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Shell, Tube & Plate Type Heat Exchanger for Marine Diesel Engine

The three basic methods for controlling the temperature of the hot fluid in a heat exchanger when the cooling medium is sea-water, are:
  1. to bypass a proportion or all of the hot fluid flow,
  2. to bypass or limit the sea-water flow;
  3. to control sea-water temperature by spilling part of the sea-water discharge back into the pump suction.

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The last of these methods could be used in conjunction with one of the other two and it was resorted to when sea water was used for direct cooling of diesel engines. It enabled the sea water to be passed through jackets at a temperature warmer than that of the sea. Very cold sea water would cause severe thermal stress. The temperature of sea water for direct cooling was kept to between 40° and 49' C, the upper limit being necessary to limit scale formation.

Automatic control equipment for the system shown above, is based on using a control valve to bypass the sea water at the outlet side of the heat exchanger. This ensures that the heat exchanger is always full of sea water and is particularly important if the heat exchanger is mounted high in the sea-water system and especially if it is above the water line. Pneumatically operated valves may be fitted for temperature control, through bypassing the sea water, The flow of hot fluid through a heat exchanger may be controlled by a similar bypass or by a control valve of the Walton wax-operated type, directly actuated by a temperature sensor.

TUBE TYPE HEAT EXCHANGER
Fig: TUBE TYPE HEAT EXCHANGER


Shell and tube coolers

Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes. A two-pass flow is shown in the diagram but straight flow is common in small coolers. The oil or water being cooled is in contact with the outside of the tubes and the shell of the cooler. Baffles direct the liquid across the tubes as it flows through the cooler. The baffles also support the tubes and form with them a structure which is referred to as the tube stack. The usual method of securing the tubes is to roll-expand them.

Tubes of alluminium brass ( 76% copper , 22% zinc, 2% aluminium ) are commonly employed and the successful use of this material has apparently depended on the presence of a protective film of iron ions, formed along the tube length, by corrosion of iron in the system. Unprotected iron in water boxes and in parts of the pipe system, while itself corroding, does assist in prolonging tube life. This factor is well known (Cotton and Scholes, 1972) but has been made apparent when iron and steel in pipe systems have been replaced by non-ferrous metals or shielded by a protective coating. The remedy in non-ferrous systems, has been to supply iron ions from other sources. Thus, soft iron sacrificial anodes have been fitted in water boxes, iron sections have been inserted in pipe systems and iron has been introduced into the sea water, in the form of ferrous sulphate. The latter treatment consists of dosing the 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.

Electrical continuity in the sea-water 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.

Premature tube failure 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 of less than 1 /sec. A more than minimum flow is vital to produce moderate turbulence which is essential to the heat exchange process and to reduce silting and settlement in the tubes. Naval brass tube plates are used with aluminium-brass tubes. The 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 when the tubes expand or contract. The tube stack is constructed with baffles of the disc and ring, single or double segmental types. 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.

Type of cooler described: This may prolong cooler life by reversing the flow so that tube entrances, which are prone to impingement damage, become outlets. Cooler end covers and water boxes are commonly of cast iron or fabricated from mild steel. Unprotected cast iron in contact with sea water, suffers from graphitization, a form of corrosion in which the iron is removed and only the soft black graphite remains.

The shell is in contact with the liquid being cooled which may be oil, distilled or fresh water with corrosion inhibiting chemicals. It may be of cast iron or fabricated from steel. Manufacturers recommend that coolers be 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 a plug are required at the bottom, for draining. Clearance is required at the cooler fixed end for removal of the tube stack,

PLATE TYPE HEAT EXCHANGER
Fig: PLATE TYPE HEAT EXCHANGER


Plate type heat exchangers

The obvious feature of plate type heat exchangers, is that they are easily opened for cleaning. The major advantage over tube type coolers, is that their higher efficiency is reflected in a smaller size for the same cooling capacity. They are made up from an assembly of identical metal pressings with horizontal or chevron pattern corrugations; each with a nitrile rubber joint. The plates, which are supported beneath and located at the top by parallel metal bars, are held together against an end plate by clamping bolts. Four branch pipes on the end plates, align with ports in the plates through which two fluids pass. Seals around the ports are so arranged that one fluid flows in alternate passages between plates and the second fluid in the intervening passages, usually in opposite directions.

The plate corrugations promote turbulence in the flow of both fluids and so encourage efficient heat transfer. Turbulence as opposed to smooth flow causes more of the liquid passing between the plates to come into contact with them. It also breaks up the boundary layer of liquid which tends to adhere to the metal and act as a heat barrier when flow is slow. The corrugations make the plates stiff so permitting the use of thin material. They additionally increase plate area. Both of these factors also contribute to heat exchange efficiency.

Excess turbulence, which can result in erosion of the plate material, is avoided by using moderate flow rates. However, the surfaces of plates which are exposed to sea water are liable to corrosion/erosion and suitable materials must be selected. Titanium plates although expensive, have the best resistance to corrosion/erosion. Stainless steel has also been used and other materials such as aluminium-brass. The latter may not be ideal for vessels which operate in and out of ports with polluted waters.

The nitrile rubber seals are bonded to the plates with a suitable adhesive. Removal is facilitated with the use of liquid nitrogen which freezes, makes


Related Information:

Heat exchanger precautions

Advantages of various heat exchangers

Sea water circulation of coolers for lubricating oil, piston cooling, jacket water, charge air, turbo-charger

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