I’ve been thinking about the burner for my larger furnace and was just looking for some feedback. I’ve been reading here and on the web and I plan to use natural gas as fuel but thought I may as well consider LP and oil in case future needs change. So I was looking at several candidates for the air source and after some back of the napkin calcs, it appears if I could reliably deliver 100cfm of air to the furnace, that would have the potential for approximately up to 500kbtu/hr. The maximum intended single melt capacity of this furnace would be ~50lbs of aluminum. The following chart seemed to have some good summary info for estimating energy potential, volumetric flow, and AFR for common fuels.
So according to this, a cubic foot of NG can produce about 1000 btu. At stoichiometric burn the air fuel ratio (AFR) to do so is approximately 10:1. So for 100kbtu/hr you’d need about 100ft3 of NG per hour or1.67ft3/min. For 10:1 AFR, that means 16.7ft3 of air per 100kbtu. You may want 10-20% excess air but in any case at most you’d need <=20cfm of air per 100kbtu/hr…..so 5x yields the 100cfm of air for 500kbtu/hr I mentioned.
For propane, the AFR is about ~2.5 times greater but so is the energy per unit of fuel so it’s in the same ball park as far as energy potential for 100cfm of air.
Oil is a little trickier on the AFR because the mist isn’t really a gas but assuming one could sufficiently atomize and completely burn the oil, it looked like a good performing oil burner would also be in the same ball park for 100cfm of air. Using the chart above, approximating the Heat of Combustion as ~20kbtu for a pound of oil, that means you need about 5lbs/hr to produce 100kbtu/hr. If the AFR is approximated at ~200ft3/lb oil, that means 1000ft3/hr or 16.7 cfm…….so all these fuels need about the same amount of air to produce 100kbtu. As an aside, if oil is ~7.5lb/ gallon, that means an oil burner that consumes .67 gal/hr is in the range of 100kbtu/hr.
Does this seem about right?
Now, as for the combustion temperatures achieved for each fuel, that’s a discussion for another day.