Redesigning Traditional Refractory Linings
A general trend is evident in iron and steel works. As alloys, products and designs are constantly improving, processes have started running hotter to keep up with the production requirements while trying to maintain the same or better process efficiencies. Equipment and process design has become more crucial than ever before.
Continuous improvements are being sought in raw-material supply as well as refractory design. We will examine two examples here. One is a redesigned reheat furnace and the other is a redesigned rotary kiln – both traditionally lined with hard refractory insulation.
Reheat furnaces (such as heat-treat, preheat and forge furnaces) were traditionally lined with hard (dense) refractory such as bricks, castable, gunnite, shotcrete, etc. Such dense refractory do not fare well when exposed to intense and continuous flame impingement, thermal cycling and alkali attack. Routine processes like ramp-ups, ramp-downs, and opening and shutting the furnace doors to charge or remove product can cause severe thermal shock and damage to the refractory lining.
Once damaged, hard-refractory repair and patchwork is cumbersome, time-consuming, labor-intensive and, therefore, expensive. Furnaces often have to be shut down for days because the refractory first needs to be completely cooled, repairs made and specific dry-out schedules followed. Finally, the furnace is very slowly ramped back up to the operating temperature (sudden ramp-ups create thermal shock). A large portion of this energy is absorbed by the refractory lining, and, due to the high thermal conductivity of dense refractories, a large percentage of this heat is conducted through to the steel casing of the furnace. Furthermore, traditional linings of hard refractory are heavy, putting undue strain on the mechanical components and structure of equipment. The resulting wear and tear of these mechanical components further adds to maintenance costs.
For these reasons, most reheat-furnace operators have, over time, migrated to using fiber-module insulation systems in their process. Fiber insulation (refractory ceramic or other low bio-persistent, alkaline-earth silicate fiber) is much easier to install, isn’t susceptible to thermal shock, has low heat storage and heat loss, and has very low thermal conductivity. Moreover, fiber-lined furnaces do not require any cure-out or ramp cycle. Fiber products are considerably lighter and often supplied in modules or entire sections, making it easier to do patchwork, repairs and new installs alike. An example of potential energy savings is exhibited in Figure 1.
Although fiber insulation is much more “user-friendly” than the dense refractories, it is more susceptible to damage from mechanical abrasion and alkali attack. Like anything else, fiber insulation also has its temperature limits, with a maximum operating temperature of 2450°F (1343°C). Most reheat furnaces today are seeing temperatures in the range of 2300-2500°F (1260-1371°C) and have issues with fiber shrinkage. While PCW (polycrystalline wool) modules can resolve this issue, full thickness and combination PCW-RCF modules tend to be very expensive and can cost up to 700% of the cost of a regular fiber lining (Fig. 2).
Silplate® Mass Module System
The Silplate® Mass module system – fiber module plus Silplate Mass (previously introduced in Industrial Heating’s April 2014 edition) – is a more viable solution to this common problem. This module system can be used at continuous operating temperatures up to 2732°F (1500°C) with ≤1% shrinkage. It can be a more cost-effective alternative to PCW modules for applications where standard fiber shrinkage, fluxing or mechanical failures occur, such as:
- Temperatures above 2450°F
- Alkali or other chemical attack
- High-velocity (over 100 feet/sec)
- Flame impingement
- Thermal shock
- Any combination of the above factors
The product is not applied simply as a hot-face coating but as a specific system that improves mechanical anchoring and adherence. Even for reheat furnaces using dense refractory, this product may be used to repair, protect or simply prolong the life of the dense refractory. Finally, this product can also be hot gunned over fiber and dense refractory alike, further reducing maintenance costs and improving production, because there is no need for ramp-downs or furnace shutdowns.
When reheat-furnace modules shrink, gaps open up and become paths for heat loss. Hot spots appear on the casing, and these furnaces have to be stopped, cooled down and then the gaps packed with expensive PCW bulk fiber. Furnace shutdown, labor costs and lost production add to maintenance costs. On average, reheat furnaces are stopped every three to six months due to hot spots on the casing. It is estimated that such stops cost furnace operators at least $30,000 per furnace annually. This includes labor, material and energy losses during start-up and shutdown. The production losses during shutdown are not accounted for in this number. If these costs are factored in as part of the acquisition calculation of a new fiber lining, the total cost is more than doubled.
Figure 3 shows that the Silplate Mass module system can actually resolve this issue in the most cost-effective way. This system initially costs 35-45% more than a regular fiber lining. Once the maintenance costs for the regular fiber lining are accounted for, the payback period for the higher initial cost of the Silplate Mass is less than a year (versus the RCF or low bio-persistent fiber modules). The furnace hot face may only need to be maintained once in 1-2 years, depending on applications and other factors. Silplate Mass may be applied directly on the hot face over existing fiber or Silplate Mass. Better yet, the product can be hot gunned over the existing surface.
The rotary kiln is another application that traditionally uses hard-refractory insulation. As the name suggests, these kilns have a rotating cylinder that’s slightly inclined to the ground. They are used to produce dolomitic lime and other higher-purity quicklime, which is used to make low-carbon steel, sintered dolomite, fiberglass and even some healthcare products. A schematic of a rotary kiln is shown in Figure 4.
Coal, petroleum coke, natural gas or waste-derived fuels are used. The offgases from the fuel are abrasive and volatile in nature. The continuous rotations (~1.2 rpm), raw materials (such as limestone, chalk or dolomite used in feed stock), the fuel gases and the combustion air act together to create an abusive environment. Intense fluxing, flame impingement and thermal cycling at temperatures between 2100-2300°F (1150-1260°C) take a heavy toll on the refractory lining.
A large amount of hard refractory is used to insulate the kiln, which also means that a large amount of energy is absorbed and transmitted through the refractory. The difficult conditions warrant the use of high-quality, wear-resistant refractory brick such as magnesia-carbon (MgO-C) or high-alumina brick in the working lining. While these bricks are very effective in maximizing the useful life of the refractory within the kiln, they have very high thermal conductivity. As a result, most rotary kilns without good backup insulation run very hot, and it is not uncommon to see hot spots on the kiln shell. The high temperatures and the weight of dense refractory put enormous stress on the rotary-kiln structure. Per ASTM and OSHA safety standards, the cold-face temperatures are very important considerations in refractory and process design. Depending on the application, steel deformation starts to become plastic or permanent between 650-750°F and can pose a very great safety hazard.
Silplate AR as Backup Board
In August 2010, Unifrax installed Silplate AR board as backup insulation behind a 9-inch MgO-C brick lining of a rotary kiln being used to produce lime for carbon-steel production. The kiln was about 164 feet long and a little over 13 feet in diameter. The cold face before Silplate AR was ~800°F. The kiln was beginning to deform, and high stress was being seen on the mechanical components (Fig. 5).
The redesigned rotary kiln was operated for a period of four years. During this time, no hot spots were observed on the kiln casing. Furthermore, the cold-face temperature was dropped from 800°F to 600°F.
Due to better insulation and low thermal conductivity of the Silplate AR board, more heat was retained by the MgO-C brick and in the kiln operations. This resulted in energy savings and higher efficiencies in the process. Hot spots on the shell and metal warping completely ceased. The annual maintenance costs for the mechanical bearings were also reduced. At the same time, production was improved by 30%. Overall, the kiln operators were able to realize savings of $1.5 million with the new refractory design.
Other rotary-kiln installations and projects utilizing Silplate AR as backup board (with other brick linings) have yielded similar or better results. On average, the cold-face temperature has been reduced by 200°F, and the net refractory weight has been reduced by ~305 pound/foot. Figure 6 includes photos and results from other such installations.
As industrial thermal processes continuously evolve, refractory designs must also improve. Now more than ever before, the industry recognizes the need to improve efficiencies while getting cleaner, greener and safer. This initiative must be taken not just to save money but also to save the environment and improve the overall standard at which industries manufacture, all while protecting those that work hard and strive to raise these standards every day.
For more information: Contact Shyam Nair, Silplate market development manager, Unifrax I LLC, Corporate Headquarters, 600 Riverwalk Pkwy, Tonawanda, NY 14150; tel: 412-841-7487; e-mail: email@example.com