Two types of heat exchanger are used to cool hydraulic oil: (1) shell-and-tube and (2) finned tube. The shell-and-tube (Fig. 8.1) has a series of tubes inside a closed cylinder. The oil flows through the small tubes, and the fluid receiving the heat (typically water) flows around the small tubes. Routing of the oil can be done to produce a single pass (oil enters one end and exits the other end) or a double pass (oil enters one end, makes a u-turn at the other end, and travels back to exit at the same end it entered).
The finned tube exchanger (Fig. 8.2) is used for oil-to-air exchange. The air may be forced through the exchanger with a fan or may flow naturally. If an oil cooler is used on a mobile machine, it is the finned tube type.
Oil coolers are not built to withstand pressure; they are mounted in the return line in an off-line loop. The two options used are shown in Figs. 8.3a and 8.3b. In Fig. 8.3a, the system pump flows oil through the heat exchanger in the return line. This arrangement works well for many circuits. The exchanger is sized to give only a small pressure drop at rated flow. The circuit shown in Fig. 8.3b has a separate low-pressure pump to flow oil through the heat exchanger.
More complex circuits can have significant pressure pulses in the return line. These pulses hammer the heat exchanger and, over time, the joints fracture and begin to leak. If significant (greater than 10 psi) pulses are measured in the return line, the circuit shown in Fig. 8.3b should be used. Here, a separate pump is used to circulate oil from the reservoir through the heat exchanger and back to the reservoir. This circulating pump does not have to build pressure (only the 15 psi or so is required to flow fluid through the exchanger); therefore, it can be an inexpensive design. Any kind of pump is satisfactory if it is rated for the needed flow rate and has seals that are compatible with the fluid properties.
