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• Hello Geoff - first my apologies for not answering your question on first posting. I was hoping that someone else would jump in. To begin with - I m going to
Dec 2 11:43 PM 1 of 4
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Hello Geoff - first my apologies for not answering your question on first
posting. I was hoping that someone else would jump in.

To begin with - I'm going to use the figures for 80/20 brass - just to make
my life easier.

1.0 Properties of the main actors:

1.1 Copper

Melting Point 1981 F
Specific Heat 0.0951
Latent heat of fusion 75.6 BTU/lb

1.2 Tin

Melting Point 449 F
Specific Heat (solid) 0.0551
(482 F) 0.05799
(2000 F) 0.0758
Latent heat of fusion 25.2 BTU/lb

(Notice that the specific heat of tin varies with its state (solid/liquid)
and its temperature. Odd, but not unusual.) To simplify things, I have
averaged the Specific Heat from 482 to 2000 and will use 0.0699

1.3 Propane 20 000 BTU/lb
Flame temp 3 600 F

1.4 Brass (80/20) MP 1830 F
Temp in crucible ~ 2 000 F

1.5 Shop temperature 100 F

1.6 Charge weight 20 lbs

Copper 16 lbs
Tin 4 lbs

OK lets see what we can do with all those figures.

2.0 Copper:

To warm it from 100 to 2000.

(2000 - 100) x 0.0951.............................. 180.7
To change its state from solid to liquid
(fancy way of saying we melt the stuff!).... 75.6
--------
sub-total 256.3 BTU/lb

3.0 Tin:

100 to 450: 350 x 0.0551 .................. 19.3
melt .... 25.2
450 to 2000: 1550 x 0.0699 .................. 108.3
--------
sub-total 152.8 BTU/lb

4.0 For a charge weight of 20 lbs

Copper 16 lb x 256.3 BTU/lb ................. 4 100.8
Tin 4 x 152.8 ............................... 611.2
---------
Total VOOMA required 4 712.0 BTU

5.0 Propane used (4 712/20 000) ................. 0.24 bs

Now, I would guess that you used much more Propane than that to melt the
charge.

Let's play some more.

6.0 EXTRAS - (in a former life these bugged me no end)

6.1 Furnace

Let's assume that the crucible and the refractory weighs in at around 80
lbs.
The specific heat of brick is around 0.22.

To heat that up to 2000 F will need
(2000 - 100) x 0.22 x 80 ........................... 3 344 BTU

Propane needed 3344/20 000 ................... 0.16 lbs

6.2 Losses

6.2.1 Useful heat

The average temperature of the furnace and contents from cold to
ready-to-pour is (2000 - 100)/2 = 950 F.
However things are never linear (it takes longer to heat that last 950 than
the first 950) so we'll fudge it by moving the useless temp up to 3/4 of the
range, say 1400 F.
All flame temperature below 1400 F is really only cooling things in the
furnace.

We have to correct the amount needed by 1 + 1400/3600 = 1.39.

So 40% of the gas that we have burnt is doing absolutely nothing. Actually
that's not true, it is warming itself!

So our calculated minimum propane requirement is

(0.24 + 0.16) x 1.4 ..................................... 0.56 lbs

6.2.2 Insulation Loss

Assume that the insulation is only 80% efficient so that's another 20%
extra.

That makes it 0.56 x 1.2 ........................... 0.67 lbs

6.2.3 Exhaust loss

Heaven only knows what the actual retention time of the gas in the kiln
actually is - I'm not even going to hazard a guess. Tell you what we'll work
that one backwards - how much propane did you use?

7.0 Estimated Propane usage

l'll climb out on a limb - but with, I hope, a well packed parachute.

I would guess that you have probably used around 6 lbs of propane - an
overall efficiency of around 10 - 15% seems to me to be all that one could
expect at such a small scale.

Enough - I have a gas burner to make and nine 1/2" holes to drill through 8"
old 20 year old concrete with a not-quite-up-to-it hammer drill. Especially
as said holes are at ankle level so I can't get any push behind the drill.

Hope all the above makes sense

cheers edi

----- Original Message -----
From: "geoffrey roberts" <rbrts1035@...>
To: <hobbicast@yahoogroups.com>
Cc: <rbrts344@...>
Sent: Friday, December 03, 2004 3:16 AM
Subject: Re: [hobbicast] BTU/BRASS?

>
> Hi, i want to know the heat required to melt brass. i can melt 20 pounds
of Silicon Brass in 35 minutes from a cold start. i'm building a new furnace
and will be changing from propane to waste oil. The new furnace will have a
number 60 crucible. If i can find the heat needed i can work backwards and
determine the relative btu needed to melt the Brass. At this point i will
see how much waste oil and how many burners i need. i think this method will
be easier and definitely than building a waste oil burner for the older,
smaller furnace. Thanks, geoff<>< Jahnke

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