Heat Ex-changers and its types used on ship...!!

Heat Ex- changers on Ship Explained

Heat exchanger is an equipment which reduces the temperature of a medium by transferring temperature of that medium to another, when both the mediums are separated by a solid membrane or wall like structure. For efficient operation, the surface area of the wall which separates the two mediums is maximized, simultaneously minimizing the flow resistance of the fluid.

Exchanging of heat in a heat exchanger can be in between- liquid and liquid, gas and liquid, liquid and gas etc. For heat transfer basically three patterns of flow are used for construction of a heat exchanger.

Opposite flow : Primary medium (to be cooled) and secondary medium (which is cooling the primary medium) enters in the heat exchanger in opposite direction to each other.

Cross flow : Primary and secondary medium enters in an exchanger perpendicular to each other.

Parallel flow : Primary and secondary medium both enter the heat exchanger parallel to each other.

 Where are Heat Exchangers Used Onboard Ship?

Each and every system in a ship is interlinked with each other in some way or the other. Even if one system fails, vessel can come to a standstill.

Heat exchanger plays an important role for efficient working of different systems, which include-

1) Propulsion Plant:

–  Main propulsion plant consists of different sub system for running of main engine like lube oil system, jacket water system (open or closed system), fuel system etc.

–  While generating energy all these systems gets heated up and the temperatures are controlled by the use of heat exchanger in the system.

–   Heat exchangers normally used in main propulsion system are – Shell and tube type and plate type heat exchanger.

2) Auxiliary Power Generation System:

– Auxiliary power generation system is similar to the main propulsion system, except that the power is generated in terms of output.

– Shell and tube type, plate type and plate fin type heat exchanger are generally used

3) Starting Air System:

– High pressure air is produced in the compressor which is further cooled in inter-cooler, which acts as a heat exchanger

–  Shell and tube type exchanger is popularly used for this purpose.

4) Fuel injection system:

–  For proper atomization, fuel is heated up in a heater with heating medium as steam. Shell and tube type heater is used for this purpose.

–  For reducing Sox emission from propulsion plant, combustible fuel is mixed with spray of water, for this direct contact heat exchanger is used.

5) Refrigeration System:

–   In refrigeration system, for meat room, fish room and vegetable room, evaporator acts as phase change heat exchanger.

–    Shell and tube type unit is used for condenser unit in refrigeration system.

6)  A.C system:

–   For maintaining the temperature, phase change heat exchanger unit is installed as evaporator.

–   Condenser unit is normally shell and tube type exchanger.

7)  Fresh Water System:

–   For generating fresh water, sea water condenser and jacket water evaporator is used. Both are types of heat exchangers.

–    Shell and tube type and plate type heat exchangers are normally used for this.

8)  Steam Turbine Unit:

–   If a ship consists of stem turbine or turbine generator, normally spiral heat exchanger is used for heat transfer.

Apart from the above mentioned places, there are several other applications where heat exchangers are effectively used for smooth running of the ship.

Types of Heat Ex-changers on a Ship

Different types of heat exchangers are used on board a ship. The type of heat exchange used for a particular usage depends on the application and requirement. In this article we talk about different types of heat exchangers used on a ship.

The types of exchangers are mainly defined by their construction and are as follows:

1)      Shell and Tube Type Heat Exchanger

This is the most popular type design with a shell accompanying several tubes and the flow of liquid to be cooled is mainly through tubes, whereas the secondary liquid flows over the tube inside shell.

Shell and tube type heat exchanger is extremely economical to install and easy to clean; however the frequency of maintenance is higher than other types.

2)      Plate Type Heat Exchanger

Plate type exchanger consists of thin corrugated plates joined parallel together, creating cavity for fluid flow inside it. Alternate sides of the plate carries two different fluids, between which, heat transfer is carried out.

Installation of this type of heat exchanger is expensive than shell and tube type, but maintenance cost is much lower.

Efficiency of plate type is higher than shell and tube type for same size of unit and can withstand high pressure.

3)      Plate Fin Heat Exchanger

Plate and fin type heat exchanger is constructed similar to a plate type exchanger but also contains fins to increase the efficiency of the system. Aluminium alloy is used as it gives higher heat transfer efficiency and lowers the weight of the unit.

Fins can be fixed in perpendicular to the direction of flow and are known as offset fins. Fins fixed in parallel to the direction of flow are straight fins. Fins can be fixed in curvature form to increase the heat exchanging effect and are thus known as wavy fins. Efficiency of this heat exchanger is slightly higher than plate type unit but installation and maintenance cost is higher

4)      Dynamic Scrapped Surface Heat Exchanger

In this heat exchanger because of the continuous scrapping of the surface long running time is achieved which helps in better heat transfer efficiency and decrease in the fouling of the system.

The scrapping is done by a blade unit operated by a motor driven shaft with timer moving inside the frame. This heat exchanger is normally used for heat transfer of highly viscous fluid by increasing the turbulence of the fluid. Maintenance cost is less as compare to other types because of the auto cleaning process.

5)      Phase Change Heat Exchanger

As the name suggests, this type of heat exchanger is used to change the phase of a medium from solid to liquid or liquid to gas by principle of heat transfer. This type is normally operated in freeze cycle and melts cycle for change in phase to happen.

The heat exchanger is normally constructed like a shell and tube type exchanger, but consists of at least two divider walls to construct upper and lower annular space for flow passage. It also consists of fins in both passage ways for efficient heat transfer.

6)     Spiral Heat Exchanger

This type of heat exchanger consists of concentric shape flow passages which help in creating a turbulence flow of a fluid which in turns increases the heat transfer efficiency. Initial installation cost is higher but highly efficient as compare to other types as space saving is much more because of the compact size. Maintenance cost is lowest as compare to other types for the same size of the unit.

The Flow of fluid in spiral type is rotary current flow which itself possesses the property of self cleaning of fouling inside the spiral body.

7)      Direct Contact Heat Exchanger

In this type of heat exchanger, there is no separating wall inside the unit. Both the mediums are in direct contact for heat transfer process.

Direct contact type heat exchangers can be further classified as

1.      Gas – liquid

2.      Immiscible liquid- liquid

3.      Solid- liquid or solid- gas

References

Marine aux machinery by H.D.McGeorge