[Stoves] Thoughts in a cold shower

[Crispin Pemberton-Pigott]

Dear Pyrologist Tom and J-Seedy Jonathan

>> It turned out the holes were not clogged at all. The holes had been 
>> made with a punching tool and then the disk was inserted upside down, 
>> so that the jagged edges of the holes faced up towards the stream of 
> >descending water while their smooth side was facing downward. 

The effect is well known and there are formulas to make a hole that while
not perfect, is above a threshold of interfering with the flow. A perfectly
sharp cornered entrance hole (not in a thin plate, more like a pipe) can
effectively reduce the diameter by 2/3 which as Jonathan noticed, is a huge
difference.

The simplest way to make a high velocity jet is to punch it undersize
(removing a circle of material) then punching a tapered pin into the hole to
enlarge it, doing so on a washer of hole that is quite a bit larger than the
original hole. This produces the nicely rounded 'jet' shape quickly and
easily.

Making a hole in a plate the right size makes the operation easy with a
hammer and tapered pin.

The 'perfect' shape for imparting the most velocity (which gives the most
mixing per millibar of draft) is a hole that is tapered evenly with the
length of the hole 6 times the exit diameter, and the exit area being 1/3 of
the entrance area. I put such a cone on a Malawian tobacco drying barn and
it made a huge difference to the draft induced by the chimney gases compared
with square-stepped reduction.  This shape has been known for a long time
and was used yonks ago to make Pelton wheels. Remember that air is a fluid
and responds the same was as water. Pelton jets are good design examples.

Getting 6 times the diameter is usually impossible with a thin plate so just
make it tapered and try to make the entrance rounded.

It is not generally true that the air flow through a round sharp edged hole
is related to the hole's area. Depending on the velocity, it is related to
that portion of the area that is pretty much undisturbed by the edge (though
all of it is, to some extent).  Because the velocity in the centre is much
higher than it is halfway to the edge, the airflow is non-linear with
diameter change.

If you have some sharp holes from the step drill (used by furnace and
testing people a lot) that are too small, instead of making them larger
(there might not be room) punch them inwards over a washer to create a
spout. The flow will increase as will the velocity.

It is possible to create this shape using a CNC punch with a recessed 'die'
with an appropriate counter-bore sitting proud above the hole.

Regards
Crispin