All engine cooling systems have an attached freshwater pump. Some installations also have a detached auxiliary pump. The attached pump serves to keep the water circulating through the cooling system. Since attached pumps are engine driven, it is impossible for cooling water to be circulated in the engine after the engine has been stopped or in the event the attached pump fails. For this reason, some engines are equipped with electric-driven (detached) auxiliary pumps, which may be used if either the freshwater pump or the seawater pump fails. An auxiliary pump may also be used as an after-cooling pump, when an engine has been secured.

The pumps used in the freshwater and seawater circuits of an engine cooling system may or may not be of the same type. In some systems, the pumps in both circuits are identical. In other systems, where pumps are of the same type but where variations exist, the principal differences between the pumps of the two circuits are in size and capacity. In the cooling system of some engines, the seawater pump has a capacity almost double that of the freshwater pump.

Centrifugal pumps are the principal types of pumps used in engine cooling systems. On some engines, a rotary type of pump in which the impeller has flexible vanes is used in the seawater circuit. The basic principles of operation of these types of pumps are discussed in Fireman, NAVEDTRA 10520 (latest edition).

Centrifugal pumps are more common in engine cooling systems, particularly in large diesel engines, than pumps of other types. In centrifugal pumps, water is drawn into the center of the impeller and thrown at high velocity into the casing surrounding the impeller where the velocity decreases and the pressure increases correspondingly. Sealing devices, usually of the mechanical seal type, are provided to prevent leakage of water, oil, grease, or air around the impeller shaft. Pump location and method of drive will vary according to engine design. Additional information on pumps can be found in chapter 13 of this manual.

COOLERS (HEAT EXCHANGERS)

As a Fireman, you learned that devices that transfer heat from one fluid to another are called HEAT EXCHANGERS. You also learned that these devices are used as either heaters or coolers and that the same device may be used for both purposes. In internal-combustion engines, heat exchangers are used primarily for cooling. For this reason, the devices in engines that remove heat from a hot fluid (liquid or gas) by transferring the heat to a cooler fluid are commonly referred to as COOLERS.

Fluids Cooled

The primary function of heat exchangers that are used in diesel engines is to remove heat from the jacket water and the lubricating oil. On some engines, coolers are used to reduce the temperatures of engine intake air and generator cooling air and to cool engine exhaust gases. In most engine installations, the fresh water in the engine cooling system is cooled by seawater. Lubricating oil and air may be cooled by seawater or by fresh water, depending on the installation. Thus, on the basis of the fluids cooled, you will encounter freshwater coolers, lubricating oil coolers, and air coolers. All coolers operate on the same principle; coolers used in various installations and for the cooling of various fluids may differ, however, in appearance and in details of design.

Classification of Coolers

Coolers may be classified in several ways: by the relative direction of flow of the two fluids (parallel flow, counterflow, and crossflow types); by the number of times either fluid passes the other fluid (single-pass and multipass types); by the path of heat (indirect-contact or surface type and direct-contact type); and by construction features of the unit (shell-and-tube type). The coolers used in cooling systems of engines are commonly identified on the basis of construction features.