Marine Cooling Systems

Marine Cooling Systems

States of Heat transfer.

Conduction - This is the transfer of heat though direct contact

Convection - This is the transfer of heat from a solid surface to a liquid or gas in motion over it.

Radiation - This is the movement of heat though and empty space.

Indirect Cooling.

Indirect cooling refers to a cooling system where the cooling source does not act directly upon the engine. Air-to-Liquid/Liquid-to-Liquid. One such system uses a raw seawater heat exchanger. The coolant flows through the engine through the core heat exchanger, then radiates out into the seawater flowing through the exchanger and over the core.

Note the cooling fins

Direct Cooling.

Direct cooling is when the cooling source acts directly on the engine. Such as air cooling which moves across blades moulded in the engine block absorbing the radiating heat from the engine. Another system involves raw seawater which is pumped through the engine and out the back of the boat.

What problems can arise in these cooling systems?
Direct cooling- When using a raw seawater system there is always the problem of corrosion. The salt water flowing though the passages of the engine starts to corrode the metal, working its way out. Engines using this sytem usually only last a couple of years as the coolant passages become severly corroded and start to leak coolant.
Indirect cooling- The heat exchanger system spoken about earlier faces the same problems. As raw water flows into the exchanger it is exposed to the corrosive salt within the water. Because of this the exchanger will need to be serviced or possibly repaired on a scheduled basis. Zinc anodes are placed within the exchanger to draw corrosion, these are replaced when serviced. There is also the possiblilty of over heating if flow of the primary coolant stops or is impaired.

What is the purpose of the thermostate?

The purpose of the thermostate is to time circulation of the coolant from the enngine to the radiator. When the thermostate is closed it forces the water to circulate within the engine, when the engine reaches operating temperature the thermostate opens allowing water to flow to the radiator and fresh coolant to circulate.

What is a pressure cap?

This is the cap were coolant is filled. As the engine heats up and the temperature of the coolant increases it expands and raises the pressure within the system. When pressure reaches the threshold the coolant pushes past the cap an into the overflow tank. As the coolant cools down it draw coolant in from the overflow tank in the pressure cap closes.

Why and what is the purpose of a core plug?

A Core or Frost plug is a metal plug in the side of the engine. This hole allows the sand to be removed after casting of the block. When plugged coolant will flow past these holes, if the coolant should ever freeze and expand the pressure will force the plug out and release the pressure without deformation of the block.

How does a heat exchanger work & What is it made of?

A heat exchanger was created as a medium, between the salt water coolant and the engine, to combat excessive engine corosion. An anitfreeze solution is circulated though the engine aborbing excess heat. When the thermostate opens water passes into the central core of the heat exchanger. Raw sea water is pumped through the heat exchanger and as it passes over the core it asborbs the heat from the antifreeze running inside. Anodes are place within the exchanger to help corosion issues.
The heat exchanger is made with various materials such as steel, copper, bronze.

How does a Centrifugal circulating water pump work?
This is an impellor that is driven from its center by an acuating shaft. This is the type of water pump uses on outboard engines. This impellor would usually be found on the gearbox drive shaft.

How does a Raw water pump work?
A raw water pump works the same as a centrifugal circulating pump. It has a central driven impellor, this pump often works via a belt or gear.
Why is it important to lubricate the impellor when fitting from new?
This is important because when the engine starts the impellor will not be covered in water and if not lubricated the implellor will rip. This is lubricated only to resist damage during the first few second of operation.

When refitting impellor what should be done before fitting the housing & Why?
It is imortant to check the woodruff key is in place. This is a small locator lug that is recesed into the shaft that turns the impellor. If the key is not fitted the impellor will not turn and the engine will overheat causing damage.
What is the purpose of an Oil cooler and how does it work?

An oil cooler works similar to the main cooling system, it also has a Liquid-to-Liquid heat exchanger. Oil is pumped through the core of the heat exchanger passing its heat through the metal and out through the raw water pumping past. If using raw water cooling, often different coolers(oil,water) would be laid in succession within one circut.





What is the process of an Intercooler/Aftercooler?
The pupose of this component is to reduce the temperature of the air, that has been compressed by the turbo or supercharger, before it enters the engine. This can be done by directing air across a radiator like component or using a raw water heat exchanger.
Why do we have Inhibitors & Antifreeze?

These are chemical compounds added to our coolant to help increase its boiling temp and lower its freezing temp. The main purpose of Antifreeze is to reduce the risk of engine deformation due to water expansion upon freezing. Antifreeze works to both lower freezing point and raise boiling temperature, it also sticks to the metal helping to prevent corrosion from cavitation.
How do you test Antifreeze?

We have antifreeze tester such as the one in this picture. This will give us a reading of freezing temperature. Also a mixture with high antifreeze content will have a sweet taste to it.
What is Cavitation corrosion?
Cavitation is surface corrosion which occurs when oxygen separates from water creating cavities, this happens due to the pressure changes in the liquid as it passes through the engine(e.g when rapidly changing direction though passage ways) or past the propellor.

What is Galvanic corrosion?
This is an electrochemical process in which corrosion attacks a metal with high negative potential leaving a metal with lower negative potential corrosion free. This is the pricipal of a sacrifial anode.

Common Terms.
Stray corrosion- This refers to corrosion that may occur due to stray voltage within a system. Often due to poor insulation of electrical components.
Stress corrosion- This is the suden and unexpected faliure of a metal subjected to tensile stress in a corrosive enviroment.
Corrosion Fatigue-  This refers to fatigue from the corrsive ennviroment. Metal will often show corrision fatigue due to the action of cyclic loading and corrsion.
Crevice corrosion- Refers to corrosion is small spaces not often disturbed. Such as between parts or within small cracks.
Electrolysis- This is a method of using direct electrical current to cause a non-spontaneous chemical reaction.

How do you prevent corrosion?
To combat corrosion we use a process called Marinisation. There are many different techniques but this could include coating surfaces that may corrode or attaching sacrificial metal in strategic places to draw corrosion. Washing down you boat or flushing with fresh water is a good way to prevent corrosion.

What are sacrifical anode made of?

Sacrificial anode and made of metals with high negative potential, such as Zinc. Sacrifical Anode are designed to draw corrosion sacrificing itself to increase life of other components.

Where would you place these in the marine industry?
Sacrificial Anode are found in many places such and the heat exchanger, or water jackets. Usually found were metal is in contact with raw water.

What is Marine bonding?
Marine bonding refers to the practice of placing extra earth cables around the engine. To places like anodes, and engine components. This helps reduce stray current throughout the vessel, in turn combating corrosion.
Reference
http://www.wikipedia.com/
http://www.google.com/
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