Lesson Four

Text Cooling water system

The high temperature of the working cycle would quickly heat up the metal forming the cylinders, cylinder covers and pistons if steps were not taken to prevent this happening.

It is necessary to keep the piston rings at temperatures sufficiently low for the lubricating oil surrounding them to survive,

and mechanisms such as fuel valves, exhaust valves and starting air valves must be kept at temperatures at which they will work satisfactorily and remain lubricated.

Also the metal of parts forming the combustion spaces must be kept at a temperature low enough to avoid losing mechanical strength(强度 ).

Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine.

The usual used coolant is fresh water. Lubricating oil is sometimes used for piston cooling since leaks into the crankcase would not cause problem.

As a result of its low specific heat ( 比热 ) however about twice the quantity of oil compared to water would be required.

The heat picked up by these cooling fluids is transferred to seawater in heat exchangers and thus rejected to the sea, the cooling fluids becoming cooled and ready to circulate the engine again.

Seawater cannot be used directly in the cooling spaces of modern engine, as it is both corrosive and liable to( 易于…的 ) leave deposits(沉淀物 ).

Fresh water cooling system

A fresh water cooling system for a slow speed diesel engine can be divided into two separate systems:

one for cooling the cylinder jacket, cylinder heads, exhaust valves and turbo-blowers; The other for piston cooling.

There are two kinds of fresh water system being used on board ship, closed systems and open systems.

In a closed fresh water system, the engine jackets, the heat exchanger and the circulating pumps form a continuous circuit which is not open to the atmosphere.

The cylinder jacket cooling water after leaving the engine passes to a sea water circulated cooler—heat exchanger—and then into the jacket water circulating pumps.

It is then pumped around the cylinder jackets, cylinder heads, exhaust valves and turbo-blowers.

However, provision has to be made for the expansion of the water due to the increase in temperature and due to small amounts of air which may enter the system and become entrained( 以气泡的状态存在 ) and for water make-up ( 补偿 ) due to leakages.

These are met by a small head tank which is open to the atmosphere and placed at a higher level than any other point in the system so that the small changes in volume can be accommodated by changing the level of the free surface of the water in the tank.

The head tank is generally connected into the system at the pump suction, as this minimizes the chance of air being drawn into the system at the pump.

The minimum height of the head tank may be based on maintaining pressure of fresh water in the heat exchanger above that of the seawater to ensure that in the event of a small internal leak developing sea water cannot enter the fresh water system.

Alternatively, it may be based on the need for a minimum pressure to be maintained within the jackets to avoid cavitation( 气穴 ) effects.

The closed systems usually need vents if steam pockets or air locks are not to cause trouble.

Vent pipes of small-bore tube are lead from any local high point in the system to the top of the head tank for the release of air from the cooling water.

A heater in the circuit facilitates warming of the engine prior to starting by circulating hot water.

The temperature of the water entering the engine jackets is regulated by means of a bypass across the jacket water cooler.

The proportions of water passing through the cooler, or bypassing it are controlled by a three-way valve.

The temperature is usually adjusted to maintain a constant outlet water temperature from the engine regardless of the load or speed or the temperature of the seawater.

The piston cooling system may be an open system which employs similar components with the closed system,

except that a drain tank is used instead of a head tank and the vents are then led to a hopper( 观 察 镜 ) or tundish( 观 察 漏 斗 ) at the high point in the machinery space.

The hopper serves as a visual flow indicator.

A separate piston cooling system is used to limit any contamination from piston cooling glands to the piston cooling system only.

Fuel injector valves require precise control of the temperature at the tip of the nozzles, too high a temperature leading to the formation of carbon trumpets, too low a temperature causing corrosion.

They are generally provided with a cooling system which is either a branch of the jacket water system or an entirely separate circuit.

In either case there will be a separate heat exchanger or a cooling tank and some means of adjusting.

The rate of flow to the injectors is independent of that to the engine cylinder jackets.

Sea water cooling system

The various cooling liquids which circulate the engine are themselves cooled by sea water.

The usual arrangement uses individual coolers for lubricating oil, jacket water, and the piston cooling system, each cooler being circulated by sea water.

Some modern ships use what is known as a central cooling system with only one large sea water circulated cooler.

This cools a supply of fresh water which then circulates to the other individual coolers.

With less equipment in contact with sea water the corrosion problems are much reduced in this system.

In a common sea water cooling system, from the sea suction one of a pair of sea water circulating pumps provides sea water which circulates the lubricating oil cooler, the jacket water cooler and the piston water cooler before discharging overboard.

Another branch of the sea water main provides sea water to directly cool the charge air (for a direct-drive two-stroke diesel).

In an emergency situation, for example in the event of damage to fresh water cooler or generator, the sea water pumps can be used to supply sea water to the fresh water cooling system, merely by removing the blind-flanges.

Reading material

The cooling systems

As a “heat engine”, heat energy is a source of power by which the diesel engine can be operated. But excessive heat will cause damage to the engine.

It is therefore necessary to constantly cool the various components of the machinery so that the engine can work properly. The separate cooling systems are designed for this purpose.

These can conveniently be grouped into sections.1. Cylinder cooling, or jacket cooling: normally fresh or distilled water. This may incorporate cooling of the turbine or turbines in a turbocharged engine and exhaust valve cooling.

2. Fuel valve cooling: this would be a separate system using fresh water or a fine mineral oil( 高级矿物油 ).

3. Piston cooling: this may be lubricating oil, distilled of fresh water. If it is oil the system is generally common with the lubrication system. If water, a common storage tank with the jacket cooling system would generally be used.

4. Charge air cooling: this is normally sea water.

Comparison of coolants1. Fresh water: Inexpensive, high specific heat, low viscosity. Contains salts which can deposit, obstruct( 阻碍、阻塞 ) flow and cause corrosion. Requires treatment.

Leakages could contaminate lubricating oil system leading to loss of lubrication, possible overheating of bearings and bearing corrosion. Requires a separate pumping system.

It is important that the water should not be changed very often as this can lead to increased deposits.

Leakages from the system must be kept to an absolute minimum, so a regular check on the replenishing—expansion tank contents level is necessary.

If the engine has to stand inoperative for a long period and there is a danger of frost, (a) drain the coolant out of the system, (b) heat up the engine room, or (c) circulate system with heating on.

It may become necessary to remove scale( 水垢 ) from the cooling spaces, the following method could be used.

Circulate, with a pump, a dilute hydrochloric acid solution ( 稀释盐酸溶液 ). A hose （软管） should be attached to the cooling water outlet pipe to remove gases.

Gas emission can be checked by immersing( 浸入 ) the open end of the hose occasionally into a bucket of water.

Keep compartment well ventilated( 通风 ) as the gases given off can be dangerous.

Acid solution strength ( 酸 溶液的浓度 ) in the system can be tested from time to time by putting some on to a piece of lime.

When the acid solution still has some strength and no more gas is being given off then the system is scale free.

The system should now be drained and flushed( 冲洗 ) out with fresh water, then neutralized( 中和 ) with a soda solution and pressure tested to see that the seals do not leak.

2. Distilled water: More expensive than fresh water, high specific heat, low viscosity.

If produced from evaporated salt water it would be acidic( 酸性 的 ). No scale forming salts. Requires separate pumping system.

Leakages could contaminate the lubricating oil system, causing loss of lubrication and possible over-heating and failure of bearings, etc.

Additives( 添加剂 ) for cooling water

Those generally used are either anti-corrosion oils or inorganic inhibitors( 无机缓蚀剂 ).If pistons are water cooled an anti-corrosion oil is recommended as it lubricates parts which have sliding contact.

The oil forms an emulsion( 乳化液 ) and part of the oil builds up a thin unbroken film on metal surfaces, this prevents corrosion but is not thick enough to impair heat transfer.

Inorganic inhibitors form protective layers on metal surfaces guarding them against corrosion.

It is important that the additives used are not harmful if they find their way into drinking water-this is possible if the jacket cooling water is used as a heating medium in a fresh water generator.

Emulsion oils and sodium nitrite( 亚硝酸钠 ) are both approved additives, but the latter cannot be used if any pipes are galvanized( 镀锌 ) or if any soldered( 堆焊 ) joints exist.

Chromates( 铬酸盐 ) cannot be used if the cooling water is used in a fresh water generator and it is a chemical that must be handled with care.

3. Lubricating oil: Expensive. Generally no separate pumping system required since the same oil is normally used for lubrication and cooling.

Leakages from cooling system to lubrication system are relatively unimportant providing they are not too large, otherwise one piston may be partly deprived( 丧失 ) of coolant with subsequent overheating.

Due to reciprocating( 往复的 ) action of piston some relative motion between parts in contact in the coolant supply and return system must occur, oil will lubricate these parts more effectively than water.

No chemical treatment required. Lower specific heat than water, hence a greater quantity oil must be circulated per unit time to give the same cooling effect.

If lubricating oil encounters high temperature it can burn, leaving as it does so carbon deposit( 积碳 ).

This deposit on the underside of a piston crown could lead to impairment of heat transfer, overheating and failure of the metal.

Generally the only effective method of dealing with the carbon deposit is to dismantle the piston and physically remove it.

Since oil can burn in this way a lower mean outlet and inlet temperature for the oil has to be maintained, in order to achieve this more oil must be circulated per unit time.

Some engines may use completely separate systems for oil cooling of pistons and bearing lubrication, the advantages gained by this method are:

1. Different oils can be used for lubrication and cooling, a very low viscosity mineral oil would be better suited to cooling than lubrication.

2. Additives can be used in the lubricating oil that would be beneficial to lubrication, e.g. oiliness agents( 油性剂 ), e.p. agents ( 抗乳化剂 ) and V.I. improvers( 粘度指数增加剂 ), etc.

3. Improved control over piston temperatures.

4. If oil loss occurs, then with separate systems the problem of detection is simplified and in the case of total oil loss in either system, the quantity to be replaced would not be as great as for a common system.

5. Contamination of the oil in either system may take place. In the event the problem of cleaning or renewal of the oil is not so great.

6. Oxidation( 氧化 ) of lubrication oil in contact with hot piston surfaces leads to rapid reduction in lubrication properties.

Disadvantages of having two separate systems are: Greater initial cost due to separate storage, additional pipework ( 管网 ) and pumps.

A sealing problem to prevent mixing of the two different oils is created and due to the increased complexity more maintenance would have to be carried out.

B. cooling water system with central cooler

The engine is closed by a closed water circuit, divided into a high temperature circuit (HT) and a low temperature circuit (LT). The fresh water is cooled by a separate central cooler.

The cooling water in the HT circuit cools the cylinder liners, cylinder heads and turbochargers. From the pump water is supplied to the engine manifold which is cast in the engine block.

From the manifold the water is distributed to all cylinder liner sections and from each liner upwards through the cooling water channels in the liner collars, hence further into the cylinder heads.

In the cylinder head the water is forced by an intermediate deck to flow along the frame plate, around the valves to the exhaust valve seats and up along the fuel injector sleeve( 套管） .

From the cylinder head the water is discharged via a connection piece into the discharge manifold. Parallel( 并联） to the flow to the cylinders part of the engine water flows through the turbocharger.

The return water from the cylinder heads and turbocharger is supplied to the HT air cooler, and then back to the inter-cooler.

In the first stage of this cooler. Most of the heat from the charge is extracted( 引 出 的 , 排出的 ).

The LT circuit water cools the charged air in the second stage and the HT cooling water in the inter-cooler( 中间冷却器 ). The necessary cooling for the LT water is gained from a central cooler.

By controlling the LT water temperature to the inter-cooler the correct charge air temperature can be obtained.

The HT cooling water system has to work on an over pressure of 3 bar and the LT cooling water system on 0.7-1.5 bar over pressure.

For preheating purposes, a heater circuit with a pump and heater is connected in the HT circuit before the engine. The non-return valves in the circuit force the water to flow in the correct direction.

Before starting, the HT circuit is heated till about 60 by a ℃separate heater. The pre-heating of the engine is , prior of starting up and loading on HFO(heavy fuel oil), of utmost important.