Thermal Insulation in Appliances

By Thomas Rebernak, Marketing Manager, Morgan Thermal Ceramics North America

Thermal insulation is widely used to improve energy efficiency and safety in cooking appliances, and designers have routinely relied on fiberglass as their material of choice. While fiberglass meets less demanding performance requirements, makers of mid-range and high performance cooking appliances, especially those with self-cleaning cycles, have recently been turning to newer alkaline earth silicate wool (AES) materials, such as Superwool® Plus^TM insulating fiber. These materials offer significant advantages in high temperature insulation applications, including low thermal conductivity and low linear shrinkage. In addition, they are widely appreciated for their low bio-persistence, which means that there are no regulations preventing their use in domestic appliances in any region of the world.

Since the latest high temperature materials can have from 5 to 20 percent lower thermal conductivity, design engineers are able to use less insulation to achieve the same thermal performance or use material with less thickness and the same density to design a larger oven cavity.

Beyond cooking equipment, combustion chambers used to heat water, particularly those for domestic hot water heaters, pool and spa heaters, may also find the insulation characteristics of the latest AES fibers beneficial. Changes in the regulatory climate surrounding efficiency standards and growing global regulations restricting the use of refractory ceramic fibers (RCF) are making use of high temperature AES fibers an option that should be considered in the future.

The basics of thermal insulation in cooking appliances

In cooking appliances, thermal insulation is used predominantly in ovens and ranges, where it prevents heat loss through conduction in gas, electric and dual fuel appliances. It is also used in some electric and dual fuel cooking tops to prevent heat transfer.

In addition to providing insulation during normal oven use at temperatures from 300 to 500°F, the material used must provide protection during the pyrolitic (self-cleaning) cycle, when the oven center can reach 900 to 950°F for a period of hours while the cycle removes combustibles as it cleans the oven. There are even sections of the oven that can get hotter, depending on how close they are to the heat source.

There are several categories of insulation used for cooking appliances. In the past, many appliances relied on air as the primary insulation. Air is composed of gases that do not transfer heat very well because the molecules are so far apart from each other. The use of only air as oven insulation has been largely curtailed in many countries due to safety considerations, but it is still used in low end appliances.

The overwhelming majority of cooking appliances use fiber insulation and Figure 1 shows the wide variety of fiber insulation types available in the marketplace. Most fiber insulation for the cooking market is made of man-made vitreous (amorphous) fibers (MMVF), primarily alkaline earth silicate fibers (AES), fiberglass, glass mat, and e-glass. Varieties of products excel with different conductivity performance in different temperature ranges. The most popular MMVF insulation materials are fiberglass and AES; these materials make up about 95 percent of the market for fiber insulation.



At the high temperatures used in self-cleaning cycles, fiberglass, other e-glass needle mats and other products exceed their use limits. At temperatures of about 1,000 to 1,100°F, these materials may start to shrink, distort and change. Such shrinkage and distortion can end up changing the oven profile over a period of time.

By contrast, higher temperature AES insulation tends not to shrink at all until it reaches well into the 1,800°F range, offering a much higher safety factor. This means that the oven profile will not change over time due to the number of cleaning cycles run. AES offers better insulation in higher ranges as the temperature shifts from the convective to radiant heat transfer range. It also offers benefits of consistency in cooking performance.

Table 1 shows a comparison of AES and fiberglass with regard to a number of properties. The analysis illustrates that AES fibers out-perform fiberglass and are a better choice for most of the factors outlined, including protecting the oven during self-cleaning and shielding sensitive electronic parts from damage due to sustained heat.

Table 1 – Comparison of Most Commonly Used Cooking Appliance Insulation Products

AES has significantly lower thermal conductivity at higher temperatures

The lower the thermal conductivity of a material, the better it is at restricting the flow of energy from hot to cold. The thermal conductivity of a material is a measure of its ability to conduct energy (heat). When selecting materials for an application where a set thickness of insulation is required, the material with a lower thermal conductivity will give a greater temperature difference between the hot and cold faces and less energy loss.

New AES materials, for example Superwool® Plus insulating fiber, contain more fiber per unit, making it more efficient for the same density or thickness. By careful control of the manufacturing process, molten glass for Superwool® Plus insulation can be made to fiberize more completely and hence the ratio of shot to fiber is improved and the size of the pieces of shot is minimized. This enhances the thermal conductivity up to 20 percent, and provides about 30 percent more fibers, making the fiber more effective in restricting thermal energy transfer.

This means design engineers may be able to use less material density and accomplish the same thermal profile, or could use less thickness and the same density to achieve a larger cavity within the unit itself, without changing the external dimensions.

Figure 2 shows that a Superwool® Plus 8 pounds per cubic foot (pcf) blanket has a thermal conductivity (as measured by ASTM C-201) significantly lower than fiberglass mat at temperatures of 500°F or above. For example, heat loss was about 13 percent less when using 1-inch thick materials at 600°F. If using the material in the most extreme temperature range, designers could substitute 1-inch 6 pcf material for 1-inch 8 pcf material and get the same result for less dense insulation, or use ½-inch or ¾-inch 8 pcf material, providing a good opportunity to reduce insulation amounts, and hence costs.

 
A look to the future

In addition to cooking appliances, combustion chambers used in the secondary heating of water are another area where insulation is essential for protecting various heat-sensitive components. Designers of water heaters used in domestic hot water, pools and spas use the materials discussed above, as well as refractory ceramic fibers (RCF).

Use limits can rule out fiberglass for newer more efficient, on-demand heaters and AES fibers have become one material of choice. In addition, the use of RCF is coming under stricter control both in the European Union, where the REACH (Registration, Evaluation and Authorization of Chemicals) regulation is now taking affect, and elsewhere. In the U.S., the state of California introduced the first regulations controlling the use of RCF in 2010. AES fiber insulation, such as Superwool® Plus insulating fiber, offers a solution that is free from regulation because of the low bio-persistence demonstrated by this family of products.

Bio: Thomas Rebernak has more than 20 years of industry experience and acts as both a participant and manager of Morgan Thermal Ceramics commercial appliance design programs.