SHELL AND TUBE TYPE HEAT EXCHANGERS :-
SHELL: The shell or cylinder is usually made of close grained alloy cast iron, with surfaces machined as required. Gun metal or fabricated steel may be used as alternatives depending upon requirements. Shell is normally is in contact with the liquid being cooled, which may be either oil or fresh water, as such there is no corrosion problem with the shell.
END COVERS AND WATER BOXES: The covers and water boxes are commonly of cast iron or fabricated from mild steel. Sacrificial anodes in the rod or plug form and an electrical contact strip are fitted to minimise corrosion due to galvanic action.
Where the end covers are coated with rubber or a bitumastic type coating or made of gunmetal, then it can provide protection for itself but makes the tubes and tube plates vulnerable for corrosion.
Easily removable covers on water boxes permit repairs and cleaning easier.
TUBE PLATES AND TUBE STACK : The tube stack is made up of stress relieved aluminium brass tubes (76% copper, 22% zinc, and 2% aluminium) expanded into Naval Brass tube plates, (62% Cu, 37% Zn, 1% Sn, Tin reduces corrosion)
One plate is fixed at one end whereas the tube plate at the other end is free to move with the expansion of the tubes. The tube stack is fitted with alternate circular disc and ring baffles made of brass to give radial flow to the fluid and also supports the tube stack. The fixed end tube plate is sandwiched between the shell and water box with jointing material. Synthetic rubber 'O' rings for the sliding tube plate permit free expansion. If the joints leak at the face end, special tell - tale hole indicates leakage and the 'tell - tale' ring will allow the liquids to escape without mixing.
Normally sea water is in contact with the inside of the tubes, tube plates and water boxes, where as outside of the tubes carry lubricating oil or water.
Premature failure of tubes can be the result of pollution in coastal waters or extreme turbulence due to excessive sea water flow rates. To avoid the impingement attack, care must be taken with the water velocity through tubes. For aluminium - brass, the upper limit is about 2.5 m/s.
Although it is advisable to design to a lower velocity than this - to allow for poor flow control - it is equally bad practice to have sea water speeds less than 1m/sec. A more than minimum flow is vital to produce moderate turbulence which is required for the heat exchanger process and to reduce silting and settlement.
INSTALLATION: Manufactures recommend that coolers are arranged vertically. Where horizontal installation is necessary, the sea water should enter at the bottom and leave at the top. Air in the cooler system will encourage corrosion and air locks will reduce the cooling area and cause overheating. Vent cocks should be fitted for purging air and cocks or plugs are required at the bottom for draining. Clearance is required at the cooler fixed end for removal of the tube stack for cleaning.
ln more recent designs of tube type heaters and coolers, the guided flow concept has been introduced i.e. a secondary heating or cooling, surface in the form of radial fins integral with the tubes between which flow is guided radially alternately out and in from section to section. This gives (a) greater heat transfer surface, (b) better heat transfer, (c) lower metal surface temperature (d) In the case of oil heaters less risk of oil cracking and hence fouling.
PROTECTION OF TUBES OF HEAT EXCHANGER :
(1) Sacrificial anodes fitted in the end covers while itself corroding while acting as anodes creates a protective film on other surfaces and protecting them. Electrical countinuity in the seawater circulating pipework is important where sacrificial anodes are installed. Metal connectors are fitted across flanges and cooler sections where there are rubber joints and 'O' rings, which otherwise insulate the various parts of the system.
(2) Iron may be introduced in the system in the form of ferrous sulphate. The treatment consists of dosing the sea water to a strength of 1 ppm for an hour per day for a few weaks and subsequently dosing again before entering and after leaving port for a short period.
(3) To avoid impingement attack due to pollution in coastal waters or extreme turbulence due to excessive sea water flow rates, the upper limit of flow is about 2.5m/sec and lower limit 1m/sec.
Shell & Tube Heat Exchanger :-
Shell or cylinder — closed grained alloy cast iron. Shell is usually in contact with the fluid to be cooled like water or oil.
Tube
Stack -
Aluminium brass (76%Cu, 22%Zn 2%A1)
Tube plate
-
Naval brass (62% Cu, 37%Zn, 1% Sn) Tin reduces corrosion.
Normally sea water is inside the tube. Tubes are expanded at the tube plate. One end of the tube plate is fixed whereas the other end of the
tube plate is free to expand with the expansion of the tubes. The tube stack is fitted with circular disc and ring baffles made of brass to give radial flow and also supports the tubes.
Protection
(l)Sacrificial anodes are fitted in the end covers. Electrical continuity in the s.w circulating pipework is to be maintained.
(2)Iron introduced into the system in the form of ferrous sulphate.
Dosing of sea water to a strength of 1 ppm for an hour per day for a few weeks and subsequently dosing again before entering and after leaving port for a short period.
(1) To avoid impingement attack, sea water flow rate should be restricted to an upper limit of 2.5 m/s and lower limit of lm/sec (too low a flow impairs the heat transfer)
GUIDED FLOW FUEL OIL HEATER
INDEX ADV ENG NEXT