Refrigeration

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Refrigeration
Plants – combine carbon dioxide from
atmosphere, moisture and traces of
chemicals from the soil to form all
constituents they need. Energy from
sunlight and green parts of plants can
effect this synthesis.
• Animals cannot do this They have to get
food from plants or other animals. From
their food they absorb what they require in
their digestive system and reject the rest.
Many micro-organisms live on the same
food as men. Our battle is to get the food
before the micro-organisms.
• In this battle the micro-organisms are well
placed – size so small they are ever
present in the atmosphere. Any food
exposed to atmosphere at ordinary
temperature – appropriate organisms will
start to grow. They consume the food like
animal – absorbing what they like and
rejecting the remainder. Rejected
remainder is often toxic – leading to food
poisoning.
• Micro-organisms can be divided in three groups
– moulds, yeast and bacteria.
• Moulds – start microscopic – can develop into
clearly visible growths.
• Yeast – reproduce into microscopic individuals –
presence known by the alcohol their digestive
process rejects – leading to fermentation.
• Bacteria - also multiply as microscopic
individuals- their waste products often giving rise
to evil smells of putrefaction as well as being
toxic.
• Each type of micro-organisms has its own
environment of temperature, atmosphere
(gaseous composition and humidity) and
food for optimum growth. Water is
required by everyone. Hence drying can
also lead to food preservation.
• Control of micro-organisms by subjecting
them to low temp. is basis of refrigerated
carriage of foodstuffs.
•
There are three main temperature group of microorganisms –
1. 36.70 C for optimal growth. This group includes bacteria.
They practically cease growth at 00 C.
2. Optimal growth at 210 C, which are capable of rapid
growth at 00 C and which will grow on unfrozen
substances below this temperature. This group includes
yeast and moulds and bacteria.
3. Micro-organisms, mainly yeasts and moulds, which will
grow on frozen substances down to about -70 C.
• Below –80 C no growth or reproduction of microorganisms occurs.
• For each living process there is an optimum
temp. at which the process occurs at the
maximum rate. As temp. falls below the
optimum, the rate of living falls off until at low
enough temperatures life ceases. As
temperature rises above the optimum, different
chemical processes take over from the normal
living reactions until at high enough temperature
life again ceases . Optimum temperature varies
for each life process, but all lie within the range
of –80 C to 1000 C, outside which life is virtually
unknown.
Dead animal products
• With dead animal produce, the primary
purpose of refrigeration is to delay or
prevent the developments of microorganisms (moulds, yeast and bacteria)
which live on the produce. Cold storage
also retards the slow chemical changes,
such as the oxidation of fats, which whilst
not rendering the produce unfit for human
consumption do adversely affect flavours.
• Dead animal products are carried as frozen
cargo below –80 C. None of the microorganisms that cause decay and putrefaction
grow and multiply below this temperature,
although some may survive in dormant state to
resume growth if temperature is raised
subsequently. Further temperature reduction is
required to slow chemical changes that impair
flavour. For beef and lamb –100 C is adequate.
For pork –12 to –150 C. Poultry and fish –180 C
is desirable.
Fruits and vegetables
• The living process continues after picking of fruits and
vegetables. After picking at immature stage the ripening
process continues until it is fully ripe and finally overripe.
Cool storage delays the ripening process. Second
benefit is delaying of onset of mouldiness as they are
likely to be attacked by moulds. Hence by refrigeration,
we keep it alive and let it ripen slowly. The water content
of fruits and vegetables is 80-90%. If brought below
freezing point, changes in physical structure will take
place and will be killed. They are to be carried in chilled
condition above their freezing point.
• Common between fruit and animal
produce –provide conditions unsuitable for
growth of organisms which live on the
produce. Chemical reaction proceed more
slowly at low temperature. 50C drop in
temperature halves the rate of reaction.
Units of Refrigeration Capacity
• Tonnage – One Ton of Refrigeration implies latent heat of
one tonne of ice or the heat required to convert one tonne
of water to ice. Equivalent to 12,000 BTUs. One BTU is
the heat required to raise one pound of water by one
degree Fahrenheit.
• One calorie is the heat required to raise one gram of
water by one degree Celsius.
• Joule is the mechanical equivalent of heat, work done
when one a force of one Newton moves through one
metre.
• One Joule = 4.2 calories
• Calculate the conversion factor from BTUs to Kilocalories
Refrigerant
• Refrigeration depends upon a substance called
the refrigerant, which can readily be converted
from liquid into a vapour (evaporation) and also
from a vapour into a liquid (condensation) within a
narrow range of pressures. Refrigerants are those
fluids, which are used as working fluids, for
example in vapour compression refrigeration
systems. These fluids provide refrigeration by
undergoing a phase change process in the
evaporator.
• A refrigerant gives up heat by condensing at high
temperatures and pressures and absorbs heat by
evaporating at low temperatures and pressures.
REFRIGERANTS
Halocarbon based
Ethane based
CFCs
Non-Halocarbon based
Methane based
HCFCs
HFCs
Primary
Secondary
Pure
Azeotropic
Blended
Near Azeotropic
Zeotropic
• Azeotropic Refrigerants: Mixture of two or more refrigerants with
similar boiling points. Where the change of phase takes place at a
specific single temperature.ex: R-500;R-502; R-503; R-507. They
can be charged as a liquid or vapour.
• Zeotropic Refrigerants: Mixture of two or more refrigerants with
dissimilar boiling points. Where the change of phase takes place
over a temperature range of over 5.56 degrees C (10 deg F).ex: R404a; 407c. They should be charged as a liquid.
• Near Azeotropic Refrigerants: : Mixture of two or more refrigerants
with dissimilar boiling points, but the change of phase takes place
over a temperature range of less than 5.56 degrees C (10 deg F). R410a. They should be charged as a liquid.
• TEMPERATURE GLIDE is the range of temperature over which
change of phase occurs. Important because fractionating of
refrigerant occurs especially when there is a leak. Also complicates
the charging process.
Generation of Refrigerants
• First Generation 1830-1930s ex: CO2;
NH3; HCs;SO2 etc. usefulness of volatile
compounds
• Second Generation: 1931-1990s ex:
CFCs; HCFCs; safety & durability
• Third Generation; 1990-2010s ex: HCFCs
and HFCs; Ozone Layer protection
• Fourth Generation; 2010 onwards ex: Pure
& Blended HCs; Global warming; high
efficiency
Refrigerant selection criteria
•
Selection of refrigerant for a particular
application is based on the following
requirements:
1. Thermodynamic and thermo-physical
properties
2. Environmental and safety properties, and
3. Economics
Thermodynamic and thermophysical properties
• The requirements are:
1. Suction pressure: At a given evaporator
temperature, the saturation pressure should be
above atmospheric for prevention of air or
moisture ingress into the system and ease of
leak detection. Higher suction pressure is
better as it leads to smaller compressor
displacement
2. Discharge pressure: At a given condenser
temperature, the discharge pressure should be
as small as possible to allow light-weight
construction of compressor, condenser etc.
3. Pressure ratio: Should be as small as possible
for high volumetric efficiency and low power
consumption
4. Latent heat of vaporization: Should be as large
as possible so that the required mass flow rate
per unit cooling capacity will be small
5. Isentropic index of compression: Should be as
small as possible so that the temperature rise
during compression will be small
6. Liquid specific heat: Should be small so that
degree of subcooling will be large leading to
smaller amount of flash gas at evaporator inlet
7. Vapour specific heat: Should be large so that
the degree of superheating will be small
8. Thermal conductivity: Thermal conductivity in
both liquid as well as vapour phase should be
high for higher heat transfer coefficients
9. Viscosity: Viscosity should be small in
both liquid and vapour phases for smaller
frictional pressure drops
10. The freezing point of the refrigerant
should be lower than the lowest
operating temperature of the cycle to
prevent blockage of refrigerant pipelines.
11. High critical temperature
Environmental properties
•
1.
2.
3.
4.
5.
The important environmental properties are:
Ozone Depletion Potential (ODP)
Global Warming Potential (GWP)
Total Equivalent Warming Index (TEWI)
Atmospheric Lifetime
Chlorine Leading Potential
Safety and Other Properties
1.
2.
3.
4.
5.
6.
7.
8.
TLV (Threshold Limit Value)
Toxicity
Flammability
Chemical stability
Compatibility with common materials
Miscibility with lubricating oils
Dielectric strength
Ease of leak detection
Ozone Depletion Potential (ODP):
According to the Montreal protocol, the ODP of
refrigerants should be zero, i.e., they should be
non-ozone depleting substances. Refrigerants
having non-zero ODP have either already
been phased-out (e.g. R 11, R 12) or will be
phased-out in near-future (e.g. R22). Since
ODP depends mainly on the presence of
chlorine or bromine in the molecules,
refrigerants having either chlorine (i.e., CFCs
and HCFCs) or bromine cannot be used under
the new regulations
Global Warming Potential (GWP)
Refrigerants should have as low a GWP
value as possible to minimize the problem
of global warming. Refrigerants with zero
ODP but a high value of GWP (e.g.
R134a) are likely to be regulated in future.
Total Equivalent Warming Index
(TEWI)
• The factor TEWI considers both direct
(due to release into atmosphere) and
indirect (through energy consumption)
contributions of refrigerants to global
warming. Naturally, refrigerants with as a
low a value of TEWI are preferable from
global warming point of view.
Atmospheric Lifetime
• HFC125, the major component of HFC
blend refrigerants, has an atmospheric life
of 29 years, while the atmospheric life of
HFC32 is only five years.
Toxicity
Ideally, refrigerants used in a
refrigeration system should be non-toxic.
However, all fluids other than air can be
called as toxic as they will cause
suffocation when their concentration is
large enough. Thus toxicity is a relative
term, which becomes meaningful only
when the degree of concentration and
time of exposure required to produce
harmful effects are specified.
Toxicity
• Some fluids are toxic even in small
concentrations. Some fluids are mildly toxic, i.e.,
they are dangerous only when the concentration
is large and duration of exposure is long. Some
refrigerants such as CFCs and HCFCs are nontoxic when mixed with air in normal condition.
However, when they come in contact with an
open flame or an electrical heating element, they
decompose forming highly toxic elements (e.g.
phosgene-COCl2).
Toxicity
•
1.
2.
3.
4.
5.
In general the degree of hazard depends on:
Amount of refrigerant used vs total space
Type of occupancy
Presence of open flames
Odor of refrigerant, and
Maintenance condition
•
Thus from toxicity point-of-view, the usefulness
of a particular refrigerant depends on the
specific application.
Flammability
• The refrigerants should preferably be nonflammable and non-explosive. For
flammable refrigerants special precautions
should be taken to avoid accidents.
Chemical stability & Compatibility
• The refrigerants should be chemically
stable as long as they are inside the
refrigeration system.
• Compatibility with common materials of
construction (both metals and non-metals)
Miscibility with lubricating oils
• Oil separators have to be used if the
refrigerant is not miscible with lubricating
oil (e.g. ammonia). Refrigerants that are
completely miscible with oils are easier to
handle (e.g. R12). However, for
refrigerants with limited solubility (e.g. R
22) special precautions should be taken
while designing the system to ensure oil
return to the compressor
Dielectric strength
• This is an important property for systems
using hermetic compressors. For these
systems the refrigerants should have as
high a dielectric strength as possible
Ease of leak detection
• In the event of leakage of refrigerant from
the system, it should be easy to detect the
leaks.
Economic properties
• The refrigerant used should preferably be
inexpensive and easily available.
Halocarbon Refrigerants
• Methane (CH4)Based will have a two-digit number ex:R22
First figure shows no. of Hydrogen atoms plus 1 Second
figure shows no. of fluorine Atoms R22=CHClF2
Chlorofluoromethane. The total number of Hydrogen and
replacement atoms should be 4; R11; R12; R13; R23 etc.
• Ethane (C2H6) Based will have a three-digit number ex:
R113. First figure shows no. of Carbon atoms minus 1;
Second figure shows no. of Hydrogen atoms plus 1; Third
figure shows no. of fluorine Atoms R113 =CCl2FCClF2
Tricholorofluoroethane; R114; R115 etc.
• R134a=C2H2F4 Tetrafluoro ethane The letter a signifies
isomer ( having same chemical composition but different
atomic arrangement)
• Refrigerants starting with 4 and 5 indicate blended
refrigerants
• The most important members of the group
have been –
• Chlorofluorocarbons (CFC)
• Hydrochlorofluorocarbon (HCFC)
• Hydrofluorocarbon(HFC)
Chlorofluorocarbons (CFC)
• CCl2F2 - Dichlorodifluoromethane (Freon
12 or R12)
• CCl3F - Trichlorofluoromethane (Freon 11
or R11)
• C2Cl2F4 - Dichlorotetrafluoroethane (Freon
114 or R114)
• C2Cl3F3 - Trichlorotrifluoroethane (Freon
113 or R113)
Hydrochlorofluorocarbon (HCFC)
• CHClF2 – monochlorodifluoromethane
(Freon 22 or R22)
• R123 – C2HCl2F3
• R124 - C2HClF4
Hydrofluorocarbon(HFC)
•
•
•
•
•
C2H2F4 - R134a
C2H4F2 - R152a
C2HF5 - R125
CH2F2 - R32
C2H3F3 - R143a
Inorganic refrigerants
• These are designated by number 7 followed
by the molecular weight of the refrigerant
(rounded-off).
Mixtures
• Azeotropic mixtures (containing two gases
with same boiling point ) are designated by
500 series, where as zeotropic (containing
two or more gases not having same
boiling point ) refrigerants (e.g. nonazeotropic mixtures) are designated by
400 series.
Pure Hydrocarbon Refrigerants
Depletion of stratospheric ozone layer
• The depletion of stratospheric ozone layer
was attributed to chlorine and bromine
containing chemicals such as CFCs,
HCFCs. If released to atmosphere, they
are broken down by photolysis to release
chlorine atoms, which catalytically destroy
ozone, the stratospheric gas which acts as
a filter to ultra violet (UV) light from the
sun.
Effect of UV light
• Scientists predict that increased UV light
on earth as a result of ozone depletion will,
amongst other possible consequences,
cause skin cancer, interfere with immune
systems and harm aquatic systems and
crops. Strong pressure was exerted to
phase out CFCs and HCFCs, resulted in
the Montreal Protocol being adopted in
1987.
Regulation 12(2) of Annex VI of
MARPOL 73/78
• New installations which contain ozonedepleting substances shall be prohibited
on all ships, except that new installations
containing hydrochloroflourocarbons
(HCFCs) are permitted till 1 January 2020.
Alternate refrigerants
•
They can be classified into two broad
groups:
1. Non-ODS, synthetic refrigerants based
on Hydro-Fluoro-Carbons (HFCs) and
their blends
2. Natural refrigerants including ammonia,
carbon dioxide, hydrocarbons and their
blends
Refrigeration oils
1.
2.
3.
4.
5.
6.
7.
Lubricating oils for refrigeration compressors
are selected for their suitability with the different
refrigerant, compressor type and the plant’s
operating temperatures. Refrigeration oils
should possess the following properties:
Good chemical stability
Good thermal stability
Low viscosity
Low wax content
Low pour point
Low Floc Point
Moisture free
Good chemical and thermal
stability
• Good chemical stability: There
should be little or no chemical
reaction with the refrigerant or
materials normally found in the
system.
• Good thermal stability: They should
not form hard carbon deposits at hot
spots in the compressor (such as
valves or discharge ports).
Low viscosity
As the oil particles are carried along with the
refrigerant through out the system, the oil is
subjected to extreme temperatures - low
temperature in the evaporator and high in the
cylinder head. Therefore it should be in a
position to flow freely at both low and high side
temperature. This is the ability of an oil to
maintain good lubrication properties at high
temperatures and good fluidity at low
temperatures, i.e. to provide a good lubricating
film at all times.
Low wax content & low pour
point
•
•
Low wax content. Plants are operating at
low evaporating temperatures, as
separation of wax particles from the
refrigerant-oil mixture may cause
problems by blocking expansion and
regulating valves.
Low pour point. Ability of the oil to remain
in a fluid state at the plant’s lowest
evaporating temperature.
Low Floc Point
The refrigerant oil may contain certain amount of
paraffin which will precipitate at low temperature.
The floc point is the temperature at which wax
will start to precipitate from a mixture of 90% of
refrigerant and 10% of oil by volume. If an oil of
high floc point is used, wax will separate at
expansion valve and restrict flow of refrigerant.
So a low floc point is necessary for an oil to be
used in the refrigeration system.
Moisture free
Any moisture added with oil may cause
corrosion, and in the case of CFC and
HCFC refrigerants would form as ice in a
choked expansion or regulating valve.
• When adding oil to a compressor, or doing an oil
change, it is therefore important that only the
type specified in the manufacturer’s operating
manual is used. The oil must be clean and have
no moisture content. Oil should always be stored
in tightly sealed containers, in a warm place, to
ensure it does not absorb moisture from the
atmosphere. It is important that the procedures
given in the compressor operating manual for
changing and topping- up the oil are strictly
followed.
Refrigeration
• Refrigeration is a process of cooling by the transfer of
heat. Heat is a form of energy and is indestructible so if
heat is removed from a space or substance to cool it to a
temperature below that of its surroundings, the heat
removed must be discarded to some substance at a
higher temperature where it is of no consequence. Since
heat will not flow freely from a body at a low temperature
to another at a higher temperature, it is necessary to
expend mechanical work, heat, or electrical energy from
an external source to achieve it. Refrigeration thus
depends on thermodynamics, heat transfer and fluid flow
for its practical achievement.
• The refrigerant, if first stored as a liquid under
pressure, then allowed to flow at reduced
pressure through an evaporator coil in the
closed system, will withdraw heat from its
surroundings during the evaporation stage. The
heat so absorbed is removed from the
refrigerated area when the vapour returns to that
portion of the refrigeration equipment designed
to cool down and compress it again to the liquid
state for reuse.
• The two main refrigeration systems in
commercial use are the absorption system and
the vapour compression system. Most marine
refrigerating plants are of the vapour
compression type.
Vapour compression cycle
• The vapour compression
cycle takes place in a
closed system,
comprising a compressor,
a condenser, a liquid
receiver, an evaporator,
and a flow control or
expansion valve,
interconnected by
discharge, liquid and
suction lines
• The liquid refrigerant e.g. R22 is stored at high
pressure in the liquid receiver. The liquid flows
from the liquid receiver through the liquid line to
the expansion valve, which regulates the rate of
flow to the evaporator to suit the rate of
evaporation. As it passes through the expansion
valve, the pressure of the liquid is reduced to the
evaporating pressure, so that the saturation
temperature of the refrigerant entering the
evaporator is below that required in the
refrigerated space. Note that as it passes
through the expansion valve, a portion of the
liquid evaporates instantly (flash gas) in order to
reduce the temperature of the remaining liquid to
the evaporating temperature.
• The liquid-vapour refrigerant mixture then flows
through the evaporator, where it extracts heat
from the refrigerated space, and changes to a
dry saturated vapour at approximately the same
temperature and pressure as that at which it left
the expansion valve.
• The evaporating pressure is maintained
constant by the action of the compressor, which
removes vapour from the evaporator at the
same rate as that at which it is formed. In
practice, the expansion valve regulating the
refrigerant flow is designed to ensure that the
vapour leaving the evaporator is slightly
superheated, thus ensuring that only dry vapour
is handled by the compressor.
• In the compressor, the temperature and pressure of the
vapour are raised by compression. The compressed
vapour flows through the ‘hot gas’ discharge line from
the compressor into the condenser, using water or air as
the cooling medium. The vapour in the condenser first
gives up its superheat as it is cooled from the discharge
temperature to the saturation temperature corresponding
to the condensing pressure, and then gives up its latent
heat as it condenses back to a liquid. The liquid then
flows from the bottom of the condenser into the liquid
receiver, thereby completing the cycle. When its
temperature is below the condensing temperature, it is
said to be subcooled.
Molliers Chart
• Diagrammatic Representation of properties
of Refrigerant. Useful for designing the
Refrigeration system
• Gives enthalpy of refrigerant at various
pressures and physical states (liquid,
vapour, mixture)
• Also called Pressure-Enthalpy Chart.
• Enthalpy is the total heat content in a
substance BTUs or KJs
• Specific Enthalpy is Enthalpy per unit mass
BTU/lb or KJ/kg
Constant
Entropy
LInes
Constant
Temperature
Lines
Pressure-enthalpy (P-H) or Mollier
diagram
•
•
•
•
•
•
•
•
Line A to B represents the change from
high to low pressure, or expansion
process
Line B to B’ represents the amount of
liquid ‘flashed-off’ in the expansion valve
cooling the remaining liquid.
Line B to C represents the evaporation
process at constant saturation
temperature and pressure in the
evaporator. At point C the refrigerant is a
dry saturated vapour.
Line C to C’ represents the superheat
absorbed by the dry saturated vapour
Line C’ to D represents the compression
process.
Line D to E represents the superheat
given up by the vapour in the condenser.
At point E the refrigerant is a dry
saturated vapour.
Line E to F represents the condensation
process at constant saturation
temperature and pressure. At point F the
refrigerant is a saturated liquid.
Line F to A represents the sub cooling of
the condensed liquid
Refrigerating effect
• The amount of heat absorbed by each unit mass of
refrigerant as it flows through an evaporator is known
as the refrigerating effect, and is equal to the
difference between the enthalpy of the vapour leaving
the evaporator and the enthalpy of the liquid at the
flow control.
• Thus, for the system shown in Fig 3, refrigerating
effect,
Refrigerating capacity
• The rate at which a system will absorb heat from the
refrigerated space or substance is known as the
refrigerating capacity, and is expressed as,
refrigerating capacity,
• where m = mass flow of refrigerant through the
evaporator (kg/s).
• To achieve a specified refrigerating capacity of
15OkW, say, the required mass flow rate is
Compressor capacity
• The capacity of a compressor must be such that it
removes the vapour from the evaporator at the same
rate as that at which it is formed. To maintain a specified
operating condition, a compressor must have a swept
volume equal to the volume of vapour formed in the
evaporator per unit time (m3/h).
• To maintain constant operating conditions and produce
the required refrigeration duty would require a
compressor with a swept volume:
• V = m x v m3
• where v = specific volume of the vapour at the
compressor suction inlet, m3/kg, and v at -25°C and
1.32 bar
• = 0.18m3/kg.
• i.e. V = 0.94 x 0.18 x 3600 = 609 m3/h.
Heat of compression
• The energy input from the compressor motor
to raise the pressure of the vapour to the
required condensing temperature is known as
the heat of compression, and is equal to the
difference between the enthalpy of the vapour
at the compressor outlet and inlet.
• Heat of compression,
Condenser duty
• The rate of heat transfer from the refrigerant
in the condenser to the cooling medium is
known as the condenser duty, and is
expressed as,
• = 0.94 (470 – 230.3) = 225.3 kW
Coefficient of performance
• The ratio of refrigerating effect to the heat of
compression is known as the coefficient of
performance (COP).
• Thus for the system shown in Fig 3,
Properties of Marine
Refrigerants
Ammonia R11
( R 717)
R12
R22
R502
- 33.3
23.6
-29.6
-40.6
-45.6
Absolute evaporator pressure 2.3
at -15°C (Bar)
0.2
1.6
3.0
3.5
Absolute condensing
pressure at 30°C (Bar)
11.6
0.9
7.4
12.0
13.0
Latent heat at -15°C (kJ/kg)
1314
194
159
217
166
Coefficient of performance at
-150C evaporator, 30°C
condensing
4.77
5.03
4.71
4.67
4.37
Evaporating temperature at
atmospheric pressure (0C)
Automatic freon system on
board ship
• Each cold compartment has a thermostatic
expansion valve, which acts as the regulator
through which the correct amount of refrigerant
is passed.
• The sketch is for a three compartment system
but shows only the detail of one. Each room has
a solenoid, thermostatic expansion valve and
evaporator. Air blown through the evaporator
coils acts as the secondary refrigerant. Regular
defrosting by means of electric heating
elements keeps the evaporator free from ice.
The automatic defrost time switch de-energizes
the solenoids to shut down the system and
diverts electrical current to the heaters, for a set
period.
“Pump-down cycle.”
• The refrigerated space temperature is monitored
by a thermostat which acts to open and close
the solenoid valve in the liquid line. The routine
starting and stopping of the compressor is
controlled by the low pressure cutout switch in
the compressor suction line. As the space
temperature is reduced to the set point of the
thermostat, the thermostat contacts open, deenergizing the solenoid valve and stopping the
flow of refrigerant to the evaporator. Continued
compressor operation reduces the suction
pressure to the set point of the low pressure
switch, stopping the compressor.
• Due to the stopping of refrigeration, the space
temperature will slowly rise. When the
temperature reaches the thermostat set point,
the contacts close, energizing the solenoid
valve, thus permitting liquid refrigerant to enter
the evaporator. The refrigerant vaporizes,
raising the suction pressure to the cut-in point of
the low pressure switch, thus starting the
compressor. There is also an HP (high pressure)
cut-out with a hand re-set which operates to shut
down the compressor in the event of excessively
high discharge pressure.
System components
1.
2.
3.
4.
5.
6.
7.
Pressure gauges
Compressors
Oil separators
Condensers
Liquid receiver
Liquid indicator
Evaporators
8. Expansion valves
9. High pressure cut-out
10. Low pressure cutin/cut-out
11. Oil pressure cut-out
12. Room temperature
control
13. Solenoid valves
14. Drier
Pressure gauges
• The pressure gauge on
the compressor suction
and discharge shows the
gas pressure and also
has marked on it the
relative condensing
temperature. It helps to
check correct pressuretemperature relationships
for refrigerants. Pressure
gauge in the lub oil line is
normally of differential
type.
Compressors
• Refrigeration compressors are usually
reciprocating type for marine refrigeration.
It may be of the rotary screw displacement
type or of the scroll type.
Reciprocating compressors
• Reciprocating compressors for systems
cooling domestic store rooms are usually
of the vertical in-line type. The larger
reciprocating compressors have their
cylinders arranged in either V or W
formation with 4,6, 8, 12 or even 16
cylinders. Compressor speeds have been
increased considerably over the years
from 500 rev/min to the high speed of
1500 to 2000 rev/min.
V-belt drive
• Compressors are
usually driven by Vbelts. Most are driven
at less speed than the
motor speed. Pulleys
must be in perfect
alignment and the
pulley shafts
(compressor and
motor) must be
parallel to each other.
• Each crank of the crankshaft for the compressor
shown carries three bottom ends. The
aluminium alloy pistons operate in cast iron
liners, which are honed internally. Piston rings
may be of plain cast iron but special rings having
phosphor-bronze inserts are sometimes fitted.
These assist when running in. Connecting rods
are H section steel forgings with white metal
lined steel top end bushes. The crankcase and
cylinders comprise a one-piece iron casting.
Main bearings are white metal lined steel shells.
• Gas from the evaporator passes through a
strainer housed in the suction connection
of the machine. This is lined with felt to
trap scale and impurities scoured from the
system by the refrigerant particularly
during the running-in period. Freons tend
to clean the circuit but the impurities will
cause problems unless removed by
strainers. Any oil returning with the
refrigerant drains to the crankcase through
holes in the diaphragm plate.
Suction and Discharge Valve
• There is a valve plate under
cylinder head with plate type
of suction and discharge
valve located in it. Large
diameter and very small lifts
of plates offer the least
resistance to the flow of
refrigerant gas. Heavy
springs on the discharge
valve cage permit a greater
valve lift to protect
compressor in case of
severe liquid refrigerant or
oil pumping.
Lubrication
• Oil is supplied to the
bearings and crankshaft seal
by means of a gear pump
driven from the crankshaft.
Oil pressure is about 2 bar
above crankcase pressure
and the differential oil
pressure gauge is necessary
to compare oil pressure with
that of the gas in the
crankcase. There is a relief
valve in the oil system set to
about 2.5 bar above
crankcase pressure.
Protection against oil failure
is provided by a differential
oil pressure switch.
Crankcase heaters
• A certain amount of refrigerant will always be dissolved
in the lubricating oil. However, large amounts of
refrigerant in the oil are undesirable. Excessive dilution
can result in inadequate lubrication. In addition, during
compressor start-up, the lowering of the crankcase
pressure will cause oil foaming due to the vaporization of
the refrigerant. In severe cases, this can disrupt
lubrication and can cause carryover of the liquid
refrigerant and oil into the cylinder. Since marine
systems typically operate on the pump-down cycle, the
low crankcase pressure at shutdown limits refrigerant
absorption by the oil. Crankcase heaters which come on
automatically during the compressor off cycle are used
to keep the oil warm and reduce refrigerant absorption.
Shaft seal
• A mechanical seal is fitted
around the crankshaft at the
drive end of the crankcase. This
prevents leakage of oil and
refrigerant from the crankcase.
All seals use two rubbing
surfaces. One surface turns with
the crankshaft and is sealed to
the shaft with an O-ring of
synthetic material. The other
surface is stationary and
mounted on the housing with
gasket. The rubbing surfaces
can be hardened steel and
bronze or ceramic and carbon.
The mechanical seal is
lubricated from the compressor
system.
Unloader mechanism
• The most common
method of varying the
capacity of multi-cylinder
compressors is to vary
the number of active
cylinders by holding the
suction valves open. The
capacity control system
unloads cylinders (i.e.,
cuts cylinders out of
operation) in response to
decreases in refrigeration
load based on suction
pressure.
• A bellows device, actuated by suction pressure
can serve to cut out one or more cylinders.
Under high loads (high suction pressures) none
of the suction valves are held open, and all the
cylinders are in operation. As the load
decreases (and the suction pressure falls off),
the cylinders are cut out in sequence. If the
suction pressure continues to fall off, the
compressor will stop on the low pressure switch.
Compressor lubricating oil is used to operate the
valve lifting mechanism. Since oil pressure is
required to load the cylinder, the compressor will
start with all controlled cylinders unloaded, thus
reducing the starting load on the compressor
motor.
Oil separators
• Oil separators may be fitted in
hot gas discharge lines from
the compressor. It is a closed
vessel fitted with a series of
baffles or a knitted wire mesh
through which the oil-laden
vapour passes. The reduction
in velocity of the vapour as it
enters the larger area of the
separator allows the oil
particles, which have greater
momentum, to impinge on the
baffles. The oil then drains by
gravity to the bottom of the
vessel where a float valve
controls flow to the compressor
crankcase.
Manual Valves
• Manual valves are installed
at various locations in the
refrigeration system to
facilitate system operation,
to isolate components for
maintenance and for other
purposes. Most of the valves
used in refrigeration
systems are of the packed
valve type. They are of the
backseating type. When in
the open position, the valve
is backseated to minimize
the possibility of leakage.
Condensers
• Most marine refrigeration condensers are of the watercooled, multipass, shell-and-tube type. Seawater is
circulated through the tubes, and the hot gas from the
compressor discharge is admitted to the shell and
condenses on the outer surfaces of the tubes. The
condenser is typically constructed of a steel shell,
copper-nickel tubes and tube plates, and bronze
waterheads. Gas inlet, liquid outlet, purge, and water
regulating valve control connections are provided.
Liquid Receiver
• The receiver collects the liquid refrigerant
draining from the condenser. It consists of a
steel shell with steel dished heads welded on
each end. Sight glasses or a liquid level
indicator is installed to permit determination of
the amount of liquid refrigerant in the receiver.
The receiver will typically have sufficient
capacity to hold the entire system refrigerant
charge and will retain a small liquid level during
full load operation. High levels indicate
overcharge and low levels indicate undercharge.
• Sometimes bottom part of condenser serves as
the liquid receiver
Liquid Indicators (Sight Glasses)
• Liquid indicators or sight glasses
are commonly installed in the
liquid line to indicate a proper
refrigerant charge. Bubbles
appearing in the liquid stream are
an indication of a shortage of
refrigerant. Some indicators also
include a moisture indicator. A
portion of the indicator will
change color based on the
relative moisture content of the
refrigerant.
Refrigerant drier
• Shell is filled with a
desiccant such as
activated alumina or silica
gel which absorbs
moisture and also acts as
a filter. Even small
amounts of moisture can
cause problems such as
frozen thermostatic
expansion valves, so it is
important to remove
sufficient moisture to
prevent the release of
water in the low pressure
portions of the system.
Evaporators
• Marine evaporators are usually forced
convection evaporators. They are in
common use in ship’s provision
refrigeration and air conditioning systems.
The units consist of a cooling coil with
finned tubes, a motor-driven fan, and drain
pan, all enclosed in a sheet metal casing.
Units designed for sub-zero temperature
applications are usually fitted with an
electric resistance defrost system.
Expansion device
• The basic functions of an expansion device used in
refrigeration systems are to:
1. Reduce pressure from condenser pressure to evaporator
pressure, and
2. Regulate the refrigerant flow from the high-pressure
liquid line into the evaporator at a rate equal to the
evaporation rate in the evaporator
• Under ideal conditions, the mass flow rate of refrigerant
in the system should be proportional to the cooling load.
It is desirable that liquid refrigerant should not enter the
compressor. In such a case, the mass flow rate has to
be controlled in such a manner that only superheated
vapour leaves the evaporator.
Thermostatic Expansion Valve
• While there are a number of devices available to control
the flow of refrigerant to the evaporator, such as the
capillary tube, the float valve, and the constant-pressure
expansion valve, the thermostatic expansion valve is the
device most commonly found in marine systems. The
thermostatic expansion valve responds to the evaporator
temperature and pressure and maintains a constant
superheat at the outlet of the evaporator. As refrigerant
is fed to the evaporator, the liquid boils off into a vapour.
By the time the refrigerant gas reaches the end of the
evaporator, it is superheated. Feeding more refrigerant
to the evaporator will lower the superheat temperature,
while feeding less refrigerant will raise the superheat
temperature.
• This consists of a feeler
bulb that is attached to
the evaporator exit tube
so that it senses the
temperature at the exit of
evaporator. The feeler
bulb is connected to the
top of the bellows by a
capillary tube. The feeler
bulb and the narrow tube
contain some fluid that is
called power fluid.
system. To ensure the
correct operation of the
valve, the bulb must be
securely clamped to the
evaporator outlet line.
• Pb - Bulb pressure on the upper
side of the diaphragm, tending
to open the valve, where Pb is
the saturation pressure of the
refrigerant in the bulb,
corresponding to the
temperature of the gas at the
evaporator outlet.
• Po - Evaporator pressure on
the lower side of the
diaphragm, tending to close
the valve, where Po is the
saturation pressure of the
refrigerant at the evaporator
inlet, and Δp is the pressure
drop between the evaporator
inlet and outlet.
• Ps - Pressure exerted by the
regulating spring, tending to
close the valve. The spring
tension, set by the regulating
spindle, controls the degree of
superheat; a typical superheat
value is 4°C to 6°C.
• At any constant operating condition, these forces
are balanced and Pb = Po + Ps
• If the superheat starts to rise, the bulb pressure
increases, Pb > Po +Ps and the valve is moved in
the opening direction, admitting more liquid and
restoring the constant operating condition.
• If the superheat falls, Pb < Po + Ps and the valve
is moved to the closing position, reducing the
supply of liquid.
• In practice, to achieve the desired degree of
superheat at the evaporator outlet, dry
expansion evaporators require up to 20 per cent
of their cooling surfaces to be available to
superheat the gas, the precise area varying with
demand.
TEV with external equalizing connection
• There is an appreciable
pressure drop in the large
evaporators. Additional
control is introduced by
incorporating a pressure
equalising connection.
This connection
eliminates further
increase in the superheat
temperature to
compensate for the
reduction in pressure,
and so allows an increase
in the effective area of the
evaporator.
Back pressure valve in vegetable
room
• If the evaporator in
vegetable room is kept at
common pressure of
meat room and fish room,
then ice formation will
take place in this
evaporator also. This ice
will be formed from the
moisture of fruits and
vegetables stored in the
room and they will
become desiccated.
• The bellows pressure
area and the valve
area are equal. The
adjustment spring and
the evaporator
pressure change can
operate the valve
motion. The valve has
a gauge opening to
check pressure of the
evaporator. For R22
system, this pressure
may be 4 bar gauge.
Solenoid valve
• The solenoid valve is
thermostat controlled valve
which provides automatic
opening of and closing of
liquid line to the
evaporator. When the coil
(3) is energised, the pilot
orifice (4) is opened and
the diaphragm (1) moves
into open position (vice
versa when coil is deenergised).
Thermostats
• Thermostats are temperature-controlled electric
switches. It is used to control the temperature in
a refrigerated space by ‘opening and closing’ a
solenoid valve in the liquid line.
• Three types of element are used to sense and
relay temperature changes to the electrical
contacts.
1. A fluid-filled bulb connected through a capillary
to a bellows.
2. A thermistor
3. A bi-metal element
High Pressure Cut-out
• There are a number of faults which cause
high discharge pressure, including loss of
condenser cooling, air in the system and
overcharge.
• In the event of overpressure on the
condenser side of the compressor the high
pressure cut-out will cause the
compressor to shut down. The device is
re-set by hand.
• The bellows in the cut-out is
connected by a small bore pipe
between the compressor discharge
and the condenser. The bellows
tends to be expanded by the
pressure and this movement is
opposed by the spring. The
adjustment screw is used to set the
spring pressure. During normal
system operation, the switch arm is
held up by the switch arm catch
and holds the electrical contact in
place. Excessive pressure expands
the bellows and moves the switch
arm catch around its pivot. The
upper end slips to the right of the
step and releases the switch arm
so breaking the electrical contact
and causing the compressor to cutout. The machine cannot be
restarted until the trouble has been
remedied and the switch re-set by
hand.
Low pressure controller
• The low pressure control stops the
compressor when low suction pressure
indicates closure of all cold compartment
solenoids. When the pressure in the
compressor suction rises again due to one
or more solenoids opening, the low
pressure control restarts the compressor.
• The controller is operated
through a bellow which
monitors pressure in the
compressor suction. A
pressure differential
between cut out and cut in
settings is necessary to
avoid hunting. The push
pin operates the switch
through a contact which is
flipped open or closed
through a coiled spring
plate. With the contacts
open the spring is coiled as
shown. Outward movement
of the pin compresses the
spring and this then flips
the contact to close the
compressor starting circuit.
Oil pressure safety cut-out
• This is used to protect
against too low oil pressure
in forced lubrication
systems. It is a differential
control, using two
connections. One side
responds to the suction
pressure of compressor
and the other responds to
the oil pressure. If the
differential oil pressure
fails, or falls below a
minimum value, the control
stops the compressor after
a certain time delay.
Carriage of refrigerated cargo
• Refrigerated cargo can be carried in
1. Specialised “Reefer Ships”
2. Refrigerated containers.
Reefer Ships
• Reefer ships are effectively large refrigerators,
heavily insulated with modern glass fibre or
similarly efficient insulation , shuttered with
bright metal that prevents taint and is easy to
clean. They are ships that tend to be divided into
many more spaces than conventional dry cargo
ships, with several ’tweendecks spaces, so that
different commodities can be separated and
carried, if required, at different temperatures.
Cleanliness and the maintenance of optimum
temperatures are the pre-requisites.
• The main features of a
modern, 450,000 ft3 reefer
vessel are as follows.
• Four holds with 4 or 5 cargo
decks, each with the same
clear head of 2.2m to minimize
lost load space when storing
standard pallets (1.2m long x
1.0 m wide x 2.1 m high
maximum). These decks are
arranged usually in eight air
tight temperature zones, with
the air coolers placed along
the bulk-heads serving one or
sometimes two decks. Variable
speed fans are placed above
the coolers forcing air through,
under the grating then
vertically from bottom to top
through the cargo and back to
the coolers.
• Four fast cranes able to handle a 40 ft
container laden, for example, with frozen
meat.
• Space for eighty or more integral
containers on the weather deck, and
space for fork lift trucks and pallet cages.
Air circulation and refreshing
• Between 90 and 120 air changes of the net
volume per hour is usually provided in holds.
Such volume flow guarantees good and uniform
cooling rate of palletized and bulk cargo and
allows for imperfect storage. This air rate will be
reduced on completion of cooling down and
when frozen cargo is carried.
• Air refreshing rates to remove carbon dioxide,
ethylene and other volatiles, can be two or three
air changes per hour.
Brine battery and air
• In this system, brine instead of primary
refrigerant is circulated through the batteries.
Air is cooled in a brine cooling system and cold
air ducted to the cargo spaces.
• Brine is relatively easy to regulate. The system
shown is arranged with three separate
refrigeration and brine circuits with connections
from both brine systems to the air cooler
batteries (or grids). Brine is inexpensive, being
made with calcium chloride and fresh water to a
gravity of about 1.25. Sodium dichromate or
lime may be added to maintain the brine in an
alkaline condition.
Calcium Chloride Brine
CALCIUM CHLORIDE / BRINE
Specific
Gravity
Hydrometer Reading
Freezing Point of Solution
(Twaddell)
°C
°F
1·20
40
-21
-6
1·21
42
-23
-9 5
1·22
44
-25
-13
1·23
46
-27
-17
1·24
48
-30
-21.5
1·25
50
-32
-26
1·26
52
-35
-31
1·27
54
-38
-37
1·28
50
-42
-44
1·29
58
-51
-60
Showing third pair of headers served by a brine heater and third pump,
so that any battery can be individually defrosted by circulating the warm
brine. Also introduced is a brine "injection cross connection from the
delivery of pump No. 1 to the suction of' pump No. 2. Brine injection is
used so that evaporator No. 1 can assist evaporator No. 2 when No. 1 is
set to deliver brine at a lower temperature than No. 2. A further
refinement of this injection is the by-pass arranged across the inlet and
outlet of evaporator No. 2 so that the cooling, of the brine circulating in
No. 2 system can be achieved entirely by injection if desired.
Additionally, there also has to be a brine make-up tank, in
which solid calcium chloride is dissolved, for topping up the
system. An overflow connection from the header tank, a safety
pressure relief line from the brine heater, and a sighting
connection to which the return from any space can be diverted,
are all arranged to terminate over this make up tank.
Decline of reefer ships
• The conventional reefer fleet shows a
negative growth rate since 1994, while the
container fleet expanded at an even higher
rate than anticipated. There are approx.
1250 reefer ships larger than 10,000 ft3 in
operation today, with an average size of
267,000 ft3.
• The largest reefer capacity in a single ship
is now found, not on a "traditional" reefer,
but aboard a large container ship.
Refrigerated containers
•
Two basic types of refrigerated containers were
developed for use in the international reefer trade.
1. The insulated box connected to the ship’s central plant
and a cold air circulation system; sometimes known as
an isotherm container, or porthole container.
2. The insulated box incorporating its own ‘plug-in’
refrigeration unit within the standard module; usually
known as an integral container.
• The containers most frequently found in practice are
20ft (6.097m) or 40ft (12.19m) long, 8 ft (2.4m) wide
and either 8ft or 8.5ft (2.56m) high.
Porthole container
•
Systems designed for the cooling
of refrigerated containers employ
trunkings arranged so that
containers stowed in stacks
between built-in guide rails, can
be connected to the suction and
delivery air ducts of the ship’s
refrigeration plant by bellows
pieces operated pneumatically.
The air is cooled either by brine or
direct expansion batteries and the
containers are arranged so that
one cooler can maintain a stack of
containers at a given temperature.
The temperature of the return air
duct for each container is
monitored. Provision of a cooler
and trunking system for
maintaining container
temperatures must also be
provided at container terminals.
Refrigeration machinery
• Refrigeration systems in porthole container ships are generally
similar to those used in conventional reefers, i.e. the primary
refrigerant-usually R22-is circulated through compact shell and tube
evaporators with refrigerant flow being controlled by conventional
thermostatic expansion valves.
• The brine, when cooled to the appropriate temperature, is circulated
through finned tube air-to-brine heat exchangers, with some form of
bypass valve to maintain the air at the prescribed temperature.
• It is advisable to keep a mean temperature difference between brine
and air of 3°C to 40C, so that relative humidity within containers is
maintained at a high level of approximately 90%.
• Defrosting of coolers is usually achieved by circulating hot brine, the
time taken being about 30 minutes.
Intermodal Refrigerated
Containers
• The number of units in operation in 2005 is
approximately 750,000, equivalent to 1,270,000
TEU. TEU “Twenty foot Equivalent Unit” refers to
a unit of volume corresponding to a twenty foot
ISO container, which used to be the standard
container. But today 40 foot containers of the
“HighCube” type dominate the market for new
reefer containers. Their size surpasses two TEU
and therefore the increase in container traffic
exceeds the increase in the number of units at
the present time.
• There are no more than 18,000 containers
of the porthole type in operation now and
no new porthole containers are built.
• About 200 new container ships and
160,000 TEU new refrigerated containers
were built annually during the last few
years.
Integral refrigerated container
• The integral container will have an independent
refrigeration unit which enables the container
operator to carry cargoes in the temperature
range -25°C to +2O0C. These units are mostly
electrically driven and are plugged in to
appropriate power points on shore or onboard
ship. Nowadays, a number of units are compact
enough to allow for a removable diesel
alternator set to be fitted when the container is
travelling on the road, or sited in areas where a
suitable 3-phase power supply is not available.
Ships for the carriage of integral
containers
• The holds of these ships are normally uninsulated. In
order to maintain the hold temperature below ambient, a
very powerful ventilation system is fitted to forward and
aft bulkheads with air intakes positioned in the vicinity of
each container integral unit. Fresh air may be supplied to
the holds at tank top level or through openings in the
vicinity of hatches. As soon as the containers are loaded
and positioned up to 6 high in guides, the refrigerating
units are plugged to the electrical power supply and
those containers having water cooled condensers are
connected to fresh or sea water cooling systems
permanently fitted onboard.
• The fresh water is circulated in a closed
circuit and cooled by sea water in heat
exchangers, its outlet temperature being 4
to 5°C higher than the sea water outlet
temperature from the heat exchanger. The
fans of air cooled condensers will not be
running. Some ships can carry over 600
integral containers under the deck.
Refrigerated container on deck
• Individual containers with their own
refrigeration plant are connected to the
440 or 220 V a.c. sockets provided on
deck. These containers may be arranged
for ships’ systems with either 440 or 220 V
by provision of a direct connection for a
22OV supply to the self-contained
refrigerator and a 440V connection
through a step down transformer.
Air cooler fans
• Fans may be either centrifugal
or of the propeller type; the air
circulation systems being
based on a pressure
requirement of about 50 mm
W.G. (water gauge). All of the
electrical energy of the fan
motors is dissipated in the form
of heat and has to be removed
by the refrigerating plant. Fan
output should be variable so
that it can be reduced as heat
load diminishes. There was no
problem with d.c. motors but
with a.c. either the motors are
two speed, or each
Controlled atmosphere
• Fruits and vegetables are actually alive,
their cells are metabolising, consuming
oxygen and carbohydrate and producing
water vapour, carbon dioxide and heat.
Also small amounts of ethylene gas (C2H2)
are produced. Therefore it is not enough to
just refrigerate the cargo, a supply of fresh
air must also be provided.
Controlled atmosphere
• If insufficient fresh air is supplied, several things can
happen:
1. Too much carbon dioxide can produce tissue damage, in
apples this condition is known as brown heart
2. Not enough oxygen can result in anaerobic respiration,
in this case the metabolic pathway is incomplete and the
final product becomes alcohol resulting in alcoholic fruit
3. Too much water vapour can encourage the development
of moulds, rots and fungi. However, not enough is just
as bad as it promotes desiccation of fruit and vegetables
4. Ethylene gas is ripening hormone, very small quantities
building up can promote premature ripening in fruit or
degreening in green vegetables
Controlled atmosphere
• Rate of breathing or respiration can be lowered
and thus ripening slowed down either by
lowering the temperature or by reducing the
amount of oxygen and increasing the amount of
carbon dioxide in the surrounding atmosphere.
• Controlled atmosphere is an inert gas system
used to extend the storage life of seasonal
perishable products and has been used for
many fruits and vegetables; primarily apples and
pears in the past, and now mainly for bananas.
Controlled atmosphere
• To successfully store fruit for long periods, the
natural ripening of the produce has to be
delayed without affecting the eating quality. This
is achieved by reducing the temperature of the
fruit to the lowest level possible without causing
damage through freezing or low temperature
breakdown. To further delay ripening, the
oxygen supply in the space is reduced to levels
below that of the natural atmosphere. This level
is below the level required to support human life.
Controlled atmosphere
• The precise levels of temperature, oxygen and carbon
dioxide required to maximize storage life and to minimize
storage disorders are extremely variable, depending on
type of produce, growing conditions and maturity.
Optimum storage conditions can vary from farm to farm
and from season to season.
• On reefer vessels, oxygen (02) and carbon dioxide (CO2)
levels and relative humidity (RH) in controlled
atmosphere zones (cargo chambers) can be
independently controlled within close tolerances,
irrespective of type, temperature and volume of cargo
carried and the length of the voyage.
• A typical modern controlled atmosphere marine
system would be expected to have flexibility to
control gas levels within the following ranges:
• 02 : < 1 – 8 %
• CO2 : 0-15%
• RH : 40-90%
• The required oxygen and carbon dioxide levels
can be achieved in a number of different ways.
Oxygen level
• The oxygen level can be decreased by:
• a ) injecting pure nitrogen as a gas or
liquid from bottles or storage tanks;
• b) burning propane in an open flame
burner, or a burner with a catalyst;
• c) generating gas (nitrogen with a low
oxygen level) on board from compressed
dry and clean air, using high pressure
membranes, etc.
Carbon dioxide level
•
•
•
•
•
•
•
The carbon dioxide level can be increased by:
a ) injecting carbon dioxide gas;
b) fruit respiration;
The carbon dioxide level can be decreased by:
a ) fresh air or gas injection;
b) hydrated lime;
c) water scrubbing.
Relative humidity
•
•
•
•
Relative humidity can be increased by:
a) injecting water mist;
b) steam;
c) evaporating water, etc.
Human body temperature
• The human body is warm-blooded and
that it must maintain a body
temperature within very close limits. It
uses food as a fuel, converting it into
energy. Some of this energy may leave
the body in the form of external work
done; the balance represents heat
production within the body and is
available for the maintenance of body
temperature. As the body must
produce heat continuously, it must also
lose it at a rate that provides control of
body temperature.
• The human body maintains a basic
minimum rate of heat production at about
75 watts during sleep and about 120 watts
when awake but sedentary. As bodily
activity increases, the rate of oxidation of
food, with its attendant release of energy,
must increase. The level of heat
production for light work will be about 190
watts, the extreme value for heavy work,
about 700 watts.
• It is possible to recognize the main ways in
which the body will lose heat and to relate them
in a simple equation as follows:
• H=S+E+R+C
• where
H = rate of internal heat production
S = rate of heat storage in the body
E = rate of loss by evaporation
R = rate of radiant energy exchange with
surroundings
C = rate of loss by convection
• Most of the heat, however, must be rejected
from the body surfaces through convection
losses to the surrounding air, by radiation
exchanges with surrounding surfaces, and by
the evaporation of perspiration from the skin
when required. The body involuntarily makes
adjustments that influence these processes by
increasing or decreasing the rate of heat loss as
required. It can attempt to induce evaporation by
pouring out perspiration on the skin when it is
too warm.
• Whether or not these measures are effective
depends on the temperature, moisture content
and motion of the air, and the temperature of the
surrounding surfaces. The amount of clothing
becomes a major factor also since it is
interposed between the skin and the influence of
the surroundings and becomes involved in the
convective, evaporative, and radiative losses for
those areas of the body that are covered.
• Increasing the air temperature tends to
reduce convective and radiative losses
and to increase evaporative losses under
sweating conditions. If air temperature
rises above skin or clothing surface
temperature there will be a heat gain
rather than a heat loss by convection, and
this must be offset by increased losses in
other ways.
Perspiration and respiration
• Perspiration and respiration components of the
evaporative loss are dependent upon the rate at which
water is actually evaporated. This depends in turn upon
the degree of saturation of the air, which may be
measured in terms of relative humidity. Thus, there can
be no evaporative loss with air saturated at 37°C,
regardless of the rate of perspiration unless the body
temperature rises above normal. On the other hand,
under conditions that lead to comfort, with only light
activity the main evaporative loss is that from the lungs,
with little or no contribution from perspiration, at least
until the upper limits of comfort are reached.
Air motion
• Air motion is another factor that can have a marked
effect. Increased air speed over the body and clothing
surfaces can increase convective losses and, when
there is perspiration, the evaporative losses as well.
Thus, under conditions of high temperature and high
humidity, discomfort can often be greatly reduced by
increasing the air flow. It is of more than passing interest
to note that under these conditions even high air speeds
can be pleasant. With cooler conditions, however, even
small localized air circulation may give rise to complaints
of chilliness.
Effective Temperature
• It is the
temperature
of still,
saturated air
which would
produce the
same feeling
of warmth.
Effect of Relative humidity
• A marked influence of relative humidity was
found. The value in summer at which 97 per cent
of the subjects were comfortable was 21.50C
Effective Temperature, which corresponds to
conditions of 27.50C degrees at 10 per cent,
24.50C at 50 per cent and 21.50C at 100 per
cent relative humidity. Correspondingly, the
value for winter conditions at which the greatest
proportion of subjects was comfortable was
about 200C, which can be obtained with 260C at
10 per cent and 25.50C at 50 per cent relative
humidity.
Air purification
• Outside air must be introduced to all living
spaces, although the amount of fresh air
necessary to sustain life is very small
indeed. They are governed by factors such
as body odours and smoking, which may
require a fresh air supply of 12 litre/s per
person or more.
Air conditioning
•
1.
2.
3.
4.
Air conditioning is the process of treating
air so as to control simultaneously its
temperature, humidity, cleanliness and
distribution to meet the requirements of
the conditioned space. Action involved:
Temperature control
Humidity control
Air filtering, cleaning and purification
Air movement and circulation
• Winter conditioning relates to increasing
temperature and humidity whilst summer
conditioning relates to decreasing
temperature and humidity. Basically the
practical difference is dependent upon
whether the air fluid is passed over a hot
grid (steam) or cold grid (brine or direct
expansion refrigerant).
Per Cent Relative Humidity
• Is the mass of water vapour per m3 of air
compared to the mass of water vapour per
m3 of saturated air at the same temperature.
This also equals the ratio of the partial
pressure of the actual air compared to the
partial pressure of the air if it was saturated
at the same temperature i.e.
Dalton’s Laws of Partial Pressures
• The pressure exerted by a mixtures of gases
and vapours is the sum of the pressure which
each would exert if it occupied the same space
alone, assuming no interaction of constituents.
• Barometer pressure = partial pressure of N2
+p.p. O2 +p.p. of H2O
• from Dalton’s Laws, viz:
1. Pressure exerted by and the quantity of, the
vapour required to saturate a given space (i.e.
exist as saturated steam) at any given
temperature, are the same whether that space is
filled by a gas or is a vacuum.
Dew Point
• When a mixture of dry air and water vapour has
a saturation temperature corresponding to the
partial pressure of the water vapour it is said to
be saturated. Any further reduction of
temperature (at constant pressure) will result in
some vapour condensing. This temperature is
called the dew point. Air at dew point contains all
the moisture it can hold at that temperature, as
the amount of water vapour varies in air then the
partial pressure varies, so the dew point varies.
• It can be seen that
cooling at constant
pressure brings the low
pressure superheated
vapour to the dew point
after which condensation
occurs. It can also be
noted that cooling at
constant temperature
increases the partial
pressure until the
saturation point is
reached thus relative
humidity can be found .
Dry and Wet bulb temperature
• Dry bulb temperature - Air temperature
indicated by a thermometer in a dry
condition.
• Wet bulb temperature – If a moist wick is
placed over a thermometer bulb, the
evaporation of the moisture from the wick
will lower the temperature reading. This
temperature is known as ‘Wet bulb
temperature’.
Psychrometric Chart
• Psychrometry is the study of the properties of mixtures of
air and water vapour. This subject is important to airconditioning because the systems handle air-water vapor
mixtures, not dry air. Some air-conditioning processes
involve the removal of water from the air-water vapor
mixture (dehumidification) while some involve the
addition of water (humidification). A convenient way to
represent the properties of air-water vapor mixtures is
the psychrometric chart. On the chart, such properties as
dry bulb temperature, wet bulb temperature, dew point,
relative humidity, humidity ratio, specific volume, and
enthalpy are presented in graphical form.
Understanding the
psychrometric chart
Reading the psychrometric chart
•
In order to locate any condition of air
on the chart, two independent
properties must be determined. The
air condition can then be plotted on
the chart, and all other properties
can be read from the chart. Dry bulb
temperatures are read along the
horizontal scale at the bottom of the
chart, humidity ratio is read along the
right-hand vertical scale, and the wet
bulb temperature, dew point
temperature, and enthalpy are read
along the diagonal scale at the
upper left. Lines of constant relative
humidity and specific volume are
labeled in the body of the chart.
• From wet and dry bulb readings the
various properties of the air-vapour
mixture can be estimated. Enthalpy is a
function of the wet bulb temperature, and
moisture content and vapour pressure are
functions of dew point. The chart gives a
quick performance check on the air
entering and leaving the cooling coil, dew
point, temperature, humidity, enthalpy, etc.
Comfort Conditions
• Comfort under summer conditions is dependent
on dry and wet bulb readings i.e. relative
humidity as well as air motion. For a given
degree of air turbulence (75 mm/s to 127 mm/s),
relative humidity between 30% and 70%,
average 50%, and thermometer readings 19°C
to 250C, average 220C, gives the best degree of
summer comfort. Air at low temperature and
high humidity can be as comfortable as air at
high temperature and low humidity.
6 psychometric zone
classification
6 zones worldwide based on temp and humidity conditions
• Hot & Dry: Temp: >33C; RH: 40-12%
• Hot & Humid: Temp: >33C; RH: 33-60%
• Warm & Dry: Temp: 27-33C;RH:4-80%
• Warm & Humid: Temp: 27-33C; RH:20-55%
• Moderate: Temp: 20-27C; RH:30-70%
• Cool: Temp: 15-20C; RH:45-70%
Comfort Conditions
Comfort Conditions
• As the temperature of air is reduced, so too is its
capacity for carrying water vapour. Air, with an initial dry
bulb temperature of 360C and relative humidity of about
60%, will, when cooled to 27°C dry bulb temperature,
have a relative humidity of 100%. The temperature drop
reduces the capacity of the air to carry moisture in
suspension. Further cooling will cause moisture to be
precipitated. Air cooled to a comfortable temperature
level of 21°C but having a relative humidity of IOO%,
would not be able to take up further moisture and
perspiration would not be evaporated. People in an
atmosphere at 21°C with IOO% relative humidity would
be uncomfortable.
• The remedy of dehumidifying the air is achieved by
overcooling to precipitate excess moisture, (removed via
the drain) so that when air is brought to the correct
temperature, its humidity will be at an acceptable level.
Thus the air could be overcooled to about 10°C dry bulb
temperature so that warming to about 210C, would bring
humidity to about 50%. The air is warmed in the trunking
or by contact with warmer air in the space.
• An adequate drain is required to remove what can be a
considerable flow of water from dehumidification of the
air.
Heating
• When the temperature of air is increased, so too is its
capacity for carrying water vapour. Air, with a very low
initial dry bulb temperature of -5°C and relative humidity
of about 5O%, will, when heated to 21°C dry bulb
temperature, have a relative humidity of about 10%. The
temperature rise increases the capacity of the air to carry
moisture in suspension. Air heated to a comfortable
temperature level of 210C but having a relative humidity
of 10% will readily take up moisture whether from
perspiration or from the nasal passages and throat.
People in an atmosphere at 21°C but 10% relative
humidity, would experience discomfort from dryness in
their nose and throat and on the skin.
• The remedy is to humidify the air with a hot
water or steam spray. This action increases
humidity towards 100% relative humidity and
also increases the temperature from -5°C and
50% relative humidity to say, + 7°C. Straight
heating by the zone heater bringing the air to
about 210C, will drop relative humidity to 40%.
The humidity will be at an acceptable level but is
kept low to minimize condensation on any very
cold external bulkheads.
Exercise
• Outside air at 35°C and
60% relative humidity is to
be conditioned by cooling
and heating so as to bring
the air to within the
"comfort zone". Using the
Psychrometric Chart plot
the required air
conditioning process and
estimate (a) the amount of
moisture removed (b) the
heat removed and (c) the
amount of heat added
Cooling to DBT27
RH100%
Over cooling to
DBT15 RH100%
Heating to DBT25
RH50%
Solution
• (a) the amount of
moisture removed
[22-10=11.5g of
H20/kg of dry air]
• (b) the heat removed
[(1)-(2), qcool 88-40=
48kJ/kg-dry-air]
• (c) the amount of heat
added [(2)-(3), qheat
50-40= 10kJ/kg-dryair].
Air conditioning circuit
Marine air conditioning unit
Types of air conditioning
systems
•
Air conditioning systems may be divided into two main
classes – the central unit type in which the air is
distributed to a group of spaces through ducting and the
self-contained type, installed in the space it is to serve.
• The central unit type is the most widely used, in one or
other of a number of alternative systems, characterized
by the means provided to meet the varying
requirements of each of the spaces being conditioned.
The systems in general use are as follows:
1. Zone control system;
2. Double duct system;
Zone control system
• The accommodation is divided into zones,
having different heating requirements.
Separate air heaters for each zone are
provided at the central unit and the
temperature of the air leaving the heater is
controlled. Air quantity control in each
room served gives individual refinement.
• The regulation of temperature by individual
air quantity control in this system can give
rise to difficulties. For instance, a
concerted move to reduce the air volume
in a number of cabins would cause
increased air pressure in the ducts, with a
consequent increase in air flow and
possibly in noise level at other outlets.
Double duct system
• In this system, two separate ducts, one
with cool dehumified air and the other with
warm humid air. These separate
airstreams are mixed just before they
reach the space to be conditioned.
Through dampers that control and balance
air, each different space in the ship can be
conditioned as needed. The air control in
this system is excellent. A single duct air
return is used.
Double duct system
SAFETY
1. Mechanical hazards
2. Electrical hazards
3. Chemical hazards
• Freon cannot be seen or smelt!
• Freon is heavier than air so it will settle and remain at
the bottom of the compartments.
• Freon is extremely harmful if it comes into contact with
the eyes.
• Freon is suffocating because it displaces air.
• If you inhale high concentrations of Freon, it attacks the
nerve system.
• When Freon comes into contact with hot surfaces and
starts to burn, it can give off poisonous gases.
• Freons, if released into the air, may cause
depletion of the Ozone Layer which contributes
to the greenhouse effect. Refrigerants are not to
be released into the atmosphere. They must be
drawn into the condenser/receiver or into a
separate cylinder.
• Most refrigerants mix with oil so oil drained from
a refrigeration system must be clearly labelled
and disposed of separately.
• Refrigerants must not be mixed.
• If you start feeling faint or dizzy as you enter
a compartment - don't think twice - evacuate!
If a refrigerant leak occurs
• Evacuate compartment immediately.
• Sound alarm and get crew in an up-wind
position.
• If leak is in engine room shut down machinery.
• Turn vessel into wind if still possible.
• Do not enter compartment without ventilating the
compartment.
• Ventilate compartment. Remember Freon sinks
to the bottom of the compartment and is very
hard to remove. Try to force airflow down into
the bottom of the compartment to force the
Freon upwards.
• Refrigerant pipes are lagged and constantly damp. This
means that pipe coatings and surface can deteriorate
relatively quickly. Check pipes regularly and make sure
the coating is maintained.
• Inspect Blower Rooms regularly and keep them clean
and dry. Often, they are neglected areas. It is a great
idea to fit an exhaust fan for the blower room and start
the same and wait for a few minutes before entering.
• When entering compressor rooms, start the exhaust fan
and wait for a few minutes before entering.
• Where flexible hoses are used, only use refrigerant
tolerant hoses. Try to avoid using flexible hoses
wherever possible.
• Maintain fittings such as valves and gauges in good
order.
• Mark pipes to show what type of refrigerant they have in
them.
• Refrigerants are supplied in metal cylinders which will
corrode in the salt environment. Make sure these are left
in dry storage (preferably ashore).
• When working on compressor crankcase, for
draining/changing oil, ensure the crankcase is totally
depressurised. Retain a firm grip on the drain plugs and
other connections so that they do not fly off
uncontrollably.
Safe handling of refrigerants.
• Ensure that personnel who handle refrigerants are
properly trained in their safe use and handling, and have
reviewed the MSDS for the refrigerant used.
• Wear safety goggles and gloves at all times when
handling refrigerants or servicing a refrigeration system.
• Wear the proper respiratory protection while working with
refrigerants. Check the MSDS for the proper level of
protection required.
• Proper ventilation or respiratory protection is required for
any work on equipment in an enclosed area where a
leak is suspected.
• Always ventilate or test the atmosphere of an enclosed
area before beginning work. Many refrigerants which
may be undetectable by human senses are heavier than
air and will replace the oxygen in an enclosed area
causing loss of consciousness.
• Inhaling refrigerants can cause sudden death. Intentional
inhalation of refrigerants to produce intoxication can
cause the heart to cease functioning properly and may
be fatal.
• Be certain that the Refrigerant Recovery Cylinder being
used is the Refillable Type and has the capacity to
contain the refrigerant to be added to its contents.
• Refrigerant cylinders should never be filled over 80% of
their capacity (liquid expansion may cause the cylinder
to burst).
• Label the cylinder with the contents using the
appropriate colour code
• Check the I.C.C. cylinder stamp to ensure the cylinder is
safe. Always check the refrigerant number before
charging to avoid mixing refrigerants.
• Always check for the correct operating pressure of the
refrigerant used. Use gauges to monitor the system
pressure.
• Always charge refrigerant into the low side of the system
to avoid damaging the compressor, or causing the
system to rupture.
• R-717 and R-764 are very irritating to the eyes and
lungs. Avoid exposure to these refrigerants.
• R-717 is slightly flammable and mixed with the proper
proportions of air may form an explosive mixture.
• Fluorocarbon refrigerants should be treated as toxic
gases. In high concentrations, these vapors have an
anesthetic effect, causing stumbling, shortness of breath,
irregular or missing pulse, tremors, convulsions, and
even death.
• Ammonia is a respiratory irritant in small concentrations
and is a life threatening hazard at 5,000 parts per million
(ppm).
• Ammonia is also flammable at a concentration of
150,000-270,000 ppm
• Always stand to one side when operating an ammonia
valve. Ammonia can burn and damage the eyes, or
cause loss of consciousness. Ammonia leaks may be
detected by their smell, or with a sulfur candle or sulfur
spray vapor.
• Refrigerant oil in a hermetic compressor is often very
acidic causing severe burns. Avoid skin contact with this
oil.
• Liquid refrigerant on the skin may freeze the skin surface
causing frostbite. If contact with the skin occurs, wash
immediately with water, treat any damaged skin area for
frostbite, and seek medical treatment.
• Never cut or drill into an absorption refrigeration
mechanism. The high pressure ammonia solutions are
dangerous and may cause blindness if the solution
contacts your eyes.
• Ensure that all liquid refrigerant is removed and the
pressure is at 0 psi before disassembling a system.
• Do not smoke, braze, or weld when refrigerant vapors
are present. Vapors decompose to phosgene acid
vapors and other products when exposed to an open
flame or hot surface.
• When soldering, brazing, or welding on refrigeration
lines, the lines should be continuously purged with low
pressure carbon dioxide or nitrogen.
• Following work, the lines should be pressure tested with
carbon dioxide or nitrogen.
• If refrigerant makes contact with the eyes, immediately
wash with mineral oil as this absorbs the refrigerant.
Then wash your eyes with a prepared boric acid solution.
• If the refrigerant is ammonia, wash with water for at least
15 minutes. Seek medical attention as soon as possible.
• Purged refrigerants must not be released into the
atmosphere. Federal law governs their disposal, and
they must be collected and disposed of properly.
• Do not allow temperatures where refrigerant cylinders
are stored to reach 125 degrees F. Temperatures can
easily exceed 125 degrees F in your vehicle during hot
weather.
• Inspect refrigerant cylinders regularly. Do not use the
cylinders if they show signs of rust, distortion, denting, or
corrosion. Store cylinders secured and upright in an area
where they will not be knocked over or damaged.
• Beware of valve spindles and other components which
can fly off because of high pressures. Enough fatal
accidents have been reported which have occurred
because of personnel coming in direct line of loose flying
off parts fitted on pressurized equipments.
• Special Note: Always check MSDS before
handling any refrigerant and follow all
safety requirements. Exposure to large
concentrations of fluorocarbon refrigerants
can be fatal. In high concentrations, these
vapors have an anesthetic effect, causing
stumbling, shortness of breath, irregular or
missing pulse, tremors, convulsions, and
even death. Take care and be safe.
Precautions when working in
Refrigerated Spaces:
• Keep refrigerated spaces dry and free of water and
condensate accumulation due to choked drains.
Moisture inside refrigerated spaces will reduce the
efficiency of refrigeration by accumulating on the
evaporator coils as frost. It will also make the floor
slippery and can cause accidents due to slips, trips and
falls.
• Take all personal safety precautions not to inhale any
refrigerant vapours which could have possibly leaked out
into the chamber which can also cause chemical
poisoning of the human system. Refrigerant Vapour
under high temperature can liberate Phosgene gas,
which is highly poisonous.
• Always vent refrigerated spaces for sufficient duration
before personnel entry.
• When working in refrigerated spaces with frozen
cargoes, wear protective warm clothing.
• When working in refrigerated spaces with chilled
cargoes, especially fruits, beware of accumulated
pockets of carbon-di-oxide and ethylene which could
cause an oxygen deficient atmosphere. This is
particularly applicable for controlled, regulated and
modified atmospheres.
• It is safer always for at least two persons to enter and
work in refrigerated spaces at a time.
• All personnel should be aware of the push button alarms,
nearest escape routes and emergency exits from
refrigerated spaces.
Mechanical hazards
• Personnel should be aware at all times
that refrigeration systems contain liquids
and vapours under pressure. Suitable
precautions must be taken when opening
any part of the system to guard against the
pressure hazard.
• Compressors must be operated within
their design parameters.
Mechanical hazards
•
Personnel must not start the compressor
until they have taken steps to verify that:
1. All guards on coupling, belts drives, and
fans are in place, and other personnel
are not in positions that might be
hazardous when the plant is in operation;
2. The compressor discharge stop valve is
open.
Mechanical hazards
• Opening up part of the system will necessitate
the loss of a certain amount of refrigerant to
atmosphere. It is essential that the amount of
refrigerant which escapes is kept to a minimum
and appropriate steps are taken to prevent
hazardous concentrations of refrigerant
accumulating. Under certain conditions, liquid
refrigerant at low temperature may be present.
Contact with this liquid must be avoided.
Electrical hazards
• Before carrying out maintenance or repair
procedures, persons concerned must
ensure that equipment is isolated from the
electrical supply and tests made to verify
that isolation is complete. Whenever
possible, precautions must be taken to
prevent the circuit being inadvertently
energised i.e. withdraw the mains fuses.
Chemical hazards
• HCFC and HFC refrigerants can present a
danger to life by excluding air. Inhalation of very
high concentrations of the vapour, even for short
periods, must be avoided since this maybe
dangerous and can produce unconsciousness or
prove suddenly fatal due to oxygen deficiency.
The refrigerant vapour is heavier than air, and in
static or poorly ventilated situations may be slow
to disperse. Anyone suffering from the effects of
inhalation of the vapour should move or be
moved to the open air.
Chemical hazards
• Care must be exercised before entering any
area where the presence of high vapour
concentration is suspected. The vapour will
displace air upwards out of cargo chambers,
ships’ engine rooms, etc., and tend to collect at
deck level and in pits and trenches.
• Should accidental escape of the refrigerant
occur indoors, adequate fan assisted ventilation
must be used to disperse the vapour, preferably
by extraction at ground level, before entering the
area. When any doubt exists it is recommended
that breathing apparatus should be worn.
Chemical hazards
• HCFC and HFC refrigerants are non-flammable,
but refrigerant vapour coming into contact with
temperatures of 315°C and above (burning
cigarettes, gas burners, electrical heating
elements, etc.), will de-compose to form
phosgene, hydrogen fluoride and hydrogen
chloride. These compounds have extremely
harmful physiological effect on human beings.
Naked flame and smoking must be prohibited in
the presence of refrigerant vapour and
refrigerant must be purged from pipes or vessels
before carrying out cutting or welding
operations.
Chemical hazards
• Approved methods of leak detection only
should be used. If a halide test lamp is
used, remember that the heating or
combustion effect will produce toxic byproducts which could be dangerous if
inhaled.
Chemical hazards
• Liquid refrigerant in contact with the eyes
or skin will cause freezing and injuries
similar to a burn. Care must be taken
when opening pipes or vessels which may
contain liquid.
• Thus it is essential when loosening a
connection on any part in which refrigerant
is confined, goggles to be worn to protect
the eyes.
Ammonia (R717)
• Ammonia is normally considered to be the most
dangerous of the primary refrigerants and has
inherent material and physiological hazards. A
limited range of ammonia/air mixtures (16 27%) ammonia by volume can be ignited by
flame and an explosion may result. Ammonia
must not be allowed to come into contact with
iodine, bromine, chlorine, hypochlorite or
mercury. There is an explosion hazard in each
case.
Ammonia Vapour
• Low concentrations may cause only
irritation and discomfort.
• High concentrations can destroy body
tissue. The action is more pronounced on
moist tissues: eyes, nose, breathing
passages, and moist areas of the skin may
be burned by high concentrations.
Ammonia Liquid
• In this form ammonia can cause severe
burning of the skin and eyes. As the eyes
are particularly delicate organs, even small
amounts of ammonia can be harmful. The
full effects of ammonia on the eyes may
not be apparent for 8-10 days but,
ultimately, blindness may result.
Ammonia
• Maintenance procedures must not be carried out
unless adequate ventilation has been provided
to avoid risk of explosion and physiological
harm. Naked flames must not be permitted in the
area. The pungency of ammonia will usually
warn personnel against remaining in locations
where dangerously high concentrations of
vapour exist. Personnel must not be permitted to
work without wearing a gas mask, even for short
periods, in a concentration which causes any
discomfort to the eyes or affects breathing.
Operation of refrigeration plant
• The refrigerating capacity of any
refrigerating plant is at its maximum when
the greatest possible quantity of refrigerant
is evaporated in the evaporator to obtain
the required room temperature or to obtain
the required brine temperature.
• The evaporating temperature and hence
pressure must be kept as high as possible
consistent with the temperatures. For a
given design this means ensuring that all
heat transfer surfaces are kept clean, so
that the temperature difference across
them is at its minimum. It is also essential
to ensure that evaporator surfaces are
supplied with liquid refrigerant at the
correct temperature and in the correct
condition.
• The condensing temperature must be kept
as low as possible so as to keep the
compressor delivery pressure to a
minimum. This again means keeping all
heat transfer surfaces clean and ensuring
the correct flow of cooling water or air
through the condenser. It is also important
to keep air out of the systems as, being
non-condensable, it will collect in the
condenser vapour space so raising the
effective compressor delivery pressure
artificially.
• As per Dalton’s law of partial pressures,
the delivery pressure is the sum of that
due to the air and that at which the
refrigerant is condensing. Air in the
condenser will be indicated by an
excessively high condenser gauge reading
in relation to the temperature of the
cooling water or air. Air can be purged out
of the top of the condenser, though some
refrigerant will also be lost with it.
• The compressor must be maintained in the
best possible condition. This means that
suction and delivery valves must not leak
and piston clearance must be kept to a
minimum. The correct operation of
unloading devices is also important, as the
partial operation of these can affect
compressor performance. Valve leakage
or excessive clearance both result in reexpansion of gas from delivery to suction
pressure, and hence in reduced pumping
capacity.
• The temperatures at inlet and outlet of each side
of the heat exchanger, if fitted, must be correct
to ensure correct superheat and sub-cooling
temperatures. The refrigerating capacity of a
plant is directly proportional to the weight of
refrigerant evaporated in the evaporator. This in
turn is directly related to the mass of vapour
pumped by the compressor. The latter is the
sum of the flash gas formed at the expansion
valve and the vapour evaporated in the
evaporator; it is therefore important to keep the
flash gas to a minimum.
• The mass of refrigerant vapour pumped by
a given compressor depends directly on
the temperature and hence pressure
difference between the evaporator and the
condenser, and this must be kept to a
minimum. Furthermore, the density of the
refrigerant vapour varies directly with the
evaporating pressure and hence
temperature, and it is therefore important
to keep the evaporating temperature as
high as possible in relation to the required
load conditions.
• The swept column of a given compressor at a
constant speed is constant, and the volume
pumped varies relatively slightly for normal
plants, although for a given condensing
temperature it drops rapidly at low evaporating
temperatures, i.e. the volumetric efficiency of a
compressor is directly related to the pressure
ratio between the compressor suction and
delivery, which in turn corresponds to the
temperature difference between the evaporating
and condensing temperatures.
• A rise in condensing temperature has
much less effect on the refrigerating
capacity than a corresponding drop in
evaporating temperature; both affect the
volumetric efficiency, but lowering the
evaporating temperature also reduces the
density of the gas entering the compressor
suction.
• It is wrong, therefore, to assume that a
lower evaporating temperature will
improve the refrigerating capacity. While
heat transfer may be improved in the
evaporator, this is much more than
counterbalanced by the reduced weight of
gas pumped by the compressor, due to its
reduced density as well as to the reduced
compressor volumetric efficiency.
MAINTENANCE
•
1.
2.
3.
4.
The plant will need a minimum of care
and maintenance if it is kept:
free of moisture;
free of impurities;
free of freon leaks;
free of frost.
• Experience has shown that most problems
with marine refrigerating plant involve
refrigerant shortage caused by leakage. In
rooms where frozen or chilled cargo below
5°C is carried, it is necessary to keep
evaporators free from frost. Cleaning of
filters is also important.
• In cases where equipment is opened up,
neither air nor moisture must enter the
refrigeration system, as either will cause
trouble, e.g. in the form of increased
condensing pressure. To avoid moisture,
filter driers are installed in HCFC/HFC
plants.
• The drying agent should be changed every
time any part of the system is opened.
This also applies when charging with oil or
refrigerant. Oil should not be filled from
vessels that have not been tightly closed.
Drop in oil level
• A leakage-free refrigerating plant does not
consume any oil. The oil which has
disappeared from the crankcase or oil
separator is always somewhere in the
system.
Oil level drops quickly at the
start
• This may be due to refrigerant being
dissolved in the oil. At evaporation the oil
is drawn with the refrigerant into the
system. Fill the system with a small
quantity of oil, as the ejected oil will
gradually come back.
Oil level drops slowly
1. the plant is operated at lower evaporating
temperature than usual or the refrigerant
charge is too small;
2. refrigerant leakage in the system, by which the
level in the evaporator has become too low;
3. condensing temperature is too low, the
minimum condensing temperature should be
maintained;
4. the cooling demand is too low, so the gas
velocity becomes too low in the evaporator and
the oil remains in the system.
• In those plants with piston compressors
which have oil separators, the shut-off
valve in the oil return line should always
be kept closed for about 1/2 hour after
compressor start in order to avoid the
carriage of condensate from the oil
separator to the crankcase.
Brine specific gravity
• In those cargo refrigerating plants where
brine serves as the heat transfer medium,
it is of great importance that the correct
brine specific gravity for the required cargo
temperature is maintained. If this specific
gravity is not checked, functional problems
may occur.
Calcium Chloride Brine
CALCIUM CHLORIDE / BRINE
Specific
Gravity
Hydrometer Reading
Freezing Point of Solution
(Twaddell)
°C
°F
1·20
40
-21
-6
1·21
42
-23
-9 5
1·22
44
-25
-13
1·23
46
-27
-17
1·24
48
-30
-21.5
1·25
50
-32
-26
1·26
52
-35
-31
1·27
54
-38
-37
1·28
50
-42
-44
1·29
58
-51
-60
Daily maintenance
•
1.
2.
3.
4.
5.
The daily maintenance for a R22 installation should be
completed as follows:
Check that condensing pressure and evaporating
pressure are correct.
Inspect the compressor unit and check that there are
no abnormal noises or vibrations.
Check the oil level.
Check the tightness of the shaft seal. Oil leakage can
be tolerated whereas gas leakage can not.
If an oil separator is installed, check that oil is returned
to the crankcase, and that the oil return line is warmer
than the crankcase.
Leak detection
• Refrigerating plants must be gas-tight to
prevent refrigerant leakage and air
entering the low-pressure side when under
a vacuum. Systems which have been
opened to the atmosphere during repairs,
must be pressure tested for mechanical
strength and leaks before charging with
refrigerant.
Pressure tests
• Pressure tests should be done with
nitrogen. Water or other fluids must not be
used as a test medium. The plant’s
compressors must not be used to
pressurise the plant. The pressure is
gradually increased by pressurising with
nitrogen gas the pressure is equal to 1.5
times the maximum working pressure of
the system. This pressure should be
maintained for about 10 minutes.
• Each joint must be examined thoroughly for
signs of gas bubbles which indicate a leak. After
sealing any leaks, pressurise the system again
with the test medium and some refrigerant as a
trace gas, and repeat the leak test using an
electronic leak detector, etc. It is imperative that
all leaks are found and sealed before the system
is charged with refrigerant, as even the tiniest of
leaks can result in the loss of the whole
refrigerant charge.
Leak detection equipment
• Electronic leak detectors
• Electronic leak detectors are the most
sensitive and accurate method of leak
detection. The detector contains an
internal pump that draws air into a probe,
or tube. If refrigerant gas is present in the
sample, the electrodes in the sensing
element generate a current, and an output
signal is obtained.
Halide lamps
• Halide lamp is used to locate leaks of
HCFC refrigerants. This method is based
on the colour of a flame that surrounds a
glowing copper element. The flame turns
blue-green if the air being consumed
contains the refrigerant.
• These lamps should only be used in well
ventilated spaces.
Routine inspections
• The high pressure side of the system may easily
be tested for leaks with the plant running, but it
may be necessary to stop the compressor, and
allow the pressure in the low pressure side to
rise sufficiently for leak test in the low pressure
side. The amount of refrigerant in the system
should also be strictly monitored as a drop in
level may indicate the presence of a leak. In the
event of a major leak, the initial leak test should
be made using the soap bubble method,
specially in areas where there are signs of an oil
leak.
Drying by evacuation
•
1.
2.
3.
4.
This method of removing moisture is based on the fact that the boiling point
of water decreases with falling pressure. In the course of evacuation, any
water or ice in the plant will evaporate, and is carried away by the vacuum
pump.
Connect a vacuum pump to the system using a short length of large bore
pipe, and open all valves in the system (expansion valves, solenoid valves,
etc., may have to be jacked open).
Evacuate the system to a pressure of 6mm Hg or less. If possible, carry out
the evacuation at ambient temperatures above 10°C.
Close the line between the system and the vacuum pump. The pressure in
the system may not rise more than 2mm Hg within five minutes. A rise of
more than 2mm Hg indicates the presence of water, and/or a leak. Where
water is present, the system will be colder than its surroundings.
Check for water and/or leaks, carry out any repairs, and repeat the
evacuation procedure until the pressure rise is less than 2mm Hg. When
this is achieved the system is free of moisture and non-condensable gases
and ready for refrigerant charging.
Charging procedures
• Refrigerating plants should not be overcharged
with refrigerant, as this may overload or damage
the compressor.
• To ensure that the correct amount is added, the
refrigerant should be weighed during charging.
• The liquid refrigerant is decanted from the
refrigerant bottle into the system via a charging
valve just after, the liquid receiver.
Charging Procedure
1.
2.
3.
4.
5.
6.
7.
8.
Weigh refrigerant bottle
Connect refrigerant bottle to charging valve with flexible charging line.
Crack bottle liquid valve before tightening line to blow out air.
Close main liquid line valve and pump down system.
Open charging valve and carefully open liquid valve on refrigerant bottle.
Liquid refrigerant will flow into the system.
Start compressor.
Continue charging until required amount of refrigerant has been
charged.
9. Check weighing scale reading and observe liquid level in receiver.
10.Close charging valve and open main liquid valve and observe liquid flow
through sight glass. Bubbles indicate the need for further charging.
11.If charge is complete, close bottle valve, and disconnect charging line.
12.Store empty refrigerant bottle for reuse.
Condensers
• Once a year, the tubes should be cleaned with a
tube brush in order to remove deposits which
would cause a high condensing pressure. The
gaskets must be glued to the condenser end
plate with good contact. If the partition wall
gasket is not properly installed, there is a risk
that it will ‘blow’, leading to an excessively high
water velocity and to damage to the tube plate.
• If a tube has become defective, it does not have
to be replaced immediately. Up to 10% in each
pass (flow direction) can be plugged.
• Corrosion plugs should be renewed if required.
Defrosting
• Another task which maybe regarded as
maintenance is to keep frost deposits on
provision and cargo refrigerating plants
under control. Frequent defrosting assures
that the plant will cause few problems.
Investigating trouble
•
1.
2.
3.
4.
5.
6.
7.
8.
In investigating trouble, there are certain things to which
attention should always be paid in the first instance:
the temperature of the refrigerated space;
evaporating pressure;
condensing pressure;
suction pipe temperature;
discharge temperature;
liquid line temperature;
compressor running time;
noise from compressor, motor, expansion valve, etc.
TROUBLE SHOOTING
• When problems are incurred in a refrigerating
plant, these can be attributed in most cases to a
shortage of refrigerant. Bearing this in mind,
always commence trouble-shooting by checking
the refrigerant charge.
• For example, in the case of HCFC plants, too
little refrigerant prevents the oil, which always
circulates in the system, from being returned as
the gas velocity is low, and this leads to various
functional troubles.
CONTAMINANTS
• If the moisture present in a refrigerating
system exceeds the amount that the
refrigerant can hold in solution it will exist
as free water. At temperatures of 0°C or
lower, the free water will freeze into ice in
the expansion valve or evaporator,
restricting the flow of refrigerant. To avoid
freeze-ups, the moisture content in low
temperature HCFC refrigerant systems
must be maintained at a very low level.
Oil
• In refrigerating systems some oil is always carried over
from the compressor into the condenser by the
refrigerant gas, from where it is carried by the liquid into
the evaporator. The presence of oil in the circulating
refrigerant reduces the heat transfer capacity of the
various heat exchangers, the problem being greatest in
the evaporator, since oil becomes more viscous and
tends to congeal at low temperature. To prevent oil
related problems, the operation of the oil separator
should be checked regularly to ensure oil is being
returned to the compressor lubrication system. The
amount of oil added to the lubrication system should also
be strictly monitored; an excessive amount indicates that
oil is being trapped in the evaporator or suction line.
Air and non condensable gases
• The presence of air and other non-condensable
gases is detrimental to the efficient operation of
a refrigerating plant, as these gases collect in
the condenser, and so increase the condensing
pressure. Abnormally high condensing
pressures cause overheating of the compressor,
excessive discharge temperatures, losses in
compressor capacity and efficiency, excessive
power consumption and possible overloading of
the drive motor.
Testing for Non-condensable
Gases
• Air and non-condensable gases, if present in the
system, are pumped through the system and
discharged by the compressor into the
condenser.
• These gases are trapped in the condenser and
cause excessive condensing pressures. In order
to check the condenser for the presence of air or
non-condensable gases, it is essential that
gauges and thermometers be accurate and that
the system has sufficient charge so that the
liquid refrigerant present in the receiver will seal
the liquid line connection.
Check for non-condensable
gases
Close liquid line valve and allow system to pump down.
•
1. Shut off compressor and close suction line valves.
2. The thermometer in the sea water outlet of the
condenser will indicate the actual condensing
temperature, when there is no further drop in
temperature.
3. Record the condensing pressure.
4. On a refrigerant pressure gauge, look up the saturation
temperature that corresponds to the condensing
pressure.
5. If the condensing temperature is less than the
corresponding saturation temperature of the condensing
pressure, it is necessary to purge.
Purging Non-condensable Gases
1. Pump down the refrigerant by shutting the
liquid valve at the outlet of liquid receiver.
2. Continue cooling the condenser for 10 to 15
minutes.
3. Open purge valve on top of condenser, and
slowly release gases.
4. Since it is difficult to tell if excessive refrigerant
is being purged with the non-condensables,
purge slowly.
Compressor Short-Cycles
Possible Cause
Low sea water
temperature.
Low refrigerant
charge.
Reduced
evaporator
capacity.
Action
Throttle condenser sea water
outlet valve. This will raise the
compressor discharge
pressure, thereby raise the
compressor suction pressure.
Check refrigerant charge. Add
as required.
Check for frosted coils.
Compressor Runs Continuously
Possible Cause
Low refrigerant
charge.
Compressor valves
leaking.
Action
Check for proper charge. If
low, repair any leaks and
recharge.
Pump down, remove cylinder
heads, and check.
Worn piston rings
Pump down, disassemble,
and/or cylinder liner and then check.
Legionella bacteria
• A type of pneumonia which may be fatal to older
people, has been blamed on the presence of a
bacteria associated with the air conditioning
plant of large buildings. Because the outbreak
which heralded the disease, occurred at a
convention for American ex-servicemen (The
American Legion), the identified cause of the
problem, was labelled legionella bacteria and
the sickness is referred to as legionnaires
disease.
• There is a risk that the bacteria could flourish in
the air conditioning systems of ships. The
organisms breed in stagnant water or in wet
deposits of slime or sludge. Possible locations
for bacteria colonies, are mentioned as being at
the air inlet area and below the cooler (stagnant
water), in the filter, in humidifiers of the water
spray type and in damaged insulation.
• Provision of adequate drainage is recommended
to remove stagnant water.
• Regular inspections and cleaning as
necessary of filters and other parts, using
a 50ppm super-chlorinated solution as the
sterilizing agent is required. The solution is
to be used also on the cooler drain area at
not more than three month intervals.
Regular sterilization is necessary for water
spray type humidifiers (steam humidifiers
being preferred).
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