# Btu's per Pound to melt different metals?

• posted

I've been casting for a while off a friend's blacksmiths forge using coke as the fuel. It's a fairly easy process, and we haven't had any trouble so far getting up to melts of about 12lbs. of bronze. Takes

15-20 minutes to do a melt from cold metal.

I also got out an old foundry text book, which has some useful tables on the energy per pound from burning coke, air consumption etc. No units given - this is a fairly old book.

My question is: does anyone know where I can find a table for the energy required to melt different alloys and metals? Not the temperature required, but the total energy required to get the melt. I'm looking at building a coke furnace, and I'd like to be able to do some ballpark estimates on how much metal could reasonably be poured.

regards,

Matt Turner Turner Racing Shells Ltd.

• posted

Matthew Turner wrote: ...

I'd say that the chances of your doing this (estimating) are really, really small. Because I don't think that you'll be able to get any kind of handle on your efficiency. When you burn a pound of coke, most of the energy goes up the flue. Most(?) of what's used heats the furnace itself. The rest heats/melts the charge. The numbers are so dependent upon your design (size, geometry, material, air flow, etc) that you'll never get a reliable estimate, even a ballpark one. IMVHO, of course.

Bob

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What Bob says. You can see this more clearly with an electric furnace, where power is easily measured (voltage * current), efficiency of the refractory can be estimated (after several hours operation, temp is such and such at this power level over this square area and thickness) and melting efficiency can be determined.

You're just interested in the last bit. That can be calculated using constants for the metal. Copper's heat of fusion is 134 kJ/kg, and latent heat is 400J/kg*K. It must be raised about 1100°C before melting, so the temp rise takes 440kJ/kg. Thus total energy is 574kJ per kg (261kJ/lb).

Returning to the electric furnace, if the melt takes three hours (3*60*60 =

10800 seconds) to melt five pounds (1.3MJ), the metal consumed an average of 1.3 x 10^6 / 10800 = 120 watts. If the furnace has a 2kW heating element, that's 6% efficiency! And considering you don't have the hot air continually coming in, being burned and then expelled at a temperature greater than the melt (if it were lower, it wouldn't be heating up!), that's about as efficient as it will EVER get!

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

• posted

Hey Matt. Haven't seen messages from you on oldtools for a while.

Take a look at "Gas Burners for Forges, Furnaces, and Kilns" by Michael Porter.