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What is heat exchanger?
Heat exchanger is an equipment, where heat is transferred from one fluid to another. A heat exchanger can be used for exchanging heat between liquid and gas, gas and gas, liquid and liquid etc. You can see small heat exchanger in your CPU just below the fan, radiator of your car and condenser of your refrigerator are also examples of heat exchanger.
What types of heat exchangers are normally used in ships?
Large size engines (normally diesel engines) are used in ships for propulsion and electricity generation. These large engines are having different systems, like water system, lube oil system, fuel oil system, starting air system etc. Different types of heat exchangers play important roles for controlling temperatures of these different systems.
Two types of heat exchangers most commonly used in ships are:
· Shell and tube heat exchanger
· Plate type heat exchanger
Shell and tube heat exchanger
As the name implies, this types of heat exchangers are made up of large numbers of tubes and a shell, covering the bundles of tubes. Shell and tube heat exchangers are useful for pressurized application. Heat is transferred between the fluid in the tubes and the fluid inside the shell. Design of the tube bundle is such that, all the entry sides of the tube could be connected to a common header and similarly all the exit sides of the tubes also form another common header.
Single pass heat exchanger: If the fluids get only one chance to exchange heat, then it is called single pass heat exchanger. Single pass heat exchangers are constructed normally by using straight tubes, where fluid enters from one side of the heat exchangers and exits from the other side.
Multi pass heat exchanger: If the design of heat exchanger is such that the fluids get more than one chance to exchange heat, then this types of heat exchangers are called multi pass heat exchanger. The shape of the tubes of multi pass heat exchangers is like “U" or multiple of “U", mans, the U shape tubes are for two pass heat exchangers as numbers of parallel sides increase, the number of pass will also increase.
Single phase heat exchangers: The heat exchangers with only one phase at each side are called single phase heat exchangers. In single phase heat exchangers, phase changes don’t take place. Lube oil cooler of ship is a single phase heat exchanger, where lube is cooled by sea water.
Two phase heat exchanger: If more than one phase of fluids exists in any side of a heat exchanger, then that is called two phase heat exchangers. Condenser is an example of two phase heat exchanger.
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Plate type heat exchanger (PHE)
Instead of tubes and shell, PHE are made up of piling corrugated plates together. These plates are the results of sophisticated modern manufacturing techniques. Two corrugated plates jointed together creates cavity to flow fluid inside the plates, alternate sides of the plate contains two different fluids, between which the heat needs to be transferred. Efficiency of PHE is higher than shell and tube heat exchangers. The approach temperature for PHE is around 1-2 degree centigrade, while that of shell and tube heat exchanger is 5-6 degree centigrade. Assembling and dismantling of PHE is easier and in case of repairing; only defected plate could be replaced.
The image below shows the picture of one such heat exchanger.
On the other hand, shell and tube heat exchangers scores at pressure withstanding capability, shell and tube heat exchanger can with stand more pressure compared to PHE.
Heat exchangers are basically used for exchanging heat between two fluids. Shell and tube heat exchangers and PHE are two main categories of heat exchangers used in ships; however technology is advancing at fast speed and lot more developments are happening in this field as well.
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Image of Shell & Tube Type Heat Exchanger - Courtesy: Technical Heat-Transfer Services Incorporated
Image of Plate Type Heat Exchanger - Courtesy: DongHwa Entec Co.