Introduction
Life on earth
has been safeguarded for thousands of years because
of a life-protecting layer in the atmosphere.
This layer, composed of ozone, acts as a shield
to protect the earth against the harmful ultraviolet
radiation from the sun. If it were to disappear,
the sun's ultraviolet radiation would sterilise
the surface of the globe, annihilating most terrestrial
life.
The Ozone Layer
Ozone is a gas
composed of three oxygen atoms, instead of the
normal two. It is formed when powerful ultraviolet
rays from the sun cause oxygen molecules to break
apart into atoms which then react with other
oxygen molecules to form ozone. Thus, ozone is
naturally converted to oxygen (and vice-versa)
by the sun's ultraviolet rays. Ozone is a trace
gas found at altitudes between 15 and 60 kilometres
which forms the ozone layer (figure 1).
The Effects
of Ozone Depletion
The ozone layer
is a vital part of the earth's atmosphere because
it efficiently screens out almost all the harmful
ultraviolet rays of the sun, by absorbing the
most dangerous radiations, and therefore, preventing
them from reaching the earth's surface. Any damage
that is done to the ozone layer will lead to
increased amounts of the dangerous ulraviolet
radiations perforating the filter and reaching
the earth's surface. Although a certain amount
of ultraviolet rays are needed to support life,
too many can create adverse effects on animal
and plant life.
An increase
in the risk of developing skin cancer and an
increase in the incidents of eye damage, for
example, cataracts and deformation of the eye
lenses, are serious effects resulting from ozone
depletion.
Increased ultraviolet
radiation would cause changes in the chemical
composition of several species of plants, resulting
in decreased crop yields and damage to forests.
The quality of certain types of crops, for example,
tomatoes, potatos and soya bean, would also be
reduced and the growth of others would be affected,
for example, rye and maize.
Ultraviolet
radiation affects ocean life and causes damage
to aquatic organisms, for example, plankton,
shrimp and aquatic plants, to a depth of 20 metres
in clear waters. These organisms form part of
the marine food web and a decrease in their numbers
may lead to a decrease in fish higher up the
food chain. Countries that rely heavily on fish
as a fundamental food source could be seriously
affected.
Materials used
in buildings, paints, packaging and other substances
could be degraded by ultraviolet radiation. Plastics
that are used outdoors are most likely to be
affected, and damage could be more severe in
tropical regions where temperatures and levels
of sunshine are high. The cost of such damage
would be immense.
In order to
contribute to ozone depletion, a substance must
contain chlorine, hydrogen, nitrogen or another
similar atom to participate in the ozone to oxygen
chemical reaction. It must also be sufficiently
stable in the lower atmosphere (troposphere)
to allow it time to reach the ozone layer.
Chlorofluorocarbons
(CFCs)
Chlorofluorocarbon
(CFC) is a chemical compound consisting of one
or more carbon atoms surrounded by chlorine and
fluorine atoms. It is a "greenhouse gas" and,
as such, contributes to the reduction of the
earth's ozone layer. CFCs are manufactured for
use in aerosol propellants, blowing agents for
plastic foams, solvents and refrigerants.
A refrigerant
is a fluid (liquid or gas) which transfers heat
away from one point to another. It is used as
a refrigeration device for chillers to cool water
or air. In a typical vapour system the refrigerant
changes from a liquid to a gas when it absorbs
heat and changes back to a liquid when it gives
up heat.
Most refrigerants
used today for vapour compression air conditioning
are called halocarbons. A halocarbon is a hydrocarbon
molecule containing one or more halogens. The
halogen elements most commonly used in refrigerants
are chlorine and fluorine, and where this is
the case, the refrigerants are CFCs.
Some CFCs have
atmospheric lives of over 100 years and therefore,
once these chemicals are emitted, they will influence
the process of ozone depletion for a long time.
Refrigerants are so stable that they do not breakdown
in the troposphere. Instead, by forces of air
circulation, they spread around the globe until
they eventually reach the stratosphere (the upper
part of the atmosphere) where they are exposed
to powerful ultraviolet radiation which break
apart the refrigerant chemical, releasing the
chlorine. As the chlorine reacts with ozone,
oxygen is created which, in turn, allows more
ultraviolet light to pass toward the earth. The
chlorine atom acts as a catalyst which means
that it is not used up in the reaction but can
continue to convert ozone to oxygen through thousands
of similar reactions.
The concentration
of CFCs in the atmosphere is increasing, mainly
as a result of industrial activity, and as more
gases are emitted, the atmospheric ozone gradually
breaks down.
The Greenhouse
Effect and Global Warming
The earth's
temperature is maintained by a balance between
heating from solar radiation flowing in from
space, and cooling from infrared radiation emitted
by the earth's warm surface and atmosphere escaping
back to space. The sun is the earth's only external
source of heat.
When solar radiation,
in the form of visible light, reaches the earth's
surface, it is partially absorbed by the atmosphere,
and partially reflected from clouds and land,
especially deserts and snow. The remainder is
absorbed by the surface which is heated and in
turn warms the atmosphere. The warm surface and
atmosphere of the earth emit invisible infrared
radiation.
Greenhouse gases
act like a blanket and prevent much of the infrared
radiation from escaping directly to space (figure
2). By slowing the release of the cooling radiation,
these gases warm the earth's surface. This process
is known as the greenhouse effect' because it
works in a similar way to a greenhouse. Glass
allows sunlight in but prevents some infrared
radiation from escaping, warming the air (and
plant life) inside it.
Although the
greenhouse effect is essential for life and keeps
the planet over 30 degrees centigrade warmer
than it would otherwise be, enhanced global warming
may have severe consequences. The presence of
greenhouse gases is growing larger, due to mankind's
activities, and produce an increasing concentration
of heat trapping gases which causes the temperature
of the earth's atmosphere to gradually rise.
Once a greenhouse
gas is released to the atmosphere, it can contribute
to global warming. The amount by which it does
so depends on the capacity of the gas to absorb
infrared energy and the amount of time that elapses
before it is removed from the earth's atmosphere.
The most obvious result of such warming would
be a rise in sea level due to the thermal expansion
of the oceans and melting of some ice land, for
example, glaciers and snowfields.
Apart from CFCs,
the greenhouse gases can all occur naturally,
for example, water vapour and carbon dioxide,
and therefore, the contribution CFCs make to
global warming can be reduced by controlling
and limiting the use of them.
The information
provided below is a summary explanation of options
available to protect the environment against
CFC emissions in refrigeration systems. The decision
to implement one of the alternatives documented
requires careful consideration of a wide range
of situation-specific parameters, many of which
are not addressed here.
Preventing Unneccessary
Release of Refrigerants
In the short
term, preventing unneccessary release of refrigerants
can only be achieved by reducing refrigerant
losses from existing systems. The three main
sources of refrigerant losses are intrinsic leaks
within the system; accidental leaks, for example,
over pressuring and release through a relief
valve; and emission due to transfer or maintenance
- emptying or filling systems - because of bad
practices.
Leak tests should
be performed at regular intervals and if a leak
is found, the refrigerant should be recovered
from the system, the leak should be repaired
and the system should be rechecked to ensure
that the repairs are effective. The whole system
should be tested and any leaks found marked so
they can be easily repaired.
New service
procedures should be introduced to avoid the
venting of any refrigerant. For example, refrigerants
should be recovered from the system prior to
repair; after the liquid refrigerant has been
removed from the system, the remaining vapour
charge must be properly recovered for reuse in
the system; refrigerant-contaminated components
must be disposed of in an environmentally acceptable
manner so they do not vent the atmosphere; and
there are new service tools available to minimise
emissions when opening refrigerant drums.
Equipment Retrofit/Replacement
Strategy
The equipment
retrofit/replacement strategy aims to eliminate
CFCs through either retrofitting CFC chillers,
that is, replacing an old refrigerant with a
new one in an existing system, or replacing CFC
chillers with newer systems that use environmentally
acceptable alternative refrigerants. Chiller
retrofits and replacements involve considerable
cost which will depend on the compatibility of
materials, the efficiency and capacity of the
system, the leakage rate and type of drive.
Refrigerant
Recycling
Refrigerant
recycling is an attempt to remove some contaminants
and return the refrigerants to the chiller cleaner
than it was when recovered by oil separation,
non-condensable removal and core filter-driers
which reduce moisture, acidity and particulate
matter.
There are two
types of recycling equipment : single pass or
multipass, which refer to whether the refrigerant
passes through the equipment once or many times.
Equipment is available to recycle in the vapour
phase or in the liquid phase.
For vapour phase
recycling, the refrigerant vapour is drawn into
the recycling device, the oil is separated out,
filtered and reintroduced into the chiller. Alternatively,
the recycled refrigerant is recycled into the
original pressure vessel, recirculating through
the recycling device several times.
For liquid phase
recycling, the refrigerant is drawn off through
the liquid valve of the recovery vessel and introduced
into the recycling device where the refrigerant
is evaporated and the oil is separated. It is
then cleaned and condensed through filters and
introduced into a clean storage vessel. In multipass
systems, the refrigerant is recycled back into
the recovery storage.
Recovery Equipment
Refrigerant
recovery is the removal of refrigerant from a
piece of equipment, such as a chiller, to a suitable
external storage container (figure 3). Three
types of recovery apparatus are available: a
self-contained recovery unit has its own compressor
to pump the refrigerant out of the chiller; a
system- dependent recovery unit relies upon the
pressure of the refrigerant in the appliance
to assist in the recovery; passive recovery refers
to a deflated bag on an activated charcoal canister
which is used to store small amounts of refrigerant
near or above atmospheric pressure.
The method of
refrigerant recovery depends upon the type of
refrigerant being recovered - high pressure where
the boiling point of the refrigerant is between
-50 and 10 degrees centigrade at atmospheric
pressure; and low pressure where the boiling
point is above 10 degrees centigrade at atmospheric
pressure and the liquid refrigerant is recovered
first.
Reclaiming Refrigerants
Reclaiming is
a process which removes contaminants and purifies
refrigerants to the same standard as new refrigerants
and is generally done at specially designated
factories.
For governments
and companies with a special interest in recycling,
recovery and reclaiming refrigerants, please
contact the United Nations Environment Programme
at the address given below for more information
on the appropriate equipment required and the
training materials available.
The Montreal
Protocol
The Montreal
Protocol, in 1987, identified the main ozone
depleting substances and set specific limits
on their production levels in the future. Amendments
made to the Protocol in 1992 established phase
out schedules for all CFCs. A ten year grace
period for developing countries was granted to
allow them to expand their economies without
hinderance by CFC phase out but by 2006 all CFCs
must be eliminated.
For further
information please contact:
United Nations
Environment Programme Industry and Environment
39-43 Quai Andre Citroen 75739 Paris Cedex 15
France
Multilateral
Fund for the Implementation of the Montreal Protocol
1800 McGill College Avenue, 27th Floor Montreal,
Quebec H3A 3JC Canada
A 25 minute
UNEP video produced by TVE for the refrigeration
trade is available at the cost of copying and
posting the video cassette.
Please contact:
Dina Junkermann, TVE Distribution Manager at tve-dist@tve.org.uk
Or write to: TVE International
Prince Albert Road
London NW1 4RZ
United Kingdom
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