Refrigeration: is a process in which the temperature of a space or its contents is reduced to below that of surroundings.
Uses of Refrigeration:
1. Domestic fridge rooms on ships for preserving foodstuffs for the crew.
2. Accommodation air-conditioning system.
3. Reefer control air dryers in engine room.
4. Refrigeration is used in the carriage of some liquefied gases like LPG and LNG.
5. Reefer containers for carrying food stuffs.
6. Reefer ships where the entire cargo space is refrigerated for carriage of perishable fruits and meat products.
7. To cool bulk CO2 for firefighting systems.
Live
& Dead Cargoes: The perishable foodstuffs carried as refrigerated
cargo or as stores on ships can be categorized as dead produce such as
meat and fish or as live produce such as fruit and vegetables.
Fruit
and vegetables are regarded as live cargoes until consumed, because
they continue to ripen though slowly under refrigerated conditions.
Fruit and vegetables continue a separate existence during which oxygen
is absorbed and CO2 is given off, with the generation of heat.
The
purpose of refrigeration in the carriage of perishable foodstuffs is to
prevent or check spoilage, the causes of spoilage are:
1 excessive growth of micro-organisms, bacterial and fungal;
2 changes due to oxidation, giving poor appearance and flavours;
3
enzymatic or fermentive processes, causing rancidity;( it refers to the
spoilage of a food in such a way that it becomes undesirable,and
usually unsafe for consumption).
4 drying out (dessication);
5 The metabolism and ripening processes of fruit and vegetables.
Principle of Refrigeration.
When a liquid evaporates a
cooling effect is produced. For example, a few drops of volatile liquid
i.e. after shave,poured on to the hand gives a cold sensation, as it
evaporates rapidly taking heat out of the skin.
Evaporation of leaked water from porous earthen pot surface keeps the water inside the pot surface cool. Similarly
if liquid CO2 is made to vaporize at a coil as shown, the heat to
vaporize the liquid CO2 will be taken from the surrounding i.e. in this
case a bottle of water.
The draw back in the example shown is that
the cylinder will soon become empty of liquid CO2 and the cooling
effect will stop unless cylinder is recharged with further liquid CO2.
Vapor Compression System
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| Image credit: http://engineering.myindialist.com |
Vapour
compression cycle is the most commonly used system of refrigeration. In
this system a gas called as refrigerant is used as a medium of heat
transfer and is alternately condensed and evaporated to remove heat from
the spaces being cooled.
(The temperature at which fluid boils or condenses, is known as saturation temperature and varies with pressure).
The system consists of 4 processes namely:
1.
Compression: Compression of the gas is carried out in the compressor,
which delivers the gas at high pressure and temperature, raising the
saturation temperature, so that it is higher than the sea water
temperature or air cooling the condenser.
2.
Condensation: The compressed high pressure gas is now condensed to a
high pressure liquid in a condenser to below saturation temperature
relating to compressor delivery pressure by circulating sea water (or
air in case of domestic refrigerators).
3.
Expansion: The high pressure liquid is then passed through an expansion
valve to reduce its pressure, after passing thru the expansion valve the
refrigerant consists of low temperature liquid and a small quantity of
vapour both at low pressures.
The pressure drop through the
expansion valve causes saturation temperature of the refrigerant to fall
so that it will boil at the low temperature of the evaporator.
4.
Evaporation: The liquid refrigerant containing small quantity of vapour
is now passed thru an evaporator which is located in the space required
to be cooled. Here the refrigerant absorbs heat from the surrounding
secondary coolant (air or brine) receives latent heat and evaporates,
cooling the surrounding space. The evaporated liquid (gas) is passed to
the compressor suction for the entire process to repeat itself.
Vapor absorption cycle
In
the early years of the twentieth century, the vapour cycle of
absorption using ammonia water systems was popular and widespread. After
the development of vapour compression cycle, the vapour absorption
cycle has lost much of its importance because of its low performance
(about one fifth of the vapour compression cycle). Today, the steam
absorption cycle is mainly used when fuel oil is present, but no
electricity.
Absorption Type Refrigeration Unit :
1.
Hydrogen vapor which is insoluble in water, leaves the absorber and
rises until it meets ammonia liquid falling into entry of evaporator.
Due to hydrogen pressure causing lowering of ammonia pressure, this
results in vaporization of ammonia.
2. Ammonia and hydrogen vapor
are carried down to the absorber where water absorbs and dissolves
ammonia and hydrogen vapor re cycles.
3. Ammonia vapor which is
highly soluble in water, rises with the water vapor from the generator
to the seperator where the water vapor and some ammonia vapor condenses.
4. Ammonia vapor then rises, is liquefied in the condenser,
reduced in pressure and vaporized in the evaporator and falls to be
absorbed in absorber. Ammonia, dissolved in water, falls down into lower
pipe to the generator.
5. Water vapor leaves the generator, is
condensed in the separator, falls through the absorber dissolving the
ammonia vapor and returning to generator.
The unit requires no
compressors or pumps and is silent and vibration less. Condenser,
evaporator and vapor liquid separator are air cooled, with fins welded
or brazed on to the piping to give extended surface heat transfer.
AIR-CONDITIONING
The
basic principles of air conditioning: 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 of air
whilst summer conditioning relates to decreasing temperature and
humidity of air.
What are the objectives of air conditioning on ships ?
1. To extract excess heat
2. To raise air temperature when required
3. To add moisture as required
4. To reduce moisture content as required
5. To maintain sufficient air flow
6. To remove dust
When
moisture evaporates from a surface, the latent heat required, is drawn
from the surface causing it to be cooled. If a thermometer bulb is
covered by a wetted fabric and exposed to the air, the rate of
evaporation will depend upon the humidity of the surrounding air. As the
heat required must come from the bulb, this results in a lower
temperature reading than if the bulb was dry.
Important definitions in air-conditioning:
Hygrometer or Psychrometer:
Hygrometer is an instrument to measure the humidity of air. This
consists of an ordinary thermometer which gives the dry bulb temperature
and a wet bulb thermometer (wetted with gauze cover).The wet bulb
reading will be less than the dry bulb reading, the difference is quoted
as the wet bulb depression.
The drier the air, the
more rapid the moisture evaporation from the gauze giving a cooling
effect. Thus greater the difference between the dry and wet bulb
readings, drier the air and lesser the relative humidity.
Relative humidity (r.h.):
The relative humidity is a measure of the amount of water vapor in the
air (at a specific temperature) compared to the maximum amount of water
vapor air could hold at that temperature, and is given as a percentage
value.
Relative humidity depends on the temperature of
the air, as warm air can hold more moisture than cold air. A relative
humidity of 100 percent indicates that the air is holding all the water
it can at the current temperature and any additional moisture at that
point will result in condensation.
A relative humidity
of 50 percent means the air is holding half the amount of moisture that
it could. As the temperature decreases, the amount of moisture in the
air doesn't change, but the relative humidity goes up (since the maximum
amount of moisture that cooler air can hold is smaller).
Dewpoint (d,p):
is the temperature to which unsaturated air must be cooled to bring it
to saturation point and to cause moisture to precipitate. (If an
unsaturated mixture of air and water vapour is cooled at constant
pressure, the temperature at which condensation of water vapour begins
is known as the Dew point.)
Or
The atmospheric temperature
(varying according to pressure and humidity) below which water droplets
begin to condense and dew can form.
The dew point is the
temperature to which the air must be cooled at constant pressure in
order for it become saturated, i.e., the relative humidity becomes 100%.
A higher dew point indicates more moisture present in the air.
Physcometry: It
is the study of properties of mixture of air and water vapour. This
subject is important to air-conditioning 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.
Comfort Zone: The condition of the air in a
space depends on its temperature, humidity and movement. The effect of
the air on people in a space varies considerably between one person and
another, so it is only possible to stipulate a fairly wide zone.
Under
summer conditions relative humidity between 30% and 70%, average about
50% and thermometer readings 19 deg to 25deg, average 22deg gives the
best degree of summer comfort.
Ventilation is defined as the circulation of air around a space to clean and refresh it, but not changing the temperature.
Air Velocity:
The early air conditioning systems were rather bulky because designs
were based on low air velocities in the distribution ducts, with
velocities in the order of 10 m/s or less. In later years with very
substantial increases in air velocities, reaching a maximum of about
22.5 m/s in the ducts and producing a large reduction in the space
occupied by the equipment. Higher velocity systems have increased
operating costs but lower installation costs.
What is Low velocity system in ventilation?
A
low velocity System is one in which the velocity of air at the
beginning of main duct is 5 to 10 m/s and successively lower there after
which results in low frictional resistance. Thus in this system we
require a fan which is having low power rating, but the only problem
with this type of arrangement is that it will require large sized and
expensive ducts and installation will be difficult.
What is High velocity system in ventilation?
In
high velocity system, velocity of air at the beginning of the main duct
is 15 to 30 m/s. Since it includes a high power fan that will produce a
high air pressure, it requires small sized ducts and would result in
economy of material,
low manufacturing and installation cost
easy installation on board ship,
considerable space saving in the ship.
This
system will have high recurring cost and will also result in high noise
levels. Also since the machinery will be running at high speed, the
frictional loses in this system will be more.
Typical Air-conditioning System
The
main components of the system, such as the oil separator filter,
condenser, expansion valve and evaporator, are explained in the
refrigeration system; The components, which are generally unique to the
air conditioners, are described below:
Compressor: It
can be stroke based or rotatory. In almost all cases, a method for
changing the amount of the feed is taken. The piston compressors
therefor is also a rotary feed unit for the speed.
Compressor
protection: Compressors are under similar protection systems as of
refrigiration plant, low pressure cooling zone, high pressure part
(manual reset) and this cuts the differential oil pressure. In addition,
it has a lock for the compressor installed, can not be started when the
air handlink unit fan is not started. When the fan stops, the
compressor is turned off.
An alternative is to mount
the solenoid valve upstream of the compressor as shown in the diagram
above, which only open when the fan is running. The compressor is
triggered with a low suction pressure. The purpose is to prevent liquid
back to the compressor.
Air handling unit
In
the diagram above a single unit contains an evaporator fed through a
gas compressor. A belt driven fan supplies air to the evaporators
through an air filter with fine mesh. This filter is removed and washed
regularly in a soapy solution containing a disinfectant. The air flows
over the evaporator, where it is cooled, and gives water vapor. The
water condenses and is transported in a collecting basin and pipelines.
The previous draft a collector was installed to remove water droplets
entrained in the air, they are not always equipped. A perforated tube is
installed after the evaporator allows the low steam quality to be
introduced into the air to improve the moisture when it is too dry.
The
fresh air is taken from the outside atmosphere and the recirculated air
is the return air housing. The air is distributed to the ships during
their stay in the port or during navigation normally, Air is
recirculated normally on tankers during port stay or during sailing when
any cargo or IG related operations are on to prevent cargo vapours from
entering the accommodation spaces. For the mass stay at loading port
air is taken back for ship carring raw materials such as coal, iron ore
etc., To avoid them from entering the air housing.
Thermostat in AC system
A
thermostat is the component of an Air-con system which regulates the
temperature of the space to be cooled so the temperature is maintained
near a desired set point temperature. The thermostat does this by
indirectly switching the compressor on or off, to maintain the correct
temperature.
As long as the desired temperature in the
accommodation space is not reached the Air con compressor keeps working
and thereby cooling the accommodation, when the desired temperature is
reached the thermostat actuates and closes the liquid line magnet
valve(solenoid valve) located on liquid line after the condenser, the
compressor then eventually stops on low suction pressure cut off.
However the air handling unit blower keeps running all the time.
Now
as the accommodation temperature starts going up above the desired set
point the thermostat energizes the liquid line magnet valve, the suction
pressure now goes up as the gas starts flowing to the compressor, the
compressor then immediately starts on L.P cut in thereby the cooling now
again commences, this cycle is repeated to maintain the desired
temperature in the accommodation spaces.
Thermostats
are normally located in the air handling unit; they sense the
temperature of Return air. Alternately they are also located in one of
the cabins on the top deck.
Recently, digital
thermostats have no moving parts to measure the temperature, and instead
rely on thermistors or other semiconductor devices, such as a
resistance thermometer (resistance temperature sensor). Each has an LCD
screen that displays the measured temperature and the set temperature.
What is Capacity Control?
Capacity
control of an air-conditioning plant can be defined as a system which
controls the output of the plant as per the load in demand.
Refrigerating capacity control with reciprocating compressors running at
constant speed consists of controlling the quantity of gas delivered to
match the fluctuating load
Holding the valves open:
This is the most common method used in unloading in multi cylinder V
& W type compressors. It is accomplished by lifting of suction
valves, usually of 2 cylinders together by means of push pins. When the
suction valve is lifted the gas drawn during suction stroke is pushed
back into the suction line during the upward stroke of the piston. No
work is done except frictional work during such idling. The push pins
are operated by oil pressure. More and more cylinders are unloaded as
the suction pressure or evaporator temperature continues to drop.
Normally
in warm weather area the AC plant is always running at full load,
ideally the air conditioning plant will start unloading when the ship
goes into colder weather. The thermostat control will come into action
only when further drop in temperature takes place, and this will stop
the air-con compressor.
#Cover Image credit: Chitre Sir (Marine Faculty) / I
f you have any problem regarding post, please contact us!
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Various books, study material and other online sources has been
refereed prior to writing this article but no part is copied or
produced from any of the source but explained same thing in better
detailed way.
Author
: Amit Article Requested by: Pranesh Devadiga
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