, you can put values in the reference box directly (e.g. k_copper refers to 390) or even a formula. For more on this topic, including how to make the range expand and contract with your data, and how to make the same named range work for different worksheets within the same workbook, get the book Excel Hacks, by David and Raina Hawley. I picked it up at my local large chain bookstore, so it shouldn't be too hard to find (and no, I'm not getting a commission!).