Cooling Water System – Onboard

  • Suman Paul 
  • 8 min read

The vessels out at sea needs to be very self-contained during the span of her voyage. She needs to produce her own power in order to carry out various operations onboard a vessel. The most vital operation with top notch standards of precision is “Propulsion”. The large sized engines burn fuel oils to converts the chemical energy stored in the fuel into heat energy which in turn converts into rotational form of energy to drive the propeller. The keyword, “HEAT”, needs to be dissipated as all the heat generated is not useful. If not done so effectively it would damage various machinery on board a ship.

water cooling system

Introduction

Heat generation is part and parcel of various systems in ships. The excess or the unused heat is usually carried away using an especially dedicated system called “Cooling Water System”. As the name suggests, the above mentioned system uses water as the coolant or the cooling medium as seawater is in abundance out there at sea. The basic components of the cooling water system onboard includes:

  • Sea chest
  • Strainers
  • Main seawater pump
  • Heat exchangers or coolers
  • Various valves
cooling water system
Fig. 1 Shows the basic block diagram of a cooling water system.

SEA CHEST: It is a reservoir or can be defined as the intake from which seawater is drawn to be supplied in the pipelines.

STRAINERS: They are used to prevent the entry of pollutants such as plastics from entering the supply line.

MAIN SEAWATER PUMP: This pump is used to draw seawater from the sea chest into the supply line and also to circulate the seawater through the entire cooling water system.

HEAT EXCHANGER: They are used to cool the hot fluid in order to draw the heat from them, as a result, the hot fluid’s temperature drops. Heat exchangers are devices which work on the principle that heat flows from a substance at higher temperature to a substance at low temperature. Heat exchangers used are basically of two types:

  • Shell and Tube heat exchangers
  • Plate type heat exchangers
Fig. 2 Shows a Shell & Tube Heat Exchanger
Fig. 3 Shows a Plate Type Heat Exchangers

VALVES: Several valves are used to regulate the flow of fluid in the different section of the cooling system such as

  • Temperature Control valve
  • Globe control valve
  • Butterfly valve etc.

Types of Cooling Water System on board:

  1. Direct Seawater Cooling System
  2. Centralized Cooling System

Direct Seawater Cooling System:

In Direct Seawater Cooling System, seawater is directly fed into each and every heat exchanger in the system. Basically, the seawater, drawn from the sea chest using the main seawater pump, is made to pass through various heat exchangers such as the lube oil cooler, the jacket water cooler, the main engine air cooler and various other coolers used in the auxiliary machinery which is a part of the cooling system. After extracting heat from the hot fluids, the seawater exits the heat exchangers or coolers and gets overboard into the sea.

Fig.4 Shows the flow of sequence of Direct Seawater Cooling System

Advantages of Direct Seawater Cooling System:

  • Less complex design
  • Installation cost is low

Disadvantages of Direct Seawater Cooling System:

  • Frequent maintenance required, high running cost
  • Seawater is highly corrosive, need to deal with corrosion very frequently
  • In case of any leak, fluids get contaminated because of seawater

Centralized Cooling System:

In Centralized Cooling System, seawater along with fresh water is used to extract the extra or the unused heat from the hot fluids. In such systems, an extra component known as the CENTRAL COOLER is also installed. Instead of making the seawater to circulate in the entire system as in Direct Seawater Cooling System, the seawater is made to pass only through the central cooler and the fresh water circulates in a close loop within the system. The fresh water is made to pass through every heat exchanger in the system where it draws heat from the respective hot fluids and since it flows in a closed circuit, it is passed through the central cooler. Eventually, the central cooler is now used to transfer the extracted heat from the fresh water to the seawater. The seawater outlet from the central cooler is then dumped into the ocean. The fresh water used in the closed circuit is provided by a Fresh water Expansion tank which is also used to refill the fresh water when required.

Fig. 5 Shows the flow of sequence of the Centralized Cooling System

Advantages of Centralized Cooling System:

  • Less corrosion in the pipelines
  • Less Maintenance cost

Disadvantages of Centralized Cooling System:

  • Initial installation cost is more
  • Complex design

Features of the Cooling Water System:

  1. Jacket Cooling Water System/ High Temperature Line
  2. Low Temperature Line

Jacket Cooling Water System/ High Temperature Line: The Jacket Cooling Water System is a closed loop circuit which mainly comprise the cooling of the jacket water of the main engine. The temperatures involved in this system are quite high. The fresh water flows in the jackets, if the vessel is fitted with central cooling system, around the cylinders to draw the excessive heat from the main engine cylinders when they burn fuel. After absorbing the heat, the fresh water transfers the heat energy to the seawater in the central cooler. Some additional features included in this system are:

  • Jacket Water Pumps: They are used to draw water from the expansion tanks to supply it to the jackets around the main engine cylinders.
  • Pre-Heater: Pre-Heaters are used to maintain the jacket water temperature especially when the heat production is not very high or when the engine is running at slow speed. They are also used to keep the main engine warmed up while the engine is stopped.
Fig.6 Shows the simple block diagram of the Jacket Cooling Water System/ High Temperature Line (in case of central cooing system).

Low Temperature Line: Low Temperature Line or Low Temperature Circuit also uses closed loops in the case of ships fitted with Centralized cooling system. A low temperature line is used for cooling small machinery or to extract heat from machinery with low temperature range. The fresh water is made to pass through various heat exchangers or coolers of various auxiliary machines in the system. After the fresh water has extracted heat from every heat exchanger in the system, it is passed through the central cooler also known as the Low Temperature Water Cooler. Eventually, the seawater after absorbing the heat from the fresh water will be dumped out into the sea. Low Temperature System can be used for:

  • Lubricating Oil coolers
  • Air compressor intercoolers
  • Main engine and auxiliary machine air coolers
  • Various auxiliary machines’ coolers
Fig. 7 Shows the simple block diagram of the Low Temperature Line (in case of central cooing system).

Conclusion

From the above study it is very much significant that there are several safety measures on board a vessel such as the cooling water system. The cooling water system enhances the level of safety by extracting the excessive heat from the machinery which, if not done, could lead to hazardous situations. Thus, Cooling water system is yet the best and the most efficient way to prevent the damages which may occur due to heat.

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