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Etching a Thermal Solution

Fri, 11/10/2006 - 10:24am

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The Project: Replace a wire heating system that was creating multiple problems, including system failure, in a hybridization oven.

The Solution: Utilize etched foil elements to enable an efficient and effective heating solution.


By Kevin Mooney
Stovall Life Science Inc., a manufacturer of laboratory instruments for the life sciences market, was developing a new hybridization oven for use in research and development by molecular biologists working on DNA (deoxy-ribonucleic acid) and RNA (ribonucleic acid) applications. The company’s early oven concepts utilized a standard, coiled-resistance wire heating system. A major problem developed when the tightly wound spirals of heat-concentrated wire kept failing. Compounding this reliability issue was the fact that the wire coil acted like a spring. When the element broke, it would touch an electrically conductive component within the oven. Not only would the heater short out, but the reliability failure became a hazard to the oven’s operator. The company approached Thermal Circuits of Salem, Massachusetts to help diagnose the cause of the problem and develop a solution.

The first order of business was to do a root cause analysis to determine what was causing the reliability problem. Thermal Circuits engineers examined the failed units and determined the standard open-wire element was not sufficient for the job. The existing wire element was running in excess of its thermal limitation. To correct the situation, Thermal Circuits recommended an etched-foil element.


The advantages of etched foil heaters

Etched-foil elements are able to operate at very high densities due to the larger surface area covered by the foil track. The element is created by acid etching a circuit in resistance alloy foil. This process ensures excellent circuit pattern repeatability and allows the design of complex heat distribution patterns within the heater area. By not confining the wattage to tightly spiraled coils, the etched-foil element heating system runs cooler and lasts longer, still with the same power. Etched-foil elements also provide faster heat transfer resulting in longer heater life in high power applications.

  Etched-Foil Heaters
in Medical Devices

While hybridization ovens present very specific heating challenges, heating assemblies play a critical role in a variety of medical instruments. Highly sensitive applications, such as blood analyzers, dialysis instrumentation, sterilizers, and incubators demand precise solutions to heating requirements.

Utilizing etched-foil heaters in these devices provide the inherent advantage of distributing wattage infinitely and exactly, making the units more reliable. By eliminating wire failure, devices are safer, run cooler, and last longer. Etched-foil heaters are available with a variety of dielectric encapsulations, to meet varying temperature requirements, as well as provide solutions for applications requiring low outgassing and resistance to radiation, chemicals, and solvents.
 
With shorter conductors covering a larger surface area of the element, the design of the etched-foil element inherently reduces the stress on the element itself, making it less likely to break and increasing reliability. In addition, the etched-foil element adds an extra level of safety because the wire is covered with dielectric insulation, thereby, not allowing the foil element to short out under any condition.Thermal Circuits engineers also determined that performance, reliability, and safety in this application would further improve if the new element was packaged as a forced convection assembly. The new forced convection packaging was implemented by retrofitting two concentric cylinders to the hybridization oven’s existing muffin fan. Specifically, the smaller inner cylinder was constructed by forming the heating element itself into a tube. The outer, larger cylinder has a dual nature. It directs the flow of air, similar to ductwork, and provides a dielectric shell for added safety.

Thermal Circuits final recommendation was to incorporate a thermal fuse as a fail-safe measure to enable UL recognition of the hybridization oven. This added feature protects the entire hybridization oven should any other component, such as the fan or temperature-controlling system, fail or should the airflow be restricted. The fuse was strategically located to trip in a fault condition, causing the unit to shut off before the oven reaches a temperature outside its normal operating range. This measure greatly enhanced the safety of the unit for a minimal cost.By solving a reliability problem with a more thermally responsive and safer design, Thermal Circuits’ solution yielded a design improvement that gave its customer a sustainable competitive advantage—significantly increased speed of heating and cooling. While other incubators could take up to 45 minutes to heat from ambient to 65°C, this new oven took as little as 5 to 15 minutes, depending on thermal load.
ONLINE
For additional information on the technologies and products discussed in this article, see the following websites:
  • www.slscience.com
  • www.thermalcircuits.com


  • Kevin Mooney, applications engineering manager, is responsible for managing new applications for all product-type and geographic markets within Thermal Circuits Inc. Previously, he managed the company’s quality assurance and design engineering departments, where he honed his design and process failure mode effects analysis (D and P FMEA) skills. Kevin holds a BSME from Villanova Univ., as well as an MBA from Boston College. He can be reached at 800-808-4328 or mooney@thermalcircuits.com.
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