Refrigeration

Importance of refrigeration
Energy cost of refrigeration
Environmental impact
Alternative technologies

History of Refrigeration
In prehistoric times game was stored in a cave or packed
in snow to preserve the game for times when food was
not available. Later, ice was harvested in the winter to be
used in the summer. Ice is still used for cooling and
storing food. The intermediate stage in the history of
cooling foods was to add chemicals like sodium nitrate or
potassium nitrate to water causing the temperature to fall.
Cooling wine via this method was recorded in 1550, as
were the words "to refrigerate." The evolution to
mechanical refrigeration, a compressor with refrigerant,
was a long, slow process and was introduced in the last
quarter of the 19th century.
Source : U.S. Department of Agriculture

Importance of Refrigeration
Refrigeration slows bacterial growth. Bacteria exist
everywhere in nature. They are in the soil, air, water, and
the foods we eat. When they have nutrients (food),
moisture, and favorable temperatures, they grow rapidly,
increasing in numbers to the point where some types of
bacteria can cause illness. Bacteria grow most rapidly in
the range of temperatures between 40 and 140 F, the
"Danger Zone," some doubling in number in as little as 20
minutes. A refrigerator set at 40 F or below will protect
most foods.

Cyclic Refrigeration
This consists of a refrigeration cycle, where heat is
removed from a low-temperature space or source and
rejected to a high-temperature sink with the help of
external work, and its inverse, the thermodynamic power
cycle. In the power cycle, heat is supplied from a hightemperature source to the engine, part of the heat being
used to produce work and the rest being rejected to a
low-temperature sink. This satisfies the second law of
thermodynamics.

Engines
Historically, people were interested in understanding the
efficiency with which heat is converted into work. This was
a very important question at the dawn of the industrial
revolution since it was easy to conceive of an engine
powered by steam, but it turned out to be quite difficult to
build one that was efficient enough to get anything done!
In an engine, there is a cycle in which fuel is burned to heat
gas inside the piston. The expansion of the piston leads to
cooling and work. Compression readies the piston for the
next cycle. A state function should have zero net change
for the cycle. It is only the state that matters to such a
function, not the path required to get there. Heat is a path
function. As we all know in an internal combustion engine
(or a steam engine), there is a net release of heat.
Therefore, we all understand that q 0 for the cycle.

A cyclic heat engine

1

2
I

IV

Phase
I.
II.
III.
IV.

II
3

4
III

Transition
12
23
34
41

Path
Isothermal
Adiabatic
Isothermal
Adiabatic

Condition
w = -q
U = w
w = -q
U = w

The work is
w = wI + wII + wIII + wIV
q = qI + qIII
= - wI - wIII
For the adiabatic steps
qII = qIV = 0
For the isothermal steps
U = 0

Work and Heat for the Cycle

Neither the work nor the heat is a state function. Neither one
is zero for the cycle as should be the case for a state function.
The work is:
w = wI + wII + wIII + wIV
=-nRThotln(V2/V1)Cv(TcoldThot)nRTcoldln(V4/V3)Cv(Thot Tcold)
w = -nRThotln(V2/V1) nRTcoldln(V4/V3) [since wII = - wIV]
w = -nRThotln(V2/V1) nRTcoldln(V1/V2) [since V4/V3 = V1/V2]
w = -nRThotln(V2/V1) + nRTcoldln(V2/V1) [property of logarithms]
The heat is:
q = qI + qIII [since wII = wIV = 0]
= - wI - wIII [since dU = 0 for isothermal steps]
q = nRThotln(V2/V1) + nRTcoldln(V4/V3)
q = nRThotln(V2/V1) + nRTcoldln(V1/V2) [since V4/V3 = V1/V2]
q = nRThotln(V2/V1) - nRTcoldln(V2/V1) [property of logarithms]

Definition of entropy
The heat is not a state function. The sum qI + qIII is not zero.
From this point on we will make the following definitions:
qI = qhot
qIII = qcold

V2
V
nRTcold ln 2 0
V1
V1
V2
V2
= nRln
nRln
=0
V1
V1

q = qhot + qcold = nRThotln

qrev qhot qcold
=
+
T
Thot Tcold

However, the heat divided by temperature is a state function.

This reasoning leads to the idea of a state function called the
entropy. We can write:
q
S = rev
T

Thermodynamics of an Engine
The cycle just described could be the cycle for a piston
in a steam engine or in an internal combustion engine.
The hot gas that expands following combustion of a small
quantity of fossil fuel drives the cycle. If you think about
the fact that the piston is connected to the crankshaft you
will realize that the external pressure on the piston is
changing as a function of time and is helping to realize
an expansion that as close to an ideal reversible expansion
as the designers can get. If we ignore friction and assume
that the expansion is perfectly reversible we can apply the
above reasoning to your car. The formalism above for the
entropy can be used to tell us the thermodynamic efficiency
of the engine.

Thermodynamic Efficiency
We define the efficiency as the work extracted divided by
the total heat input.

efficiency = work done

heat used
|w | |nR(Tcold Thot)ln (V2 / V1)| Thot Tcold
= qtotal =
=
Thot
nRThotln (V2 / V1)
hot
The efficiency defined here is the ideal best case. It assumes
a reversible process with no losses due to friction. The
temperature Thot is the temperature of the expansion in the
engine. The temperature Tcold is the temperature of the
exhaust. Tcold cannot be less than the temperature of the
surroundings.

Coefficient of Performance
The coefficient of performance, or COP (sometimes CP),
of a heat pump is the ratio of the output heat to the supplied
work or
qhot
COP =
|wtotal|
where q is the useful heat supplied by the condenser and w
is the work consumed by the compressor. (Note: COP has
no units, therefore heat and work must be expressed in the
same units.)
According to the first law of thermodynamics, in a reversible
system we can show that qhot = qcold + w and w = qhot qcold,
where qhot is the heat taken in by the cold heat reservoir
and qcold is the heat given off by the hot heat reservoir.

Coefficient of Performance
Therefore, by substituting for w,
qhot
COPheating = q q
hot
cold

Therefore,
Thot
COPheating =
Thot Tcold

temperature Thot is the temperature of the expansion in the

engine. The temperature Tcold is the temperature of the
exhaust. Tcold cannot be less than the temperature of the
surroundings. For refrigeration, the COP is:

qcold
Tcold
COPcooting = q q =
Thot Tcold
hot
cold

Cyclic Refrigeration
The refrigeration cycle uses a fluid, called a refrigerant, to
move heat from one place to another. The key to
understanding how it works is recognizing that at the same
pressure, the refrigerant boils at a much lower temperature
than water. For example, the refrigerant commonly used in
home refrigerators boils between 40 and 50F as compared to
water's boiling point of 212F. Let's look at the process to see
how boiling and condensing a refrigerant can move heat. The
process is the same whether it is operating a refrigerator, an
air conditioner or a heat pump. This example illustrates a
closed-loop system.

Cyclic Refrigeration
We'll begin with the cool, liquid refrigerant entering the indoor
coil, operating as the evaporator during cooling. As its name
implies, refrigerant in the evaporator "evaporates." Upon
entering the evaporator, the liquid refrigerant's temperature is
between 40 and 50F and without changing its temperature, it
absorbs heat as it changes state from a liquid to a vapor. The
heat comes from the warm, moist room air blown across the
evaporator coil. As it passes over the cool coil, it gives up some
of its heat and moisture may condense from it. The cooler,
drier room air is re-circulated by a blower into the space to be
cooled.

Cyclic Refrigeration
The vapor refrigerant now moves into the compressor, which is
basically a pump that raises the pressure so it will move
through the system. Once it passes through the compressor,
the refrigerant is said to be on the "high" side of the system.
Like anything that is put under pressure, the increased
pressure from the compressor causes the temperature of the
refrigerant to rise. As it leaves the compressor, the refrigerant
is a hot vapor, roughly 120 to 140F.
It now flows into the refrigerant-to-water heat exchanger,
operating as the condenser during cooling. Again, as the name
suggests, the refrigerant condenses here. As it condenses, it
gives up heat to the loop, which is circulated by a pump.

Cyclic Refrigeration
The loop water is able to pick up heat from the coils because it
is still cooler than the 120 degree coils.
As the refrigerant leaves the condenser, it is cooler, but still
under pressure provided by the compressor. It then reaches
the expansion valve. The expansion valve allows the high
pressure refrigerant to "flash" through becoming a lower
pressure, cooled liquid. When pressure is reduced, as with
spraying an aerosol can or a fire extinguisher, it cools. The
cycle is complete as the cool, liquid refrigerant re-enters the
evaporator to pick up room heat. In winter, the reversing valve
switches the indoor coil to operate as the condenser and the
heat exchanger as the evaporator.

Cyclic Refrigeration
In summary, the indoor coil and refrigerant-to-water heat
exchanger is where the refrigerant changes phase, absorbing
or releasing heat through boiling and condensing. The
compressor and expansion valve facilitate the pressure
changes, increased by the compressor and reduced by the
expansion valve.

Vapor-compression cycle
A simple stylized diagram of the refrigeration cycle:
1) condensing coil
2) expansion valve
3) evaporator coil
4) compressor.

Cyclic Refrigeration
A refrigeration cycle describes
the changes that take place in
the refrigerant as it alternately
absorbs and rejects heat
as it circulates through a
refrigerator. Heat naturally
flows from hot to cold. Work
is applied to cool a space by
pumping heat from a lower temperature heat source into
a higher temperature heat sink. Insulation is used to reduce
reduce the work and energy required to achieve and
maintain a lower temperature in the cooled space.

Cyclic Refrigeration
The operating principle of the refrigeration cycle was described
mathematically by Sadi Carnot in 1824 as a heat engine.
The most common types of refrigeration systems use the
reverse-Rankine vapor-compression refrigeration cycle
although absorption heat pumps are used in a minority of
applications.
Cyclic refrigeration can be classified as:
1. Vapor cycle, and
2. Gas cycle
Vapor cycle refrigeration can further be classified as:
1. Vapor compression refrigeration
2. Gas absorption refrigeration

Vapor-compression cycle
The vapor-compression cycle is used in most household
refrigerators as well as in many large commercial and industrial
refrigeration systems. In this cycle, a circulating refrigerant
such as Freon enters the compressor as a vapor.

Vapor-compression cycle
From point 1 to point 2, the vapor is compressed at constant
entropy and exits the compressor superheated. From point 2
to point 3 and on to point 4, the superheated vapor travels
through the condenser which first cools and removes the
superheat and then condenses the vapor into a liquid.

Vapor-compression cycle
Condensation removes additional heat at constant pressure and
temperature. Between points 4 and 5, the liquid refrigerant
goes through the expansion valve (also called a throttle valve)
where its pressure abruptly decreases, causing flash
evaporation and auto-refrigeration of, typically, less than half
of the liquid.

Vapor-compression cycle
The step from 5 to 1 is the expansion (and cooling step).
During this step all of the remaining liquid is converted to
vapor while extracting heat from the surroundings. This
usually takes place in a heat exchanger coil so that the air
passing through coil is cooled.

The working fluid

The ideal refrigerant has good thermodynamic properties, is
noncorrosive, and safe. The desired thermodynamic
properties are a boiling point somewhat below the target
temperature, a high heat of vaporization, a moderate density
in liquid form, and a relatively high density in gaseous form.
Since boiling point and gas density are affected by pressure,
refrigerants may be made more suitable for a particular
application by choice of operating pressure.
American engineer Thomas Midgley developed chlorofluorocarbons (CFC) in 1928 as a replacement for ammonia (NH3),
chloromethane (CH3Cl), and sulfur dioxide (SO2), which are
toxic but were in common use at the time as refrigerants.

The Invention of CFCs

CFCs have a low boiling point and be non-toxic and are
generally non-reactive. In a demonstration for the American
Chemical Society, Midgley demonstrated all these properties
by inhaling a breath of the gas and using it to blow out a
candle. Midgley specifically developed CCl2F2 (CFC-12). One
of the attractive features is that there exists a whole family
of the compounds, each having a unique boiling point which
can suit different applications. such as propellants in aerosol
cans, cleaning solvents for circuit boards, and blowing agents
for making expanded plastics (such as the expanded
polystyrene used in packaging materials and disposable
coffee cups).

Chlorofluorocarbons
Principal uses of CFCs are for coolants in refrigeration systems
and air conditioners, as solvents to clean electronic
components, as blowing agents in the production of plastic
foams, and as propellants in air conditioners. These uses are
reviewed in the chapter "Controlled Substances" of The
Montreal Protocol 1991 Assessment (United Nations
Environment Programme 1991). Of the 682 million kilograms
of chlorofluorocarbons consumed globally during 1991, the
DuPont Corporation estimates the use for various applications
as follows: 32 percent for refrigerants, 28 percent for blowing
agents, 20 percent for cleaning agents, and 18 percent for
propellants. The chlorine-containing products are CFC-11,
CFC-12, CFC-113, CFC-114, CFC-115, and the
hydrochlorofluorocarbon HCFC-22.

Chlorofluorocarbons and the

Ozone Layer
Chlorofluorocarbons (CFCs), along with other chlorineand bromine-containing compounds, have been implicated
in the accelerated depletion of ozone in the Earth's
stratosphere. CFCs were developed in the early 1930s and
are used in a variety of industrial, commercial, and
household applications. These substances are non-toxic,
non-flammable, and non-reactive with other chemical
compounds. These desirable safety characteristics, along
with their stable thermodynamic properties, make them
ideal for many applications--as coolants for commercial
and home refrigeration units, aerosol propellants,
electronic cleaning solvents, and blowing agents.

Montreal Protocol
http://www.ciesin.columbia.edu/docs/011-494/011-494.html
Chlorofluorocarbons (CFC) are compounds containing
chlorine, fluorine and carbon only, that is they contain no
hydrogen. They were formerly used widely in industry, for
example as refrigerants, propellants, and cleaning solvents.
Their use has been prohibited by the Montreal Protocol,
because of effects on the ozone layer. They are also a
powerful greenhouse gas, in terms of carbon dioxide
equivalence because of their persistence in the upper
atmosphere.

Alternatives to CFCs
Work on alternatives for chlorofluorocarbons in refrigerants
began in the late 1970s after the first warnings of damage to
stratospheric ozone were published in the journal Nature in
1974 by Molina and Rowland (who shared the 1995 Nobel
Prize for Chemistry for their work). Adding hydrogen and thus
creating hydrochlorofluorocarbons (HCFC), chemists made the
compounds less stable in the lower atmosphere, enabling
them to break down before reaching the ozone layer. Later
alternatives dispense with the chlorine, creating
hydrofluorocarbons (HFC) with even shorter lifetimes in the
lower atmosphere. The effect of HFCs is about 10% of the
effect of CFCs on the upper atmosphere.

Alternatives to CFCs
Hydrofluorocarbons (HFCs) contain no chlorine. They are
composed entirely of carbon, hydrogen, and fluorine. They
have an even lower global warming potential than HCFCs, and
no known effects at all on the ozone layer. Only compounds
containing chlorine and bromine are thought to harm the
ozone layer. Fluorine itself is not ozone-toxic. However, HFCs
and perfluorocarbons (PFCs) do have activity in the entirely
different realm of greenhouse gases, which do not destroy
ozone, but do cause global warming. For this reason HFCs and
PFCs are targets of the Kyoto Protocol.
Polymer haloalkanes
Chlorinated or fluorinated alkenes are used to create resistant
polymers. Important examples include polychloroethene
(polyvinyl chloride, PVC), and polytetrafluoroethylene (PTFE,
Teflon)

The Ozone Layer

Importance of the ozone layer
Location in the atmosphere
The problem of ozone depletion
Solutions

Importance of the Ozone Layer

The ozone layer is a layer in Earth's atmosphere which
contains a few parts per million of ozone (O3). This layer
absorbs 97-99% of the sun's high frequency ultraviolet
light, which is potentially damaging to life on Earth.
Over 90% of ozone in earth's atmosphere is present here.
It is mainly located in the lower portion of the stratosphere from approximately 15 km to 35 km above Earth's
surface, though the thickness varies seasonally and
geographically. The ozone layer was discovered in 1913
by the French physicists Charles Fabry and Henri Buisson.
Its properties were explored in detail by the British
meteorologist G. M. B. Dobson, who developed a simple
spectrophotometer that could be used to measure
stratospheric ozone from the ground.

Ozone in the Atmosphere

The origin of ozone

The photochemical mechanisms that give rise to the ozone
layer were worked out by the British physicist Sidney
Chapman in 1930. Ozone in the earth's stratosphere is
created by UV light striking O2, and splitting it into individual
oxygen atoms (atomic oxygen); the atomic oxygen then
combines with unbroken O2 to create ozone, O3. The ozone
molecule is also unstable (although, in the stratosphere,
long-lived) and when ultraviolet light hits ozone it splits into
a molecule of O2 and an atom of atomic oxygen, a continuing
process called the ozone-oxygen cycle.

Ozone Depletion
Not until 1973 was chlorine found to be a catalytic agent in
ozone destruction. Although catalytic destruction of ozone
was known to be potentially damaging to the ozone layer,
only in 1984 was there conclusive evidence of stratospheric
ozone loss. Announcement of polar ozone depletion over
Antarctica in March 1985 prompted scientific initiatives to
discover the ozone depletion mechanism, along with calls to
freeze or diminish production of chlorinated fluorocarbons. A
complex scenario of atmospheric dynamics, solar radiation,
and chemical reactions was found to explain the anomalously
low levels of ozone during the polar springtime. Recent
expeditions to the Arctic regions show that similar processes
can occur in the northern hemisphere, but to a somewhat
lesser degree due to warmer temperatures and erratic
dynamic patterns.

Alternative Technology
Importance of the ozone layer
Location in the atmosphere
The problem of ozone depletion
Solutions

Vapor-absorption cycle
In the early years of the twentieth century, the vapor
absorption cycle using water-ammonia systems was popular
and widely used but, after the development of the vapor
compression cycle, it lost much of its importance because of
its low coefficient of performance (about one fifth of that of
the vapor compression cycle). Nowadays, the vapor
absorption cycle is used only where waste heat is available
or where heat is derived from solar collectors. The
absorption cycle is similar to the compression cycle, except
for the method of raising the pressure of the refrigerant
vapor.

Vapor-absorption cycle
In the absorption system, the compressor is replaced by an
absorber which dissolves the refrigerant in a suitable liquid, a
liquid pump which raises the pressure and a generator
which, on heat addition, drives off the refrigerant vapor from
the high-pressure liquid. Some work is required by the liquid
pump but, for a given quantity of refrigerant, it is much
smaller than needed by the compressor in the vapor
compression cycle. In an absorption refrigerator, a suitable
combination of refrigerant and absorbent is used. The most
common combinations are ammonia (refrigerant) and water
(absorbent), and water (refrigerant) and lithium bromide
(absorbent).

Heating and pressurization

During this period, the adsorber receives heat while being
closed. The adsorbent temperature increases, which induces
a pressure increase, from the evaporation pressure up to the
condensation pressure. This period is equivalent to the
"compression" in compression cycles.

Heating and desorption

During this period, the adsorber continues receiving heat
while being connected to the condenser, which now
superimposes its pressure. The adsorbent temperature
continues increasing, which induces desorption of vapour.
This desorbed vapour is liquified
in the condenser. The condensation
heat is released to the second heat
sink at intermediate temperature.
This period is equivalent to the
"condensation" in compression cycles

Cooling and depressurization

During this period, the adsorber releases heat while being
closed. The adsorbent temperature decreases, which
induces the pressure decrease from the condensation
pressure down to the evaporation pressure.
This period is equivalent to the
"expansion" in compression cycles.

Cooling and absorption

During this period, the adsorber continues releasing heat
while being connected to the evaporator, which now
superimposes its pressure. The
adsorbent temperature continues
decreasing, which induces
adsorption of vapour. This
adsorbed vapour is vaporised in
the evaporator. The evaporation
heat is supplied by the heat source
at low temperature. This period
is equivalent to the "evaporation"
in compression cycles.

Solar refrigeration: Summary

The cycle is intermittent because cold production is
not continuous: cold production proceeds only during part of
the cycle. When there are two adsorbers in the unit, they
can be operated out of phase and the cold production is
quasi-continuous. When all the energy required for heating
the adsorber(s) is supplied by the heat source, the cycle is
termed single effect cycle. Typically, for domestic
refrigeration conditions, the coefficient of performance (COP)
of single effect adsorption cycles lies around 0.3-0.4.
In double effect cycles or in cycles with heat regeneration,
some heat is internally recovered between the adsorbers,
which enhances the cycle performance.

Examples of Solar Refrigerators

Silica Gel + Water

Activated Carbon + Methanol