heat exchangers and specific heat
Wednesday, December 7 2005
I was at Sears and Best Buy today looking at refrigerator options. You'd think that a hip (if annoying) newish company calling itself Best Buy would have better deals than a dreary old brand like Sears, but you'd be wrong. Mind you, there are definitely things not to get at Sears, chief among them auto tires (a couple years ago I went through that day-ruining hell, as did our friend Susan only a week ago). But they have an enormous variety of refrigerators right there on the floor for you to look at, and there are even some good scratch-and-dent options (though none met my particular requirements). I've been to Lowes, Home Depot, and Best Buy, but Sears definitely has the best refrigerator options in the greater Kingston area.
After that I went to P&T Surplus in the Rondout to look for copper shapes that might easily be turned into a heat exchanger, one that would be sort of like a tiny indirect hot water heater. An ideal shape would have been some sort of tank, but the best I could do was a length of fat copper pipe. I opted for the one with a three inch diameter instead of the four incher, just because it was so much cheaper. You see a lot of weird-looking guys trawling the aisles at P&T Surplus. You know a lot of crazy Catskill survivalists go there for all their off-grid needs, as do unbathed artists, perverts needing iron for that all-important Turkish-style basement dungeon, and straight-up retarded people who collect scrap brass just because it's shiny. Some months ago I saw a retarded guy go to the counter with something shiny and the P&T Surplus dude (it's always the same guy) said he could have it for free.
Once home, I immediately regretted having not gone for the four inch copper pipe. A four inch pipe holds twice the volume of a three incher (area=πr2), and for my heat exchanger to avoid constantly calling on the boiler, I knew I'd be needing more than a single gallon of volume.
This being the early planning stages of my heat exchanger, I did a little research on what materials have the highest specific heat, that is, the ability to store heat per unit of mass. Surprisingly, all the densest materials have low specific heats. Lead, for example, has such low specific heat that you can dump 25 grams of lead heated to a temperature of 212 degrees Fahrenheit into your hand and experience no discomfort at all. (Try doing that with 25 grams of boiling water!) It turns out that protons play no role whatsoever in specific heat, so the lightest atoms have the greatest ability to store heat on a per-mass basis. Thus the way to transfer the most heat from one place to another through a limited conduit is to fill that conduit with liquid hydrogen. Your second-best option is ammonia, which doesn't require full cryotech to achieve, but why bother? Water isn't far behind. The upshot of this is as follows: it doesn't pay to swaddle my heat exchanger with heavy metals underneath its outer layer of insulation; doing so would do little to increase its thermal mass.
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