Heating and Refrigeration.
- When designing a dovetail log
cabin, especially in the arctic conditions of
Alaska, it is important to consider the various
aspects of transferring heat into the place and
holding the heat as efficiently as possible.
Heat transfer comes in three different forms,
conduction, convection, and radiation.
Convection - Heat transfer by
motion of fluids or materials.
Conduction -
Heat transfer through coupled vibrations and
collisions between particles.
Radiation - Heat transfer via
electromagnetic waves.
- It is common for rustic
dovetailed log cabins to have a stone fireplace
that heats the cabin with a cozy wood fire. The
flames from the fire are producing heat through
radiation by emitting electromagnetic waves
which transfer energy from the fire to the skin
of the person who is being warmed by the fire.
As the wood fire continues to burn, the outside
of the stone fireplace, or steel chimney will
slowly start heating up. The heating of the
fireplace comes as a result of many electrons
vibrating back and forth and colliding with each
other as a result of the added energy from heat
on the inside of the chimney. The increased
vibrations and collisions, then cause friction
between particles as they bounce around. As the
particles bounce around, and cause friction, the
friction from the particles heats the fireplace
and the heat is transferred through the
fireplace towards the the outside of the
fireplace through the coupled vibrations. The
heat felt on the outside of the fireplace then
is a result of conduction.
- In order the hold the heat from
the fire inside of the cabin, it is imperative
that the cabin be properly insulated. Insulation
as defined by The New Oxford American Dictionary
is “The action of being insulated electrically
or physically; the condition of being insulated
by non-conductors, so as to prevent the passage
of electricity, heat, or sound.” In other words,
the insulation in houses is designed to act as a
barrier from the warm air inside to the cold air
outside by utilizing material that has a very
low thermal conductivity, so that heat cannot
pass through the material very quickly.
Dovetailed log cabins are different from
conventional cabins in that they are constructed
with solid wooden beams with gaps between them
that are filled with synthetic insulation (such
as fiberglass insulation) or natural insulation
such as mud or moss.
The Physics Behind Rising Hot Air
In addition to insulation the walls, it is
important to also insulate the floor, and the
roof. This is due to a phenomena that heat rises
and cold air sinks. The reason that hot air
rises, is because the hotter air has more
energy, and that extra energy reduces the
density of the air, and thus the weight of the
air. Since the colder air is denser, it
naturally falls towards the center of the earths
gravitational field, and in doing so
displaces the hotter air, which causes the
hotter air rise. With that in mind, it is
important to insulate the floor, so that no cold
air seeps in from the ground outside. The roof
is similarly important to insulate, because as
the heat rises it will concentrate near the
ceiling, and so insulation is needed to keep the
hot air from escaping. The following pictures of
the author's cabin shows the floor being
insulated with fiberglass insulation as it is
being constructed, the roof framer spacing the
rafters at an even 16" O.C. for pre-cut
fiberglass insulation to fit between, and the
rafters on the roof before the roofing or
insulation is added.
 
Photo courtesy of the author.
- As mentioned in the electricity
section, incandescent light bulbs create
a lot of energy loss due to heat. The heat that
is a byproduct of the light bulb comes in the
form of radiation in the same way that the fire
heats the house. Since there is a significant
amount of heat loss from the light-bulbs, the
smart homeowner will capitalize on that detail,
and turn the stove down when there are lots of
lights running.
Refrigeration
It is important to utilize some sort of
refrigeration in a log cabin in order to keep
groceries cold or frozen. The historical way to
keep groceries cold over the hot summer months,
was to harvest a lot of ice in the wintertime,
and haul it to an ice-house, where it was stored
with lots of sawdust to keep the ice and
groceries cold. Today, commercial refrigerators
have become the norm. Refrigerators such as an Ideal
gas refrigerator use work to transfer
heat from a colder object to a hotter object.
The way refrigerators work, is by compressing a
gas in order to increase it's temperature and
pressure by lowering the gas's volume by the
ideal gas relation PV=nRT where
P=pressure, V=volume, n=number of moles of gas,
T=temperature (in Kelvins) and R is the ideal
gas constant. Once the gas is compressed, the
gas flows through a heat exchanger, where the
pressure is decreased, which causes the gas to
cool down. The gas is then flows through the
refrigerator, and expels a proportion of it's
cold air. The remaining warmer air is then
compressed and heated, and ready to start the
cycle again. There is a physical limit to the efficiency
of a refrigerator though. The limit is that the
coefficient of performance (K) must be
smaller than or equal to the cold temperature
divided by the hot temperature minus the cold
temperature. The actual coefficient
of performance is measured by the heat out
divided by the work imputed. In other words, the
coefficient of performance is what you get out
divided by what you pay (Physics pg#547).
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