REVERSE OSMOSIS DESALINATION

Osmosis:

When fluid of different concentration in a vessel are separated by a membrane, the dilute solution will flow through the membrane into the concentrated solution.

Osmotic pressure:

The level of dilute solution drops and the level of concentrated solution rises until an equilibrium is reached. The pressure difference between the two levels is called osmotic pressure.

Reverse osmosis:

If a pressure excess to a osmotic pressure is applied to the concentrated solution, the flow is reversed from the concentrated solution to  the diluted solution.

Membranes used:

• Hollow fine fibre
• Spirally wound

For sea water application membranes used:

• Polyamide
• Polysulphonate

In reverse osmosis desalination:

Pic: Reverse osmosis desalination

• Sea water feed is pressurized using high pressure pump.
• The high pressure sea water is passed through a chamber containing a permeable membrane.
• The membrane stops the passage for salt and allows pure water to pass.
• The clean water is then sent to a potable water tank.

BASIC REFRIGERATION - Vapour compression, Vapour absorption

Definition: -

Refrigeration is defined as a process in which the temperature of the space or its content is reduced below its surrounding temperature.

Purpose of refrigeration on ships:

• To prolong the life of perishable food stuffs.
• Liquefaction of boil off on LNG carrier.
• To maintain sub zero temperature for certain chemicals.
• Removal of moisture from control air.
• Air conditioning for the comfort of personnel and prolonging life of electronic components.
• For cooling the drinking water.
• For making ice on passenger ships.
• For making interference fit of small components such as exhaust valve seat and valve guide etc.

Classification of refrigeration system:

Classification based on the uses

• Vapour compression
• Vapour absorption
• Steam ejection method
• Air refrigeration method
• Unconventional method

Classification as per the size of the plant

• Primary system
• Primary/secondary system

Classification based on the refrigerant 

• Freon-22
• R-134a
• R-404
• R-507
• Ammmonia

Different refrigeration system:

• Ice refrigeration.
• Vapour compression refrigeration system.
• Vapour absorption refrigeration system.
• Air refrigeration 
• Steam jet refrigeration system
• Spray refrigeration system
• Dry ice refrigeration
• Thermo-electric refrigeration

Vapour Compression Refrigeration system: -

Pic: Vapour compression refrigeration system

Components of vapour compression system:

• Evaporator
• Compressor
• Condenser
• Expansion valve

Operation: -

• In vapour compression system, the phase will change from liquid to vapour in evaporator and vapour to liquid in condenser.
• The refrigerant absorbs the latent heat from evaporator and reduces the temperature of the space from its surrounding thus change its phase from liquid to vapour.
• This high temperature refrigerant is then compressed in a compressor to make the pressure equal to the condensing pressure.
• In condenser, the refrigerant is cooled thus leaving the latent heat. The phase of refrigerant changes from vapour to liquid.
• This high pressure low temperature refrigerant is then passed through the expansion valve where its pressure is reduced and made equal to evaporating pressure.
• This refrigerant is thus again used for the refrigerant purpose.

FUNDAMENTALS OF ELECTRICITY

Unit of charge:

Coulomb is used as the unit of charge.

Charge of an electron is,

e = -1.60217733 x 10-19 C

Electric current:

Directed flow of free electrons(charge)is called electric current.

Actual direction of current is from negative terminal to positive terminal.

Current flowing from positive terminal to negative terminal is called conventional current.

Types of electric current:

Electric current is classified into the following: -

1. Steady current
2. Varying current
3. Alternating current

Steady Current: When the magnitude of current does not change with time, it is called as steady current.

Varying current: When the magnitude of current changes with time, it is called as varying current.

Alternating current: When the magnitude of current changes continuously with time and direction changes periodically, it is called as alternating current.

Electric Potential:

The capacity of a charged body to do work is called as electric potential.
Electric potential, V = Work-done/Charge = W/Q
Unit of Electrical potential, V = Joule/Coulomb = Volt

Potential Difference:

The difference in the potentials of two charged bodies is called potential difference.

It is also called as voltage.

The current will flow in a circuit if a potential difference exists in the circuit.

Resistance:

The opposition of the flow of electric current is called its resistance.

Unit of resistance is Ohm and is denoted by Ω.

Factors upon which resistance depends:

The resistance of a conductor,

• Is directly proportional to its length(l).
• Is inversely proportional to its cross sectional area(a).
• Depends upon the nature of the material.
• Changes with temperature.

i.e.,

Rα l/a

R=pl/a

Where,

P=Resistivity

Resistivity or specific resistance:

It is defined as the amount of resistance offered by a material of length ‘l’ and a cross sectional area ‘a’.

It is denoted by p.

Effect of temperature on resistance:

1. The resistance of pure metal (copper, aluminum), conductors increases with increase in temperature. It has a positive temperature coefficient of resistance.
2. Resistance of electrolytes, insulators, semiconductors (silicon, germanium) decreases with increase in temperature. It has a negative temperature coefficient of resistance.
3. Resistance of alloys increases with increase in temperature but this increase is very small and irregular.

Ohm’s law:

It states that the ratio of potential difference (v) across the end of a conductor to the current (I) flowing between them is constant, provided the physical condition (temp) does not change.

V/I = Constant = R

Electric Power: Rate at which work is done in an electrical circuit is called electric power.

Electric power = Work done in electric circuit/Time

P = VIt/t = VI = I²R = V²/R

Unit of power is Joule/Sec or watt.

1KW = 1000 watts

1MW = 10⁶ watts = 10³ KW

Electrical Energy:

The total work done in an electrical circuit is called electrical energy.

Electrical energy = Electric power x time

= VIt

=I²Rt

=V²t/R

Its unit is Kwh (Kilowatt- hour)

1Kwh of electrical energy is called Board of time (BOT) unit.

Power Rating:

Ability of a resistor to dissipate heat without destructive temperature built up

MARINE SEWAGE TREATMENT PLANT

Principle of sewage treatment

• The breakdown of raw sewage in water is effected by aerobic bacteria if there is a relatively ample presence of oxygen, but by anaerobic bacteria if oxygen has been depleted.
• Aerobic bacteria require free oxygen to survive.They breakdown the organic matter to produce safe products such as water, carbon dioxide. Final discharge has a clean and clear appearance.
• Anaerobic bacteria can only multiply in the absence of free oxygen.They breakdown the organic matter into water, carbon dioxide,methane, hydrogen sulphide and ammonia which are noxious and toxic.

Pic: Sewage treatment plant

Components/Chambers/Tanks

• Wire mesh
• Primary tank
• Aeration chamber
• Settlement tank
• Chlorination and collection tank
• Blowers
• Chlorinator
• Low level, high level alarm

Operation

• Sewage water is introduced into a primary vessel through a wire mesh.
• It is then passed through a wire mesh which grinds the sewage and breakdown into smaller particles.
• A plate is provided which makes the sewage to rise and flow into the aeration chamber.
• Air is forced through the diffuser into the air chamber which helps the aerobic bacteria to grow and attack the sewage.
• Constant pressure of around 0.3-0.4 bar is kept for proper agitation and formation of bubbles.
• The sewage is decomposed into carbon dioxide, water and inorganic sewage.
• The mixture of liquid and sludge is passed to the settling tank from the aeration chamber.
• The sludge settle at the bottom and  clear liquid on the top, sludge is not kept in settling tank as it may produce anaerobic bacteria and foul gases are produced.
• The sludge formed is recycled by sending it back to the aeration chamber.
• The clear liquid produced from the settling tank is overflown and is disinfected with the help of chlorine.
• This is done due to the presence of E-coli present in liquid which has to be eliminated.
• The liquid during chlorination is kept for a period of at least 60 min.
• The collected liquid is discharged overboard when the high level alarm raises and the vessel is in proper geological location. 

Effluent quality standards

• The bio-chemical oxygen demand (BOD) is determined by incubating at 200 degree Celsius, a sample of sewage effluent which has been well-oxygenated. The amount of oxygen absorbed over a five-day period is then measured. The test measures the total amount of oxygen used for complete breakdown of organic matter. This indicates the strength of the sewage. IMO recommends BOD of less than 50 mg/L after treatment through sewage treatment plant.
• Suspended solids - this can give rise to silting problems. Suspended solids are measured by filtering a sample through a pre-weighed pad which is then dried and re-weighed. IMO recommends 50 mg/ litre after treatment.
•  Coliform count - The e-coliform is a family of bacteria which live in the human intestine. The result of this test is called the e-coli count and is expressed per 100ml. Presence of these organisms in water is an indication of pathogen (disease causing bacteria responsible for cholera, dysentery, typhoid). IMO recommends fecal coliform count of less than 250/100 ml. of affluent after treatment.

The rules which has to be followed for the discharge of sewage is given in annex IV as below.

Marpol Annex IV: -

• Vessel should be equipped with a certified sewage treatment system or holding tank.
• Within 3 miles of nearest land, sewage discharges are to be treated by a certified marine sanitation that is a sewage treatment plant device prior discharge.
• Between 3 miles and 12 miles from shore, sewage discharges must be treated by no less than maceration or chlorination.
• Sewage discharge beyond 12 miles from shore is unrestricted.
• The discharge of sewage should be such that the ship is en route and is proceeding not less than 4 nautical miles.

DIFFERENCE BETWEEN 2 STROKE ENGINE AND 4 STROKE ENGINE & 2 STROKE TIMING DIAGRAM

Difference between 4 stroke engine & 2 stroke engine

Serial No.	4 Stroke Engine	2 Stroke Engine
1	It has one power stroke for every two revolution of crankshaft.	It has one power stroke for every revolution of crankshaft.
2	Heavy flywheel is required and engine is balanced.	Lighter flywheel is required and engine is unbalanced.
3	Engine is heavy.	Engine is lighter.
4	Engine design is complicated due to valve mechanism.	Engine design is safe due to absence of valve mechanism.
5	Less mechanical efficiency due to more friction in parts.	More mechanical efficiency.
6	Engine is usually water cooled.	Engine is air cooled or water cooled.
7	Less fuel consumption and complete burning of fuel.	More fuel consumption and air is mixed with burnt gases.
8	Engine requires more space.	Engine requires less space.
9	Engine consist of inlet and exhaust valves.	Engine consist of inlet ports and exhaust ports or valves.
10	Thermal efficiency is more.	Thermal efficiency is less.

2 Stroke timing diagram

Operation of a 2-stroke diesel engine

Pic: 2 Stroke timing diagram

Numbers	Actions
0	Scavenge ports are open.
0-1	Air is sucked in, which pushes out the residual exhaust gases.
1	Piston is at BDC.
1-2	Completion of scavenge process and filling of fresh air for combustion.
2	Scavenge ports are closed.
2-3	Post scavenging takes place.
3	Exhaust valves closes.
3-4	Compression of air.
4	Fuel injection commences.
5	Fuel injection commences, near TDC
6	Fuel injection and combustion completion.
6-7	Expansion of the heat energy from combustion, being converted into work energy to push the piston downward.
7	Exhaust valve opens.
7-0	Blowdown of exhaust gases seen as a sudden rapid pressure drop on the P.V.diagram.

DIESEL ENGINE - 2 STROKE & 4 STROKE

Diesel Engine - Strokes

A Diesel engine is a machine which produces power by burning the oil or fuel in a body of air which has been squeezed to a high pressure by a moving piston.

2 Stroke Diesel engine

2-Stroke Diesel engine has the following strokes: -

1. Power stroke
2. Scavenging/Compression stroke   

The 2 stroke cycle is completed in two strokes of the piston or in one revolution of the crankshaft.
The fresh air must be forced in under pressure and is used to clean out or scavenge the exhaust gases and is then filled with fresh air.

Pic: 2 Stroke Cycle

The 2 strokes are explained as follows: -

Power Stroke: -

• The piston is at the top of the cylinder i.e, TDC* where fuel injection and combustion has taken place.
• Due to combustion, the piston is pushed down to the BDC** thereby producing power.
• Due to this movement, exhaust ports are opened where burnt gases escapes out and continues to move downward till scavenge ports are opened. 

Scavenging/Compression: -

• Pressurized fresh air then enters the cylinder through scavenge port and thereby driving out the remaining exhaust gases.
• The piston then moves from BDC to TDC thereby closing all the ports.
• The air is then compressed as the piston moves from BDC to TDC thereby completing the cycle. 

4 Stroke Diesel engine

4-Stroke Diesel engine has the following four strokes: -

1. Suction stroke
2. Compression stroke
3. Power or expansion stroke
4. Exhaust stroke

Pic: 4 Stroke cycle

Suction Stroke: -

• At starting the piston will be at top dead center, the inlet valve is opened from where fresh charge of air comes in.
• The crankshaft rotates towards its right and pulls down the piston from TDC to BDC.
• Due to this movement, fresh air is drawn inside the cylinder through the inlet valve.
• As the piston reaches BDC, the inlet valve closes and the intake or suction stroke is completed.

Compression Stroke: -

• As the crank turns to its left and thereby pushing the piston towards TDC.
• Since all valves are closed, it is forced into a smaller space.
• This increase its pressure and also temperature, when piston reaches TDC air occupies 1/16th of its original space and rises its temperature to nearly 540 degree celsius or more.

Power or Expansion Stroke: -

• At this point, fuel is injected to the cylinder where it instantly ignites due to the presence of hot compressed air.
• The oil burns quickly as it is mixed well with the hot air and also due to the burning of fuel, the mixture becomes more hotter.
• Due to this high temperature and pressure being created due to combustion pushes the piston from TDC to BDC thereby expanding it.
• This push of the piston from TDC to BDC produces the power thereby this stroke is also known as power stroke.
• As the piston reaches BDC, exhaust valve opens and the burnt gases escapes to the atmosphere till its pressure becomes as same as that of atmosphere.

Exhaust Stroke: -

• At this stroke, exhaust valve is opened and the exhaust gas are released or escapes until its pressure falls to near to the atmosphere pressure.
• The piston moves from BDC to TDC thereby expelling the burnt gases through the exhaust valve.
• As the piston reaches TDC, all the exhaust gases are expelled and exhaust stroke is then completed.

* TDC - Top Dead Center

** BDC - Bottom Dead Center