➤ Propulsion Services And 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 of 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 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 seawater 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 aluminiumbrass. 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 brittle and causes contraction of the rubber seal which is then easily broken away. Other methods of seal removal result in plate damage.
Nitrile rubber is suitable for temperatures of up to about 110°C. At higher temperatures, the rubber hardens and loses its elasticity. The joints are squeezed when the plates are assembled and clamping bolts are tightened after cleaning.
Overtightening can cause damage to the plates, as can an incorrect tightening procedure. A torque spanner can be used as directed when clamping bolts are tightened; cooler stack dimensions can also be checked.
Titanium
Charge air coolers
The charge air coolers fitted to reduce the temperature of air after the turbocharger and before entry to the diesel engine cylinder, are provided with fins on the heat transfer surfaces to compensate for the relatively poor heat transfer properties of air. Solid drawn tubes with a semi-flattened cross-section, have been favoured.
Cooling of the air results in the precipitation of moisture which is removed by a progressive increase in the temperature difference between the two fluids, and a change of pressure. Fouling on the seawater side is the most common cause of deterioration in performance. The method of cleaning the seawater side surfaces depends on the type of deposit and heat exchanger.
Soft deposits may be removed by brushing. Chemical cleaning by immersion or in situ is recommended for stubborn deposits. With shell and tube heat exchangers the removal of the end covers or, in the case of the smaller heat exchangers, the headers themselves, will provide access to the tubes. Obstructions, dirt and scale can then be removed, using the tools provided by the heat exchanger manufacturer.
Flushing through with fresh water is recommended before a heat exchanger Is returned to service. In oil coolers or heaters, progressive fouling may take place on the outside of the tubes. Manufacturers may recommend a chemical flushing to remove this in situ, without dismantling the heat exchanger.
Plate heat exchangers are cleaned by unclamping the stack of plates and exposing the surfaces. Plate surfaces are carefully washed using a brush or dealt with as recommended by the manufacturer to avoid damage.
If the plate seals require replacement they may be removed with the method described in the section on plate coolers. Pricing seals from their bonding, e.g. with sharp tools, causes plate damage.
Corrosion by seawater may occasionally cause perforation of heat transfer surfaces with resultant leakage of one fluid into the other. Normally the sea water is maintained at a lower pressure than the jacket water and other liquids that it cools, to reduce the risk of sea water entry to engine spaces.
Leakage is not always detected initially if header or drain tanks are automatically topped up or manual top-up is not reported. Substantial leaks become evident through the rapid loss of lubricating oil or jacket water and the operation of low-level alarms.
The location of a leak in a shell and tube cooler is a simple procedure. The heat exchanger is first isolated from its systems and after draining the sea water and removing the end covers or headers to expose the tube plates and tube ends, an inspection is made for evidence of liquid flow or seepage from around tube ends or from perforations in the tubes.
The location of small leaks is aided if the surfaces are clean and dry. The fixing arrangement for the tube stack should be checked before removing covers or headers to ensure that the liquid inside will not dislodge the stack. This precaution also underlines the need for the isolation of a cooler from the systems.
To aid the detection of leaks in a large cooler such as a main condenser, in which it is difficult to get the tubes dry enough to witness any seepage, it is usual to add a special fluorescent dye to the shell side of the cooler.
When an ultraviolet light is shone on to the tubes and tube plates leaks are made visible because the dye glows. Plate heat exchanger leaks can be found by visual inspection of the plate surfaces or they are cleaned and sprayed with a fluorescent dye penetrant on one side.
The other side is then viewed with the aid of an ultraviolet light to show any defects. Leaks in charge air coolers allow seawater to pass through to the engine cylinder. This can be a problem in four-stroke engines because there is a tendency for salt scale to form on air inlet valve spindles and this makes them stick. The charge air manifold drain is regularly checked for salt water.
The location of the leak may be achieved by having a very low air pressure on the air side and inspecting the flooded seawater side for air bubbles. Soapy water could be used as an alternative to having the seawater side flooded.
If a ship is to be out of service for a long period, it is advisable to drain the seawater side of heat exchangers then clean and flush through with fresh water, after which the heat exchanger should be left drained, if possible until the ship reenters service.
Venting and draining
It is important that any heat exchanger through which sea water flows should run full. In vertically mounted single-pass heat exchangers of the shell and tube or plate types, venting will be automatic if the seawater flow is upwards.
This is also the case with heat exchangers mounted in the horizontal attitude, with single or multipass tube arrangements, provided that the seawater inlet branch faces downwards and the outlet branch upwards.
With these arrangements, the water will drain virtually completely out of the heat exchanger when the remainder of the system is drained.
With other arrangements, a vent cock fitted at the highest point in the heat exchanger should be opened when first introducing seawater into the heat exchanger and thereafter periodically to ensure that any air is purged and that the seawater side is full. A drain plug should be provided at the lowest point.
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