Wednesday, December 5, 2018

The Top Four Reasons Why Cartridge Heaters Fail

Cartridge heaters fail because either the heat generated in the internal resistance wire is not efficiently dissipated or moisture (or a foreign substance) seeps inside the protective sheath, creating a short circuit.

Inadequate heat dissipation results in an elevated internal temperature, which can rapidly breakdown the heating element. Inadequate heat dissipation occurs for several reasons: when machined tolerances are outside of an accepted range (improper fit); if the watt density is too high; or when powered by too high a supply voltage.

1) Loose Fit


Loose fit is the most common cause of premature cartridge heater failure. The bore hole within which they are inserted must be held to tight tolerances. High watt density cartridge heaters are even more sensitive as the internal temperature of the heater can rise rapidly and jeopardize the life of the heating element. To ensure adequate thermal dissipation, the recommended hole diameter is no more than 0.002 in. greater than the nominal diameter of the heater.

Typical allowable watt densities for swaged cartridge heaters are based on fit and operating temperature.
Watt density graph
Max. allowable watt density vs. fit. Click image for larger view.
Graph courtesy of Backer Hotwatt.

2) Too High a Watt Density


The watt density of the heater is vital to its performance. This is a measure of thermal power density and the higher the watt density, the greater the needs are for thermal dissipation. High watt densities can lead to premature failure when thermal dissipation needs are not met, as the internal temperature of the heater will exceed the limits of the resistive heating element.

3) Too High a Supply Voltage


In a resistive circuit, since the resistance is fixed, when the voltage is doubled, the current doubles as well as quadrupling the wattage output. Incorrectly specifying the supply voltage can lead to premature heater failure, as voltage has a dramatic effect on the wattage and the amount of heat generated as can be seen in the following formula.
Heater voltage calculation
Voltage calculation. Courtesy of Backer Hotwatt.

4) Moisture or Contaminant Ingress


Even when cartridge heaters feature heliarc-welded end caps, they are prone to failure when the air surrounding the heater contains impurities or has a high moisture content and the heater’s leads are not adequately sealed. This is due to the nature of MgO insulation: it is a highly hydroscopic white powdered mineral, and when the heater undergoes thermal cycling a vacuum is created, drawing in moisture or other contaminants such as oil, which can result in internal shorting.

Watt Density Selection and Thermal Cycling


Suggested watt density is based on several factors including the fluid medium to be heated, the desired operating temperature and process variables such as flow rate. In general, operating temperature is inversely related to the suggested watt density. Additional considerations are taken when heating a fluid to a point near its boiling point, as phase changes drastically reduce its heat transfer capabilities. Highly viscous fluids or fluids that tend to coke or carbonize also require a low watt density. Highly corrosive solutions also need a low watt density, as the increased watt density increases the potential for corrosion, drastically reducing the life of the heater’s sheath.

Selecting an incorrect watt density can have adverse effects to the response of a thermal system, but it is not the only factor to consider. There are four basic elements to any thermal system, including the thermal load, the heat source, the heat transfer device and the temperature controller.

Thermal power delivered by a heating element is a function of wattage, and a correctly sized heating element will provide an ideal thermal response without rapid cycling of the element. The optimal wattage results in a 50/50 off/on cycle, which prevents or minimizes hunting or temperature overshooting. For more precise thermal control, variable voltage devices or solid-state controllers may be used.

For more information on properly applying cartridge heaters, contact BCE by calling 510-274-1990 or by visiting the cartridge heater section of the BCE website.

Monday, November 26, 2018

Electric In-line Clean Fluid Heater Provides On-demand Heat in a Small Package

clean fluid, flow-through heater
Clean fluid, flow-through heating system.
Many original equipment manufacturers require precise temperature control and a very compact package to heat clean fluids. Equipment examples are semiconductor gas processing equipment, kidney dialysis (hemodialysis) machines, process gas analyzers, ink preheating systems, photoresist coating equipment, and parts cleaning equipment.

Electric heaters used in these applications must be compact, lightweight, and made of materials that won't contaminate samples. They also must be fast responding and provide large amounts of power when required.

BCE, a Northern California manufacturer of custom electric heating elements, developed their "Mini Clean Flow" heater for these types of applications. The Mini Clean Flow is a very compact, fast responding electric heating element designed for applications where the heating of clean fluids is required, most often in the semiconductor, medical, and laboratory equipment industries. Designed with high power ratings wrapped in a small package, these specialized heaters offer ultra-fast heat-up and precise, accurate temperature control.

Mini Clean Flow heaters combine inlet and outlet connections along with a baffled stainless steel enclosure, creating a turbulent flow pattern for efficient heat transfer. Sealed resistance heaters are used to isolate process fluids from having contact with the elements directly. Internal thermocouples provide for outlet temperature regulation as well as for maximum sheath temperature control.  The Mini Clean Flow's advanced mechanical design, along with its high power density and overall low mass, provides the end-user with a very efficient and precise heating solution.

For more information, contact BCE.
Phone: 510-274-1990
Web: https://bcemfg.com/minicleanflow




Tuesday, November 13, 2018

Consistent, Repeatable and Efficient Results from a 300mm Semiconductor Chuck Heater with Mirror Finish

300mm Hot Chuck Heater
300mm Hot Chuck Heater with Mirror Finish
Semiconductor fabrication involves numerous processes, materials, and specialized equipment. The base semiconductor material from which integrated circuits and microchips are manufactured comes in the form of round, thin crystalline disks referred to as wafers. During semiconductor production, wafer temperature uniformity is one of the most critical factors affecting product quality and consistency.

To increase the speed at which a physical or chemical reaction takes place, heat is applied at various stages of the process. Heat is applied through the use of heated chucks which provide precise thermal uniformity which, as mentioned above, is critical for production.

Semiconductor Chuck Heater
Click image for larger view.
To summarize, the goal of the heated chuck is to provide a very high level of temperature uniformity across the entire wafer surface so that processing output is highly consistent, repeatable and efficient.

BCE, a manufacturer of custom electric heaters and thermal systems was asked by a customer

to replace a cast aluminum heater plagued with surface finish problems. Poor surface finish equates to poor thermal uniformity and heat transfer. To add insult to injury, the vendor was quoting extremely long lead-times for replacements.

Using their broad experience in semiconductor chuck heater design, BCE knew immediately the best solution would be a vacuum brazed-in 6061 T6 aluminum heater with a 2-3 Ra μin surface.

Specification to be met:
  • Temperature uniformity +/- 1%
  • 350°C - 450°C Temperature range
  • Surface finish of 2-3 Ra μin
  • Hard coat Anodize
  • 6061 T6 Aluminum
  • Be able to heat ceramic top plate
  •  20 Meg-ohm isolation at 1000 VDC
  • Hi-pot 2E + 1K at 3mA
Results

BCE designed a chuck heater using their own proprietary internal element patterns, notches, and thermocouple holes, as well as adding additional manufacturing processes to provide the 2-3 Ra μin finish. Vacuum integrity for the customer has been greatly increased as well as contact with the ceramic workpiece and temperature uniformity.

For more information, contact BCE by calling 510-274-1990 or through visiting their website at https://bcemfg.com.



Saturday, October 27, 2018

Important Safety and Performance Precautions When Using Electric Heating Elements

Important Safety and Performance for  Electric Heating ElementsThe safety and performance of electric heating elements is dependent upon the user's proper handling, installation, control, application, and maintenance. While it is impossible to anticipate all the operating conditions for electric heaters, the following items are universal precautions that must be considered in every situation.

Electric heater element handling, installation, application, and maintenance precautions:
  1. Always have a qualified person install the heating element in accordance with the National Electrical Code and/or local codes.
  2. Always use extension wire rated for the current, voltage, and exposure temperatures suitable for the application.
  3. Always use the proper environmentally rated electrical connection and housing for the type of  service the heater will see.
  4. Use temperature controlling and/or limiting devices with electric heaters.
  5. Use ground fault protection where required.
  6. Do not apply higher voltages than the marking on the heater indicates.
  7. Do not operate heaters in thermally insulated conditions where sheath temperatures may exceed the recommended maximum.
  8. Do not expose heaters to conditions, substances or contaminants that can damage, change, or destroy the integrity of the heater's sheath or electrical insulation.
  9. Heaters by their nature can absorb moisture which can cause high leakage current. A megohm test to the manufacturers specification should be performed to ensure moisture levels are within acceptable standards.
  10. Do not apply heaters with operating sheath temperatures that exceed the safe exposure temperature of the process media.

Thursday, October 18, 2018

Understanding Vacuum

Understanding Vacuum
At sea level, the earth's atmosphere exerts a standard pressure upon us of 14.7 pounds per square inch absolute (PSIA), or 29.92" of mercury (Hg), or 760 mm of mercury (Torr). All of these values refer to "standard atmosphere" which is measured at sea level.

Vacuum is a term used to describe an area, zone, or compartment containing less pressure than atmospheric pressure.

Vacuum is measured in inches of mercury (Hg) in the United States. There are two different scales to measure vacuum.

One scale is referred to as "inches of mercury gauge vacuum" ("HgV), where the measuring range starts at 0 inches of mercury (atmospheric pressure) and goes up to 29.92 inches of mercury, known as perfect vacuum (not achievable on Earth).

The alternative scale is "inches of mercury absolute vacuum" ("HgA), which reverses the "HgV scale, having it instead read 29.92 inches of mercury at atmospheric pressure and 0 inches of mercury at perfect vacuum.

Here's an example of the relationship between inches of mercury gauge and inches of mercury absolute:

24 inches of Hg gauge = 5.92 inches of mercury absolute.
Calculation: 29.92 - 24 = 5.92

It is very important to  determine what scale you are using, meaning gauge or absolute. A misunderstanding can result in large errors.

The unit "Torr" is used when working in higher vacuum ranges (low absolute pressure). 1 Torr equals 1 millimeter (mm) of mercury in absolute pressure.

Considering one linear inch equals 25.4 mm, and atmospheric pressure (at sea level) is 29.92 inches of mercury, then using the equation 29.92 inches x 25.4 mm = 760 Torr. 760 Torr is 0 vacuum, while 0 Torr is perfect vacuum.

BCE is a leading manufacturer of electrical, fiber optic, and tubing feedthroughs for use in vacuum applications. Visit https://bcemfg.com or call (510) 274-1990 to learn more.

Tuesday, October 16, 2018

BCE In-house Heater and Feedthrough Machining Capabilities

BCE, a California manufacturer of electric heating elements and vacuum feedthroughs, has extensive in-house machining capabilities providing BCE a competitive advantage with customized products and quick prototype turnaround.

BCE produces machined components for their specialized cartridge heaters, vacuum feedthroughs, and custom thermal assemblies. A good example of their in-house capabilities is their complete machining process for wafer chuck heaters used in the semiconductor industry. BCE machines the complete assembly, with grooved patterns to accommodate interference fit, vacuum brazed, or welded-in heaters and / or cooling tubes. BCE can both deliver fast prototypes as well as handle production manufacturing in both standard and exotic materials.

https://bcemfg.com
(510) 274-1990