[Stoves] Calc. void space and pellet particle density

ajheggie at gmail.com ajheggie at gmail.com
Sun Mar 9 06:22:02 CDT 2014


[Default] On Sat, 8 Mar 2014 14:53:30 -0800,"Frank Shields"
<frank at compostlab.com> wrote:

>This is a sample from a local US pellet source. The bulk density is 0.638 g/cc and the particle density of the pellets is 1.338 g/cc
>
>Void space for air movement in a TLUD filled with these pellets is 52.3% 

I haven't checked your maths but the numbers seem good, I wasn't
expecting the pellets to be quite so dense, perceived wisdom is that
the density of wood with no pores is 1.5g/cc so the press has got
quite close to the theoretical maximum.

If we consider a 6mm cylinder of dry wood 10mm long then to achieve
this density it will have needed to be squeezed down to 3mm.

Of course we are starting with sawdust which is 1/3 the bulk density
of dry wood so in fact we have to squeeze a 30mm column of sawdust
down to 3mm the average force times 27mm is the energy necessary to do
that, neglecting friction, or are my numbers too far out? Tom Miles
said the pressure necessary is 2000psi so that seems to be an average
weight on the 6mm column of only 40kg (~400N) moved through 0.027m ??
I make that ~11Joules. The pellet has a mass of 0.378 gramme and a
calorific value of 7036 Joules so the which seems to give a very low
figure of 0.16% of the heat energy  is required in motive energy to
compress the sawdust. Unless my suppositions are wildly out the losses
in the pellet making system are high as we worked on 2.5%.

Also the air space being greater than the solid space is more than I
expected.




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