Refrigeration cycle Refrigeration system Refrigerant properties Safety devices Air conditioning Troubleshooting

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Fundamentals
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Refrigeration
Refrigeration is a process in which the temperature of a space or its contents is reduced to below
that of their surroundings. Air conditioning is the control of temperature and humidity in a space
together with the circulation, filtering and refreshing of the air. Ventilation is the circulation and
refreshing of the air in a space without necessarily a change of temperature. With the exception
of special processes, such as fish freezing, air is normally employed as the heat transfer medium.
As a result fans and ducting are used for refrigeration, air conditioning and ventilation. The three
processes are thus interlinked and all involve the provision of a suitable climate for men,
machinery and cargo.
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Refrigeration Cycle
The transfer of heat takes place in a
simple system: firstly, in the evaporator
where the lower temperature of the
refrigerant cools the body of the space
being cooled; and secondly, in the
condenser where the refrigerant is cooled
by air or water. The usual system
employed for marine refrigeration plants
is the vapor compression cycle, for which
the basic diagram is shown
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The pressure of the refrigerant gas is increased in
the compressor and it thereby becomes hot. This
hot, high-pressure gas is passed through into a
condenser. Depending on the particular application,
the refrigerant gas will be cooled either by air or
water, and because it is still at a high pressure it
will condense. The liquid refrigerant is then
distributed through a pipe network until it reaches a
control valve alongside an evaporator where the
cooling is required. This regulating valve meters the
flow of liquid refrigerant into the evaporator which
is at a lower pressure
790
Refrigeration Cycle

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Air from the cooled space or air
conditioning system is passed over the
evaporator and boils off the liquid
refrigerant, at the same time cooling the
air. The design of the system and
evaporator should be such that all the
liquid refrigerant is boiled off and the
gas slightly superheated before it returns
to the compressor at a low pressure to be
recompressed.
791
Refrigeration Cycle

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Refrigeration Cycle

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Refrigeration Cycle

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Desirable properties of a refrigerant
1. Low boiling point  otherwise operation at a high vacuum becomes necessary!
2. Low condensing pressure  to avoid a heavy machine and to reduce leakage risk
3. High specific enthalpy of vaporization  to reduce the reduce the quantity of
refrigerant in circulation  lower machine speeds and sizes
4. Low specific volume in vapour phase  reduces the plant size & increase efficiency.
5. High critical temperature (temperature above which vapour cannot condensed by
isothermal compression).
6. Non-corrosive and non-solvent
7. Stable under working conditions
8. Non-flammable & non-explosive
9. No action with oil
10.Easy leak detection
11.Non-toxic, non-poisonous and non-irritating
12.Cheap and easy to store.
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Refrigerant properties
Refrigerant Type Mass * Formula
Boiling
point
C at Atmos
Freezing
point
C at Atmos
Critical
temp (C)
Critical
pressure
(kpa)
Liquide
density
(kg/m3)
ODP ** GWP ***
R-11
CFC
137.37 CCl3F 23.7 -111.1 198 4408 1447 1 3800
R-12 120.91 CCl2F2 -29.75 -160 112 4136 1486 1 8100
R-22 HCFC 86.46 CHClF2 -40.81 -160 96.1 4990 1413 0.05 1500
R134a HFC 102.03 C2H2F4 -26.06 96.67 101.08 4060 1206 0 3260
* The unified atomic mass unit or dalton (symbol: u, or Da) is a standard unit of mass that quantifies mass on an atomic or molecular scale
(atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically
equivalent to 1 g/mol
** The ozone depletion potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with
trichlorofluoromethane (R-11 or CFC-11) being fixed at an ODP of 1.0. Chlorodifluoromethane (R-22), for example, has an ODP of 0.05. CFC
11, or R-11 has the maximum potential amongst chlorocarbons because of the presence of three chlorine atoms in the molecule.
*** Global warming potential (GWP) is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the
amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is
calculated over a specific time interval, commonly 20, 100, or 500 years. GWP is expressed as a factor of carbon dioxide (whose GWP is
standardized to 1)
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Refrigerant properties
• The production of R12 and R11 has now stopped under the
Montreal Protocol and EU regulation on ozone depleting
gasses. A short term solution has been conversion to
HCFC's such as R22 (HCFC's have an Ozone Depletion Rate
{ODP} 2-15% of CFC's) but this refrigerant also has a
harmful effect on the environment, although far less
damaging than R12. HCFC's are also targeted for eventual
production phase out as controlled substances, with usage
totally banned by the EU in new equipment rated at 150kW
and over from the 1st Jan 2000. In some countries such as
Germany and Sweden tighter restrictions are in force.
• An uncertainty over the long term future of HFC's has led
to growing interest in old natural refrigerants such as
ammonia and carbon dioxide or hydrocarbons such as
propane and iso-butane. Using the refrigerants, however,
dictates more stringent safety measures which are being
drafted by the appropriate classification societies.
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Refrigerant properties
New refrigerants such as
R134a and R404A, which are
HFC's may offer a longer term
solution against harmful
emissions. They contain no
chlorine atoms and thus do
not attack the ozone layer but
they are GHGs and may be
subject to future legislation.
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Cycle
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Refrigeration system
799

15. Compound gauge
800

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Refrigeration Compressor
801

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Refrigeration Compressor

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Refrigeration Compressor

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Refrigeration Compressor

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Refrigeration Compressor

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Refrigeration Compressor

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Compressor valve assembly
807

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Cylinder Head
808

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Compressor Safety Devices
809

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Over-pressure devices

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Unloader start-up operation
811
The compressor starts with the
inlet valve lifted, reducing the
compressor load
When up to speed, the unload
pins drop setting the compressor
on-load

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Compressor Mechanical Seal
812

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Shaft gland
813

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Refrigeration Compressor lubrication
814

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Charging Connection
815

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Charging
816

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Thermostatic Expansion Valve
817

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Thermostatic Expansion Valve

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Automatic Expansion Valve (Constant Pressure)
819
Also known as a constant pressure
expansion valve acts in such a manner so
as to maintain a constant pressure and
thereby a constant temperature in the
evaporator. The spring force controls the
location of the needle with respect to the
orifice and hence its opening.
When the compressor starts after an off-
cycle period, the evaporator pressure
increases as a result to the needle
movement downward and the valve opens.

35. M a r i n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G2 January 2021
Automatic Expansion Valve (Constant Pressure)
820

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Pressure Controllers
821

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Pressure Controllers
822

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Oil trap
823

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Refrigeration Compressor
824

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Condenser
825

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Common faults
826

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Air Conditioning
827

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Air Temperature
828

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Dew point
• The dew point is the temperature of air which is needed for condensation or dew (at that particular temperature).
• If you take a glass of ice water and it develops condensation on the glass surface, the air on the glass has condensed to
its dew point and created dew.
• Dew point actually measures how much water vapor is in the air.
829
Relative Humidity

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Dry bulb thermometer and a wet bulb thermometer
mounted together
Dry bulb tells actual temperature
Wet bulb shows how much water can be evaporated
– temperature lowers as water is evaporated
The difference in temperature on the 2
thermometers is an indication of the
amount of water vapor in the air.
830
Relative humidity
Moist air: little water will evaporate from the wet-bulb and the temperature decrease will be small.
The difference between the wet bulb and dry bulb will be small.
Dry air: the water will evaporate quickly and cause a large drop in the wet-bulb temperature.
This makes the difference in readings on the 2 thermometers greater.

46. M a r i n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G2 January 2021
Relative humidity
Relative humidity is expressed as a percentage of how much moisture the air could
possibly hold at the temperature it happens when you measure it.
831
When the Wet bulb temperature = the dry bulb temperature………
100% HUMIDITY!!!

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Air movement
832

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Psychometric Curves

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Psychometric Curves

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Psychometric Curves

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Psychometric Curves

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Psychometric Curves

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Psychometric Curves
838

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Single duct

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Single duct

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Single duct
841

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Single duct
842

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Twin duct
843