Lesson 2.10 Refrigeration
Overview
This lesson deals with the refrigeration cycle. On
completion of the lesson, you should be able to describe
how a refrigerator works and explain the concept of a
"heat pump".
MINI
LAB
CHOICE OF
ACTIVITIES
1) Measure the temperature
of the air in a container (or the surface of the
container) Use a pump to compress air into a container.
Show that the temperature increases when the air is
compressed.
2) Identify the different
components o a refrigerator or air conditioner and
discuss their design and operstion.
Thermodynamic
Systems
A thermodynamic system is a part of the universe that can
be defined by some boundary or is under consideration.
The boundary can be real or imaginary and this boundary
separates the system from the surroundings. The type of
boundary often determines the type of system being
considered.
Isolated systems
are completely isolated from their surroundings. Neither
heat nor matter can pass between the system and the
surroundings. An example of an isolated system would be
an insulated container. (A system can never be completely
isolated from its environment. There is always some
slight coupling).
Closed systems can
exchange heat between the system and the surroundings,
but matter cannot be exchanged.
Open systems can
exchange both heat and matter with their surroundings.
Compression &
Heating
When a gas is compressed, the molecules are pushed closer
together and they collide more often with each other and
with the surface of the container. The kinetic energies
of the molecules increase and the temperature of the
system increases. If the pressure is reduced, the
temperature decreases.
Adiabatic Compression
When a gas is compressed adiabatically, the system does
not lose any energy to the surroundings and the
temperature increases. A non-adiabatic compression occurs
if the gas can lose thermal energy to the surroundings
while it is being compressed. In most mechanical
compression processes – in a compressor or in a
motor vehicle engine – the time taken to compress
the gas is so short that very little energy can be lost
during the compression process. The process is regarded
as adiabatic.
Compression &
Liquifaction
If a compressed gas is allowed to cool by transferring
heat to the surroundings, the gas (or vapor) may condense
to form liquid. This depends on the type of gas or vapor.
Gases such as oxygen and nitrogen need to be cooled
considerably before they will liquefy whereas other gases
such as propane and Freon, can exist as liquids at normal
room temperatures with a minimum amount of compression.
Evaporation &
Cooling
When the pressure on the vapor above a liquid such as
water, Freon or propane, is reduced, the liquid
evaporates rapidly or boils. When this happens, latent
heat of vaporization needs to be provided by the system
and the temperature of the system drops.
Refrigeration
The heart of the refrigerator is the compressor.
The electrically powered
compressor is usually sealed inside a canister. Its very
important not to lose fluid from the compressor and the
best way to do this is to enclose the motor and the
compressor completely.
When a gas is compressed,
its temperature increases. The compressor compresses the
working fluid in the refrigeration system (Freon or
ammonia) into a coil situated outside the refrigerator.
This increases the temperature of the gas. The coil is
very much like the radiator of a car. It has a large
surface area in contact with the air because air does not
absorb energy very easily from a solid surface.
As the fluid cools inside
the coil, it turns to liquid.
The liquid is then allowed
to expand again as it passes through a small hole (the
expansion valve) and into a coil that is at a lower
pressure. The other end of the coil is connected to the
inlet of the compressor and this keeps the pressure low.
As the liquid vaporizes, it cools the coil and whatever
is on the outside of the coil. The cycle repeats itself
as long as the compressor keeps running.
Questions
- What is a
thermodynamic system?
- How does an open
system differ from a closed system?
- Explain what happens
when a gas is compressed adiabatically.
- How does the average
kinetic energy of atoms and molecules in a
material change when its temperature is
increased?
- Does the average
kinetic energy of gas particles increase if the
gas is compressed adiabatically?
- If a compressed gas
is allowed to cool, will some of the gas liquefy?
- Will the temperature
of a gas that is allowed to expand adiabatically
increase or decrease?
- If a liquid with a
low boiling point is kept under pressure and then
allowed to evaporate rapidly by reducing the
pressure, will the temperature of the system
increase or decrease?
- Describe the function
of each of the following in a refrigeration
system:
a) The cooling coil outside the refrigerator
b) The compressor
c) The expansion valve.
- Is it possible to
cool a room on an ongoing basis by leaving the
door of the refrigerator open?
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