Cooling Systems

Jacket Water System

HT/LT systems

Sea Water Cooling system

Jacket cooling system

Jacket Water System

Jacket Water Heater. Unheated engine can lead to poor combustion, poor lubrication and thermal shock, hence a water heater. A modern variation on this is the "blend" water from the stand-by auxiliary alternator engines into the main engine circuit increasing plant efficiency. The water enters and leaves the engine via a series of cylinder isolating valves. In this way each cylinder may be individually drained to prevent excessive water and chemical loss. In addition dual level drains may be fitted which allow either full draining or draining of the head only. A portion of the water is diverted for cooling of the turbocharger. De aerator. Air or gas entering the system can lead to unstable and even total loss of cooling water pressure as the gas expands in the suction eye of the circulating pumps. In the event of gas leakage via the head or cracked liner rapid loss of jacket water pressure can occur. The deaerator is a method to try to slow this process sufficiently. This system also allows the vessel to operate with minor gas leakage. .

Jacket Water System• Jacket Water Cooler The hot water leaving the engine passes to a

temperature control valve were a portion is diverted to a cooler. Temperature is controlled using both a feedback signal (temperature measured after the cooler) and a feed forward signal (temperature measured at outlet from the engine). In this way the system reacts more quickly to engine load variations.

• Evaporator Increases plant efficiency . Modern systems sometimes rely on the evaporator to supplement a reduced size main cooler.

• Expansion or header tank Maintains a constant head on the circulation pumps reducing cavitations at elevated temperatures. Allows the volume of water in the system to vary without need for dumping. Acts as a reserve in the event of leakage

Scaling of Jacket Water SystemScale and deposit formationIn areas of deposit formation, dissolved solids, specifically Calcium and magnesium hardness constituents can precipitate from cooling water as the temperature increases. Deposits accumulate on the heat transfer surfaces as sulphates and carbonates, the magnitude of which is dependent on the water hardness, the dissolved solid content, local temperatures and local flow characteristics. Scales can reduce heat transfer rates and lead to loss of mechanical strength of component parts, this can be exacerbated by the presence of oils and metal oxides

Temperature solubility curves for CaSO4

Scale and deposit formationThe degree and type of scaling in a cooling water circuit are determined by;

System temperatures, Amount of leakage/makeup, quality of make upquality of make up, quality of treatment

Calcium CarbonateAppears as a pale cream, yellow deposit formed by the thermal decomposition of calcium bi-carbonate ; Ca(HCO3)2 + Heat becomes CaCO3 + H2O + CO2 Magnesium SilicateA rough textured off-white deposit found where sufficient amounts of Magnesium are present in conjunction with adequate amounts of silicate ions. Silicate deposit is a particular problem for systems which utilize silicate additives for corrosion protection. This is typical of system with aluminum metal in the cooling system. The silicate forms a protective barrier on the metal surface. A high pH (9.5 - 10.5) is required to keep the silicate in solution. In the event of sea water contamination or some other mechanism that reduces the pH the silicate is rapidly precipitated and gross fouling can occur. CopperThe presence of copper within a cooling system is very serious and it can lead to aggressive corrosion through galvanic action. Specific corrosion inhibitors are contained with cooling water system corrosion inhibitors.

Effects of scale deposition

• The effects of scale deposition can be both direct or indirect, typically but not specifically

• Insulates cooling surfaces leading to; – increased material temperatures as the temperature

gradient must increase to ensure maintain heat flow. – Loss of efficiency as exhaust gas temperatures from

cylinders increases – Increased wear due to lubrication problems on overheated

surfaces • Indirectly;

– Lead to caustic attack be increasing the OH- ion concentration

Corrosion inhibitors used in Jacket Water System

In order to maintain mechanical strength the components surrounding the combustion zone must be cooled. The most convenient cooling medium is water, the use of which could lead to possible problems of corrosion and scaling if not properly treated. Within the jacket water system a number of corrosion cells are available but the two most common and most damaging are due to dissimilar metals and differential aeration. In both types of cell there exists an anode and a cathode, the metals which form part of the jacket system, and an electrolyte which is the cooling water. The rate at which corrosion takes place is dependent upon the relative areas of the cathode and the anode and the strength of the electrolyte. It is the anode that wastes away. Corrosion due to temperature differences is avoidable only by the use of suitable treatments. Dissimilar metals-a galvanic cell is set up where two different metals and a suitable liquid are connected together in some way. All metals may be placed in an electro-chemical series with the more noble at the top . Those metals at the top are cathodic to those lower down. The relative positions between two metals in the table determined the direction and strength of electrical current that flows between them and hence, the rate at which the less noble will corrode

Galvanic Action

Corrosion within cooling systems can occur if the coolant, i.e. water, has not been properly treated. The corrosion can take the form of acid attack with resultant loss of metal from a large area of the exposed surface, or by Oxygen attack characterized by pitting. A primary motive force for this corrosion is Galvanic action

The metals closer to the anodic end of the list corrode with preference to the metals towards the cathode end. A galvanic cell can occur within an apparently Homogeneous material due to several processes on of which is differential aeration where one area is exposed to more oxygen than another. The area with less oxygen becomes anodic and will corrode.

The Galvanic Series.   Cathode

1 Gold and Platinum

2 Titanium

3 Silver

4 Silver solder

5 Chromium-Nickel-Iron (Passive)

6 Chromium-Iron (Passive)

7 Stainless Steel (Passive)

8 Copper

9 Monel

10 70/30 Cupro-Nickel

11 67-33 Nickel-Copper

12 Hydrogen

13 lead

14 Tin

15 2-1 Tin lead Solder

16 Bronzes

17 Brasses

18 Nickel

19 Stainless-Steel 18-8 (Active)

20 Stainless Steel 18-8-3 (Active)

21 Chromium Iron (Active)

22 Chromium-Nickel-Iron (Active)

23 Cadmium

24 Iron

25 Steel

26 Cast Iron

27 Chromium

28 Zinc

29 Aluminum

30 Aluminum Alloys

31 Magnesium  AnodeMetals closer to the anodic end of the list corrode with preference to the metals towards the cathode end

Galvanic ActionGalvanic action within metal

Galvanic action due to temperature gradient

This situation can exist in cooling water systems with complex layout of heat exchangers and passage ways within the diesel engine. Systems containing readily corrodible metals such as zinc, tin and lead alloys can complicate and intensify problems by causing deposit formations

Differential Aeration

-Where only a single metal exists within a system corrosion can still take place if the oxygen content of the electrolyte is not homogenous. Such a situation can occur readily in a jacket water system as regions of stagnant flow soon have the oxygen level reduced by the oxidation of local metal. The metal adjacent to water with reduced levels of oxygen become anodic to metals with higher oxygen content electrolyte in contact with it.. Generally, the anodic metal is small in comparison the cathode i.e. the area of stagnant flow is small compared to the area of normal flow of electrolyte, and high rates of corrosion can exist. One clear case of this is the generation of deep pits below rust scabs.

Water treatment

To remove the risk of corrosion it is necessary to isolate the metal surface form the electrolyte. One method would be by painting, but this is impractical for engine cooling water passages. A better solution would be a system which not only searchsearch out bare metal, coating it with a protective barrier, but also repair any damage to the barrier.

For corrosion to occur four conditions must be met;

There must be an Anode There must be a cathode An electrolyte must be present An electron pathway should exits

Corrosion Inhibitors• Corrosion inhibitors are classified on how they affect the corrosion

cell and are placed into three categories; – Anodic Inhibitors – Cathodic Inhibitors – Combination inhibitors/organic inhibitors

• Common Corrosion Inhibitors

Principally Anodic Inhibitors

Chromate Nitrite Orthophosphate Bicarbonate Silicate Molybdate

Principally Cathodic Inhibitors

Carbonate Polyphosphate Phosphonates Zinc

Both Anodic and Cathodic Inhibitors

Soluble Oils Mercaptobenzothiazole (MBT) Benzotriazole (BZT) Tolytriazole (TTZ)

Anodic InhibitorsNitrite (NO2- )- These are the most commonly used form of treatment and operate by oxidizing mild steel surfaces with a thin, tenacious layer of corrosion product (magnetite Fe3O4). Relatively high volumes of treatment chemical are required so this method is only viable on closed systems Sodium Nitrite- (sometimes with Borate added)-effective with low dosage, concentration non-critical. It is non toxic, compatible with anti freezes and closed system cooling materials. It does not polymerize or breakdown. However protection for non-ferrous materials is low. An organic inhibitor is thus required. Although will not cause skin disease it will harm eyes and skin. Approved for use with domestic fresh water systems. Sodium Nitrite is a passivator which chemically produce an insulating layer on the metal surface. Whenever corrosion takes place the corrosion products including bubbles of gas, are released from the metal surfaces. Passivating chemicals act on the corrosion products preventing release from the metal surface and thus stifling further corrosion. If the insulating layer becomes damaged, corrosion begins again and the passivator acts on the new products to repair the layer. Sodium Chromate which was an excellent inhibitor. Not allowed. Due to its toxicity. Sometimes used as a biocide in such places as brine in large Reefer plants.

Anodic Inhibitors• Silicates- react with dissolved metal ions at the anode. The

resultant ion/silicate complex forms a gel that deposits on anodic sites. This gel forms a thin, adherent layer that is relatively unaffected by pH in comparison to other inhibitors. The inhibiting properties increase with temperature and pH, normal pH levels are 9.5 to 10.5.

• Care should be taken with the use of silicates, which are often used for the protection of systems containing alumiinium. In the event of boiling increased concentrations and lead to aggressive corrosion due to the high pH.

• Orthophosphate Forms an insoluble complex with dissolved ferric ions that deposit at the anodic site. It is more adherent and less pH sensitive than other anodic inhibitors. The film forms in pH of 6.5 to 7.0. Dosage is typically 10ppm in neutral water

Cathodic Inhibitors

Cathodic InhibitorsPolyphosphate- Forms complexes with Calcium, Zinc and other divalent ions, this creates positively charged colloidal particles. These will migrate to the cathodic site and precipitate to form a corrosion inhibiting film. The presence of calcium is required at a typical minimum concentration of 50ppm. Extreme variations in pH can upset the film and a reversion to orthophosphate will occur with time and temperature. Positively charged zinc ions migrate to the cathodic site and react with the free hydroxyl ions to form a zinc hydroxide stable film at pH 7.4 to 8.2. If the water is too acidic the film will dissolve and not reform. If it is too alkaline the zinc hydroxide will precipitate in bulk and not at the cathodic site. Phosphonates Initially introduced as scale inhibitors to replace polyphosphates, they exhibit absorption at the metal surface especially in alkaline hard water. Generally used with other inhibitor types

Both Anodic and Cathodic InhibitorsBenzotriazole and Triazole Specific corrosion inhibitor for copper. They break the electrochemical circuit by absorbing into the copper surface. They are generally added to standard treatments.

Soluble and dispersible oils. Petroleum industry recognized that emulsifying cutting oils (erroneously called soluble oils) were able to reduce corrosion on metals by coating the surface. There were disadvantages though, if the coating became too thick then it could retard the heat transfer rate. Adherent deposits form as organic constituents polymerize or form break down products which can accumulate and disrupt flow. MAN-B&W recommend it not to be used. It is effective in low dosages, safe to handle and safe with domestic water production. Effectiveness is reduced by contamination with carbon, rust, scale etc. Difficult to check concentration, overdosing can lead to overheating of parts

Oils are classed as a barrier layer type inhibitor. The surfaces being coated in a thin layer of oil.

Modern treatmentNitrite-Borate treatment is most effective with a high quality water base. This treatment has no scale prevention properties and its effectiveness is reduced by high quantities of dissolved solids. A modern treatment will be a Nitrite -Borate base, with a complex blend of organic and inorganic scale and corrosion inhibitors plus surfactants, alkali adjusters, dispersants and foam suppressers. A high quality water supply is still strongly recommended.

Electrolytic protection for the whole system by the use of sacrificial anodes is impractical. Parameters such as water temperature, relative surface area of anode and cathode, activity of metals in system and relative positions in galvanic series come into play. Anodic protection has become out of favor for cooling water systems as it can lead to local attack, causes deposits leading to flow disturbance and it has no scale protection

Preparation for cooling water treatment-All anodes should be removed and the system inspected. No galvanized piping is to be used (old piping can be assumed to have had the Galvanizing removed). High quality water should be used and chemicals measured and added as required. A history log should be kept Microbiological FoulingUnder certain conditions bacteria found in cooling water systems can adapt to feed on the nitrite treatment. This can lead to rapid growth, formation of bio-films, fouling and blockages.Typical evidence is a loss of nitrite reserve but a stable or rising conductivity level as the nitrate formed still contributes to the conductivity, Problems of this sort are rare due to the elevated temperatures and pH levels. Should it occur treatment with a suitable biocide is required.

Piston coolingPiston crowns attain a running temperature of about 450oC and in this zone there is a need for high strength and minimum distortion in order to maintain resistance to gas loads and maintain the attitude to the rings in relation to the liner. The heat flow path from the crown must be uniform otherwise thermal distortion will cause a non-circular piston resulting in reduced running clearance or even possible contact with the liner wall. Efficient cooling is required to ensure the piston retains sufficient strength to prevent distortion. For medium and high speed engines the weight of the material becomes important to reduce the stresses on the rotating parts. The high thermal conductivity of aluminum alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into the crown, although this may be omitted on smaller engines. Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface.

Piston coolingWater Cooled Oil Cooled High specific heat capacity therefore

removes more heat per unit volume Low specific heat capacity

Requires chemical conditioning treatment to prevent scaling

Does not require chemical treatment but requires increased separate and purification plant

Larger capacity cooling water pump or separate piston cooling pump and coolers although less so than with oil

Larger capacity Lube oil pump, sump quantity and coolers

Special piping required to get coolant to and from piston without leak

No special means required and leakage not a problem with less risk of hammering and bubble impingement.

Coolant drains tank required to collect water if engine has to be drained. Increased capacity sump tank required

Pistons often of more complicated design Thermal stresses in piston generally less in

oil cooled pistons

Cooling pumps may be stopped more quickly after engine stopped

Large volumes of oil required to keep oxidation down and extended cooling period required after engine stopped to prevent coking of oil

Pistons may be cooled by oil or water. Oil has the advantage that it may be supplied simply from the lubrication system up the piston rod. Its disadvantage are that maximum temperatures is relatively low in order to avoid oxidized deposits which build up on the surfaces. In addition the heat capacity of oil is much lower than that of water thus a greater flow is required and so pumps and pipe work must be larger. Also, if the bearing supply oil is used as is mainly the case a greater capacity sump is required with more oil in use. Water does not have these problems, but leakage into the crankcase can cause problems with the oil (such as Microbial-Degradation). The concave or dished piston profile is used for most pistons because it is stronger than the flat top for the same section thickness

Sulzer water-cooled piston (rnd)Sulzer water-cooled piston (rnd)Increasing section thickness would result in higher thermal stress. Sulzer piston require a flat top because of the scavenging and exhaust flow arrangement (loop scavenging of RND etc). in order to avoid thicker sections internal support ribs are used. However these ribs cause problems in that coolant flow is restricted. With highly rated engines overheating occurred in stagnant flow areas between the ribs and so a different form of cooling was required. The cocktail shaker effect has air as well as water in the cooling cavity as the piston reciprocates water washes over the entire inner surface of the piston just as in a cocktail shaker. Unfortunately air bubbles become trapped in the water and flow to outlet reducing the air content and removing the cocktail shaker effect. To avoid this problem air must be supplied to the piston some engine builders use air pumps feeding air to the inlet flow. The sulzer engine allows air to be drawn into the flow at a specially designed telescopic transfer system. The telescopic arrangement is designed to prevent leakage and allows air to be drawn into the coolant flow to maintain the cocktail shaker effect.. Small holes allow connection from the main seal to the space between the nozzles. Water flowing through the lower nozzle is subject to pressure reduction and a velocity increase. The space between the nozzles is therefore at a lower pressure than other parts of the system. Any water which leaks past the main seal is drawn through the radial holes into the low pressure region and hence back into the coolant flow. The pumping action of the telescopic draws air past the lower seal and this is also drawn through the radial holes into the coolant flow..

B&W LMC oil cooled piston

The piston has a concave top. This is near self supporting and reduces the need for internal ribbing. It prevents the cyclic distortion of the top when under firing load. This distortion can lead to fatigue and cracking

Sulzer water-cooled piston (rnd)The sulzer water cooled piston differs from that of the Oil cooled variety by the method it uses for distributing the cooling medium. In this case the piston is not continually flooded but instead contains a level governed by the outlet weir. Cooling of the crown occurs during change of direction at the top of the stroke by so called 'Cocktail shaker' action.

Thermal distortion of Piston

Shell and Tube Heat exchangers• Tubes

aluminum –brass (Cu-76%,Zn-22%,Al-2%)• Naval brass end plate (alpha-beta brass containing tin) • End covers

cast iron (unprotected suffers graphitization)• Shell

CI or MS

water velocity max.2.5 m/s

PHE• Corrugation promotes turbulence and good heat

transfer due to breaking of boundary layer• Corrugation makes the plates stronger• The corrugation increases plate area

• Titanium and SS plates are the most common based on duties.

• Nitrile rubber is found suitable up to 110’C, re- tightening as advised by makers are very important as this can damage rubber, plate etc.

Heat exchanger maintenance

• Maintain cleanliness at heat transfer surfaces w/o any flow restriction

• Corrosion of heat transfer surfaces due to sea water is not uncommon. Sea water pressure is normally maintained lower than the liquid being cooled to avoid major problems

• Un protected steel will, in the presence of sea water shall waste due to galvanic corrosion.Electroplating with nobler metal, cathodic protection by sacrificial anodes, impressed current system are the remedies

Cooling water protection• Effective protection against corrosion by

adding corrosion inhibitor• Maintaining correct water quality• Effective air vents• Check and monitor of the water during service• Using the correct procedure of cleaning and

maintenance

Corrosion

Cooling water quality• Distilled water is common

This prevents deposits on hot surfaces which can lead to very high operating temperatures

Water losses and overhauling

Metal Specific Heat

Thermal Conductivity Density Electrical

Conductivity

      cp           cal/g° C

k        watt/cm K   g/cm3 1E6/Ωm

Brass 0.09 1.09 8.5  

Iron 0.11 0.803 7.87 11.2

Nickel 0.106 0.905 8.9 14.6

Copper 0.093 3.98 8.95 60.7

Aluminum 0.217 2.37 2.7 37.7

Lead 0.0305 0.352 11.2  

alpha-beta brass - a brass that has more zinc and is stronger than alpha brass; used in making castings and hot-worked products

Sp heat capacity of some metals

Oil s

ystem

System Volume in relation to centrifuging process

System Volume in relation to centrifuging process

1. High operating temperature

2. Air Admixturethe total oil quantity is such that it circulates 15-18 times per hr.

3. Catalytic action due to copper, iron, varnish, lacquer rust etc being present in oil