Lesson Four Text Cooling water systemThe high temperature of the wor
king cycle would quickly heat up the metal forming the cylinders,
cylinder covers and pistons if steps were not taken to prevent t
his happening. It is necessary to keep the piston rings at temper
atures sufficiently low for the lubricating oil surrounding them
to survive, and mechanisms such as fuel valves, exhaust valves an
d 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.Cooling of engine
s is achieved by circulating a cooling liquid around internal pas
sages within the engine. The usual used coolant is fresh water (淡
水). Lubricating oil is sometimes used for piston cooling since le
aks (泄漏) into the crankcase would not cause problem.As a result o
f its low specific heat (比热) however about twice the quantity of
oil compared to water would be required.The heat picked up by the
se cooling fluids is transferred to seawater in heat exchangers(热
交换器) and thus rejected to(排向) the sea, the cooling fluids becomin
g cooled and ready to circulate the engine again. Seawater cannot
be used directly in the cooling spaces of modern engine, as it i
s both corrosive and liable to(易于…的) leave deposits.Fresh water c
ooling systemA 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-bl
owers; The other for piston cooling. There are two kinds of fresh
water system being used on board ship, closed systems(闭式系统) and
open systems(开式系统).In a closed fresh water system, the engine jac
kets, the heat exchanger and the circulating pumps form a continu
ous circuit which is not open to the atmosphere. The cylinder jac
ket 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, prov
ision has to be made for the expansion of the water due to the in
crease in temperatureand due to small amounts of air which may en
ter the system and become entrained(以气泡的状态存在) and for water make-
up due to leakages.These are met by a small head tank which is op
en to the atmosphere and placed at a higher level than any other
point in the systemso that the small changes in volume can be acc
ommodated(使适应) by changing the level of the free surface of the w
ater in the tank.The head tank is generally connected into the sy
stem at the pump suction, as this minimizes the chance of air bei
ng drawn into the system at the pump. The minimum height of the h
ead tank may be based on maintaining pressure of fresh water in t
he heat exchanger above that of the seawater to ensure that in th
e event of a small internal leak developing sea water cannot ente
r the fresh water system. Alternatively, it may be based on the n
eed for a minimum pressure to be maintained within the jackets to
avoid cavitation(气穴) effects.The closed systems usually need ven
ts if steam pockets or air locks are not to cause trouble. Vent p
ipes 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 t
he cooling water. A heater in the circuit facilitates warming of
the engine prior to starting by circulating hot water The tempera
ture of the water entering the engine jackets is regulated by mea
ns of a bypass(旁通) across the jacket water cooler. The proportion
s of water passing through the cooler, or bypassing it are contro
lled by a three-way valve. The temperature is usually adjusted to
maintain a constant outlet water temperature from the engine reg
ardless of( 无论，不管) the load or speed or the temperature of the se
awater. The piston cooling system may be an open system which emp
loys similar components with the closed system, except that a dra
in tank is used instead of a head tank and the vents are then led
to a hopper(带支架的观察镜) or tundish(漏 斗) at the high point in the ma
chinery space. The hopper serves as a visual flow indicator. A se
parate piston cooling system is used to limit any contamination f
rom piston cooling glands to the piston cooling system only.Fuel
injector valves require pre’cise control of the temperature at th
e tip of the nozzles, too high a temperature leading to the forma
tion of carbon trumpets(喇叭), too low a temperature causing corros
ion. They are generally provided with a cooling system which is e
ither 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 t
o the injectors is independent of that to the engine cylinder jac
kets. Sea water cooling systemThe various cooling liquids which c
irculate 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 circulate
d by sea water. Some modern ships use what is known as a central
cooling system with only one large sea water circulated cooler. T
his cools a supply of fresh water which then circulates to the ot
her individual coolers. With less equipment in contact with sea w
ater the corrosion problems are much reduced in this system.With
less equipment in contact with sea water the corrosion problems a
re much reduced in this system.In a common sea water cooling syst
em, from the sea suction(吸入口) one of a pair of sea water circulat
ing pumps provides sea water which circulates the lubricating oil
cooler, the jacket water cooler and the piston water cooler befo
re discharging overboard. Another branch of the sea water main(主管
线) provides sea water to directly cool the charge air (for a dire
ct-drive two-stroke diesel).In an emergency situation(紧急情况), for
example in the event of damage to fresh water cooler or generator
(造水机、发电机), the sea water pumps can be used to supply sea water to
the fresh water cooling system, merely by removing the blind-fla
nges(盲板法兰).Reading material The cooling systemsAs a “heat engine”
, heat energy is a source of power by which the diesel engine can
be operated. But excessive heat(过热) will cause damage to the eng
ine. It is therefore necessary to constantly cool the various com
ponents of the machinery so that the engine can work properly. Th
e separate cooling systems are designed for this purpose. These c
an conveniently(便利地) be grouped into sections.Cylinder cooling, o
r jacket cooling: normally fresh or distilled water(蒸馏水). This ma
y incorporate cooling of the turbine or turbines in a turbocharge
d engine and exhaust valve cooling. Fuel valve cooling: this woul
d be a separate system using fresh water or a fine mineral oil(高级
矿物油).Piston cooling: this may be lubricating oil, distilled or fr
esh water. If it is oil the system is generally common with the l
ubrication system. If water, a common storage tank with the jacke
t cooling system would generally be used.Charge air cooling: this
is normally sea water.A. Comparison of coolantsFresh water: Inex
pensive, high specific heat, low viscosity(粘度). Contains salts wh
ich can deposit(沉淀), obstruct(阻碍、阻塞) flow and cause corrosion. Re
quires treatment. Leakages(泄漏) could contaminate lubricating oil
system leading to loss of lubrication, possible overheating of be
arings 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 m
ust be kept to an absolute minimum, so a regular check(定期检查) on t
he replenishing-expansion tank contents level(水位) is necessary.If
the engine has to stand inoperative(不工作的) for a long period and
there is a danger of frost(冻), (a) drain the coolant out of the s
ystem, (b) heat up the engine room, or (c) circulate system with
heating on.It may become necessary to remove scale(水垢) from the c
ooling spaces, the following method could be used. Circulate, wit
h a pump, a dilute(稀释) hydrochloric acid solution (盐酸溶液). A hose
should be attached to the cooling water outlet pipe to remove gas
es. Gas emission(散发、排出) can be checked by immersing(浸入) the open
end of the hose occasionally(偶尔) into a bucket of water. Keep com
partment(舱室) well ventilated as the gases given off can be danger
ous. Acid solution strength (酸溶液的浓度) in the system can be tested
from time to time(有时) by putting some on to a piece of lime(石灰).
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 neutra
lized(中和) with a soda solution and pressure tested to see that th
e seals do not leak.Distilled water: More expensive than fresh wa
ter, high specific heat, low viscosity. If produced from evaporat
ed(蒸发) salt water it would be acidic(酸性的). No scale forming salts
. Requires separate pumping system. Leakages could contaminate th
e lubricating oil system, causing loss of lubrication and possibl
e over-heating and failure of bearings, etc.Additives(添加剂) for co
oling waterThose generally used are either anti-corrosion oils or
inorganic inhibitors(无机防锈剂).If pistons are water-cooled an anti-
corrosion oil is recommended as it lubricates parts which have sl
iding 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 c
orrosion but is not thick enough to impair(削弱、削减) heat transfer.I
norganic inhibitors form protective layers on metal surfaces guar
ding them against corrosion.It is important that the additives us
ed are not harmful if they find their way into drinking water(饮用水
)-this is possible if the jacket cooling water is used as a heati
ng medium(媒介) in a fresh water generator. Emulsion oils and sodiu
m nitrite(亚硝酸盐钠) are both approved(认可的) additives, but the latter
cannot be used if any pipes are galvanized(镀锌) or if any soldere
d(钎焊) joints exist. Chromates(铬酸盐) can not be used if the cooling
water is used in a fresh water generator and it is a chemical th
at must be handled with care.Lubricating oil: Expensive. Generall
y no separate pumping system required since the same oil is norma
lly used for lubrication and cooling. Leakages from cooling syste
m to lubrication system are relatively(相对地) unimportant providing
(倘若) they are not too large, otherwise one piston may be partly d
eprived(丧失) coolant with subsequent overheating. Due to reciproca
ting(往复的) action of piston some relative motion between parts in
contact in the coolant supply and return system must occur, oil w
ill lubricate these parts more effectively than water. No chemica
l treatment required. Lower specific heat than water, hence a gre
ater quantity oil must be circulated per unit time to give the sa
me cooling effect. If lubricating oil encounters(遭遇) high tempera
ture it can burn, leaving as it does so carbon deposit(积碳). This
deposit on the underside(内面) of a piston crown could lead to impa
irment of heat transfer, overheating and failure of the metal.Gen
erally the only effective method of dealing with the carbon depos
it is to dismantle(拆卸) the piston and physically remove it. Since
oil can burn in this way a lower mean outlet and inlet temperatu
re for the oil has to be maintained, in order to achieve this mor
e oil must be circulated per unit time.Some engines may use compl
etely separate systems for oil cooling of pistons and bearing lub
rication, the advantages gained by this method are:Different oils
can be used for lubrication and cooling, a very low viscosity mi
neral oil would be better suited to cooling than lubrication. Add
itives can be used in the lubricating oil that would be beneficia
l to lubrication, e.g. oiliness agents(润滑剂), e.p. agents (抗乳化剂) a
nd V.I. improvers(粘度指数增加剂), etc.Improved control over piston temp
eratures. If oil loss occurs, then with separate systems the prob
lem of detention(阻止，滞留) is simplified and in the case of total oi
l loss in either system, the quantity to be replaced would not be
as great as for a common system. Contamination of the oil in eit
her system may take place. In the event the problem of cleaning o
r renewal of the oil is not so great. Oxidation of lubricating oi
l in contact with hot piston surfaces leads to rapid reduction in
lubrication properties.Disadvantages of having two separate syst
ems 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 comple
xity more maintenance would have to be carried out. B. COOLING
WATER SYSTEM WITH CENTRAL COOLERThe engine is cooled by a closed
fresh 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 co
ols the cylinder liners, cylinder heads and turbocharger. From th
e pump water is supplied to the engine manifold which is cast in
the engine block. From the manifold the water is distributed to a
ll cylinder liner sections and from each liner upwards through th
e 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 flame plate, around the va
lves to the exhaust valve seats and up along the fuel injector sl
eeve. From the cylinder head the water is discharged via a connec
tion piece into the discharge manifold. Parallel to the flow to t
he cylinders part of the engine water flows through the turbochar
ger. 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 air is extracted.The LT circuit water cools the charge air in the second stage and the HT cooling water in the inter-cooler. 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 60oC by a separate heater. The pre-heating of the engine is, prior of starting up and loading on HFO, of utmost importance. |
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