Marine Engineering

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