CHP Stove Heat Exchanger

=Design=

Concept of a CHP stove for running with a steam engine:

=Questions=
 * Do you think if I have a 5x2 foot heat exchange coil as shown, in rectangular geometry and chimney on top - that I would get effective heat extraction from the fire?
 * Should there be more than 10 layers, 14 feet each, for a total of 140 feet of pipe?
 * I'm thinking 3/4" tubing. That still gives me about 3 gallons of inside tube volume. Would that pose explosion risk, or are monotube coils like that safe at 300 PSI?