Spinning Down a Beer
Here’s the story I like to tell when I’m explaining the role of air flow in thermal change rates. I’m remembering back to my college days. I’m an Engineering sophomore. Finals are over, and I am experiencing a remarkable string of good luck. I’ve been invited to a summer party (Wow! They let in nerds!) AND – I have a date! We get to the party, and, miracle of miracles, my date actually wants a beer! I hurry over to the ice tub to grab a few cold ones, and am dismayed to find the tub empty, and being refilled with warm cans. Time is critical – I don’t know how narrow the beer window is for my date, but I don’t dare bring her a warm can. Fortunately, I have just finished getting a 94 on my Thermodynamics final, and I remember that the rate of temperature change between two infinite bodies at different temperatures and in thermal contact is proportional to the temperature difference between the bodies. And as soon as the bodies begin to change temperature, the rate of change goes down, because the temperature differential goes down. Depending on the thermal conductivity of the bodies, it can take a long time for that decaying exponential to yield a suitably cold beverage. So, I grab a can by its top, hold it so it is covered with ice water, and rotate it quickly back and forth. This agitates the contents, ensuring that the warmest possible contents are always in contact with the inside of the can, and it agitates the ice water, ensuring that the coldest possible water is in contact with the outside of the can. The aluminum can transfers heat quite well, and in about 3 minutes I have two ice cold beers – well within my date’s beer window. All is well. I have ‘spun down’ the beer.
The same thing happens in a thermal chamber. Without high speed air flow, a ‘boundary layer’ of mid-temperature air exists around the product, and the rate of thermal change goes down. But, if the air is moving very quickly, that boundary layer is continuously stripped away, so the coldest (or warmest) possible air is always moving across the product. You can actually get faster change rates below the surface of a product using high speed air flow and high capacity heating or cooling (liquid nitrogen is a very aggressive coolant…) than you can with liquid-to-liquid thermal shock, if the liquid isn’t agitated.
To get an idea of how much air is moving inside of a Qualmark chamber, take a look at this video. Then, get ready to impress your kids when your engineering expertise yields a cold can of soda in record time.
And remember – if it isn’t broken, you’re not done yet!