Thermal Management Techniques for Medical and Laboratory Equipment

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI  January 1998 Column

THERMAL MANAGEMENT

Choosing the right thermal control element helps ensure accurate functioning of complex electronic, imaging, and processing systems.

Equipment engineers seeking to develop faster and more accurate medical and laboratory products are beginning to recognize the importance of proper thermal management. The speed or accuracy of sensitive electronic devices such as microprocessors and lasers can be affected by thermal conditions, and cooling generally has a positive effect on equipment reliability. Chemical reaction rates are proportional to temperature, and the working time or shelf life of a biological sample or laboratory reagent can be increased by keeping the substance at an optimal temperature. Instruments such as DNA cyclers, tunable laser diodes, and thermal-stress analyzers all require a capacity for cycling an object or sample through a range of temperatures with speed and precision.

Heat sinks can be used with or without fans and offer considerable installation flexibility, but they cannot cool components below ambient temperature. Photo courtesy of Melcor Corp. (Trenton, NJ)

There are many different tools and methods for transferring heat. Which method is best depends on the temperatures and tolerances of the application. Simpler devices might function well enough with passive cooling elements such as heat sinks, while devices that operate in more demanding environments might require an active cooling method such as a compressor-based or thermoelectric system. A fan, for example, can be used to remove the heat generated inside an electronics cabinet. If the cabinet is sealed, a heat sink or heat pipe is needed. If the cabinet's temperature must be controlled, a heat pump or air conditioner is indicated. The best design will be determined by system needs and limitations. System needs would address the amount of heat to be added or removed to achieve the required temperature. Limitations might involve space, cost, allowable vibration, and available power. Once these factors are defined, the thermal engineering choices become apparent. The following is an overview of the thermal management technologies readily available to the engineer, listed from the simplest to the most sophisticated.

FANS AND BLOWERS

Fans operate by passing air over a hot component, absorbing the component's heat. Overall cooling effectiveness is determined by the air's flow rate and temperature together with the component's size and output. Typically, fans and fan trays are used in cabinets for bulk cooling of electronics. Fans and blowers are relatively inexpensive and provide a high measure of flexibility in installation. On the other hand, the constant exchange of air raises the potential for contamination from dust and moisture. Moreover, fans and blowers can prove ineffective for high-power devices and cannot cool an object at or below ambient temperature.

HEAT SINKS

Generally, heat sinks are made from aluminum because of the metal's relatively high thermal conductivity and low cost. They are either extruded, stamped, bonded, cast, or machined to achieve a shape that will maximize surface area, facilitating the absorption of heat by the surrounding cooler air. Most have a fin or pin design. When used with fans (forced convection), heat sinks can dissipate large amounts of heat while keeping the targeted components at 10°—15°C above ambient temperature. Heat sinks without fans (free convection) result in a higher component temperature because of the decreased impingement of air. Like fans, heat sinks are inexpensive and offer installation flexibility but cannot cool components at or below ambient temperature. Also, heat sinks do not permit temperature control.

Thermoelectric coolers made from semiconductor pairs between ceramic plates (Melcor Corp., Trenton, NJ).

LIQUID COLD PLATES

Liquid cold plates are typically made from copper, aluminum, or aluminum-clad copper tubing. Heat is absorbed by a liquid pumped through the plate, which is attached directly to the object being cooled. In an open-loop system, the liquid (usually tap water) runs through the plate and out through a drain. In a closed-loop system, a pump recirculates the liquid through a heat exchanger or radiator. Liquid cold plates are characterized by a small size (at point of attachment), and they offer effective heat dissipation. The devices are limited by the fact that they cannot cool below ambient (liquid) temperature and permit no temperature control. The potential for leakage is also a concern, and the availability of liquid sources can sometimes pose a problem.

The best design will be determined by system needs, such as the amount of heat to be removed, and limitations, such as space, cost, power, and permissible vibration.

HEAT PIPES

A heat pipe is a sealed vessel that transfers heat by the evaporation and condensation of an internal working fluid. Ammonia, water, acetone, or methanol are typically used, although special fluids are used for cryogenic and high-temperature applications. As heat is absorbed at the evaporator, the working fluid is vaporized, creating a pressure gradient within the heat pipe. The vapor is forced to flow to the cooler end of the pipe, where it condenses, giving up its latent heat to the ambient environment. The condensed working fluid returns to the evaporator via gravity or capillary action within the wick structure. Because heat pipes exploit the latent heat effects of the working fluid, they can be designed to keep a component near ambient conditions. Though they are most effective when the condensed fluid is working with gravity, heat pipes can work in any orientation. Using forced air at the condenser allows for larger amounts of heat dissipation. Heat pumps are typically small and highly reliable, but they can't cool objects below ambient temperature and do not permit temperature control.

COMPRESSOR-BASED COOLING

Compressor-based cooling systems, found in commercial refrigerators and air conditioners, contain three fundamental parts: an evaporator, a compressor, and a condenser. In the evaporator, pressurized refrigerant is allowed to expand, boil, and evaporate, absorbing heat as it changes from a liquid to a gas. The compressor acts as the refrigerant pump and recompresses the gas to a liquid. The condenser expels the heat absorbed (along with the heat produced during compression) into the ambient environment. Compressor-based refrigeration is effective for large heat loads (300 W or more) and can cool components far below ambient temperature. The technique also allows users to control temperature. These refrigerators must be used in their designed orientation, which limits installation flexibility. Maintenance and reliability are also compromised by moving parts. Compressor-based systems also tend to be bulky and noisy.

THERMOELECTRIC COOLERS

Thermoelectric coolers (TECs) are solid-state heat pumps made from semiconductor materials. They have no moving parts but comprise a series of p-type and n-type semiconductor pairs or junctions sandwiched between ceramic plates. Heat is absorbed by electrons at the cold junction as they pass from a low energy level in a p-type element to a higher energy level in an n-type element. At the hot junction, energy is expelled to a heat sink as the electrons move from the high-energy n-type element to a low-energy p-type element. A dc power supply provides the energy to move the electrons through the system. A typical TEC will contain up to 127 junctions and can pump as much as 120 W of heat. The amount of heat pumped is proportional to the amount of current flowing through the TEC; therefore, tight temperature control (<0.01°C) is possible. By reversing the current, TECs can function as heaters, which can be useful in controlling an object in changing ambient environments or in cycling at different temperatures. Sizes range from 2 to 62 mm, and multiple TECs can be used for greater cooling. Because of the relatively large amount of heat being pumped over a small area, TECs require a heat sink to dissipate the heat into the ambient environment. The modular units can be used in any orientation and are compatible with heat sinks, cold plates, and heat pipes. On the down side, TECs require a dc power source and are more expensive than passive components.

THERMAL COMPOUNDS

When mounting a cooling device to a component or assembling a cooling system, designers must select a thermal bonding material that will allow heat to flow out of the device with minimal resistance. Designers should take into account mechanical stresses at the interfaces caused by differing material coefficients of thermal expansion. The idea is to eliminate any air pockets between the two surfaces.

The most common interface material is thermal grease, typically made from zinc oxide in a silicon or petroleum base. There are also pastes available with thermal conductors such as aluminum oxide and aluminum nitride. Pads and foils are less messy to apply and can be cut to match the component footprint. Some pads are available with adhesive surfaces to allow permanent attachment. Aluminum oxide and aluminum nitride are used in thermal pads, as are sheets made from graphite, indium, and aluminum.

Thermal epoxies create rigid, permanent bonds. They are typically supplied as two-part systems comprising a hardener and a resin filled with silver, aluminum, aluminum oxide, or aluminum nitride. Because they are permanent, epoxies should be used only in areas that will not require future disassembly. Rigid bonds can also be achieved using solder. Eutectic and noneutectic formulations are available for use in a wide temperature range. Soldered surfaces offer a good rigid thermal joint and can be disassembled by simply reflowing the solder. As with all rigid joints, the effects of differing thermal expansions should be considered.

CONCLUSION

High-performance electronics, sensitive imaging equipment, and sample-processing systems all require proper thermal control to ensure accuracy and functionality. Design engineers need to identify temperature-sensitive components in order to create an integrated system with parts that are both compatible and economical. They should do this early in the design process; the sooner thermal limitations are identified, the more flexibility the engineer has in choosing from the available options. In the final analysis, retrofitting thermal products is usually not as effective and economical as generating a solid thermal design from day one.

Robert Smythe is vice president of sales and marketing at Melcor Corp. (Trenton, NJ).

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