I have a hot tub that contains approximately 500 gallons of water and would like to try heating it with a wood fire. I currently have a 20 pound propane tank that I have cut holes in that I was thinking I could place over the fire and have water run through it. Another thing I have seen is a big spiral of soft copper tube placed in the fire. How I understand it, it is simply the surface area that I care about. To heat the water in any sort of efficient way, do I need to go buy copper tube, or will the propane tank work. I understand that the water coming out of the copper spiral would be much hotter, but would they eventually heat the hot tub in about the same amount of time?
As you probably know, there are several factors to account for, but let's talk about the major ones. Since you're going to use the same circulator pump with either the tank or the tubing, let's assume that you'd have roughly the same flow rate for each option. And let's say I also take your statement at face value: I understand that the water coming out of the copper spiral would be much hotter . . . The circulating water is the carrier for the heat calories from the fire to the tub. If it's much hotter coming out of the tubing than the tank, then, for a given amount of time, the tubing will carry many more calories than the tank solution would. So, roughly speaking, if the water output from the tubing is twice as hot as that from the tank, then it will be transporting twice as many calories per unit time, and the tub will heat up twice as fast--i.e. in ~ half the time. Regardless of the material you use, the ideal geometry for a heat exchanger will expose each molecule of the conducting fluid to the heat source as long as it takes to make them the same temperature before it leaves the exchanger. That's why automobile radiators are built like a fine honeycomb mesh--a molecule of water has a good chance of being exposed to the cooling air before it exits the radiator. That's where the surface area guideline comes in: The more the surface area of the exchanger (for a given flow rate), the more heat will be exchanged simply because of physical contact. In a sense, the tank you're proposing to use would be one of the worst exchangers, because very few of the water molecules would have direct contact with the wall of the tank before they exit the other end. The second factor is the thermal conductivity of the exchanger, which measures how fast heat flows through the material. Here copper, with a k = 400, has a big advantage over, say, steel, where k = 43. (See Source 1.) .
