Marine Engineering

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Basic Theory

Distance
Viscosity
Size of particle
Gravitational Force

Rate of sedimentation

When a particle (solid or liquid) moves through a viscous
medium under the influence of gravitation, it will attain a constant velocity after a certain time.
This is known as the sedimentation velocity
or derived from Stokes' Law, the rate of sedimentation
can be calculated if the following physical parameters
are known:
Particle diameter d (m)
Particle density Pp (kg/m3)
Density of the continuous phase PI(kg/m3)
Viscosity of the continuous phase 1'/(kg/m,s)
Gravitational acceleration g (9.81 m/s2)



Simple centrifuging

A heat exchanger allows the transfer of heat from one fluid (liquid or gas) to another fluid
 
Reasons for heat transfer

To heat a cooler fluid by means of a hotter fluid
To reduce the temperature of a hot fluid by means of a cooler fluid
To boil a liquid by means of a hotter fluid
To condense a gaseous fluid by means of a cooler fluid
To boil a liquid while condensing a hotter gaseous fluid

Methods of Heat Transfer

Conduction
Convection
Radiation

Heat Exchanger Types

Recuperative
Regenerative
Evaporative

Evaporative Type

Cooling Tower
Direct contact heater

Heat Transfer depends on

Area
Mean Temperature difference between the liquids
Material through which heat transfer takes place
Time in contact
Thickness

Types of Flow

Parallel
Contra
Cross

Types of Shell & Tube Heat Exchanger

Expansion Bellow in shell
Floating Tube Arrangement
Fixed ‘U’ Tubes
Bayonet Tube ( Guided Flow)

Heat Exchanger Materials

Tube –Aluminium Brass,Cupro Nickel or Stainless Steel
Tube Plate – To suit tube material-Naval Brass
Shell & End Covers – Fabricated Mild Steel or Cast Iron

Shell &Tube Heat Exch. Tube faults

Cavitation corrosion -Vapour bubbles are produced in regions of high velocity and low pressure caused normally by constriction of flow, when these bubbles move into a region of high pressure where they increase in size and then implode, if the bubble is remote from the tube surface then the pressure is transmitted as a shock wave.
Dezincification this is a form of wastage which takes place in brasses that contain more than 15% zinc. Zinc is removed leaving a weak porous copper. Tubes often contain a small amount of arsenic to act as an inhibitor.
Impingement especially near inlet tube ends. This impingement causes the protective oxide film to be removed and corrosion takes place since this small area becomes anodic whilst the rest of the tube is cathodic. Reduced by fitting plastic inserts at tube ends.
Acidic water prevents the formation of a ferrite oxide film forming, thus the tube surface is under continuous attack and it thickness is gradually reduced until perforation takes place.
Erosion by abrasive solids such as sand in the sea water and high sea water velocities can cause metal from the tubes to be removed
Graphitisation occurs in cast iron or mild steel water boxes, iron is removed leaving a soft layer of carbon.
Tube vibration.can cause failure of tubes due to fatigue, tubes should be adequately supported. Additionally tubes may come into contact with one another, this hammering effect causing flats to be formed at the points of contact, eventually wearing through the tube.

Plate type heat exchanger

PROPERTIES

Elasticity
The elasticity of a metal is its power of returning to its original shape after deformation by force. Many materials behave to some extent like powerful elastic, and within limits, will recover their shape when the load on them is removed.

Hardness

The hardness of a metal is a measure of its ability to withstand scratching wear and abrasion, indentation by harder bodies, marking, by a file etc.
The Brinnel hardness of a metal is found by pressing a ball on to the surface of the metal, the hardness number being found by dividing the load on the ball by the surface area of the impression.

Malleability

This is the property of permanently extending in all directions without rupture by pressing, hammering, rolling, etc. It requires that the metal shall be plastic but is not so dependent on strength e.g. lead is malleable.

Plasticity

This is a rather similar property to malleability and involves permanent deformation without rupture.It is the extreme opposite to elasticity.

Strength

The strength of a metal is its ability to resist the application of force without rupture.In service a material may have to withstand tension,compression,or shear forces.The strength of material is measured by loading it in a testing machine until fracture.

Toughness

Is the amount of energy a material can absorb before it fractures or measure of toughness of a metal may be obtained by nicking it, placing it in a vice and striking it hard with a hammer

Steering Gears
The direction of the ship is controlled by the steering gear. As the ship moves through the water, the angle of the rudder at the stern determines the direction it will move. Modern ships are so big that moving the rudder necessitates the use of hydraulics or electrical power.

The steering starts at the Bridge. The required rudder angle is transmitted hydraulically or electrically from the steering wheel at the Bridge to the telemotor at the steering gear, just above the rudder.
There are a few common arrangements for using hydraulic power. There are the 4-rams, 2-rams, and rotary vane types. The heart of these hydraulic systems is the variable delivery pump. This type of pump can be controlled by just moving a spindle. The pump is driven by an electrical motor at constant speed. By moving the control spindle away from the central point, the pump stroke increases, and the hydraulic fluid is pumped in one direction. Moving the spindle more from the central point will cause more fluid to be pumped and consequently more pressure is generated to drive the rams. Moving the control spindle back to the original position and then away in the opposite direction causes the hydraulic fluid to be pumped in the reversed direction. The rams will also move in the reversed direction.

By using a floating lever feedback mechanism, when the rudder stock has reached the desired angle, the pump control lever moves back to the original position, and the pumping action stops. The rudder is stopped at the required angle. Moving the steering wheel to the opposite direction will cause the rudder to come back to the original zero position.