Machine Design

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.

Liquid-to-liquid exchangers 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 Removable bundles can be withdrawn from the shell as an assembly for maintenance, but fixed bundles must remain in the shell.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.

Liquid-to-air heat exchangers transfer heat from hydraulic fluid to ambient air. Working much like an automobile radiator, they allow air to be passed over finned tubes containing the hot liquid. The finned tubes can be made of aluminum, copper, steel, or stainless steel, and are brazed or roller expanded to the header tank. Air is moved through the core by forced or induced-draft fans.

Air-cooled exchangers are most commonly used where water is costly or unavailable in sufficient quantities to dissipate the required heat, or where a portable heat exchanger is required. In some instances, they have been used to help supply plant heating requirements during winter months. These liquid-to-air heat exchangers are available in sizes to 100 hp, operating at pressures to 300 psi. Units up to 600 hp are available on special order.

Typically, liquid-to-air exchangers are larger, heavier, and noisier than liquid-to-liquid units. In return, they operate without necessity for water and they are portable. They require ambient air at least 10 to 15°F below the required oil output temperature for efficient operation. The only requirement for long life is that fins must be protected from clogging and dirty environments; a single mesh (window screen) overlay avoids fin clogging and provides for easy cleaning.

The heat pipe is a high-performance thermal conductor. In its basic form, it consists of an enclosure containing a fluid that can be vaporized and a material or structure, called a wick, that provides capillary action. In an essentially isothermal process, heat at the input, or evaporator, end causes the fluid to evaporate. Vapor travels through the center of the container to the output, or condenser, end. There the vapor condenses, giving up its latent heat, returns to a fluid state, and travels back to the evaporator end through the wick.

Although called heat pipes, not all of these devices are cylindrical. Tubular heat pipes predominate because most are fabricated from stock tubing materials. Any geometry that preserves the essential evaporation-condensation cycle can be used. Flat heat pipes are being made, and flexible heat pipes are used where the evaporator and condenser cannot be in line or where both heat-pipe ends oscillate independently.

Pipe materials are usually metal, such as copper or aluminum, but for some electrical applications they can be made from a dielectric material.

Working fluid depends on the temperature range over which the heat pipe is to operate, and the amount of heat to be moved. For cryogenic applications, hydrogen, which operates between -259 and -248°C, may be used. For high temperatures, sodium, lithium, and silver may be the working fluids. Sodium, for example, functions in the 550 to 1,000°C range. Most commonly used tubes operated in the 0 to 500°C range using water, one of the Freons, or a variety of organic fluids such as Dowtherm A.

The wicking material or structure performs two functions: channeling liquid through the tube, and wetting the tube interior. The wick design used depends on the working fluid, the tube orientation and desired heat flow. Slits or spirals, cut or formed into the tube wall may be sufficient in some cases. Sometimes, special wicking material, such as sintered metal, mesh, or various types of cloth, are necessary.

Location: All heat exchangers should be installed in the low-pressure side of a hydraulic circuit. This location eliminates the need for a high-pressure unit, which may be comparatively expensive. Heat exchangers should be protected against damage from high-pressure surges by a relief valve.

For large hydraulic systems operating at high pressures, a separate cooling circuit from a reservoir to the heat exchanger may be used to circulate the oil independent of changing flows in the main circuit.

For systems that are to be used outside, a system bypass line should be provided around the heat exchanger. Such a bypass line permits efficient year-round operation; the heat exchanger can be bypassed during cold weather starting until fluid has reached the operating temperature. Such a bypass line also permits maintenance of the heat exchanger without shutdown of the hydraulic system.

Experts recommend that line filters be installed upstream of the unit to protect the exchanger from excessive accumulations of dirt and scale, which can degrade thermal efficiency.

Protection: Generally, the colder the chilling fluid (air or water), the more heat will be removed from the oil -- up to a point. At temperatures below a certain level, the fluid may be too cold for efficient operation of the exchanger. Most hydraulic fluids tend to form viscous layers on contact with an extremely cold surface, and this stagnant fluid can create a thermal barrier within the heat exchanger. The colder the chilling fluid, the thicker is the viscous layer in the hydraulic fluid. For example, a 1-in. layer of noncirculating fluid has the insulating quality of a … -in.-thick layer of rock-wool insulation. Therefore, the heat-transfer capability of an exchanger under cold-weather operation may be improved by restricting the temperature or supply of the chilling fluid.

Clean, soft water should be used in water-type heat exchangers to prevent corrosion and scaling in the tube bundle. If the only available water is hard (with excessive minerals) or brackish (with excessive salt), scale and dirt deposits can form in the small-diameter tubes. These deposits cut heat efficiency. Where fouling is possible, low cooling-fluid velocities should be avoided.

Another potential hazard to cooling efficiency is tube corrosion. Corrosion restricts coolant flow and can eventually perforate the tubes, permitting water to pollute the hydraulic fluid. Experts recommend use of a zinc anode or chemical inhibitor in the cooling water circuit to prevent or reduce this deterioration in the tube bundle.

Doubling up: If the flow requirements of the hydraulic system exceed the capacity of standard heat exchangers, two smaller standard units of equal capacity can be connected in parallel to dissipate the heating load. In this arrangement, the thermal load is shared equally by each small exchanger at a total system cost much less than a single large unit custom built for a system.

Many hydraulic circuits generate high thermal loads for relatively short periods. During normal operation, normal loads can be carried away by a standard liquid-air exchanger but, under extreme operating conditions, a small liquid-to-liquid unit is required to carry the additional load. In such circuits, the liquid-to-liquid exchanger is installed in series with, but downstream of, the liquid-to-air unit. With a bypass line around it, the water-type exchanger is held in standby during normal system operation. Again, this "ganged" arrangement is simpler and more efficient than use of a single, very large exchanger capable of handling peak loads.

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