January 1994– The combustion chamber is an important part of any heating appliance, since it is the place where the mixture of fuel and air is brought together and combustion takes place. The heat produced during combustion is distributed through the adjoining heat exchanging devices; finally, to be delivered to the desired destination or media (most commonly air or water). Proper functioning of a heating appliance in a safe manner is greatly influenced by many factors, such as air to fuel ratio, combustion flue gas velocity, design, adequate operation of safety hardware and, of course, the combustion chamber. As a manufacturer of Gemcolite Combustion Chambers for the past thirty years, and one of the pioneers in the field of combustion related refractory products, we would like to take this opportunity to address some of the issues faced by field sales representatives, distributors, and service technicians, in relation to the Refractory Ceramic Fiber (RCF) Gemcolite Combustion Chambers.
In the past, combustion chambers were typically made from clay based kiln-fired cordierite refractory material, which adequately served the purpose of the combustion needs of that time. These chambers were composed of clays, silica, alumina and mullite, depending on the manufacturer and the service temperatures. The advent of the Refractory Ceramic Fiber-making process in the late 1950’s allowed Ceramists to convert the same refractory materials (kaoline clay, silica, alumina and mullite) into fiber, with further enhancement of refractory properties through higher purity. These fibers are engineered to handle service temperatures up to 3000°F. This revolutionized the designs of heating appliances by providing a “soft” fibrous chamber that provided the following distinct advantages over the kiln-fired cordierite refractory chambers:
- Better insulation value
- Higher resistance to thermal shock, allowing more frequent on-off cycling
- Faster heating and cooling of the combustion chamber
- Light weight – reduced mass
- Higher surface area for combustion with highly porous structure, allowing more complete combustion
- Greater overall combustion efficiency
Some of these characteristics inherently influence the Product Life Cycle through which an RCF Gemcolite Chamber goes, which greatly differs from that of the kiln-fired cordierite refractory chamber’s that we have all come to know so well. A typical kiln-fired cordierite refractory chamber is more suitable in applications where there is no need for the frequent on-off cycling and insulation value and combustion efficiency are not a primary concern. If a kiln-fired cordierite refractory chamber is exposed to more frequent on-off cycling, the only way it can relieve its mechanical stresses (caused by rapid expansion and contraction of the refractory material under heat), is through cracking. These cracks are through the cordierite refractory material, making it essential to replace the combustion chamber. RCF Gemcolite Chambers are highly porous in their structure and behave differently. Porous structure with low expansion properties allows the soft fibrous refractory to tolerate more frequent on-off cycling. Also, the fireside of the RCF Gemcolite Chamber is exposed to temperatures in excess of 1900°F – 2100°F in typical applications; whereas, the outside remains closer to the ambient temperatures. This difference in temperature causes the inside to shrink at a greater rate than the outside, causing the inside to crack on the surface. This cracking is essentially different from that of the kiln-fired cordierite refractory chamber in that the cracks are typically 1/8” or less deep (although they may go as deep as ¼”), depending on the intensity of the inside temperature; and they do not propagate through the material. Fibers in the outside layer of the material, with the help of high temperature refractory binders, adhere to the cracked surface, thus deterring further propagation of the surface cracks. In our experience, this happens in the first few firings of the appliance and does not alter the performance of the appliance, as long as the other normal operating procedures are followed and the rest of the hardware in the appliance is in good working order. During servicing of the appliances, we recommend service technicians not to disturb or move the Gemcolite Chamber, provided the cracks are no deeper than ¼” and the rest of the chamber is intact. Factors which have been known to reduce the life of the RCF Gemcolite Chamber are improper operation of furnace, condensation and/or water damaging the chamber, abuse during cleaning, high sulphur content fuels, malfunctioning burners and/or controls and improperly adjusted burners. In such conditions, the cracks may be deeper, which warrants the replacement of the chamber. When replacing chambers in such appliances, care must be taken to properly diagnose the cause of the condition(s) and remedy it, as well as replacing the chamber.
DISCLAIMER: The information presented herein is based on data considered to be accurate as of this date. However, no warranty or representation, express or implied, is made as to the accuracy or completeness of the foregoing data. In addition, no responsibility can be assumed by vendor for any damage or injury resulting from abnormal use, from any failure to adhere to recommended practices, or from any nature of the product.