Heat exchanger types and calculations on ship

Heat exchanger for ship

Types of Industrial Heat Exchangers

There are many types of heat exchangers used in a wide variety of industrial applications including power plants, boilers for industrial steam, chemical plants, and many types of manufacturing facilities. Several different configurations are used to accomplish the basic function of transferring heat from one fluid to another without mixing the two fluids together. The types of heat exchangers described and discussed in these articles include the two most common industrial heat exchangers, the shell and tube heat exchanger and the double pipe heat exchanger, along with information about spiral heat exchangers, flat plate heat exchangers, fin-tube heat exchangers, and condensers.

Heat Transfer Coefficients and Heat Transfer Fundamentals

Convection heat transfer coefficients and log mean temperature differences are needed for heat exchanger design calculations. Natural convection heat transfer coefficients are calculated using a correlation of dimensionless numbers, typically Nusselt number in terms of Grashof number and Prandtl number. Forced Convection heat transfer coefficients are calculated from correlations of Nusselt number in terms of Reynolds number and Prandtl number. These articles provide information, equations and example calculations for natural convection and forced convection heat transfer coefficients, overall heat transfer coefficients and log mean temperature differences. There is also information about heat exchanger flow patterns and equations and calculations for combined convection and conduction heat transfer.

Heat Exchanger Design – Theory and Calculations

Heat exchanger design is an iterative process. A preliminary value for the required heat transfer area can be calculated using an estimate of the overall heat transfer coefficient together with information about the required rate of heat transfer, the flow rates of the two fluids, and the inlet and outlet fluid temperatures. Preliminary details of the heat exchanger can then be calculated, including items such as diameter, length, and number of tubes for a shell and tube heat exchanger, diameter and length of the shell, number of tubeside passes, and number of shellside passes. The allowable pressure drop across the heat exchanger can also be calculated and used in the preliminary design. With a preliminary detailed design completed, a better estimate of convective heat transfer coefficients and the overall heat transfer coefficient can be made and adjustments can then be made to the preliminary detailed design as necessary. The use of Excel spreadsheets for the preliminary design calculations is included in some of these articles.

Heat Exchangers on Ships

Heat exchangers are used on ships in the power production process, such as for diesel engines, boilers, and economizers, as well as in production of steam for accommodation and deck equipment. The heat exchangers used include common types used as industrial heat exchangers, such as the shell and tube heat exchanger, the plate heat exchanger, and the fin tube (radiator) type.

Heat Exchangers in Everyday Life

Although most people are not aware of them, heat exchangers are very prevalent all around us in our everyday life. For example, heat exchangers are crucial to operation of furnaces, air conditioning systems, refrigerators and freezers. Automobiles require a heat exchanger in the form of the radiator to provide cooling for the engine and a smaller one for heating of the passenger cabin. High performance engines use heat exchangers in turbochargers.

Types of heat exchangers used on ships

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.

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.

Shell and Tube Type Heat Exchanger.

Shell and Tube Type Heat Exchanger
Shell and Tube Type Heat Exchanger

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.

Plate type heat exchanger
Plate type 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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