Design considerations for heat exchange systems in off-highway equipment.

Heat exchangers and other cooling systems components used in off-highway equipment have a dual role. They must provide the cooling performance required for the individual application, as well as maintain a high degree of mechanical integrity, as they are routinely subjected to shocks and stresses

Over the years, manufacturers of heat exchange systems have developed a range of designs and materials to meet the unique demands of heavy-duty off-road equipment.

Even under normal operating conditions, heat exchangers must withstand a high degree of thermal stress as the system cycles to meet cooling demands. Areas of particular stresses include where the light core sections are attached to the large headers that are used to collect and distribute cooling fluids. The thinner metal used in the core sections expands and contracts to a greater degree than the thicker material used in the header, which can lead to failure at the joining of the two sections.

Nested fin cores, a patented technology developed by Karmazin Products Corp., are designed to minimize thermal cycling problems, as two core tubes are connected to a single header opening through a manifold. This strategy provides a gradual mass increase from the lighter core to the heavier header.

The cores consist of a double wall approximately 0.015 in. thick, attached to the manifold bottom. The manifold section, which is manufactured from 0.03 in. thick steel, is mechanically clinched and copper brazed to the manifold top, which is constructed of 0.06 in. thick steel. That section is then copper brazed into headers that can range from 0.060 to 0.3 in. thick.

While the gradual increase in mass allows the unit to be much more resistant to thermal cycling stresses, that is not the only advantage the design offers. By bringing two core tubes into a single header opening, the straight in-line forces that may be encountered with conventional designs are dramatically reduced. The offset arrangement in the manifold also allows the manifold to act as a shock absorber, bending slightly at the end of each core tube, minimizing thermal stress concentrations and substantially extending the service life of the unit.

Beside thermal cycles, pressure cycles are also a major contributor to heat exchanger failure. Pressure cycles are normally generated by system pumps and are not damaging when they are considered in the design process. When they are generated by circuit conditions that create rapid flow changes which convert dynamic energy flow into instantaneous pressure energy, water hammer failures can occur.

Light core sections expand under internal pressure, while rectangular or oval shaped sections try to round out. The greatest area of potential failure is at the headers, where the lighter core material is prevented from rounding out. One way to reduce or eliminate such problems is through the use of round core tubes, which are better able to distribute pressure stresses in a uniform manner.

Another important consideration in heat exchanger design for off-highway equipment is structural resistance to vibration. This varies depending on the vehicle type, and the manufacturer must take into account the presence of mechanical vibration and inertial loads in multiple axes, all of which can result from the equipment traveling on uneven terrain in loaded and unloaded conditions.

The use of isolation mounts can minimize some of these mechanical stresses, but the fundamental method for ensuring a particular heat exchanger system design will operate in the equipment environment is through thorough field testing at the prototype stage.

There are a number of other factors that impact the viability of heat exchanger system components in equipment. Erosion is a good example. Erosion can occur in vehicles using blower fans with minimal clearances between the fan blades and the heat exchanger core. The acceleration of particles by the fan blades creates a "sandblasting" effect, which can literally wear core sections away.

There are several strategies that can be used to reduce erosion. The distance between the fan and heat exchanger core can be increased and a heavy gauge screen or grid assembly can be placed between the blower and the core to physically impede the travel of airborne particles while not inhibiting airflow significantly. Finally, stronger, more wear resistant components, such as steel fins and tubes on the leading face or all-steel heat exchangers, can be used.

Another kind of erosion can be the result of strong concentrations of acids, bases or other caustic substances in the air or soil where the machine is operating. This type of erosion can be addressed through the use of corrosion resistant coatings.

For example, Karmazin produces steel, copper and aluminum heat exchangers which are designed for high reliability in heavy erosion applications. The corrosion protection process incorporates a special metal cleaning pretreatment, followed by an epoxy-based coating, along with painting and paint curing. This E-coat process is designed to provide a tough, hard, uniform epoxy coating over a fine grain zinc phosphate base which is intended to minimize the spread of corrosion should the coating be penetrated. This process has also been found to be resistant to most chemical environments.

In applications such as agricultural, earthmoving and forestry, air side fouling can be an issue. While heat exchangers can perform well in wind tunnel testing, they can still foul rapidly in actual operating conditions. In such applications, cores designed to minimize air side fouling and facilitate easy cleaning, should be considered.

Nested fin cores, designed with nominal 0.375 in. i.d. round in-line tubes or 1 in. centers with six or eight fins per inch core fin densities, have proven resistant to air side fouling when compared to oval or flat tube cores on 0.5 in. tube centers. The core's steel fins and brazed core design also reduce the thermal decay that can occur when air side cleaning processes that can loosen the fin-to-tube bonds or damage fins, creating higher air resistance.

Most recently, attention has been paid to the development of non-ferrous materials for nested fin heat exchangers to provide higher cooling efficiencies and greater options for the equipment manufacturer. Other strategies, such as the incorporation of integral bypass valves on oil coolers to allow improved cold weather performance, are also being studied. These and other innovations are likely to become more commonplace as cooling system manufacturers respond to new demands for efficiency and reliability.

Dana Dent is director of marketing & sales and Ivan Woodhull is vice president, engineering at Karmazin Products Corp., a heat exchange equipment manufacturer in Wyandotte, Mich.