2004 water maker thermal efficiency

8/8/2019 2004 Water Maker Thermal Efficiency

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SPEC_FWG_Cruise_Ship_E_Rev 1 Page 1 of 4 SERCK COMO GmbH/25.02.04

How to define the Thermal Efficiency of Evaporators for Cruise Ships

This very short description shows in a simplified manner and with typical data how to specify the thermal efficiency of a fresh water generator in dependence of thedesign philosophy of a cruise ship.

The first step is the design of the engine power and the required fresh water produc-tion. According to the cruise profile, it has to be defined at which speed of the vesselthe nominal required water production has to be maintained only by using the ex-haust heat of the HT motor water (19 knots/hr in this example). The recoverable heatat this operating point defines the thermal efficiency of the evaporator (as kWh/kg ofwater produced).

Nominal Speed: 22 knots/hrPower for Propulsion at nominal Speed: 40000 kWBase Load Power Plant (zero speed): 5000 kW

Power Requirements at nominal speed: 45000 kW100% Engine Load: 50000 kWEngine Load at nominal Speed: 90 %Number of Engines: 2 x 3Water Production: 2 x 600 t/dCritical Speed for 100% Water Production: 19,0 knots/hr = 30800 kW Power

(incl. Base Load)3 Engines are linked to one evaporator via a common cooling water circuit

Diagram 1 shows the approximate operating characteristics of the vessel as kw ofelectric power versus the speed, considering a base load of 5000 kW for the ship.

max. Speed

Nominal Speed

Critical Speed

for evaporator design

CONSTANT = 3,757

BASELOAD = 5000 kW

EXPO = 3,0

Power/Speed Characteristics of Cruise Ship

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 5 10 15 20

Speed (knots/hr)

TotalElectricPower(kW)

POWER = BASELOAD + CONSTANT * SPEEDEXPO

BASELOAD

Diagram 1: Typical Speed / Power-Characteristics of a Cruise Ship

This document, and more, is available for download at Martin's Marine Engineeirng Page - www.dieselduck.net


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Diagram 2 shows the typical exhaust heat characteristics of a Diesel Engine, as thedimensionless relation between recoverable HT motor water heat and shaft power.

Exhaust Heat in HT Motor Water

y = -3,538E-12x6

- 8,774E-09x5

+ 2,022E-06x4

- 1,399E-04x3

+ 4,548E-03x2

-

1,993E-02x + 2,202E+01

20

25

30

35

40

0 20 40 60 80 100

x = Engine Load (%)

y=Heat(in%ofShaftPower)

Diagram 2: Typical Exhaust Heat Charcteristics of Diesel Engine

After knowing the capacity of the installed engines, the diagram 3 can be provided,

showing the heat as kW versus the load of the engines.

HT-Motor Water Heat (kW)(all engines running at same Load)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 20 40 60 80 100Engine Load (%)

Heat(k

W)

Diagrams 3: Exhaust Heat in HT Motor Water of selected Diesel Engines(typical)



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The combination of diagram 1 and diagram 3 results in diagram 4, showing the ex-haust heat versus the speed of the ship. The point marked in yellow colour is thecritical design point of the evaporator.

Heat available from HT Motor Water (all engines running)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 5 10 15 20

Speed of Ship (knots/hr)

Heat(kW)

Diagram 4: Exhaust Heat in HT Motor Water versus Speed of Ship (typical)

Diagram 5 shows how 3 Diesel Engines can be combined with one evaporator via acommon cooling water system. For calculation of the net utilisable motor water flowthe motor water temperatures at engine outlet as well as behind engine cooler haveto be considered.

The resulting design requirements for the Fresh Water Generators are as follows:

QDesign = 8146 kW (from 2 x 3 engines) = 4073 kW per Engine Set

Water Production = 2 x 600 = 1200 t/d

Resulting QSPEC = 162.9 kWh/t

Motor Water Temperature: TMOT = 90 C (indivudually controlled in each engine)

Motor Water return Temperature: TMOT-RETURN = 73 C

Motor Water Flow at design point): FLOWMOT = 206 t/h (*) per engine set !

(*) to be calculated as follows (FLOWMOT = QMOT * 3.6 / 4.18 / (TMOT TMOT-RETURN)

Remarks:

It has to be noted that full heat utilisation is only maintained in case of the abovegiven motor water return temperature of 73 C is low enough for safe cooling of theengines. Some types of engines especially such with low motor water flow rates

through the engines require lower inlet temperatures. In such cases additional cool-ing takes place in the engine cooler system, the utilisable motor water flow drops,and a certain portion of the available exhaust heat thus can not be recovered !



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Diesel Engine 1

Heat Exchanger

Diesel Engine 2

Diesel Engine 3

Engine Cooler

Seawater

Seawater return

TIC

M

TIC

M

TIC

M

M

TIC

Constant Flow,

variable Temperature

Diagram 5: Control of Motor Water Temperatures

(1 Engine set with 3 engines)

Constant Temperature,variable Flow

Battery Limit

Please note: In case of the setpoint ofthis controller has to be set lower thanthe motor water return temperaturefron the evaporator, a certain portionof heat gets lost in the engine cooler !!!


2004 water maker thermal efficiency

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