1. Heat Exchangers
- Heat exchangers on board ship are mainly coolers where a hot liquid is cooled by sea water. There are some instances where liquid heating is required, such as heavy fuel oil heaters and sea water heaters for tank cleaning.
- The heat exchange process is accomplished by having the two liquids pass on either side of a conducting surface.
- The heat from the hot liquid passes to the cold liquid and the conducting surface, i.e. the tube wall, is at a temperature between the two.
- For marine heat exchangers to have the two liquids flowing in opposite directions, i.e. counter or contra flow. This arrangement provides a fairly constant temperature difference between the two liquids and therefore the maximum heat transfer for the available surface area.
1.1. Types of Heat Exchangers
The heat exchangers at sea fall into two groups, shell and tube and the plate type.
1.1.1. Shell and tube
Most marine heat exchangers are of the shell-and-tube type, an example of which is shown in Figure 1.
Figure 1 Shell and tube heat exchanger
- A tube stack is inserted inside a shell, whose branches are connected into the circulating system of the hot fluid. The stack comprises a number of tubes secured into a tube-plate at each end, and a series of baffles to direct the flow of hot fluid back and forth across the tube bundle.
- At each end of the heat exchanger is a header. These headers may be designed to give a single pass through the tubes or, as in figure 1, two passes. Removable covers or inspection door are normally provided on the headers, to facilitate access to the tubes for cleaning.
- At one end of the heat exchanger, gaskets are fitted between the tube-plate and both the shell and the header.
- At the other end, the tube plate is free to move with seals fitted either side of a safety expansion ring. This is, in the event of leakage past the seal it will pass out of the cooler and be visible with out intermixing or contamination.
- This arrangement also permits movement of the tube-plate to differential expansion between tube stack and shell.
- The shell is usually made of closed grained cast iron. Gun metal or fabricated steel may be used as alternatives depending upon requirements.
- End boxes with end access covers are of the same material as the shell. Sacrificial anodes in rod form and an electrical contact strip are fitted to minimize corrosion. The tube stack is made of stress relieved aluminum brass tubes.
1.1.2. Plate Type
- The plate-type heat exchanger is made up of a number of pressed plates surrounded by seals and held together in a frame, figure 2 & 3.
- This consists of a variable number of gasketed, titanium (stainless steel or aluminum brass) plates, clamped together, between a closing pressure plate and a frame.
- The surface of the plates is corrugated to give strength and additional heat transfer surface.
- Gaskets are usually nitrile rubber bonded to the plate and arranged so that in the event of failure the two fluids cannot mix.
- The inlet and outlet branches for each liquid are attached to one end plate.
- The arrangement of seals between the plates provides passage ways between adjacent plates for the cooling liquid and the hot liquid (Figure 2.b).
- The plates have various designs of corrugations to aid heat transfer and provide support for the large, flat surface.
- The seals between the plates are so arranged that one fluid flows in alternate passages, usually in the opposite direction.
- The plates are usually of titanium providing very high resistance to corrosion by sea water.
- Principal advantages of the plate heat exchanger are:
1- Compact and space saving.
2- Easily inspected and cleaned.
3- Variable capacity
4- With titanium plates there is virtually no corrosion or erosion risk.
5- The turbulent flow which takes place between plates increases the heat transfer and also enables fewer plates to be used.
Figure 2 Plate-type heat exchangers: (a) construction, (b) operation
Figure 3 Plate heat exchanger assemblies
- Temperature control of coolers is usually achieved by adjusting the cooling liquid outlet valve.
- The inlet valve is left open and this ensures a constant pressure within the cooler.
- Vents are provided in the highest points of coolers which should be opened on first filling and occasionally afterwards. Also, it is important because Air remaining in a cooler will considerably reduce the cooling effect.
A) Shell and Tube
1- Clean heat transfer surfaces are the main requirements for satisfactory operation. With sea water cooling the main problem is fouling of the surfaces, i.e. the presence of marine plant and animal growth.
2- With shell and tube coolers the end covers are removed to give access to the tubes for cleaning.
3- Special tools are usually provided by the cooler manufacturer for cleaning the tubes.
4- Tube leakage can result from corrosion. This can be checked for, or identified, by having the shell side of the cooler circulated while the cooling water is shut off and the end covers removed. Any seepage into the tubes will indicate the leak.
5- It is also possible to introduce fluorescent dyes into the shell-side liquid: any seepage will show under an ultraviolet light as a bright green glow.
6- Leaking tubes can be temporarily plugged at each end or removed and replaced with a new tube.
B) Plate Type Coolers
They develop leaks present a more difficult problem.
1- The plates must be visually examined to detect the faulty point. The joints between the plates can present problems in service or on assembly of the cooler after maintenance.
2- Where coolers are out of use for a long period, such as during surveys or major overhauls, they should be drained on the sea water side, flushed through or washed with fresh water, and left to dry until required for service.
2. Oil/Water Separator
Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard when pumping out bilges, de-ballasting or when cleaning oil tanks.
The international legislation of IMO relating to pollution has limited the requirement to fit the separators.
Inshore discharge of oil can cause damage to fish and bird life and mass pollution of beaches.
The legal maximum oil particle discharge quantity is 100 parts per million (PPM) of water.
Oil/water separators can only achieve 100 PPM.
Filtering systems are further required to provide an effluence of no more than 15 PPM under all inlet conditions.
Figure 4 Simplex-turbolo oil/water separator with coalescer
2.1. Oil/water separator; System Description
- It consists of a vertical pressure vessel containing a number of inverted conical plates.
- The oily water mixture is pumped through the separator inlet pipe into the cause separating compartment. Hence some oil, as a result of its lower density, will separate and rise into the oil collection space.
- The remaining oil/water mixture now flows down into the fine separating compartment and moves slowly between the conical plates.
- More oil will separate out on to the underside of these plates and travel outwards until it is free to rise into the oil collecting space. The almost oil-free water passes into the central pipe and leaves the separator unit.
- The purity at this point will be 100 PPM or less. Where greater purity is required, the almost oil-free water passes to a filter unit.
- An automatically controlled valve releases the separated oil to a storage tank. Air is released from the unit by a vent valve.
- Before initial operation, the separator must be filled with clean water. To a large extent the conical plates are self-cleaning but periodically the top of the vessel should be removed and the plates examined for sludge build up and corrosion. It is important that the separator is not run at over capacity, to avoid deterioration of the effluent quality.
Figure 5 Simplex-turbolo oil/water separator
Figure 6 Oil/water separator
2.2. Fuel and lubricating oil treatment
- Both fuel oils and lubricating oils require treatment before passing to the engine. To ensure good combustion in diesel engines and reduces wear and corrosion in this type of engines and turbines it may be necessary to remove solid particles, impurities, from fuel and lubricating oils. The most widely method used at sea is the centrifuging.
- Separation is speeded up by the use of a centrifuge and can be arranged as continuous process.
- Where a centrifuge is arranged to separate two liquids, it is known as a 'purifier'.
- Where a centrifuge is arranged to separate impurities and small amounts of water from oil it is known as a 'clarifier'.
- A centrifuge consists of an electric motor drive to a vertical shaft on the top of which is mounted the bowl assembly. An outer framework surrounds the assembly and carries the various feed and discharge connections. The bowl can be a solid assembly which retains the separated sludge.
- The dirty oil is admitted into the centre of the bowl, passes up through a stack of discs and out through the top.
- The bowl contains a stack of conical discs up to 150 and are separated from are another by a small gap.
- The action of centrifugal force causes the lighter components (the clean oil) to flow inwards and the water and impurities flow outwards.
Figure 7 Bowl centrifuge
Table (1) Difference between Purifying and clarifying processes
- Separation of two liquids such as oil and water.
- Separation of impurities and small amounts of water from oil.
- There is cylindrical interface between the two liquids.
- No interference is formed.
- It is necessary to use a dam ring or gravity discs at the outlet of the centrifuge.
- No gravity disc is necessary.
Figure 8 Purifying bowl arrangement
Figure 9 Clarifying bowl arrangement
2.3.1. Non-continuous operation
- Certain designs of centrifuges are arranged for a short period of operation and are then shut down for cleaning.
- After cleaning and removal of the sludge from the bowl, the machine is returned to service.
- Two different designs are used for this method of operation;
a- The narrow-bowl machine:-
It has to be cleaned after a shorter running period and requires dismantling in order to clean the bowl.
b- The wide-bowl machine:-
It can be cleaned in place. The complication of the stack of conical discs must be cleaned.
2.3.2. Continuous operation
- Modern wide-bowl centrifuge designs enable continuous operation over a considerable period of time.
- This is achieved by an ejection process which is timed to discharge the sludge at regular intervals.
- The sludge deposits build up on the bowl periphery as separation continues, and the ejection process is timed to clear these deposits before they begin, to affect the separation process.
- To start the ejection process the oil feed to the centrifuge is first shut off and the oil remaining in the bowl is removed by admitting flushing water into the hydraulic system in the bottom of the bowl to open a number of spring-loaded valves. This 'operating' water causes the opening of discharge ports in the bowl periphery. The sludge is discharged through these ports by centrifugal force (Figure 10).
- Closing 'operating' water to close the discharge ports, the oil feed reopened, and separation continues.
Figure 10 Sludge discharge
1) The bowl and the disc stack will require periodical cleaning.
2) Care should be taken in stripping down the bowl, using only the special tools provided.
3) The centrifuge is a perfectly balanced piece of equipment, rotating at high speeds: all parts should therefore be handled and treated with care.
3. Stabilizers and stabilizing systems
There are two basic stabilising systems used on ships- the fin and the tank. A stabilising system is fitted to a ship in order to reduce the rolling motion. This is achieved by providing an opposite force to that attempting to roll the ship.
3.1. Fin stabilizer
- One or more pairs of fins are fitted on a ship, one on each side, see Figure 11. The size or area of the fins is governed by ship factors such as breadth, draught, displacement, and so on.
Figure 11 Fin stabilizer
The fins may be retractable, i.e. pivoting or sliding within the ship's form, or fixed.
- They act to apply a lighting moment to the ship as it is inclined by a wave or force on one side. The angle of tilt of the fin and the resulting moment on the ship is determined by a sensing control system. The forward speed of the ship enables the fins to generate the thrust which results in the righting moment.
- Control of fin movement is automatic and is usually driven as a result of a hydraulic power unit incorporating a type of variable displacement pump. When it reaches the desired value, the ship is brought to rest.
- A compensated control system for stabilizing is generally used in large installations, see figure 12
Figure 12 Control system of stabilisers
- The control system which signals the movement of the fins utilises two gyroscopes, one which senses movements from the vertical and the other the rolling velocity. As a result of this control system, fin movement is a function of roll angle, roll velocity, roll acceleration and natural list.
- The effectiveness of the fins as stabilisers depends upon their speed of movement, which must be rapid from one extreme point to the other. The fins are rectangular in shape and streamlined in section.
- Fin stabilisers provide accurate and effective roll stabilisation in return for a complex installation, which in merchant vessels is usually limited to passenger ships. It is to be noted that at low ship speeds the stabilising power falls off, and when stationary no stabilisation is possible.
3.2. Tank Stabiliser
- Tank stabilizers are independent of the forward speed of the vessel. They generate a righting or anti-rolling force as a result of the delayed flow of fluid (water or reserve fuel, etc) in transverse tanks installed at suitable heights and distances from the ship's center line.
- Consider a mass of water in an athwart ships tank. As the ship rolls the water will be moved, but a few degrees after the ship (see figure 13). Thus when the ship is finishing its roll and about to turn, the still moving water will oppose the return roll. The water mass thus acts against the roll at each ship movement. The system is considered passive, since the water flow is activated by gravity.
Figure 13 Air controlled tank stabiliser
- A wing tank system arranged for controlled passive operation is shown in Figure 14. The greater height of tank at the sides permits a larger water build-up and thus a greater moment to resist the roll. The rising fluid level must not however fill the wing tank.
- The wing tanks are connected by cross ducts. The air ducts contain valves which are operated by a roll sensing device. The differential air pressure between tanks is regulated to allow the fluid flow to be controlled and 'phased' for maximum roll stabilisation.
- A tank system must be specifically designed for a particular ship by using data from model tests. The water level in the system is critical and must be adjusted according to the ship's loaded condition.
- Free surface effect resulting from the moving water which effectively reduces the stability of the ship.
- The tank system does, however, stabilise at zero speed and is a much less complex installation than a fin stabiliser.
Figure 14 Brown-NPL Passive tank stabiliser
H. D. Mcgeorge, ''Marine Auxiliary Machinery'', 7th Edition
D. A. Taylor, ''Introduction to Marine Engineering'', 2nd Edition