Heat Exchanger How It Works
Flame Heat Exchangers
In any flame heat exchanger, there are 3 main routes to heat transfer:
- Conduction from plasma (flame) directly to metal. Here, plasma molecules physically touch (through their electron clouds) the metal of a heat exchanger, and give up their energy via conduction
- Radiation - energy is given off proportional to the fourth power of temperature (blackbody radiation).
- Convection - this is a form of conduction of hot gas to metal, where gas flow moves the heat from the flame area to the heat exchanger.
Thus, it is reasonable to logic that:
- All energy of flame is produced as motion of charged particles (the flame is a plasma). Moving particles emit radiation.
But: the grand question is: in a flame, what percentage of energy is in the form of plasma kinetic energy, and what percentage is in the form of electromagnetic radiation? This answer influences burner design profoundly. Understanding this question allows one to gain a deeper insight into burner design, and potentially to make breakthroughs in design.
What we do know, however, is that the heat of the flame is transfered completely to surrounding matter by radiation or conduction (directly from plasma particles or indirectly from plasma particles hitting intermediary hot particles. If air intake is controlled carefully (no excess air), then 100% of the gases are hot.
If the heat exchanger were located right beyond the flame, then 100% of the heating occurs by conduction from plasma. If it is farther, then the plasma radiates its electromagnetic energy and itself slows down. The slowdown is proportional to the velocity change of bumping into other particles. Therefore, does radiation happen only upon collision?
In practice, do we design a heat exchanger for radiative or conductive heat transfer?