influencia das condições de temperatura do ambiente

8/2/2019 Influencia das condies de temperatura do ambiente

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Influence of Ambient

Temperature ConditionsMain engine operation of MAN B&W two-stroke engines


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2 Influence of Ambient Temperature Conditions


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MAN B&W Diesel3Influence of Ambient Temperature Conditions

MAN B&W Diesel3

Contents

Introduction .................................................................................................5

Chapter 1 ....................................................................................................5

Temperature Restrictions and Load-up Procedures at Start of Engine ...... ...... .. 5

Start of warm engine normal load-up procedures ..................................5

Start of cold engine exceptional load-up procedures .............................6

Preheating during standstill periods .........................................................6

Jacket cooling water systems with a built-in preheater .............................7

Preheater capacity ..................................................................................7

Chapter 2 ....................................................................................................8

Engine Room Ventilation ...............................................................................8

Air temperature ...... ....... ...... ...... ...... ...... ...... ...... ...... ....... ...... ...... ...... ...... .8

Air supp ly ........ ...... ....... ...... ...... ...... ...... ...... ...... ...... ....... ...... ...... ...... ...... .9

Air pressure ......... ...... ....... ...... ...... ...... ...... ...... ...... ....... ...... ...... ...... ...... . 10

Chapter 3 ..................................................................................................11

Ambient Temperature Operation and Matching .. ...... ...... ....... ...... ...... ...... .... 11

Standard ambient temperature matched engine ....................................11

Non-standard ambient temperature matched engine .............................12

Design recommendations for operation at extremely low air temperature ....... 15

Closing Remarks ........................................................................................17


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Influence of Ambient Temperature ConditionsMain engine operation of MAN B&W two-stroke engines

Introduction

Diesel engines used as prime movers

on ships are exposed to the varying cli-

matic temperature conditions that pre-

vail in different parts of the world, and

must therefore be able to operate un-

der all ambient conditions from winter

to summer and from arctic to tropical

areas.

As the temperature variations on the

surface of the sea are rather limited,

the diesel engine will not normally be

exposed to really extreme tempera-

tures. However, the changes that do

occur in the ambient conditions will,

among other things, cause a change

in the specific fuel oil consumption, the

exhaust gas amount and the exhaust

gas temperature of the diesel engine.

These changes are already describedin our Project Guides and will therefore

not be discussed in this paper.

Also the scavenge air, compression and

maximum firing pressures of the diesel

engine will change with climatic chang-

es and, at very low ambient air temper-

atures, unrestricted engine operation

requires adjustments of individual en-

gine parameters.

This paper describes our recommenda-

tions of load-up procedures on engine

start-up, the supply of ventilation air to

the engine room and engine operation

under normal, high and extremely low

ambient temperature conditions.

The paper is divided into three chapters

which, in principle, may be read inde-

pendently of each other. Thus, Chapter

3 is more or less a copy of our paper

Ambient Tempair temperature as a

common parameter.

The three chapters are entitled:

Temperature Restrict ions and Load-

up Procedures at Start of Engine

Engine Room Ventilation

Ambient Temperature Operation and

Matching

Chapter 1

Temperature Restrictions and Load-up

Procedures at Start of Engine

In order to protect the engine against

cold corrosion attacks on the cylinder lin-

ers, some minimum temperature restric-

tions and load-up procedures have to be

considered before starting the engine.

Below stated load-up procedures are

valid for MAN B&W two-stroke engines

with a cylinder bore greater or equal to

80 cm, and may with benefit also be

applied for engines with a smaller bore.

However, if needed, the existing load-

up programme recommendation (from

90% to 100% in 30 minutes) is still valid

for engines with bore sizes from 70 cm

and down.

Note: The below recommendations are

based on the assumption that the en-

gine has already been well run in.

Start of warm engine normal load-

up procedures

As a summary, the load-up procedures

recommended for normal start of en-

gine are shown in Fig. 1.

Recommended start of engine at normal en-

gine load operation

Fixed pitch propellers

Normally, a minimum engine jacket wa-

ter temperature of 50oC is recommend-

ed before the engine may be started

and run up gradually from 80% to 90%

of specified MCR speed (SMCR rpm).

during 30 minutes.

SMCR = Specified Maximum

Continuos Rating.

rpm = revolutions per minute


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Start of warm engine (normal load-up procedures)

Required jacket water temperature at normal start of engine: minimum 50oC

FPP:

CPP:

Fixed Pitch Propeller

Controllable Pitch Propeller

Recommended start of engine

1. at normal engine load operation

A. Run up slowly minimum

temp. 50oC

FPP From 0% up to 80% SMCR speed

CPP From 0% up to 50% SMCR power

B. Run up slowly,

(minimum 30 min)



C. Run up slowly,

(minimum 60 min)



2. at normal very low engine load operation

A. Run up slowly

If normally 10% to 40% engine low load operation

(slide fuel valves needed) extra slowly load-up proce-

dure is recommended: minimum 30 min from 10% to

40% load and minimum 60 min from 40% to 75% load

For running-up between 90% and

100% of SMCR rpm, it is recommend-

ed that the speed be increased slowlyover a period of 60 minutes.

Controllable Pitch Propellers

Normally, a minimum engine jacket wa-

ter temperature of 50oC is recommend-

ed before the engine may be started

and run up gradually from 50% to 75%

of specified MCR load (SMCR power)

during 30 minutes.

For running-up between 75% and

100% of SMCR power, it is recom-

mended that the load be increasedslowly over a period of 60 minutes.

Recommended start of engine at normal very

low engine load operation

For engines normally running at 10%

to 40% engine low load operation an

extra slowly load-up procedure is rec-

ommended compared with above de-

scribed load-up procedures, and is

also shown in Fig. 1.

Start of cold engine exceptional

load-up procedures

As a summary, the load-up pro-

cedures recommended for ex-

ceptional start of cold engine are

shown in Fig. 2.

Fixed pitch propellers

In exceptional circumstances where it is

not possible to comply with the above-

mentioned normal recommendations,

a minimum of 20oC can be accepted

before the engine is started and run up

slowly to 80% of SMCR rpm.

Before exceeding 80% SMCR rpm, a

minimum jacket water temperature of

50oC should be obtained before the

above-described normal start load-up

procedure may be continued.

Controllable Pitch Propellers

In exceptional circumstances where it is

not possible to comply with the above-

mentioned normal recommendations,

a minimum of 20oC can be accepted

before the engine is started and run up

slowly to 50% of SMCR power.

Before exceeding 50% SMCR power,

a minimum jacket water temperature

of 50oC should be obtained before the

above described normal start load-up

procedure may be continued.

The time period required for increasing

the jacket water temperature from 20C

to 50C depends on the amount of wa-

ter in the jacket cooling water system,

and on the engine load.

Preheating during standstill periods

During short stays in ports (i.e. less than

4-5 days), it is recommended to keep

the engine preheated, the purpose be-

Fig. 1: Temperature restrictions and load-up procedures at normal start of engine

Start of cold engine (exceptional load-up procedures)

Required jacket water temperature at start of cold engine: minimum 20oC

FPP:

CPP:

Fixed Pitch Propeller

Controllable Pitch Propeller

Recommended start of engine at normal engine load operation

A. Run up slowly Minimum

temp. 20oC



B. Run up slowly, Minimum

(minimum 30 min) temp. 50o

C


CPP From 50% up to 75% SMCR powerC. Run up slowly,

(minimum 60 min)



Fig. 2: Temperature restrictions and load-up procedures at start of cold engine in exceptional cases


7/20


ing to prevent temperature variations in

the engine structure and correspondingvariations in thermal expansions, and

thus the risk of leakages.

The jacket cooling water outlet temper-

ature should be kept as high as possi-

ble (max. 75-80C), and should before

start-up be increased to at least 50C,

either by means of the auxiliary engine

cooling water, or by means of a built-in

preheater in the jacket cooling water

system, or a combination of both.

Jacket cooling water systems with a

built-in preheater

For two different jacket water preheater

systems, A and B, the positioning of a

preheater in the jacket cooling water

system is shown schematically in Figs.

3 and 4, respectively.

For system A, the circulating water flowis divided into two branches, one go-

ing through the engine and one going

through the cooling water system out-

side the engine. As the arrows indicate,

the preheater water flows in the oppo-site direction through the engine, com-

pared with the main jacket water flow.

As the water inlet is at the top of the

engine, the engine preheating is more

effective in this way.

Preheater

pumpPreheater

Preheater

bypass

Diesel engine

Jacket water main pumps

Fig. 4: Preheating of jacket cooling water system System B

For system B, the preheater and circu-

lating pump are placed in parallel with

the jacket water main pumps, and the

water flow direction is the same as for

the jacket cooling water system.

In both cases, the preheater operation

is controlled by a temperature sensor

after the preheater.

Preheater capacity

When a preheater is installed in the jacket

cooling water system, as shown in Figs.

3 and 4, the preheater pump capacity,

should be about 10% of the jacket water

main pump capacity. Based on experi-

ence, it is recommended that the pres-

sure drop across the preheater should

be approx. 0.2 bar. The preheater pump

and the jacket water main pump should

be electrically interlocked to avoid therisk of simultaneous operation.

The preheater capacity depends on

the required preheating time and the

required temperature increase of the

Direction of main water flow

Direction of preheater circulating water flow

Preheater

pump

Preheater

Preheater

bypass

Diesel engine

Jacket water main pumps

Fig. 3: Preheating of jacket cooling water system System A


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Fig. 5: Preheating of diesel engine

Preheater capacity in %

of nominal MCR power

Preheating time

0 10 20 30 40 50 60 70

0

10

20

30

40

50

60

oC 1.50% 1.25% 1.00% 0.75%

The temperature increase and corresponding

preheating time curves are shown for the different

preheater sizes indicated in % of nominal MCR power

hours

Temperature

increase

of jacket water

engine jacket water. The temperature

and time relationship is shown in Fig. 5.

The relat ionship is almost the same for

all engine types.

If a temperature increase of for example

35C (from 15C to 50C) is required, a

preheater capacity of about 1% of the

engines nominal MCR power is required

to obtain a preheating time of 12 hours.

When sailing in arctic areas, the re-

quired temperature increase may be

higher, possibly 45C or even higher,

and therefore a larger preheater capac-

ity is required. The curves in Fig. 5 are

based on the assumption that, at the

start of preheating, the engine and en-

gine room are of equal temperatures.

It is assumed that the temperature will

increase uniformly all over the engine

structure during preheating, for which

reason steel masses and engine surfaces

in the lower part of the engine are also

included in the calculation.

The results of the preheat ing calcula-

tions may therefore be somewhat con-

servative.

Chapter 2

Engine Room Ventilation

In addition to providing sufficient air for

combustion purposes in the main en-

gine, auxiliary diesel engines, fuel fired

boiler, etc., the engine room ventilation

system should be designed to remove

the radiation and convection heat from

the main engine, auxiliary engines, boil-

ers and other components.

A sufficient amount of vent ilat ion air

should be supplied and exhausted

through suitably protected openings

arranged in such a way that these

openings can be used in all weather

conditions. Care should be taken to

ensure that no seawater can be drawn

into the ventilation air intakes.

Furthermore, the ventilation air inlet

should be placed at an appropriate dis-

tance from the exhaust gas funnel in or-

der to avoid the suction of exhaust gas

into the engine room.

Major dust and dirt particles can foul

air coolers and increase the wear of

combustion chamber components.

Accordingly, the air supplied to the

engine must be cleaned by appropri-

ate filters. The size of particles passing

through the air intake filter should not

exceed 5m.

An example of an engine room vent ila-

tion system, where ventilation fans blow

air into the engine room via air ducts, is

shown in Fig. 6.

Air temperature

Measurements show that the ambient

air intake temperature (from deck) at sea

will be within 1 to 3C of the seawater


9/20


temperature, i.e. max. 35C for 32C

seawater, and max. 39C for 36C sea-

water.

Measurements also show that, in a nor-

mal ventilation air intake system, where

combustion air is taken directly from

the engine room of a ship, the engine

room temperature is normally 10-12C

higher than the ambient outside air tem-

perature. This temperature difference is

even higher for winter ambient air tem-

peratures, see Fig. 7. In general, the en-gine room temperature should never be

below 5C, which is ensured by stopping

one or more of the air ventilation fans,

der normal air temperature conditions,

the air inlet temperature to the turbo-

charger is only 1-3C higher than the am-

bient outside air temperature.

This means that the turbocharger suc-

tion air temperature will not be higher

than about 39 + 3 = 42C (ref. 36C

S.W.), say 45C.

For arctic running conditions, a ducted

air intake system directly to the turbo-

charger can be an advantage in order to

maintain sufficiently high temperatures

for the crew in the engine room. With

a ducted air intake, the turbochargers

intake air temperature may be assumed

to be approximately equal to the ambi-

ent outside air temperature.

Air supplyIn the case of a low speed two-stroke

diesel engine installed in a spacious en-

gine room, the capacity of the ventilation

system should be such that the ventila-

tion air to the engine room is at least

1.5 times the total air consumption of

the main engine, auxiliary engines, boil-

er, etc., all at specified maximum con-

tinuous rating (SMCR).

As a rule of thumb, the minimum en-

gine room ventilation air amount corre-

sponds to about 1.75 times the air con-

sumption of the main engine at SMCR.

Accordingly, 2.0 times the air con-

sumption of the main engine at SMCR

may be sufficient.

On the other hand, for a compact engine

room with a small two-stroke diesel

engine, the above factor of 1.5 is rec-ommended to be higher, at least 2.0,

because the radiation and convection

heat losses from the engine are relatively

Air inletAir inlet

Engine room

ventilation fans

Air outlet

AEAE AE

Main ducts for supply

of combustion air

ME: Main engine

AE: Auxiliary engines

ME

Fig. 6: Engine room ventilation system

thus reducing the air supply to and

thereby the venting of the engine room.

This means that the average air tem-

perature in a ventilated engine room will

not be lower than 5C and not higher

than 39 + 12 = 51C, say 55C (ref.

36C S.W.), as often used as maximum

temperature for design of the engine

room components.

Since the air ventilation ducts for a nor-

mal air intake system are placed nearthe turbochargers, the air inlet temper-

ature to the turbochargers will be lower

than the engine room temperature. Un-


10/20


greater than from large two-stroke

engines, and because it may be difficult

to achieve an optimum air distribution

in a small engine room.

To obtain a correct supply of air for the

main engines combustion process,

about 50% of the ventilation air should

be blown in at the top of the main en-

gine, near the air intake to the turbo-

chargers, as shown in Fig. 6.

Otherwise, this can have a negative ef-

fect on the main engine performance.

Thus, the maximum firing pressure will

be reduced by 2.2% for every 10C theturbocharger air intake temperature is

raised, and the fuel consumption will go

up by 0.7%.

Furthermore, a correct air supply near

the turbochargers will reduce the dete-

rioration of the turbocharger air filters

(from oil fumes, etc., in the engine room

air), and a too draughty engine room

can be avoided.

Moreover, a sufficient amount of air

should be supplied to areas with a high

heat dissipation rate in order to ensure

that all the heat is removed, for instance

around auxiliary engines/generators

and boilers. Ventilation ducts for these

areas are not shown in Fig. 6.

In the winter time, the amount of airneeded to remove the radiation/con-

vection heat from the engine room may

be lower.

Fig. 7: Engine room temperature

20 0 20 40

0

10

20

30

40

50

60

oC

The engine room temperature T and the engine

room/ambient air temperature difference T are shownas functions of the ambient air temperature T

ER

amb

oC

Amb. air temp. Tamb.

Engine room temperature Tand difference T

ER

DT = Tamb.TER

TER

D

D

D

Air pressure

The air in the engine room should have

a slightly positive pressure, but should

not be more than about 5 mm WC (Wa-

ter Column) above the outside pressure

at the air outlets in the funnel.

Accommodation quarters will normally

have a somewhat higher over-pressure,

so as to prevent oil fumes from the en-

gine room penetrating through door(s)

into the accommodation.

The vent ilat ion air can be supplied, for

example, by fans of the low-pressure

axial and high-pressure centrifugal or

axial types. The required pressure head

of the supply fans depends on the re-

sistance in the air ducts.

All vent ilat ion air is normally deliveredby low-pressure air supply fans which,

to obtain sufficient air ventilation in all

corners of the engine room, may re-

quire extensive ducting and a pressure

head as stated below.

Low-pressure fans,

p = 60-100 mm WC

For further information, please consult

engine room ventilation standard ISO

8861: 1998 (E).


11/20


Fig. 8: Influence on SFOC of the cooling water

(scavenge air coolant) temperature

Engine shaft power40 SMCR50 60 70 80 90 100%

SFOCg/kWh

36C C.W

10C C.W2 g/kWh

Turbocharger air intake temperature: 10C

Chapter 3

Ambient Temperature Operation and

Matching

Standard ambient temperature

matched engine

Standard unrestricted service demands

For a standard main engine, the en-

gine layout is based on the ambient

reference conditions of the Interna-

tional Standard Organization (ISO):

ISO 3046-1:2002(E) and

ISO 15550:2002(E):

ISO ambient reference conditions

Barometric pressure: 1,000 mbar

Turbocharger air intake

temperature: 25C

Charge air coolant tem-perature:

25C

Relative air humidity: 30%

With this layout basis, the engine must

be able to operate in unrestricted serv-

ice, i.e. up to 100% Specified Maximum

Continuous Rating (SMCR), within the

typical ambient temperature range that

the ship is exposed to, operating from

tropical to low winter ambient conditions.

According to the International Associa-

tion of Classification Societies (IACS)

rule M28, the upper requirement, nor-

mally referred to as tropical ambient ref-

erence conditions, is as follows:

IACS M28 (1978):

Tropical ambient reference conditions


Air temperature: 45CSeawater temperature: 32C


The above tropical ambient relat ive hu-

midity of 60% at 45C is theoretically

the absolute limit at which it is possible

for humans to survive. The correspon-

ding wet bulb temperature is 36.8C.

MAN Diesel & Turbo has never mea-

sured levels above 50% at 45C, and

humidity levels above standard tropical

ambient conditions will never occur.

When applying the central cooling wa-

ter system which, today, is more com-

monly used than the seawater system,

the corresponding central cooling wa-

ter/scavenge air coolant temperature is

4C higher than the seawater tempera-

ture, i.e. equal to 36C.

The winter ambient reference condi-

tions used as standard for MAN B&Wtwo-stroke engines are as follows:

Winter ambient reference conditions


Turbocharger air intake

temperature:

10C

Cooling water temperature:

(minimum for lub. oil cooler)

10C


Shipyards often specify a constant

(maximum) central cooling water tem-

perature of 36C, not only for tropical

ambient conditions, but also for winter

ambient conditions. The purpose is to

reduce the seawater pump flow rate

when possible, and thereby to reduce

the electric power consumption, and/

or to reduce the water condensation in

the air coolers.

However, when operating with 36C

cooling water instead of for example

10C (to the scavenge air cooler), the

specific fuel oil consumption (SFOC)

will increase by approx. 2 g/kWh, see

Fig. 8. Any obtained gain in reduced

electric power consumption, therefore,

will be more than lost in additional fuel

costs of the main engine.

The above ISO, tropical and winter

ambient reference conditions are used

by MAN Diesel & Turbo for ships, and

MAN B&W two-stroke engines com-

ply with the above rules. MAN B&W

engines matched according to the

above rules are able to operate con-

tinuously up to 100% SMCR in the

air temperature range between about

-10 and 45C.

Often the engine room temperature is mi-

staken for being equal to the turbocharger

air intake temperature. However, since theair ventilation duct outlets for a normal

air intake system are placed near the

turbochargers, the air inlet temperature

to the turbochargers will be very close

to the ambient outside air temperature.

Under normal air temperature condi-

tions, the air inlet temperature to the

turbocharger is only 1-3C higher than

the ambient outside air temperature.


12/20


engine load resulting in a scavenge

air temperature below the level of the

scavenge air temperature alarm.

Nevertheless, the engines obtainable

load level will in all cases be much

higher than required to ensure a safe

manoeuvrability (4-6 knots) of the ship

even at an extreme seawater tempera-

ture of for example 42C.

When sailing in, for example, the har-

bour area during manoeuvring, the en-

gine load will normally be relatively low

(15-30% SMCR), and the correspond-

ing scavenge air temperature will then

only be slightly higher than the scav-

enge air coolant temperature. There-

fore, a seawater temperature as high

as for example 42C in harbour areas

is not considered a problem for themain engine, and a special temperature

matching is not needed under these

operating conditions.

In general, when sailing in areas with a

high seawater temperature, it is pos-

sible to operate the standard ambient

temperature matched main engine at

any load as long as the scavenge air

temperature alarm limit is not reached.

If the alarm is activated, the engine load

has to be reduced.

Non-standard ambient temperature

matched engine

If unrestricted loads are desired in a

temperature range different from the

standard, different matching possibili-

ties are available.

Engine matching for non-standard air tem-

perature conditions

Usually, higher or lower turbocharger

air intake temperatures may result in

lower or higher scavenge air pressures,

respectively, and vice versa.

An increase of, for example, 5C of the

tropical air temperature from standard

45C to special 50C will result in a too

low scavenge air pressure at 50C.

However, the pressure reduction can

be compensated for by specifying a

correspondingly higher (turbocharger)

scavenge air pressure at ISO ambient

reference conditions. This involves that

the engine, instead of being matched

for the ISO-based design air tempera-

ture of 25C, has to be matched for the

25 + 5 = 30C turbocharger air intake

temperature.

The orig inal ISO-based heat load con-

ditions will then almost be obtainedfor this higher design air temperature.

The principles for standard and spec ial

high (or low) ambient air temperature

matched engines are shown in Fig. 9.

At the other end of the air temperature

range, the increase of 5C of the de-

sign air intake temperature will involve

a too high scavenge air pressure when

operating at -10C. Operation below

-10 + 5 = -5C will then only be pos-

sible when installing a variable exhaust

gas bypass valve system for low air

temperatures, as described later.

Fig. 9 may in a similar way also be

used to explain a special low tempera-

ture matched engine. For example, if

the standard tropical air temperature

needed is reduced by 10C, from 45C

to 35C, the engine matching designair temperature can be reduced to

2510 = 15C.

The classification society rules often

specify an engine room air temperature

of 0-55C as the basis for the design

of the engine room components. The

55C is the temperature used when

approving engine room components.

This, however, must not be mistaken

for the above tropical air intake tempe-

rature of 45C specified when related to

the capacity or effect of the machinery.

In recent years, owners/shipyards have

sometimes required unrestricted serv-

ice on special maximum ambient tem-

peratures higher than the tropical am-

bient temperatures specified by IACS

M28. In such cases, the main engine

has to be special high temperature

matched, as described later in this pa-

per.

Furthermore, operation in arctic areas with

extremely low air temperatures has also

sometimes been required by owners/

shipyards, and the measures to be taken

are also described later in this paper.

Operating at high seawater temperature with

standard matched engine

An increase of the seawater tempera-

ture and, thereby, the scavenge air

temperature has a negative impact on

the heat load conditions in the combus-

tion chamber. Therefore, all MAN B&W

two-stroke engines for marine applica-

tions have an alarm set point of 55C

for the scavenge air temperature for pro-

tection of the engine, as described later.

For a standard ambient temperature

matched engine operating at an in-

creased seawater temperature existingin some inland, gulf, bay and harbour

areas, the maximum power output of

the engine should be reduced to an


13/20


Turbocharger

air intake temperature

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

55

Max.

45 C

ISO

25 C

Min.

-10 C

ISO

design

layout

Normal

tropical

temperature

For engine loads

higher than 30% SMCR

a low scavenge air

coolant temperature

is recommended

(Giving low SFOC and

low scav. air press.)

Normal min.

ambient air

temperature

Standard

ISO temperature

matched engine

Possible low ambient

air temperature

exhaust gas bypass for

operation under

extremely low ambient

temperature conditions

ISO based

design

layout

Special

tropical

temperature

Lowest

ambient air

temperature

Low ambient air

temperature exhaust

gas bypass will be

needed below min.

temperature

Max.

Special

design

temperature

Min.

Special

High temperature

matched engine

Up to 100% SMCR running is not allowed

Up to 100% SMCR running is allowed

Up to 100% SMCR running only allowed when low

ambient temperature exhaust gas bypass (C1+2)

is installed

Standard

design

Temperature

ISO based

design

layout

Special

tropical

temperature

Lowest

ambient air

temperature

Max.

Special

design

temperature

Min.

Special

Low temperature

matched engine

-50

-45

60

65

Fig. 9: Principles for standard and special high (or low) ambient air temperature matched engines


14/20


This involves that the exhaust gas tem-

perature will increase by about 16C

compared with a standard ISO tem-

perature matched engine, whereas the

SFOC will increase.

Engine matching for high tropical seawater

temperature conditions

For long time operation in an area with

high tropical seawater temperatures,

the following should be observed.

An increase in the seawater tempera-

ture and, thereby, of the scavenge air

coolant temperature will involve a simi-

lar increase in the scavenge air tem-

perature, which has a negative impact

on the combustion chamber tempera-

tures. Therefore, for all marine applica-

tions, an alarm set point of 55C for the

scavenge air temperature is applied for

protection of the engine.

The standard marine scavenge air cooler

is specified with a maximum 12C tem-

perature difference between the cooling

water inlet and the scavenge air outlet

at 100% SMCR, which gives a maxi-

mum scavenge air temperature of 36 +

12 = 48C for the scavenge air cooler

layout and, accordingly, a margin of 7C

to the scavenge air temperature alarm

limit of 55C.

Temperature C

42

44

46

48

50

52

54

56

Maximum

scavenge air

temperature

at 100% SMCR

ISO based

scavenge air

coolant

temperature

ISO

design

layout

Standard ISO temperaturematched engine

Standard air cooler design

ISO based

design

layout

High scavenge

air coolant

temperature

Maximum

scavenge air

temperature

at 100% SMCR

Special high temperaturematched engine

Special air cooler design

Max. 48 CStandard 48 C

26

28

30

32

34

36

38

40

22

24

Standard tropical

seawater

temperature

Standard

tropical

scavenge air

coolant

temperature

High tropical

scavenge air

coolant

temperature

High tropical

seawater

temperature

Max. 40 C

Max. 36 C

Max. 29 C

Standard 36 C

Standard 32 C

Standard

basis 25 C

Max. 55 CStandard 55 CScavenge air temperature limit Scavenge air temperature limit

Up to 100% SMCR running is

not allowed (scavenge air)


allowed (scavenge air)


allowed (scavenge air coolant/central

cooling water)


allowed (seawater)

A temperature difference of 8C is con-

sidered to be the lowest possible tem-

perature difference to be used for a

realistic specification of a scavenge air

cooler. Accordingly, the 488 = 40C is

the maximum acceptable scavenge air

coolant temperature for a central cool-

ing water system, see the principles for

layout of the scavenge air cooler in Fig. 10.

The demand for an increased tropi-

cal scavenge air coolant (central cool-

ing water) temperature of up to 40C,

therefore, can be compensated for by a

reduced design temperature difference

of the scavenge air cooler. This can be

obtained by means of an increased wa-

Fig. 10: Principles for layout of scavenge air cooler for standard and special high scavenge air coolant temperature (illustrated for a central cooling water system)


15/20


ter flow and/or a bigger scavenge air

cooler.

Design recommendations for operation

at extremely low air temperature

When a standard ambient temperature

matched main engine on a ship occa-

sionally operates under arctic condi-

tions with low turbocharger air intake

temperatures, the density of the air will

be too high. As a result, the scavenge

air pressure, the compression pressure

and the maximum firing pressure will be

too high.

In order to prevent such excessive

pressures under low ambient air tem-

perature conditions, the turbocharger

air inlet temperature should be kept as

high as possible (by heating, if possi-

ble).

Furthermore, the scavenge air cool-

ant (cooling water) temperature should

be kept as low as possible and/or the

engine power in service should be re-

duced.

However, for an inlet air temperature

below approx.10C, some engine de-

sign precautions have to be taken.

Main precautions for extreme low air tem-

perature operation

With a load-dependent exhaust gas

bypass system (standard MAN Diesel &

Turbo recommendation for extreme low

air temperature operation), as shown

in Fig. 11, part of the exhaust gas by-

passes the turbocharger turbine, giving

less energy to the compressor, thus re-

ducing the air supply and scavenge airpressure to the engine.

For the electronically controlled ME

engine (ME/ME-C/ME-B), the load-de-

pendent bypass control can be incor-

porated in the Engine Control System

(ECS) as an add-on.

Engine load, fuel index and scavenge

air pressure signals are already availa-

ble for the ME software and, therefore,

additional measuring devices are not

needed for ME engines.

In general, a turbocharger with a nor-

mal layout can be used in connection

with an exhaust gas bypass. However,

in a few cases a turbocharger modifica-

tion may be needed.

The exhaust gas bypass system ensures

that when the engine is running at part

load at low ambient air temperatures,

the load-dependent scavenge air pres-

sure is close to the corresponding pres-

sure on the scavenge air pressure curve

which is valid for ISO ambient condi-

tions. When the scavenge air pressure

exceeds the read-in ISO-based sca-

venge air pressure curve, the bypass

valve will variably open and, irrespec-

tive of the ambient conditions, ensure

that the engine is not overloaded. At the

same time, it will keep the exhaust gas

temperature relatively high.

Fig. 11: Standard load-dependent low ambient air temperature exhaust gas bypass system

D2

C1+2

B

D1

1

2

B Exhaust gas bypass valve

Controlled by the scavenge air pressure

C1+2 Control device

Ensures that the loaddependent scavenge air pressure

does not exceed the corresponding ISO based pressure

D Required electric measuring device

D1 Scavenge air pressure

D2 Engine speed and engine load

Diesel engine

Scavenge

air receiver

Scavengeair cooler

Compressor

Turbocharger

Turbine

Exhaust gas

receiver

Exhaust gas system

Air intake casing

Exhaust gas bypass


16/20


The latest generations of turbochargers

with variable flow, e.g. the VTA (Variable

Turbine Area) system from MAN Diesel

& Turbo, can replace the variable by-

pass and ensure the same scavenge air

pressure control.

Other low temperature precautions

Low ambient air temperature and low

seawater temperature conditions come

together. The cooling water inlet tem-

perature to the lube oil cooler should

not be lower than 10C, as otherwise

the viscosity of the oil in the cooler will

be too high, and the heat transfer inad-

equate. This means that some of the

cooling water should be recirculated to

keep up the temperature.

Furthermore, to keep the lube oil vis-

cosity low enough to ensure propersuction conditions in the lube oil pump,

it may be advisable to install heating

coils near the suction pipe in the lube

oil bottom tank.

The following additional modifications

of the standard design practice should

be considered as well:

Larger electric heaters for the cylin-

der lubricators or other cylinder oil

ancillary equipment

Cylinder oil pipes to be further heat

traced/insulated

Upgraded steam tracing of fuel oil

pipes

Increased preheater capacity for

jacket water during standstil l

Different grades of lubricating oil for

turbochargers

Space heaters for electric motors

Sea chests must be arranged so that

blocking with ice is avoided.

Ships with ice class notation

For ships with the Finnish-Swedish ice

class notation 1C, 1B, 1A and even

1A super or similar, all MAN B&W

two-stroke diesel engines meet the

ice class demands, i.e. there will be no

changes to the main engines.

However, if the ship is with ice class

notation 1A super and the main engine

has to be reversed for going astern

(Fixed Pitch Propeller), the starting air

compressors must be able to charge

the starting air receivers within half an

hour, instead of one hour, i.e. the com-

pressors must be the double in size

compared to normal.

For other special ice class notations,

the engines need to be checked indi-

vidually.

40

2,500

kg/h

Engine shaft power

60 80 100%

Surplus steam

Extra steam needed

Total steam production,

without bypass

Total steam production,

with exhaust gas bypass

Steam consumption

2,000

1,500

1,000

500

0

Steam production

SMCR50 70 90

6S60MC-C7/ME-C7

SMCR = 13,560 kW at 105 r/min

Air intake temperature: 0 C

Cooling water temperature: 10C

Fig. 12: Expected steam production by exhaust gas boiler at winter ambient conditions (0 C air) for main

engine 6S60MC-C7/ME-C7 with/without a load-dependent low air temperature exhaust gas bypass system


17/20


The exhaust gas bypass system to be

applied is independent of the ice class-

es, and only depends on how low the

specified ambient air temperature is

expected to be. However, if the ship is

specified with a high ice class like 1A

super, it is advisable to make prepara-

tions for, or install, an exhaust gas by-

pass system.

Increased steam production in wintertime

During normal operation at low am-

bient temperatures, the exhaust gas

temperature after the turbochargers will

decrease by about 1.6C for each 1.0C

reduction of the intake air temperature.

The load-dependent exhaust gas by-

pass system will ensure that the exhaust

gas temperature after the turbochargers

will fall by only about 0.3C per 1.0C

drop in the intake air temperature, thus

enabling the exhaust gas boiler to pro-duce more steam under cold ambient

temperature conditions.

Irrespective of whether a bypass sys-

tem is installed or not, the exhaust gas

boiler steam production at ISO ambient

conditions (25C air and 25C C.W.) or

higher ambient temperature conditions,

will be the same, whereas in wintertime

the steam production may be relatively

increased, as the scavenge air pressure

is controlled by the bypass valve.

As an example, Fig. 12 shows the influ-

ence of the load-dependent exhaust gas

bypass system on the steam production

when the engine is operated in winter-

time, with an ambient air temperature of

0C and a scavenge air cooling water

temperature of 10C.

The calculat ions have been made for a

6S60MC-C7/ME-C7 engine equipped

with a high-efficiency turbocharger, i.e.

having an exhaust gas temperature of

245C at SMCR and ISO ambient con-

ditions.

Fig. 12 shows that in wintertime, it is

questionable whether an engine with-

out a bypass will meet the ship's steam

demand for heating purposes (indicat-

ed for bulk carrier or tanker), whereas

with a load-dependent exhaust gas by-

pass system, the engine can meet the

steam demand.

Closing Remarks

Diesel engines installed in ocean-going

ships are often exposed to different cli-

matic temperature conditions because

of the ships trading pattern, but as the

temperature variations on the sea sur-

face are normally relatively limited, the

engines will normally be able to operate

worldwide in unrestricted service with-

out any precautions being taken.

Even if the ship has to sail in very cold

areas, the MAN B&W two-stroke en-

gines can, as this paper illustrates, also

operate under such conditions without

any problems as long as special low

temperature precautions are taken.

The use of the standard load-dependent

low ambient air temperature exhaust

gas bypass system may as an ad-ditional benefit also improve the ex-

haust gas heat utilisation when running

at low ambient air temperatures.

Furthermore, at the other end of the

temperature scale, if the ship should

need to sail in unrestricted service in ar-

eas with very high ambient air tempera-

tures, higher than 45C, this will also be

possible provided a high temperature

matching of the engine is applied. Even

when sailing should be needed at very

high seawater temperatures, this will be

possible provided a specially designed

scavenge air cooler is installed on the

diesel engine.


18/20


19/20


20/20

MAN Diesel & TurboTeglholmsgade 41

2450 Copenhagen SV, Denmark

Phone +45 33 85 11 00

Fax +45 33 85 10 30

[email protected]

www.mandieselturbo.com

Alldataprovidedinthisdocumentisno

n-binding.Thisdataservesinformationalpurposesonlyan

disespeciallynotguaranteedinanyway.

Dependingonthesubsequentspecificindividualprojects,therelevantdatamaybesubjecttocha

ngesandwillbeassessedanddetermined

individuallyforeachproject.Thiswilldependontheparticularcharacteristicsofeachindividualproject,especiallyspecificsiteandoperational

conditionsCopyrightMAN

Diesel&T

urboSubjecttomodificationintheinterestoftechnicalp

rogress.5510-0074-00pprAug.2010

PrintedinDenmark

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