Refrigeration

With reference to refrigeration system answer the following:

(a)Purpose of having condenser & evaporator. (6)
(b)Explain why the refrigerant temperature changes from one side of
the expansion valve to other Side. (5)
C) With reference to thermostatic expansion valve, explain what is
meant by term super heat Setting. (5)
See digram for condenser and evaporator in the note
Condenser: The condenser removes heat given off during the liquefaction of
vaporized refrigerant.Heat is given off as the temperature drops to
condensation temperature. Then, more heat (specifically the latent heat of
condensation) is released as the refrigerant liquefies. There are air-Cooled
and water-cooled condensers, named for their condensing medium. The
more popular is the Air-cooled condenser for smaller units. The condensers
consist of tubes with external fins. The Refrigerant is forced through the
condenser. In order to remove as much heat as possible, the tubes Are
arranged to maximize surface area. Fans are often used to increase air flow
by forcing air over the Surfaces, thus increasing the condenser capability to
give off heat. The heat removed from the Refrigerant in the condenser of a
refrigeration plant is the heat of compression- popularly known as Latent
heat of condensation. The coolant used for a finned condenser is air. The
accumulation of air And other non-condensable gases in a refrigeration
system will collect in the condenser. After the Freon leaves the condenser,
the storage space for the liquid refrigerant is called as receiver.

Evaporator: In a refrigeration system the purpose of an evaporator is to

absorb latent heat of Vaporization. This is the part of the refrigeration
system that is doing the actual cooling. Because its Function is to absorb
heat into the refrigeration system (from where you don’t want it), the
Evaporator is placed in the area to be cooled. The refrigerant is let into and
measured by a flow Control device, and eventually released to the
compressor. The evaporator consists of finned tubes, Which absorbs heat
from the air blown through a coil by a fan. Fins and tubes are made of
metals With high thermal conductivity to maximize heat transfer. The
refrigerant vaporizes from the heat it Absorbs in the evaporator.

Explain why the refrigerant temperature changes from one side of the
expansion valve to other check notes
With reference to thermostatic expansion valve, explain what is meant by
term super heat setting.
• Super heating is the process of increasing the temperature above
saturation temperature
• Super heating is provided in low pressure side of the refrigeration cycle
because liquid refrigerant Should not go to the inlet of compressor. Liquid is
in-compressible if it reaches the compressor it Damages the crank shaft.
• Super heating is done in refrigeration by placing outlet line from the
evaporator is placed adjacent To the outlet line form the condenser. So due
to the high temperature in HP side compared to LP Side, heat transfer is
happening. Simply this act as a heat exchanger. Check diagram in notes

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Sketch and describe the principles of operation of a Shipboard domestic
reotsfrigeration system Main Components of Refrigeration plants
Check diagram in notes
Main components of refrigeration cycle
compressor: Reciprocating single or two stage compressor is Commonly
used for compressing and supplying the refrigerant to The system.

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 condenser: Shell and tube type condenser is used to cool down The
refrigerant in the system.
Receiver: The cooled refrigerant is supplied to the receiver, Which is
also used to drain out the refrigerant from the system for Maintenance
purpose.
Drier: The drier connected in the system consists of silica gel to
Remove any moisture from the refrigerant

Solenoids: Different solenoid valves are used to control the flow Of

refrigerant into the hold or room. Master solenoid is provided In the main
line and other solenoid is present in all individual cargo Hold or rooms.

Expansion valve: An Expansion valve regulates the refrigerants To maintain

the correct hold or room temperature.

evaporator unit: The evaporator unit acts as a heat exchanger to

Cool down the hold or room area by transferring heat to the Refrigerant.
Control unit: The control unit consist of different safety and Operating
circuits for safe operation of the refer plant.

Working of Ship’s Refrigeration Plant

The compressor acting as a circulation pump for refrigerant
Has two safety cut-outs- Low pressure (LP) and High Pressure (HP) cut outs.
When the pressure on the suction side drops below the set valve, the
control unit Stops the compressor and when the pressure on the discharge
side shoots up, the Compressor trips.
LP or low pressure cut out is controlled automatically i.e. when the Suction
pressure drops, the compressor stops and when the suction pressure rises
Again, the control system starts the compressor.

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HP or high pressure cut out is provided with manual re-set.
The hot compressed liquid is passed to a receiver through a
Condenser to cool it down. The receiver can be used to collect the
Refrigerant when any major repair work has to be performed.
The master solenoid is fitted after the receiver, which is controlled
By the control unit. In case of sudden stoppage of compressor, the master
Solenoid also closes, avoiding the flooding of evaporator with refrigerant
liquid.
The room or hold solenoid and thermostatic valve regulate the Flow of the
refrigerant in to the room to maintain the temperature Of the room. For
this, the expansion valve is controlled by a diaphragm Movement due to the
pressure variation which is operated by the bulb sensor Filled with
expandable fluid fitted at the evaporator outlet.
The thermostatic expansion valve supplies the correct amount of
Refrigerants to evaporators where the refrigerants takes up the heatFrom
the room and boils off into vapors resulting in temperature Drop for that
room.

Temperatures to be maintained in different cold rooms

Meat and Fish room = -12℃ to -20℃
Veg and Lobby = +4℃ to -5℃

B) Describe pressure switches, the tubular heat exchanger, oil Separator and
the drier system.
Pressure Switches
A device which opens/closes an electrical circuit when pressure in The
system has reached the value preset on this device .These devices are very
similar in construction and operation but perform different functions

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In the refrigeration system.
The high-pressure switch is a safety device. It is actuated by the Compressor
discharge pressure and stops the compressor in the event of high pressure.
The Low-pressure switch is actuated by the compressor suction pressure. It
is the primary Control for stopping and starting the compressor during
normal system operation when Operating on the pump-down cycle. The
sensing line for the low pressure cutout switch for a Refrigeration system is
typically connected at the suction side of the Compressor when the suction
pressure has been pumped down to the desired level (the cut-out setting),
The low-pressure switch opens and stops the compressor. When the
pressure rises to the Desired level (the cut-in setting), the switch closes, and
compressor starts. The low-pressure Switch is designed to close on high
discharge pressure and open on low system pressures, While the high-
pressure switch is designed to open on high system pressure and close on
low System pressures.
The Tubular Heat Exchanger
A typical shell and tube heat exchanger consists of a set of tubes Forming a
nest. The tube nest is also called as tubestack and is Embedded within a
shell. Cupro-nickel alloy is commonly used in Heat exchangers and possesses
a high resistance to corrosion.
Expansion of the tube bundle in a shell-and-tube type cooler may
Be provided for by the floating end tube sheet.One fluid flows through these
tubes, while the second fluid flows Inside the shell over the tubes. The
cooling or heating medium Flows through the tubes in one or more passes
depending on the Requirement.
In one pass, the coolant enters through one side and leaves from
The other side. In double pass, one set of tubes carries the coolant
In a direction, while the other set of tubes passes the coolant in the
Reverse direction. Directing the flow causes the heating or cooling
Medium to stay longer inside the tube stack. This is known as Dwell time.

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Oil Separator
To recover a substantial quantity of oil and to make it return to the
Compressor as soon as possible Oil separators are almost always made of
steel. As oil-laden discharge gas enters the oil Separator’s very large internal
volume, it immediately slows down its velocity. This low Velocity is the key
to good oil separation. The oil is mixed with the discharge gas in the Form of
a fog. This refrigerant/oil fog now runs into internal baffling, which forces
the fog Mixture to change direction. At the same time, this fog mixture is
slowing down rapidly On the surface of these baffles. Very fine oil particles
collide with one another and form Heavier particles. Finally, fine mesh
screens separate the oil and refrigerant even farther, Causing larger oil
droplets to form and drop to the bottom of the separator. Often, a Magnet
is connected to the bottom of the oil sump to collect any metallic particles.
When The level of oil gets high enough to raise a float, an oil return needle is
opened and the oil is Returned to the compressor crankcase through a small
return line connected to theCompressor crankcase. The pressure difference
between the high and low sides of theRefrigeration or air conditioning
system is the driving force for the oil to travel from the oilSeparator to the
crankcase.
The Drier System
Refrigerant driers are recommended for all refrigerating systems employing
a hydrocarbon refrigerant.Moisture in the Freon system should always be
avoided as it would Lead to ice formation at the expansion valves. Also, it
may causeCorrosion of the steel parts the product of which may also block
The expansion valves and form sludge in the lube oil.The most common
drying agents used are silica gel, alumina gel and synthet Silicates as these
are excellent moisture absorbent. Driers absorb water faster at lower
temperature; therefore the drier should be Installed in the liquid line just
before the refrigerant flow control device.A non-refillable drier has an
arrowstamped or cast on the body to indicate the Direction that the

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refrigerant should flow through it. One must ensure that it is Installed
properly when replaced

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. With reference to main refrigeration plants give reason for
Each of the following operational irregularities and state how
Are these dealt with
a. Rapid loss of lubricating oil form the crankcase of a vee
Block compressor
Reasons
1. Oil separator faulty..then oil can carry over and lead to loss
2. Low loads-more running time -more loss,
3. Short cycling cause compressor to start and stop frequently can .
4. Worn piston rings-can cause oil to leak past then and into the
compression chamber-loss
5. Crankcase pressure-high crankcase pressure can cause lub oil to be
forced out of the crankcase and into refer system-maintain pressur
6. Crankcase overfill-overfilling can cause excessive foaming and result in
loss-drain and fill to correct amount.
7. High superheat tempreature-when compressor gets to hot ,the sump
oil overheats and breaks down..loss

B) Steady fall off in refrigeration effect over a

Comparatively short period of time
– Reasons -
1. Leakgae-even small leaks in the system can cause a gradual reduction
in the amount of refrigerant in the system -result reduce cooling
capacity over time ..check and arrest leak.
2. Decrease in compressor efficiency-due to wear and tear of
components,compressor becomes less efficient.

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 3. Faulty expansion valve-can cuse reduction in the amount of refrigernat
flowing through a evaporator coil-reduced cooling effect
4. Evaporator coils fouling
5. Reduced evaporator fan speed
6. Indeqyate refrigernat in the system
Excessive icing up at compressor suction
Oil in the evaporator -
Oil acts as an insulator inside the evaporator tubes causing reduced heat
transfer -lead to frosting-clean the evaporator and check for pil seprator
functioning
Defectvive defrosting system-
If it is not functioning ,it can cause ice buildup on the evaporator
Dirty evaporator
Less heat transfer-lce up
Faulty expansion valve-
Higher refrigerant flow rate
Overloading of refrigent
1. Evaporator tubes frosted
– Defrost
2. Expansion valve is not adjusted properly
– Adjust for proper superheat
3. Sub cooling is high (in case of cold countries)
– Throttle condenser outlet
4. Bulb unattached; senses atm temp instead of line temp,
Resulting in sending large amount of refrigerant through
– Attach bulb to its original place
Short cycling
Frequent start and stop of compressor on LP cut out is Called short cycling

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Reasons
1. LP cut out difference too low
2.Compressor valves leaking
2. Low charge-
3. Filter/Drier choked-if the suction filter is chocked --
4. Leaky solenoid valves
5. Moisture in the system

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Sketch and describe a high pressure cut-out in a refrigeration
System The refrigeration compressor has stopped due to operation of
The HP cut out.
Hp cutout explanation in the notes

a. Possible causes
Occurs due to
a. Faulty pressure switch-inspect and repair
b. Overcharge of refrigerant-drain
c. Low sw pressure in condenser
d. Clogged filter drier -clean or replaced
e. Malfunctioning expansion valve-inspect and repair or replace
f. Condenser coolant failure
g. Dirty or choked condenser tubes
If the refrigerant cannot liquefy rapidly, The discharge
Pressure will abnormally raise and high pressure cut out will
Be triggered.

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 b. How these causes would be found and possible Remedies
1. Condenser coolant failure
– Observe flow in CW pipe gauge glass
2. Low coolant pressure
– Observe CW pump discharge pressure
3. Dirty or choked condenser tubes
– Observe differential pressure
1. Refrigerant fills condenser and reduces its effective area.
Draw-off coolant.
2. Regulate water/air supply or reduce compressor capacity, if
Called for. Check condenser as per instructions for same
3. Blow air out of condenser. Follow instructions for
Condenser.
What steps are taken if the compressor “short cycle” on low
Pressure cut out
1. To provide sufficient suction pressure control difference
According to the system loading and frequency of room
Inspection
2. Refrigerant charges should be adequate, system should be
Without leaks. Suction line filter to be kept cleaned with no
Obstruction in suction line.
3. Leaky solenoid valve to be replaced. Evaporator coil to be
Defrosted regularly and inner surface to be ensured clean.
4. Piston rings, cylinder liner, discharge valve, by-pass valve and
Safety valve are to be maintained in good condition.
5. Compressor capacity to be selected according to the system
Requirement and nature of loading.

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Q TEV CONSTRUCTION AND WORKING..CHECK NOTES
UNDERCHARGIG OVERCHARGING -CHECK NOTES
GAS LEKAGE DETECTION-NOTES
Q9 -CHECK THE APP

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