Plate Type Heat Exchanger
Heat exchanger transfer heat from one fluid to another. They can be coolers, heaters, condensers or evaporators. All these types of heat exchangers are used on board ships. Some of them are main engine jacket water cooler, auxiliary engine jacket water cooler, lubricating oil cooler, central fresh water cooler, camshaft lube oil cooler, jacket water preheater, main engine fuel oil heater, auxiliary engine fuel oil heater, refrigeration gas condenser, a/c condenser, atmospheric condenser or dump steam condenser and freshwater generator evaporator.
Heat exchangers used in ships are of two types; plate type and shell and tube type. Here we discuss about the former one. Principle of plate type heat exchanger is simple. Consider three metal plates pressed together with a gap in between them. When we group three plates we have two spaces in between. These two spaces are separated by one central plate. When we keep liquids of two different temperature in two spaces, heat transfer occurs from one space to the other through the central plate.
In a plate type heat exchanger, a series of corrugated plates are pressed together to form a compact structure. These corrugations are of chevron patterns. Since the plates are very thin, these corrugations provide support as well as increases surface area for better heat transfer. Also chevron pattern offers a turbulent flow rather than laminar. All these factors together increases heat transfer efficiency. Turbulent flow causes breaking up of boundary layer thereby allowing mixing up of liquid to increase heat transfer. But turbulence can give rise to erosion problems to the plates. Hence suitable materials to be used for plate manufacturing.
Normally titanium is the plate material used for manufacture. This is very expensive, but has excellent corrosion and erosion resistance properties. Aluminium-brass also used for manufacture, but were failed to provide useful service life. Also aluminium brass has a threshold limit for the liquid flow velocity, beyond which its erosion resistance drastically reduces. Apart form this stainless steel plates also found not suitable for all types of applications. Titanium is successfully experimented and are used widely which has advantage over all other materials. But providing optimum turbulence and flow velocity increases the service life of the titanium plates.
'Nitrile Rubber' is used as the joint material between two plates. This is bonded firmly to one plate with suitable bonding adhesive such as 'Plibond' or high temperature adhesive like compressed asbestos fiber (CAF). Plate heat exchanger to be used only for designed temperatures and pressures given by the manufacturer. Using in high temperatures than rated causes the 'Nitrile Rubber' to get hardened and loosing its elastic properties. Also higher temperature reduces the effect of bonding between the plates and nitrile rubber if proper bonding adhesive is not used. Plates are compressed together in between two end plates, using clamping bolts.
Loosely packed plates cause leakage of the liquids. Similarly over tightening than desired increases pressure drop at the heat exchanger and damages to the chevron corrugations. Before dismantling for cleaning and inspection, measure the length of clamping bolts from end plates and is recorded. The cooler to be boxed back with same measurements. Fluid inlet and outlet connections are made with the end plate. Maintenance of the plate type cooler is very easy can be cleaned without removing plates. Care to be taken not to mix up the plates.
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