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Providing
energy from heat: |
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Heat Transfer. The Never
Ending Destiny
Heat transfer,
or heat exchange, is a process of (what some people nickname
it as) "heat
migration" from a point, to another point.
Come again? From what point to what point?
From a point of higher temperature to a point of
lower temperature.
Tell me
about heat transfer from the beginning, please.
What if, I start at energy?
Energy,
as defined by my Physics teacher, Mr Ismail, back in high
school, is the ability to do work.
Work, in simple terms, is
defined as force (F) applied to move any
object, by a distance (x).
Hence, if the force required to move
the object is larger, i.e. moving a train (due to its
steel-laden weight), then the work done per distance
moved is much larger than moving a bag of cotton.
That was just an example.
The indestructible nature of
energy:
As
explained in the compression section, every atom has an
internal energy.
This is due to the continuous motion
of electron/s orbiting the neutron/s and proton/s. Electron
has its own mass to move, so force is required.
Electron has its own distance to move. Hence, energy
is existent.
As long as the basic of all building
blocks, namely electron, proton, and neutron exist, energy
can neither be destroyed nor created.
Therefore, energy can only be
transformed from one state, to another:
Consider a
sample of gas, with a specified internal energy.
Consider another sample of
gas with lower internal energy.
Now, due to the internal energy from
the electrons, atoms or molecules will move in a specified
random motion. Speed of which these particles (atoms
or molecules) move, will depend on the internal energy.
The higher its internal energy, the higher the speed of
these particles.
Back to our samples of gas.
When these gases are brought into
contact, the gas particles will start to collide with
each other. Gas particles with higher internal energy, will
collide with gas particles with lower internal energy.
Speed of these particles will be
traded. The lower particles’ speed will gain its speed,
and the higher gas particles’ speed will loose its speed.
Analogy
It is analogous to what
happens when a white ball in snooker game
hits a pack of snooker balls. Initially, the
white ball will have a very high speed. As it
impacts the pack of balls (which are initially at a
standstill), the white ball will loose its speed
tremendously, and the rest of the pack will gain
some speed.
This is an example of how energy is transferred from one
state, to another.
So, how can you say that heat
is a form of energy?
A
specific sample of gas, with temperature above 0
Kelvin, and a finite mass; has particles
vibrating, rotating, or travelling at a high speed. In
other words, these particles are moving.
Since these particles are in
motion, the gas particles are doing work!
Therefore, heat
is a form of energy.
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Change gas
to liquid giving you cold : |
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Manufacturers of
air conditioners use refrigerants, since it has an
attractive boiling temperature for heat exchange between air
that we live in, and the refrigerant.
But, our
atmospheric condition will not allow refrigerant to be in
liquid state. Hence, we need to compress it enough, coupled
with condensing it, to change the state from gas, to liquid.
The energy from compression and condensation overcomes the
internal energy of the refrigerant. Hence the state is
changed from gas to liquid through compression and
condensation.
This stage is coupled with compression, for two reasons,
-
to change the
refrigerant’s state from high pressure
gas
to high pressure
liquid
-
to avoid
having very large compressor to compress refrigerant
beyond critical point – i.e. change gas to liquid
without going through liquid-vapour phase. In other
words, ensuring air conditioners are designed properly
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Although
compression alone
may
change the state of refrigerant from vapour to
liquid, it is deemed unpractical for air
conditioners’ design. You’ll only waste your
electricity bills, and you might
not get
cool air in the end. So, condensation is required.
Condensation happens mainly in a heat
exchanger, through a heat exchange process. That
heat exchanger is known as condenser.
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Sneak peek into the basics:
Every atom
and molecule of matters has a specific internal energy, as
explained in compression page. And due to this specific
internal energy, different matter
exists in different
state (solid, liquid, or vapour) at a given pressure
and temperature.
Solids will have atoms or molecules
packed together very closely, hence the movement only
involves rotation about each atom or molecule’s axis.
Liquid however, will have a close
formation of molecules or atoms. The movement is much more
flexible than solids, but still restricted to short
distances. Liquid has higher molecule or atomic energy
compared to solid, but lower energy compared to vapour.
Vapour has all the freedom in this
world, to move in all direction possible, with large
distances between each atoms or molecules, high speed
and random in motion. It has the highest molecule or atomic
energy between solid and liquid.
Our interest is in vapour and
liquid phases, and the phase in between. The phase in
between? Keep reading.
Condensation wishes to introduce
itself:
Condensation is
defined as the state, when a vapour starts to change phase
into liquid state, as a result of
temperature drop.
It starts when a superheated vapour reaches its
saturation point.
We have to bear in mind that condensation does not occur
at a singular temperature. It occurs at different ranges of
temperature. Reason being, the pressure of the vapour
itself.
Compression page has explained that increasing the
pressure will
reduce the distance between the vapour molecules. Thus,
vapour molecules will have lower net energy.
Decreasing the pressure will have a
reverse effect, where the distance between the molecules
will increase, and net molecular energy will increase.
So, if a gas or vapour has a
higher net energy, more energy needs to be removed to
reduce the distance between the molecules.
Similarly, lower net energy of
vapour requires less energy removal for molecules
distance to close.
What’s with all the molecule
distance and energy removal?
As explained earlier, the molecule
distance is a characteristic to different states of a
matter. Solid, liquid, or vapour.
Energy can be in terms of work, or
heat. And when condensation is in the picture, energy
removal is in terms of heat.
Hence, as the pressure of the vapour
increases, the heat removal required is smaller to condense
it. This means that condensation of the matter starts to
happen at a higher saturation (or boiling) temperature. This
is good, as any temperature lower than the saturation
temperature, means we will have a condensed liquid.
The flip side occurs for low vapour
pressure.
Let us consider Refrigerant 12 as an
example.
Referring to Rogers’ and Mayhew’s
“Thermodynamic and Transport Properties of Fluids”:
- the saturation temperature at 1
bar is about -30 oC (-22 oF)
- whereas the saturation
temperature at 9.6 bar is about 40 oC
(104 oF)
This means that we have to have
ambient air at -30 oC (-22 oF) for
condensation of the refrigerant to occur at ambient
pressure.
Well, if we have that kind of
ambient temperature, we wouldn’t need cooling anymore don’t
we?
You see, the ambient temperature for
the compressed refrigerant does not have to be very
low for condensation to occur. In fact the saturation
temperature is higher than most ambient temperatures during
hot summer. Hence heat exchange occurs from the refrigerant,
to ambient air.
This is why we have to couple
condensation with compression in air conditioner systems.
Bring the pressure up, close the gap
between the gases, and remove the heat from the refrigerant
to close the gap even more – liquid refrigerant we have!
Short meeting with vapour and
liquid, and the phase in between:
This
meeting occurs when heat is continually removed from it.
If all of the matter is in vapour
form, then we call it superheated vapour, as there is no
condensation.
Once the temperature drops to
saturation line, we will have a mix of vapour and liquid.
This, is the phase in between. |
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At this
time, temperature will
not drop.
This is due to energy consumption of the atoms or
molecules to bond and change phase from vapour, to
liquid. |
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Temperature will not decrease until all vapour within
an enclosed space, turn into liquid.
Once in liquid form, temperature will
start to decrease again, until it meets freezing point. But
that’s a different story.
Condensation wishes to conclude
with real life example:
-
Order a glass of iced lemon tea
- Watch condensation of water
vapour happening on the glass
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