OPEN COOLING SYSTEM
An open system means that the liquid that is used to carry heat away from the engine is drawn directly from the water in which the boat or ship operates. This liquid is moved through the system and then discharged overboard. In the open system, there is no freshwater circuit.
The open cooling system is not used on most marine diesel engines for several reasons. The most important reason is that the open cooling system exposes the engine to scale formation, marine growth, and dirt deposits in the piping. You may, however, find the open cooling system in use on some small gasoline engines, such as out-board motors and P-250 fire-fighting pumps. On engines such as these, you must flush the cooling system with fresh water to remove any traces of seawater that might cause corrosion and fouling. For the most part, our discussion of cooling systems throughout this chapter will be limited to closed cooling systems.
THE CLOSED COOLING SYSTEM
A cooling system is classified as closed if it has a freshwater circuit (system) that is self-contained and used continuously for the cooling of the engine. Closed cooling systems are normally operated at pressures greater than atmospheric pressure so that the boiling point of the coolant is raised to a temperature that is higher than 212°F.
Cooling of an internal-combustion engine is accomplished by the use of either a cooler (heat exchanger), keel cooler, or radiator and fan. We will continue our discussion of the closed type of cooling system with a general description of these three types of closed systems. We will then take a more detailed look at the components that make up the closed type of cooling system.
Heat Exchanger Cooling System
The heat exchanger cooling system combines two separate cooling systems-a jacket-water (freshwater) system and the raw-water (seawater) cooling system. The principal components that comprise the freshwater system are an engine coolant pump, one side of the heat exchanger, and the expansion tank and piping.
In the jacket-water (freshwater) cooling circuit, the fresh water is reused continuously for cooling the engine. The order of the parts through which water flows in the freshwater circuit of a cooling system is not always the same. In the majority of installations, however, the coolant is circulated throughout the engine cooling spaces by an attached circulating freshwater pump. The direction of flow of engine coolant must always be such that the temperature of the fresh water will increase gradually as the fresh water passes through the engine. So that thermal shock of the hottest parts of the engine can be prevented, the coolant is directed to flow around the cylinder liners and cylinder head last before it is directed out of the engine. The coolant then flows to a freshwater cooler (heat exchanger) where it is cooled by the seawater cooling circuit. After it leaves the cooler, the fresh water may or may not, depending on the installation, go through the lubricating oil cooler to act as a cooling agent for the lubricating oil. The coolant finally returns to the suction side of the freshwater pump, completing the circuit.
The seawater circuit of the heat exchanger cooling system consists of a centrifugal pump (usually similar to the freshwater pump). On most small engines, the seawater pump is attached. However, on large engines that are used for propulsion or power generation, the seawater pump is a motor-driven pump remote from the engine. The seawater cooling system for a large engine is often equipped with some means for providing emergency cooling water, such as from the firemain system. The centrifugal pump draws seawater through a sea chest, strainer; and sea valves. The pump then discharges the seawater through the freshwater cooler (heat exchanger). (In some installations, an additional strainer is located in the pump discharge.) From the freshwater cooler, the seawater is discharged over-board. The overboard discharge performs varying functions, depending on the individual installation. Normally, it serves to cool the engine exhaust piping and the silencer.
On some engine-generator units, the attached seawater pump furnishes seawater to the generator air coolers as well as to the freshwater coolers and returns the water to the overboard discharge. Throttling valves, or orifice plates, which are frequently placed in lines of the seawater cooling system and the outlet of a generator air cooler, serve to control the flow rate of the water that passes through these heat exchangers. For prevention of scale formations on heat transfer surfaces, the temperature must not exceed 130°F with a minimum flow rate to reduce the effects of erosion.
Figures 7-1 and 7-2 illustrate the freshwater and seawater systems of an engine that employs the heat exchanger type of cooling system. As shown in these illustrations, an aftercooler is part of this system because the engine is a turbocharged unit. In the system in figure 7-1, part of the coolant flows through the cooler and is directed back to the lower passage of the flywheel housing. This coolant is then directed to the cylinder banks on each side of the cylinder block. The coolant (fresh water) circulates around the cylinder block, around the liners, and upward through the cylinder heads to the exhaust manifold. (See directional arrows.) From the exhaust manifold, the coolant flows to the expansion tank where temperature regulators control the flow of coolant to the heat exchangers. When the temperature of the coolant is not high enough to open the regulators, coolant bypasses the heat exchangers to ensure quick warm-ups. A portion of the coolant is bypassed at all times.
In the jacket-water system we have just described (fig. 7-1), note the heater. (The jacket-water heater is also commonly referred to as the keep-warm heater.) This device is used on some engines prior to starting to preheat the engine by heating the jacket water.
In the seawater circuit shown in figure 7-2, the seawater is drawn through the sea chest to a strainer by the seawater pump. The pump, through piping, directs the flow to the aftercooler for cooling the air and to the heat exchanger for cooling the fresh water. (See directional arrows.)
Figure 7-1.-Jacket-water (heat exchanger) cooling system.
Figure 7-2.-Seawater cooling system.