Multi-tubed Heat Exchanger

With reference to sea water cooled multi-tubed heat exchangers state

a) The materials used for the construction of tubes, tube plates & water boxes.

Tubes - Cupro nickel(70% Cu-30%Ni) or 90%Cu-10%Ni

or Aluminium Brass (76% Cu-22% Zn -2% Al)

Tube Plate - Admiralty Brass (70% Cu - 29%Zn - 1% Sn)

Naval Brass ( 69% Cu-39%Zn - 1% Sn)

Water Boxes - Cast Iron or steel

90-10 Cupro-Nickel, Cast Ni-Al Bronze

b) Various types of corrosion that the parts in (a) are subjected to

General Corrosion - The static corrosion rate is of about 0.1 mm/year. This would increase with increase in velocity of 3 m/sec to 1 mm/ year for steel pipes. For stainless pipes, greater supply of oxygen improves the protective effect of surface films and above 1m/sec the dangers of pitting also decreases.

Impingement attack - Copper base alloys are not affected by increase in velocity but eventually a critical velocity is exceeded and an intensive localised corrosion called impingement attack occurs.

The metals and alloys have a well established surface water speed above which their protective film is removed and corrosion accelerated because a galvanic couple is set up between the exposed and unexposed metal.

Cavitation Attack - These remove oxide skin and also removes the metal until a hole is worn in metal. This is due to water hammering effect of steam of vapour bubbles caused by turbulence. The bubbles burst over small area of metal surface and chip away the metal.

Selective Corrosion - 

a)De-zincification of Brass - Removal of Zinc from brass, leaving behind porous & weak copper. These can be inhibited by addition of small amount of arsenic but duplex alloys as 60-40 brass cannot be inhibited.

b)Graphitisation - In sea-water, the iron in matrix form can be selectively corroded leaving behind a fragile shell largely constituting graphite. This is soft and easily get fractured. 

c)Measures employed to reduce or prevent above corrosion

Chlorine treatment - Chlorine or hypochlorite is sometimes added to sea-water systems to prevent marine fouling

Cathodic protection - 

Sacrificial anodes

When dissimilar metals are connected together and immersed in an electrolyte, a galvanic cell is set up. Hence corrosion would take place at anode. Sacrificial anodes which are more electro-positive than the other material(to be protected) are placed on water boxes making good electrical contact with it.

Zinc is used to protect Iron in sea water.

Ferrous Solution - Presence of iron or iron oxides has a beneficial effect on the tubes. Breakdown of protective layer may occur due to turbulence. To aid the formation of protective layer. FeSO4 solution is injected in sea water system.

Coating - Bituminous or epoxy coatings, rubber linings etc. may be used. They are electrically insulated from sea water which are in contact with other parts.