Fig. 1. Brick kiln insulated with Microtherm®


Keeping the heat inside of the furnace can speed production, save energy, reduce cost and improve product quality. The best way to keep most of the heat inside of the furnace is to use an efficient insulation material. The type of insulation chosen depends largely on the temperature inside the furnace. Industrial furnaces are, at the very least, lined with refractory brick and then backed up with an insulator such as ceramic fibers. In recent years, a more efficient insulation is being used which can improve process time, increase capacity and decrease energy consumption (and energy bills).

Improving the process of heat generation, heat transfer, heat containment and heat recovery can result in savings with typical payback periods from only one month to two years. More information on these savings as well as other energy savings ideas can be found at www.eere.energy.gov. There are several workshops offered, including process-heating assessments, which provide insight into the different aspects of evaluating energy use and efficiency in any industrial plant.

Heat Transfer

Heat lost through the furnace walls is transferred from the interior of the furnace to the exterior by three primary methods: conduction, gaseous convection and infrared radiation. The better an insulator is at blocking each of these, the more efficient the insulation. All three methods combine to give the heat-transfer effect. The ability of a material to block heat transfer is its thermal conductivity. Microporous insulation effectively blocks all types of heat transfer at temperatures up to 1200°C (2192°F) and is the most efficient insulation on the market today.

Convection

Convection of heat from the outer surface of the furnace occurs when the mass movement of molecules transfers heat. Convection is heat transfer by bulk movement within a heated fluid such as a liquid or gas. Microporous insulation is effective in blocking convection because the pores are too small to allow heat transfer by this method.

Conduction

Conduction of heat occurs when the atoms in a material are in contact with each other. Energy is passed along chemical bonds from atom to atom through the structure, leading to a transfer of energy away from the heat source. Solids are the most effective conductors of heat while gases are poor conductors of heat. Increasing the energy in a material also increases the kinetic energy of the atoms, resulting in a higher chance of conduction. Microporous material minimizes this by providing a low-density material with a high ratio of gas to solid. The core of the material is composed of amorphous materials with a low intrinsic thermal conductivity. The material particles are very small and randomly packed, resulting in long heat paths through the material. Since heat flux is inversely proportional to the distance of the path, heat transfer is reduced and the overall thermal conductivity of the material is reduced.

Fig. 2. Thermal conductivity comparison

Radiation

Radiation is the dissemination of electromagnetic energy from a source. Most heat is lost from a hot object by infrared radiation. As the temperature increases in the furnace, the effect of radiation also increases and the effects of convection and conduction become less important. While most insulation materials effectively control heat transfer by gaseous and solid conduction, they are relatively poor at blocking direct infrared radiation. As a result of this, the thermal conductivity values rise at elevated temperatures. Microporous insulation maintains its low thermal conductivity values at these temperatures by the addition of opacifiers - small particles of a metallic oxide that make the material almost opaque to heat transfer by radiation. These opacifiers reflect the heat back toward the heat source. Figure 2 shows typical thermal conductivity values of several different types of insulations.

In microporous insulation, the void volume is 90% of the total volume of the material. Because the voids are so small, the collisions between the gas molecules are almost entirely eliminated, thus controlling the effect of gaseous conduction. Effectively, each air molecule is trapped in a box and unable to interact with its neighbors. Air under these conditions has a far lower thermal conductivity than free air. This is known as the microporous effect and is the primary reason why materials such as Microtherm®, a microporous solid, has a thermal conductivity value even lower than that of still air.

Insulation Options

In choosing an insulation material, several factors must be considered. The material must reduce heat loss (resulting in energy and cost savings) as well as improve process control (resulting in a more refined final product). The material must be dimensionally stable, meaning it must not shrink at elevated temperatures. It must also be easy to handle and install and be safe to the environment and installers. It is preferable that the material be as thin as possible in order to increase capacity and/or reduce the overall footprint of the furnace. It can also be important to reduce the overall weight of the furnace. The material should also not introduce any sort of impurities into the furnace. A microporous material offers solutions in all of these areas. The savings and improvements can be seen in furnaces as well as many other industrial-heating applications.

Once the material is installed, it is imperative that it does not shrink, causing gaps between the edges. One of the key benefits of Microtherm® Super G insulation is the low percentage of shrinkage at elevated temperatures. Shrinkage has been measured to be less than 3% over a 2,000-hour usage at 1000°C (1832°F). A reduction in product shrinkage not only increases furnace efficiency, it has the added benefit of preserving the refractory brick as well as the shell of the furnace. If the heat is allowed to escape through "hot spots", it is possible that the spots in the brick and the lining will experience drastic heat flux (compared to the insulated portions) and thus result in more wear and tear, e.g. cracking brick, in those spots.

Savings

Energy costs typically represent 5 - 25% of the cost of production. By improving the efficiency of the system, a total of 10 - 30% average savings can be seen in the plant. With the rising costs of energy, labor and materials, improving the energy efficiency may be the best way to cut costs in order to keep your plant profitable. Improvements in only the heat-containment system can result in as much as 15% energy savings with as little time to implement as four weeks and a typical payback period in only three months to one year.

Fig. 3. Comparison of conventional insulation (ceramic fiber) to Microtherm

Thickness Considerations

The microporous insulation manufactured by Microtherm is completely safe to handle and safe for the environment. The material has been certified non-combustible and contains less than 1% organic matter. Compared to conventional insulations, microporous products are at least three to four times more effective at higher temperatures. This means that only a fraction of the thickness required of conventional insulation is required of microporous insulation. Typically, the thickness can be reduced by three to four times that of conventional insulations. This can result in an increase in furnace capacity or a decrease in the overall footprint of the furnace, depending on the desired result. The thickness can also be kept the same with a resulting dramatic drop in cold-face temperature. Figure 3 shows different options and benefits.

Environmental Impact

Selecting an efficient insulation is also important to improve process control and reduce harmful emissions into the environment as well as into the product in the furnace. Maintaining correct air temperatures inside the furnace reduces the need for additional oxygen for combustion, reducing the emissions of harmful gases. It also eliminates the need to fire excess fuel to elevate the temperatures, decreasing the chances of introducing impurities into the product.

Flexibility

For many years, microporous insulation has been used in the storage-heater industry due to its capability to hold the heat for long periods of time. Lining the furnace with microporous insulation will also result in heat being stored inside the furnace, thus reducing startup time in many cases. The flexible forms of microporous insulation can be used on piping in the industrial-heating system, and with proper installation has a much lower incidence of sagging than conventional insulations.

Microporous insulation is offered in several forms. The fit of the material is essential in improving the operating efficiency. All of the material forms offered can be produced to standard and custom sizes in order to make the best fit for the application. Table 1 shows the different product forms currently available as well as the standard density range.

Summary

There are, of course, many factors that go into calculating the efficiency of furnaces and other industrial-heating applications. An energy expert can visit your facility and help make a total assessment. This article was meant only to discuss the savings opportunities provided by using microporous insulation in furnaces. In addition to offering improvements in furnace efficiency, microporous insulation has been used to increase efficiency or capacity in torpedo ladles, off-take ducts, troughs, blow pipes, tundishes and a variety of other applications requiring a thinner, lighter, better-performing insulation.