[Stoves] Stoves Digest, Vol 11, Issue 38

Tue Aug 2 02:16:23 CDT 2011

Dear Dr Tom

I have given this some thought. You always inspire me and I learned a great
deal from you over the years so I felt if needed to reflect a little to give
you a quality answer.

Andrew pointed out that preheating takes perhaps 1% of the energy. That is
not much.  Hardly measurable.  The question is whether or not this adds
something to the value of the burn. 

Your observation seems to be focussed on the volume/density of air and what
effect that can have in penetrating the volume of gases as they are burned.
It is true that I have worked on getting the penetration of the rising gases
balanced with the incoming secondary air and wrote about it several years
ago. 

In terms of penetration the above photo is a good example of full coverage.
The secondary air reaches the centre and the burn is pretty clean. Stove:
Vesto, fuel: 7-8mm Switchgrass pellets

I see two topics that need to be covered: the velocity angle and the total
heat content in the fire. If the air blown by a fan and is cold, then the
expansion of the preheating air will have the effect of increasing the
velocity passing through the air holes, which we presume have correctly
sized to allow entry. The fan is the power source, which we presume to have
been chosen to be able to hold the necessary pressure.

Suppose we compare two systems with well chosen fans and hole sizes. At
first glance there is no difference because for a given power input, they
seem to have similar jet profiles. The cold air is denser but slightly less
viscous, and there is only half as much volume of it. The holes will be
smaller and the jet velocity will be 'A'. When heated, air is less dense,
slightly more viscous, larger in volume and uses hole chosen to turn the
pressure into a jet velocity that is also 'A' because the hole will be
larger to deliver the higher volume. 

So: small holes, cold air, velocity 'A', large holes, hot air and velocity
A.

The question is which has more energy? If there is a difference in total
energy, then the description of an 'equal system' (based on constant
electrical fan power used) is not correct.

Energy = 1/2mv2

Even if the volume is doubled by heating the mass of the air remains the
same, the energy is the same if the velocity is the same. The only traceable
difference between the two systems, given similar output jet speeds, is the
viscosity difference which is very small.

The remaining difference is the hole diameter. If I need 24 x 6mm holes in a
flat (rolled) sheet for cold air and 24 x 8.5 mm holes for hot air (to get
the same area per unit volume) they are not equal systems. The larger hole
will deliver disproportionally more air per sq mm because that is how air
passes through holes. Hence I have to refer to equal systems, not ones that
are produced by simply jiggering with the dimensions.

The larger diameter holes (whatever size they are) passing hot air will
definitely produce a different mixing pattern compared with the small ones,
given a similar combustion chamber.

I found that having a smaller number of larger holes produces better
penetration than a large number of small holes (i.e. the big jets reach the
centre) and leave a triangular space between the holes/jets for combustible
gases to pass into. So I tend to favour, as the fire chamber diameter
increases, to have larger and fewer jets for any given draft available. An
example of this is the charcoal gasifier that Kobus was working on doing his
Masters degree. When he put in a single large hole it penetrated to the far
side of the chamber. It is my understanding that he increased the number of
holes and reduced their diameter until he got a good air distribution in
adequate amounts. Still, the hole size would have to be matched to the
combustion chamber diameter on condition that the fire will be vertically
short (as you described).

So I end with a question passed back to you: Suppose we have two identical
systems, with fans and holes, one with preheating and one without. The holes
for the cold system are punched in a flat sheet which is rolled into a
cylinder and forms the combustion chamber.

The holes for the hot air are not larger in this case, but are shaped in a
properly calculated tapering cone so that the whole volume of hot air can
pass through the holes with the same fan pressure and electrical input. This
option is open to anyone so it represents a viable alternative to changing
the hole size.

Now, which system will invariably or inevitable have better mixing and
better combustion?  I think the answer is that the hot air with twice the
velocity will always perform better.

Now, change the holes in your mind to a system whereby the reverse is true:
the cold are jets are smaller and shaped to deliver higher velocity; the hot
air ones are larger and 'flat'. Same power, same fan, same input volume.
With this comparison I am not so sure. The cold jets will heat very rapidly
by radiant heat after emerging but they might not reach the centre of the
chamber because of the severe local turbulence (a puffy ball) created at the
interface where the jet hits the fire by both the 'impact' and the fact that
the air is expanding rapidly as it heats.

As the system is dynamic (everything is in vertical motion) it seems that
the larger hot jets, containing the same motion energy (but not the same
heat energy I agree, but which I am ignoring) will tend to create pipes of
hot air penetrating into the flame space and because they are larger in
diameter, they will tend to move into and upward with the flames. My
subjective experience is that it creates a pretty good mix but it is
definitely taller (the flame).

As the hot air fire is more diffuse (in vertical space) I think it 'needs'
the extra heat from the preheating to maintain a CO-burning temperature. The
cold air jets, creating a locally turbulent, rapid mix with a shorter flame,
will achieve a higher temperature in that confined space. It is arguable
that it would be even better using hot air and a slightly stronger fan, but
I want to leave the systems 'equal'.

So there are arguable two sets of conditions in which cold and hot secondary
air can produce equal results with respect to CO, and they have quite
different combustion chamber shapes. One would want to know in advance what
the final height should be, for example, before choosing one approach over
the other.

Another element though, is the 'Time' of the three T's (time, turbulence and
temperatures). It seems they both get turbulence, and temperature, but the
time is not the same at all. If the cold air combustion chamber is 1 inch
high, and the hot air chamber is 5 inches high, there is far more time to
break down complex molecules in the hot air system - perhaps 2-3 times as
much (it is not linear!) and this means the hot air system should be able to
burn a wider range of fuels and a wide range of moisture contents for a
given fan input energy.

In practise it is interesting that the short flame, cold air fan stoves tend
to be gasifiers and pyrolysers - gases being the easiest thing to burn with
the least demand for 'Time'. Is this generally true for cold secondary air
burners like the Campstove? When I look at the natural gas stove in my
kitchen I see the designer only planned for a short flame with cold
secondary air. Interesting.

Perhaps this is a question for Andrew: if the fuel to be burned completely
has more moisture and higher tars (Pines, Gums) is it true that more
preheating and more burn time are required to get a clean result? We can
assume the temperature in both options is the same. This is a question about
the flame length, basically. If your fuel demands a lot of time to burn, you
would be better off with a lazier, longer hotter secondary air pattern, if
not, cold air will do and expect a short, hot flame.

I am searching for violations of the above generalisations and I think the
FSP (Free State Paraffin) stove qualifies. It has preheated fuel (to the
boiling point) and preheated air and a very short flame with extremely low
CO. I think it qualifies as the 'hot air into easily ignited fuel' model
described above. As Andrew said, it is recycled heat that otherwise would
have escaped the stove body (what I call 'free heat').

In conclusion, I can't think of any example where preheating the secondary
air is a disadvantage. I can think of several where cold secondary air is a
problem, but that does not include pyrolysers/gasifiers that are guaranteed
to burn highly combustible gas and the burner is suited to short flames. In
those cases it is not necessary to go to the trouble of collecting heat into
the secondary air. 

It would be very interesting, Tom, to measure the CO in your stove with cold
air and then fabricate a preheater (for the secondary air only) and run it
again. I think it would help. The question remains whether anything truly
beneficial would be gained. If it is already really good, there is perhaps
no health or energy benefit.

I did not address the heat transfer issue. To be very brief, if the pot is
large, there will be no meaningful difference on heat transfer. For a small
pot (like a little tea kettle), concentrated, small hot fires will
outperform larger systems, however as noted above, require a narrower range
of fuels.

Regards

Crispin

-----Original Message-----
Crispin and All

Let's explore the plus and minus of preheating secondary air.  

Certainly, preheating increases efficiency by adding Joules/gram-degree
(heat capacity), about 0.7 J/g-C, back into the combustion process.  If you
could preheat the air to 300C, that would be 210 J/g.

Consider that the heat of combustion of 10% moisture wood is about 20,000
J/g.  So, this is about 1% of the energy being released.  That's the plus.

<><><><> 

One of the beauties of the TLUD forced draft stove is that all the volatiles
are burned in a disk 4" in diameter and about 1" thick, so that all of the
heat is available to the pot, no unburned gases are quenched into emissions.

If you preheat the air 300C, 573K, degrees to capture 1% of the heat of
combustion, you reduce its density by a factor of about 2.  This means that
the jets of secondary air intended for combustion can only penetrate the
gases half as far, and there will be a lot more unburned gases exiting the
pot area.  

So, in my (moderately) humble opinion, preheating secondary air is
counterproductive.  

You can preheat the primary air, but that is only 20% of the incoming air,
so, again not worth it.

<><><> 

I believe you are one of the most diligent and practical practitioners of
TLUD stoves, so could balance the theoretical vs practical advantages in
each situation, and look forward to your comments on this issue...

Your co-stover,

Tom Reed

Dr Thomas B Reed

President, The Biomass Energy Foundation  <http://www.Woodgas.com>
www.Woodgas.com

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