By W. John Bilski, Senior Engineer, Thermacore, Lancaster, PA
Selecting the right heat exchanger depends on the application and how and where it is used.
Medical devices require reliable and effective thermal solutions to control the heat that results from electronics enclosed in confined spaces. Incorporating heat exchangers into the design of medical devices and equipment can boost these instruments’ accuracy, longevity, and efficiency.
Heat exchangers are typically only used for sealed enclosures or devices. If the enclosure does not need to be sealed, though, a fan circulating ambient air through it will usually cool the device better and less expensively than a heat exchanger. Most medical devices, however, are sealed to maintain cleanliness and minimize contamination.
Heat exchangers are typically rated NEMA 12 for their ability to prevent dust from getting into the enclosure, or NEMA 4 for preventing liquid and dust intrusion into the enclosure. They can be a suitable thermal management choice for medical devices.
In this extruded heat sink, the fin spacing is relatively close, so it is designed for forced convection (requires a fan) and would be difficult to manually clean and sterilize.
They typically work by circulating the hot air inside the device or enclosure over some form of extended surface area, such as internal fins, and circulating cooler external air over external fins. As a result, the inside temperature will always be warmer than the outside temperature. If the device must be kept cooler than the outside ambient temperature, something like an air conditioner or Peltier device (thermoelectric cooler) must be used.
Heat exchange vendors use several types of fins to transfer the heat from within the electronic enclosure to the outside ambient air for rejection, including folded fin impingement cores, double-sided extrusions or heat sinks, and passive heat pipe technology.
The simplest heat exchanger is a single-sided extrusion with fins on only one side mounted through the wall of the enclosure. The hot components are mounted directly to the heat sink base. The heat conducts through the base plate of the extrusion out of the enclosure to the fins. The cool external air then removes the heat from the fins either through natural convection or through forced convection through a fan.
Double-sided extrusion type heat exchangers which include fins on both sides, transfer the heat from the inside air to internal fins. The heat travels from the inside half of the extrusion by conduction; for aluminum, the thermal conductivity is 180-200 W/m-°K. Outside air then circulates over the outer fins to remove the heat. Because heat is taken from the internal air and transported to the external air, these heat exchangers are called “air-to-air” heat exchangers.
Some extruded heat sinks have larger spacing between the fins. This example is designed for natural convection but adding a fan would increase the performance. It obviously would be easier to manually clean. For both extrusions, the electronic components that are generating the heat are mounted directly to the base of the extrusion.
The double-sided impingement heat exchanger uses a folded fin core that separates the enclosure inside and outside. A set of inside fans draws in the hotter, inside air and blows it toward the fin core. This internal impingement efficiently transfers the heat to the fin core. Similarly, a set of outside fans draws in the cooler, ambient air and blows it toward the outer side of the fin core removing the waste heat. This type of air-to-air heat exchanger moves the heat by conduction as well. The difference between a double-sided extrusion type of heat exchanger and an impingement style heat exchanger is that instead of conducting the heat inches away (from an internal fin, through the center plate, to an external fin), the impingement heat exchanger conducts the heat only a few thousandths of an inch (through the thickness of the aluminum used to make the folded fin). Heat pipe-based air-to-air heat exchangers also use fins to absorb (internally) and dissipate (externally) the heat from an enclosure. The difference is that the heat pipe transports the heat from the internal fins to the external fins.
Consider key components
Various components work together in air-to-air heat exchangers to effectively cool medical devices. All heat exchangers need a steady air supply. Components that generate a greater amount of heat need a larger supply of air and often an attached heat sink to cool them. The heat exchanger’s fans often provide the air to cool the individual components. Ducting may help channel the air toward the exterior of the component or device. Once the heat is dissipated by the component to the air stream, the heat must be removed from the air by the heat exchanger.
Heat pipes are filled with a small quantity of working fluid (water, acetone, nitrogen, methanol, ammonia, and sodium for example). Heat is absorbed by vaporizing the working fluid in the heat pipe. The vapor transports heat to a cooler (condenser) region where the condensed vapor releases its latent heat of vaporization. The condensed working fluid is returned to the evaporator by gravity or by the heat pipe’s capillary wick structure. Heat pipes have a higher effective thermal conductivity (often exceeding 10,000 W/m-°K) enabling them to transfer heat more efficiently and evenly than through pure conduction. They are totally passive heat transfer systems, having no moving parts to wear out and requiring no energy to operate. Highly versatile, heat pipes can transport heat anywhere from a few inches to a few feet. Likewise, they can be designed to work with gravity, against gravity, or in gravity-neutral orientations.
The HX-400 heat exchanger has 10-20 fins/in. It is representative of a typical enclosure cooling heat pipe heat exchanger. The fin pitch for any heat exchanger is optimized by taking into account the fan’s performance as well as the resistance to air flow of the enclosure. The need to clean or sterilize the fins may necessitate a fin pitch that is less than the optimum.
Tips for incorporating heat exchangers
When it comes to incorporating heat exchangers into medical device designs, keep these tips in mind.
• Collaborate closely with customers and inform them of any FDA or other regulatory restrictions that affect the design of the device.
• Consider thermal management as early as possible in the development process. If the device reaches the testing stage before a cooling solution is developed, the most effective product may not fit in the space allotted or it may be too expensive.
• If the design will require a fan, make sure the power supply of the device can accommodate the additional power needed for the fan.
• Consider appropriate materials for the application. For example, heat pipes are normally copper, which can be toxic to the human body or can corrode in certain environments. In most medical applications, customers ask designers to use gold or nickel plating on heat pipes to preserve conductivity and improve patient safety as well as to resist environments and cleaners that might be corrosive to copper.
• Consider sterilization needs. Heat exchangers with heat pipes will not be able to handle certain forms of thermal sterilization (for example, heating the devices in an autoclave). However, there are a number of sterilization alternatives for heat exchangers with heat pipes, including radiation and chemical solutions (alcohols, ethylene oxide, bleach, and so on). Also, heat exchangers that cool high heat-generating components feature closely spaced fins, which can be difficult to clean between uses. Changing the design of the heat exchanger to widen the space between the fins can ensure that the device can be more easily sterilized; however, this can increase the size and cost of the exchanger.
Meeting application specs
Single-sided extrusions work best when there are just a few components generating sufficient heat to require cooling and they can be mounted directly to the base of the extrusion. If the heat generated is small and the thermal requirements are not stringent, the extrusion may be cooled by natural convection. If a significant amount of heat must be dissipated and the heat sink cannot be large enough for natural convection cooling, a fan may be added to the external side of the extrusion to produce forced convection.
Double-sided extrusion heat exchangers would be the next step up in sophistication and cost. They are well suited for small devices that do not generate much heat and do not have stringent thermal requirements. This type of heat exchanger can handle the heat output from many devices but will not have the thermal performance of the more sophisticated types of heat exchangers.
Impingement and heat pipe styles of heat exchangers can be effective and efficient. The type of heat exchanger chosen is usually based more on the application. Although exceptions certainly exist for all of these generalities, heat pipe heat exchangers usually extend further into the cabinet, which makes it easier to transport the cool air back to the electronics. If there is minimal room inside the cabinet, impingement type heat exchangers tend to intrude into the cabinet less but they require a larger footprint on the enclosure surface.
If the medical device will be used in a clean room environment, heat exchangers with fans may give rise to too much dust and particles into the air. In this case natural convection cooling may be the only option. If a large amount of heat must be dissipated, the heat exchanger will be relatively large and pure conduction methods will be inefficient. Heat pipes, with their highly effective conductivity, will make the large, natural convection external half of the heat exchanger much more efficient.