Heat exchangers on ships
Heat exchangers on ships are important as heating and cooling of liquids and gases are essential on all ships. Examples of this include cooling the main and auxiliary engines, heating water for domestic purposes, conditioning fuels and in the case of vessels fitted with waste heat recovery systems for power production.
Heat exchange is also needed in air conditioning systems so as to circulate air at the required temperature to ensure maximum comfort. For all of these purposes some form of heat exchangers are needed. Heat exchangers are not always referred to by that name with some being referred to by function such as boilers and condensers.
While they can be complex pieces of equipment, heat exchangers are merely a means of benefitting from one of the basic principles of physics whereby energy in the form of heat always flows from a hotter medium to a cooler medium.
In the heat exchangers, the two mediums are brought into indirect contact using tubes or fins as appropriate. The greater the common surface area between the two mediums, the more efficient the heat transfer between them will be.
In a ship the two mediums are generally liquids or gases but the sources can be very diverse. For example, the cooling system of the main engine involves the fresh water circulating inside the engine block coming into contact with sea water at ambient temperature passing through the heat exchanger of the engine cooler and returning to the engine to continue to remove the heat generated during the combustion process.
Obviously both the mediums need to be moving and there are various ways this can be done producing flow patterns that provide the most efficient heat transfer for a specific purpose. In a parallel flow model both mediums enter the exchanger at the same point and flow in the same direction, in an opposite flow the mediums enter at different ends and flow past each other horizontally, a cross flow is similar but the mediums move in a perpendicular direction.
As well as the method of operation, heat exchangers can be referred to by construction. Most common on board ships are shell and tube or plate. In a shell and tube, one medium flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa.
The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use. Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangers. A boiler in a steam turbine is a heat exchanger and once the steam exits the turbine it enters a condenser heat exchanger so that the water condensate can be recirculated in the system.
A plate heat exchanger uses metal plates to transfer heat between the fluids. This allows the fluids to be exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change. The high heat transfer efficiency for such a small physical size is useful on board ships especially for cooling main and auxiliary engines.
Plate and fin type heat exchangers are similar to a plate type exchanger but also contains fins to increase the efficiency of the system. 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.
A specialist heat exchanger that is found in the air conditioning and ventilation systems of some ships is the enthalpy wheel which allows both heat energy and moisture energy to be exchanged. Enthalpy wheels are usually made of porous materials to increase surface area which aids in energy transfer.
In most cases a matrix core material is coated with a desiccant such as silica gel or other molecular sieves to increase latent transfer.
The enthalpy wheel is rotated between the incoming fresh air and the exhaust air. Heat and moisture are given up to the wheel. When the space is in the heating mode, the heat and desirable humidity is used to pre-condition the incoming, cold, dry air. In the cooling mode, the incoming air is pre-cooled and dehumidified.
Because the cost to remove moisture can represent 30 to 50% of the cost to condition air, substantial additional savings are available with enthalpy wheels over conventional air-to-air exchangers.