Post by jliebler on Aug 15, 2014 14:39:58 GMT -8
I've been thinking about a rocket mass heater for the house I hope to build. My knowledge is limited and I'm always trying to learn. First a not so obvious fact is that all mass stores the same amount of heat! One pound of perlite heated 100 degrees has exactly the same amount of stored heat as one pound of concrete or copper! The difference between materials is how quick the heat can be added or removed. There ought to be a simple way of describing this difference, I'll give my attempt after a bit more introduction. Materials of different composition conduct heat at different rates and the measure of how well they conduct at is called the thermal conductivity ( the English units are BTU * inches/square feet * hours * degrees Fahrenheit). Materials also have different amounts of mass per unit volume AKA density ( the English units of density are pounds/cubic feet) I'm trained as an electrical engineer so my inclination is to think in electrical terms. Heat behaves much like electric charge and the electronic circuit element that stores charge is the capacitor, electrical conductivity is also analogous to thermal conductivity. Most electronics is kind of one dimensional but similar methods are used for 3 dimensional problems, which heat transfer always is. Most electronic analysis deals with "lumped elements" but there are techniques developed for "distributed circuits" as well. One of the first "distributed circuits" I met as a student was the "transmission line". Transmission line "parameters" are expressed on a per unit of length basis and analysis requires integration over the length. Transmission lines store energy! While it's pretty useless, in electronics, a transmission line with only resistance per unit length and capacitance per unit length was the very first we studied and that is exactly what heat storage and conduction resembles, but in 3 dimensions of course. We also learned a lot about "transient response" of electrical energy storage systems and one very useful concept was the " time constant" which describes the initial "rate of change" after a disturbance and that after several, about 10 time constants, "steady state is achieved. So any way I decided that the "time constant per unit length would be useful way of understanding the nature of different materials. Well it's certainly not on any data sheets! But I did a quick dimensional analysis and Density/conductivity has units of hours/inch with English units! So I looked up a few of the more likely materials. Here are a few results: 4:1 perlite concrete, 43hour/inch; 8:1 perlite concrete 39 hour/inch; light fire brick 40 hour/inch; denser insulating fire brick 19 hours/inch; ordinary concrete 20 hours/inch; mineral wool board 32 hours/inch. While I didn't do the calculations for steel, because of it's really high conductivity and its density it's thermal time constant is very small, on the order of 0.1 minute/inch.