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 high-

temperature 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 dq 0 for the cycle.

A cyclic heat engine

I

II

III

IV

1 2

34

The work is

w = wI + wII + wIII + wIV

q = qI + qIII

= - wI - wIII

For the adiabatic steps

qII = qIV = 0

For the isothermal steps

DU = 0

Phase Transition Path Condition

I. 12 Isothermal w = -q

II. 23 Adiabatic DU = w

III. 34 Isothermal w = -q

IV. 41 Adiabatic DU = w

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(Tcold–Thot)–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 dwII = dwIV = 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

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 = qhot + qcold = nRThotlnV2

V1

– nRTcoldlnV2

V1

0

qrev

T=

qhot

Thot

+qcold

Tcold

= nRlnV2

V1

– nRlnV2

V1

= 0

DS =qrev

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.

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.

efficiency = work doneheat used

=|wtotal|qhot

=|nR(Tcold – Thot)ln (V2 / V1)|

nRThotln (V2 / V1)=

Thot – Tcold

Thot

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

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.

COP =qhot

|wtotal|

Coefficient of Performance

Therefore, by substituting for w,

Therefore,

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:

COPheating =qhot

qhot – qcold

COPheating =Thot

Thot – Tcold

COPcooting =qcold

qhot – qcold=

Tcold

Thot – Tcold

The Joules-Thompson coefficient

The differential of the enthalpy can be expressed as:

Where m is the Joules-Thomson coefficient. The enthalpy

change be written:

Therefore:

The derivation of the above expression relies on the

permutation relation:

dH = – mCPdP + CPdT

TP H

PH T

HT P

= – 1

HP T

= – TP H

HT P

= – mCP

dH = HP T

dP + HT P

dT

Modern measurement of the

Joules-Thompson coefficient It is relatively difficult to perform measurements under

constant H (isenthalpic or adiabatic) conditions. Therefore,

the modern measurements are at constant temperature.

In other words,

is measured. Based on the expression on the previous

slide:

HP T

= mT

mT = – TP H

HT P

= – mCP

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

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 5 and 10°C as compared to

water's boiling point of 100°C. 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.

Vapor-compression cycle A simple stylized diagram of the refrigeration cycle:

1) condensing coil

2) expansion valve

3) evaporator coil

4) compressor.

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 5 and 10°C 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.

Vapor-compression cycle A simple stylized diagram of the refrigeration cycle:

1) condensing coil

2) expansion valve

3) evaporator coil

4) compressor.

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 50 to 60°C.

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.

Vapor-compression cycle A simple stylized diagram of the refrigeration cycle:

1) condensing coil

2) expansion valve

3) evaporator coil

4) compressor.

Cyclic Refrigeration

The loop water is able to pick up heat from the coils because it

is still cooler than the 50 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.

Vapor-compression cycle A simple stylized diagram of the refrigeration cycle:

1) condensing coil

2) expansion valve

3) evaporator coil

4) compressor.

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

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 chlorofluoro-carbons (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 chlorine- and 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 strato-sphere 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 adsorption

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