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Ventilation of Beehives
Includes Airflow
in Beehives |
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Bees are capable of fanning their wings to cause a draught,
or movement of air, for cooling the hive or evaporating moisture from
nectar.
A beehive with it's regular spaced rows of frames is
convenient for the beekeeper, but may not be so suited to the bees
needs as regards ease of ventilation.
If we can take more notice of the bees requirements then we
will reduce the need for some of the fanning bees, thus releasing
them for other duties and improving overall productivity.
Ventilation during clustering... There are two schools of
thought on this matter one is to provide for a through daught of air
and the other is to close off all forms of ventilation at the top of
a hive (as indeed the bees do in the wild) and insulate the top cover.
I am at risk of upsetting many American beekeepers that
"swear by" upper entrances which are deeply rooted in American
management.
Wedmore wrote a book "the ventilation of beehives". He
made many errors in his book simply because he had decided on the
answer before doing the experimental work. He proposed the adoption,
and deliberate inclusion of top ventilation by matchsticks being
placed under crownboards, leaving feed holes open, I am not sure
whether he mentioned upper entrances, but they fall into the same
category.
He obviously did not do much experimentation, or observe
the way the bees do it, otherwise he would have found that the
majority bees in hollow trees have a single entrance, of around 100
square mm, well below the halfway point of the nest.
If you give bees a mesh panel over an upper vent they will
propolise it shut when the do not want such ventilation and will
unblock the holes at times when they do want it. (I would like to
understand how they "know" where a blocked up hole is so that they
can unblock it, but that is a side issue.)
If you have a closed box with a heat source in it, a
convection current will be set up. If that box is roughly cubic in
form and the heat source is in the centre the air will rise up the
centre and spread accross the inside of the top like a mushroom, then
fall down all outer surfaces before combining again at the bottom to
replace the air that was displaced by the original rise. This is a
dynamic process, the speed of which is governed by the energy input
(heat from the bees), the density and viscosity of the gas, the "U"
value of the box walls and the surface roughness of those walls.
Now our beehive is a little different... let us consider
one with a full width entrance slot at the bottom of the "front", a
thin plywood coverboard (inner Cover) and all upper holes propolised
shut with a wintering cluster of bees in the centre of the box. The
whole hive probably has a telescoping roof, but this does not impinge
on the conditions we have postulated as the outer surface of the thin
plywood is at ambient temperature.
Our convection current will be set up as before, but most
of the heat will be lost through the thin plywood top causing much
condensation of the exhaled water vapour... this causes a circular wet
patch that is wettest in the centre (sometimes the periphery) and
drips will fall directly on the clustering bees causing disturbance
and a greater saturation of moisture in the rising air. The
convection current continues around the circuit and a small portion
of it will be "exchanged" with fresh air at the entrance.
Now consider the same circumstances, but with an insulated,
sealed roof containing at least 50 mm of expanded polystyrene foam...
The moist air will rise, but after a very minimal amount of
condensation the temperature on the underside of the top will rise to
close on the temperature of the rising stream of air and thus will not
lose moisture due to condensation. Some condensation will occur over
the large area of the sidewalls, but much will stay in suspension in
the warmer air and so the air that is exchanged at the entrance
contains a higher percentage of this suspended water.
To take this further we can consider replacing our solid
floor with an open mesh one... As in the last case our air reaches
the floor carrying much more water vapour and so a greater exchange
of wet and dry air occurs at our screen barrier, than would have been
the case with our single entrance slot.
In a hollow tree a different mechanism is at work... By the
very nature of hollow trees the cavity is roughly cylindrical, but
the upper and lower ends of it are rough and fibrous. Our colony
produces a rising stream of moist air in exactly the same manner as
our colony in the beehive, but this rising stream condenses on a
fibrous end grain "roof" and is absorbed by capilliary action, then
widely disseminated throughout the structure of the tree. Any
moisture that is still left in suspension by the time the airflow
reaches the floor will encounter a rough "forest" of torn fibres that
are cold and damp and yet more of the moisture will condense out and
wick down the fibres adding water to the natural recycling/composting
action that takes place under the nest. This leaves little extra water
vapour in the air by the time it reaches the entrance where a portion
of what is left will be exchanged for "fresh".
Deliberate ventilation at the top of the hive will cause
some of the moist air to be lost, giving the appearance of improving
the situation, but it will destroy the natural circulation of air
within the hive, replacing it with a chimney effect.
I have been using solid, insulated roofs for 19 or 20 years
now (without a separate coverboard). I have used them with solid
floors, mesh floors, small entrances, large entrances, in damp wooded
areas and on a city rooftop 130 feet up... I have not seen any
condensation during this time.
The above "sets the scene" for the effect that this has on
evaporation of nectar... As the circulation in winter is driven by
heat from the cluster so summer circulation is driven by the
difference in temperature of the brood nest and the rest of the hive,
but this is not enough for the bees to shift the moisture caused in
this artificial environment that we call a bee hive. The bees are not
wastful and so have discovered that the easiest and most efficient
way to shift this moisture is to increase the natural circulation by
fanning. The relative humidity of air within this circulation governs
the amount of additional moisture the air can carry as vapour.
Convection & flow within the individual cell. As the cell
is deeper than it is wide there is a space near it's bottom that has
relatively stationary saturated air. This stagnant air is only moved
by a bee entering the cell and displacing it. Near the mouth of the
cell small circulating currents will be set up that are caused by the
airflow past the cell concerned and this will homogeonise the
moisture levels of the air in the cellmouth and the main airflow.
The local temperature variation in bee hives. Bees have a
remarkable abilty to regulate temperature very accurately. But this
does not mean that the temperature within a beehive is the same
thoughout. Bees will locally regulate temperature and humidity
according to what is going on in the part of the hive concerned. For
instance queen cells within a hive will receive "special" temperature
and humidity control. I suspect that under emergency conditions that
the bees may well raise the "normal" queen incubation temperature to
hasten emergence (regardless of what other effects that this may
produce.
Brood & Super Comb Spacing
"Rational" boxes and loop scavenging, Murray's Email.
Top & Middle Entrances
through draughts.
Wedmores Ventilation Ideas
re-read book first.
Open Mesh Floors
link later.
Static Aerofoils
turbine blade theory.
High Insolation
Desert floor problems.
Written... September 2000
Revised... 18 December 2001
