Heat Exchangers

Unwanted heat is a problem for all hydraulic systems. Even a well-designed system operating at top efficiency converts about 20% of its input power to heat. An inefficient system, or one poorly matched to its task, may convert nearly 100% of input power to heat at certain times in the cycle. The heat input can be dissipated through natural cooling; if this is insufficient, a heat exchanger is added to the system. draw heat from the hydraulic fluid and transmit it into a cooling fluid, usually water. Most liquid-to-liquid exchangers use a shell-and-tube package, consisting of a bundle of small tubes inserted into a shell. The coolant flows through the small tubes, and the hydraulic fluid passes around and between the tubes. These units are compact, reliable, and are often less expensive to install and maintain than other types. There are two basic types of shell-and-tube exchangers: the U-tube (or hairpin) type and the straight-tube type. Either type can have either a fixed or removable tube bundle. Removable bundles can be withdrawn from the shell as an assembly for maintenance, but fixed bundles must remain in the shell. A normal rule of thumb is that U-tube exchangers are best suited for high-temperature, high-pressure applications, with the straight-tube units most thermally efficient and least expensive. Liquid-to-liquid exchangers are available in single or multiple-pass, parallel, or reverse-flow arrangements. The multiple-pass, reverse-flow units provide the greatest heat transfer for a given size. Standard liquid-to-liquid units have a working pressure of 150 psi and can handle temperatures to 300°F, although the actual temperature difference between oil and water should not exceed 200°F. Special units are available to operate at pressures to 300 psi. transfer heat from hydraulic fluid to ambient air. Working much like an automobile radiator, they allow air to be passed over finned