A Note on Heat
Whenever we talk about cooking and cooling, we are talking about the
movement of heat. We are talking about the devices we make gaining,
losing, and storing heat. Heat storage is the same as thermal mass.
A massive, heavy object, like a steel anvil, a barrel of water, or
a brick wall, can absorb and store a lot of heat. It takes a lot of
heat gain to heat them up, and a lot of heat loss to cool them down.
In contrast, air has very little thermal mass, and heats or cools
easily. (This is also why being in air at 100°C is merely uncomfortable,
while being in water at 100°C is lethal. There is a lot more heat
stored in each litre of water than in air, and the water conducts
that heat into your body more easily.)
Heat moves through convection, conduction, and radiation.
•Convection is the movement of heat by circulation
in a fluid (a gas or liquid). Air over a heater warms and rises, which
draws in cool air from nearby, which also warms and rises. As the
air gradually gives its heat to the ceiling, it sinks down to start
again. Many people think that heat rises, but heat itself will go
in any direction; it is warm fluids, like air, which tend to rise.
The heat within those fluids is then released into the cooler things
that they touch.
•Conduction is the direct movement of heat
through an object. When you stick a metal rod into the fire, heat
moves along it by conduction, and gradually heats up the other end.
•Radiation is the movement of heat directly
from a warm body outward, like the light from the sun, or the heat
from a fire.
This is important because we want to alter the movement of heat in
our devices. When you want to stop the movement of heat away from
the food you are cooking to conserve fuel, or the movement of heat
into food you want to keep cool, consider how heat moves and how you
can slow its movement. To slow heat loss by convection, minimize or
eliminate air pockets or gaps in which air can circulate. (Air will
not circulate in gaps less than about 1 cm or 3/8? across.) To slow
heat loss by conduction, insulate with a material that has many tiny
air pockets, which slow the movement of heat. To slow heat loss by
radiation, use a “radiant barrier” like aluminum foil,
which reflects heat.
When designing a device for heating or cooling, we want to consider
the concepts of heat gain, heat loss and heat storage. Heat gain just
means heat is moving into or being generated in the device, heat loss
means heat moving out of it. For cooking, we want to maximize heat
gain and heat storage (within reasonable levels) and minimize heat
loss. For cooling food to keep it fresh, we want to maximize heat
loss and heat storage, and minimize heat gain.
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