Download REFRIGERATION TECHNOLOGIES 4. Two ways of preparing

REFRIGERATION TECHNOLOGIES
Introduction: How to generate cold
1. Cold – heat - temperature
Heat is a form of energy which every substance and body contains more or less.
Touching a body we can feel if it is hot, warm, lukewarm, cool or cold. In doing so we
characterize its heat status, which is called temperature. Our sensation is a bad
calorimeter!
When we go in the winter from a heated room into an unheated one, we think it is cold.
When we arrive in the same room after having been outside in a severe cold, it appears
to us to be warm. In reality, however, the room had the same temperature.
2. Temperature measurement
For that reason thermometers are used for measuring the temperature. For their
calibration, however, different temperature scales are used. The most common scale ist
the centigrade scale (°C). The Swedish astronomer Celsius assigned as fixing points the
melting point of ice (0°C) and the boiling point of water under standard atmospheric
pressure (1013 hPa). This temperature scale is continued upwards and downwards
correspondingly. For temperatures which are higher (warmer) than 0°C plus is used, for
those which are smaller (colder) minus.
In the USA temperatures are measured in degrees Fahrenheit. The German physicist
Fahrenheit established as his freezing point on his scale the deepest temperature of the
American winter in 1709 (-17,8°C). The melting point of ice was established as 32 F and
the boiling point of water at 212 F. For food freezing the freezing temperature has been
set to 0 F for reasons of simplicity. As the freezing technology comes from the USA, this
storage temperature was simply taken over. This explains why -18°C was considered as
being the necessary storage temperature. It goes without saying, however, that the
deep-freezing of foodstuffs can take place at, for example, 15°C, too.
3. Methods of refrigeration generation
Cooling means extracting heat. So there is no refrigeration, but extraction of heat. The
result of it is a lower temperature. If you want to cool down a room, the heat must be
moved from a location where it is unwanted to a location where it does not do any harm
nor where it disturbs. Owing to chemical and physical laws there is no refrigeration, but
heat extraction.
4. Two ways of preparing refrigeration
A. Freezing mixture (Application: The heat from solution – spreading salt in the
winter)
A practical application of a freezing mixture is spreading salt in the winter.
Ice and snow are mixed with salt by road salt. The melting point of this mixture is depending on the concentration - far deeper than 0°C. The ice-salt or rather the snowsalt mixtures are dissolved as salt water.
Many mixtures (for instance salt water solutions) show a certain concentration, which
is called eutecticum. In a eutectic mixture an extremely cold melting point is reached
which is far below the one of the single components. Since with the help of these
chemical solutions heat can be extracted from the environment they are called freezing
mixtures.
Example: A mixing ratio of 1 component of sodium chloride/table salt and 3 components
of ice has the effect that the salt in the ice dissolves as saltwater and the temperature of
the freezing mixture goes down to -21°C.
Project task: How much refrigeration can you prepare?
We prepare several freezing mixtures and measure the temperature.
Take one of the substances given and prepare 3 different mixtures with ice at least.
Proposed quantity of ice: ca. 30-50 grams each
Substances you can choose and proposed quantities for the experiments:
sodium chloride NaCL
5g-20g
calcium chloride CaCl
15g-75g
potassium chloride KCl
50g-120g
Draw a graph of your experiment:
Temperature (y-axis) depending on the amount of substance (x-axis) and determine the
eutectic point, i.e. the lowest melting point or rather freezing point
_____________________________________________________________________________________________________
B: Thermoelectric refrigeration with Peltier elements
(Application in mobile battery operated cooling boxes)
Thomas Johann Seebeck (1770-1831): Discovery of thermoelectricity (1821) Seebeckeffect
Jean Charles Athanase Peltier (1785-1845): Discovery of the inverse of thermoelectricity
(1843) Peltier effect
The Seebeck effect: When two dissimilar conducting materials are connected and when
there is a temperature difference between these junctions, an electric current can flow.
Explanation: At the warm side there are more electrons and only a few with low
energy. Owing to diffusion (like in gasses) the electrons flow to the cold place
/cold surface. This means it is a heat conduction with electron flow. The result is
a charge separation many electrons at the cold surface; only a few electrons at
the heated surface. This generates a voltage, the so-called thermoelectric voltage.
In order to measure this voltage two different metals are needed. The voltage
depends on the temperature difference and the interference in the electron flow.
Peltier effect: The Peltier effect is characterized by inverse processes. When an electric
current flows from material A to material B at a junction, heat is generated. Depending
on the direction of the current one side will be warm and the other will be cold.
Explanation: electrons, which are moved, do not only transport an electric
charge, but also energy. High energy electrons are characteristic of the current to
a higher extent, but they also transport more energy. Moving from one material
to the other the energy transported in the electrons is changed. The energy is
either released or absorbed. Elements which show this effect are called Peltier
elements or thermoelectric elements.
Project task:
1. Simple experiments with the Peltier elements
A. Materials: Peltier-Element (2V), two alligator clip cables, a 9V-battery, LabQuest
with temperature sensors
Performance of the experiment:
a.) Tape down the temperature sensors at one side.
b.) Connect the red cable of the Peltier–elements with the positive pole and the
black cable with the negative pole by using alligator clips
c.) Measure the temperature with the help of the LabQuest. Wait for some time until
the temperature does not change. Make sure that that you put the side which is
to be measured onto the table.
d.) Do the same with the other side of the Peltier-elements. As regards the Peltierelements usually the warm side is heated more than the cool side cools down.
When you want to have a colder temperature you must cool down the warm side.
B. Materials: Peltier element (12), 4 cables with alligator clips, two 9V-batteries,
LabQuest with temperature sensors, extractor fan, a heat conducting paste.
Performance:
a) Tape down the temperature sensors at the side which is to be cooled down with
adhesive tape.
b) Connect the red cable of the Peltier element with the positive pole and the black
cable with the negative pole by using alligator clips. Do the same at the cooler
with the other battery.
c) Put down the Peltier element with the cold side and put some of the heatconducting paste onto the other side . Use the extractor fan on this side.
d) Measure the temperature with the help of the LabQuest. Wait for some time until
the temperature does not change. Make sure that that you put the side which is to
be measured onto the table.
C. Materials: Peltier element, two cables with alligator clips, voltage source, LabQuest
with temperature sensors
Performance:
a) Tape down the temperature sensors at one side with adhesive tape.
b) Connect the red cable of the Peltier element with the positive pole and the black
cable with the negative pole by using alligator clips. Look out: Never use a higher
charge than indicated on the Peltier-element.
c) Measure the temperature with the help of the LabQuest. Wait for some time until
the temperature does not change. Make sure that that you put the side, which is
to be measured, onto the table.
d) Do the same with the other side of the Peltier element. In case some time is left:
Measure the Seebeck effect.
2. We build a cooling-box for cooling down a small beverage can:
Materials: Peltier elements, polysterene, cooling elements, temperature sensors
Film instruction:
https://www.youtube.com/watch?v=I-cdhGbGjKE
At first make a sketch using the exact scale.
The box is fitted together and a beverage box is put into it.
The temperature is measured for half an hour and represented in a graph.