Tag Archives: Heat Exchangers

Shell and Tube Heat Exchangers

Shell and Tube Heat Exchangers for engine jacket water and lubricating oil cooling are normally circulated with seawater. The seawater is in contact with the inside of the tubes and the water boxes at the cooler ends. The oil or water being cooled is in contact with the outside of the tubes and he shell of the cooler. Baffles direct the liquid across the tubes as it flows through the cooler. he baffles also support the tubes.

shell_and_tube_heat_exchangers

Tubes of Aluminium-brass (76% copper; 22% zinc; 2% aluminium) are commonly used. Ordinary brasses and other cheap materials have been used with unsatisfactory results. Successful use of aluminium-bass has apparently depended on the presence of a protective film formed along the tube length by corrosion of iron in the system. Thus unprotected iron in water boxes and other parts, while itself corroding, has prolonged tube life. This was made apparent when steel was replaced by  other corrosion resistant materials or protected more completely. The remedy in these systems is to fit sacrificial soft iron or mild steel anodes in water boxes or to introduce iron in the form of ferrous sulphate fed into the seawater. The latter treatment consists of dosing the seawater to a strength of 1 ppm for an hour per day over a few weeks and subsequently to dose before entering and after leaving port for a short period.

Early tube failures may be due to pollution in coastal waters or to turbulence in some cases. Many coolers are fitted with tubes of 70/30 Cupro-Nickel. More expensive materials are available. Tubes are expanded into tube plates and may be further bonded by soldering.

Tube Plates

Naval brass tube plates are used with aluminium bass tubes. Tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end which is free to move with the expansion of the tubes. Other materials found in service are gunmetal, aluminium bronze and sometimes special alloys.

Tube Stack

The tube stack is fitted with alternate disc and ring baffles. The fixed end tube stack is sandwiched between the casing and the water box. If the joints leak at the other end the special ‘tell-tale’ ring will allow the liquids to escape without mixing. The joint rings are of synthetic rubber.

Water Boxes & Covers

Easily removable covers on water boxes permit repairs and cleaning to be carried out. The covers and water boxes are commonly of cast iron or fabricated from mild steel. Where they have been coated with rubber or a bitumastic type coating, the iron or steel has been protected but has provided no protection for tubes and tube plate. Uncoated ferrous (iron) materials in water boxes provided a protective film on the tubes as the unprotected iron itself corrodes, products of corrosion coating the tubes. The iron also gives some measure of cathodic protection.

Water boxes of gunmetal and other materials are used but these, like the coated ferrous metals, give no protection. Soft iron or mild steel anodes can be fitted in the water boxes and provided they cause no turbulence, will help to give cathodic protection and a protective film.

Shell

The shell or cylinder is fabricated or cast. It is in contact with the liquid being cooled. This may be oil, with which there is no corrosion problem, or water, which is normally inhibited against corrosion. The material is not critical (provided it is not reactive with any inhibiting chemicals) because it is not in contact with seawater.

REFERENCES

“General Engineering Knowledge” by H.D. McGeorge