As manufacturing “re-shores” from countries outside the U.S. back to the 50 states, older, previously mothballed furnaces – many built decades ago – are being brought back from the dead.
When reconditioning heat-treating furnaces, thermal oxidizers, boilers and the like, refractory lining redesigns should be considered along with changes in burners, electric heating elements and control systems.
The surge in American energy production and resulting decline in energy prices over the past decade have contributed to a resurgence of manufacturing processes such as metal heat treating, hydrocarbon conversion and VOC destruction, all of which require the use of refractory materials. Many firms in need of additional heating capacity in their manufacturing processes turn to older furnaces that can be obtained through auction or other means instead of buying new. Breathing new life into these furnaces often means taking a careful look at the condition of the refractory lining.
Cast, Monolithic or Mortared Brick Linings
Many furnaces built from 1960 into the 1990s were lined with hard or insulating firebrick on the walls and roof. Often, an original roof brick arch in an aging furnace has partially or totally collapsed. Bricks may have cracked due to thermal cycling (Fig. 1). Finding a qualified refractory brick mason is difficult in many parts of the country.
Furnace owners often look to cast, monolithic or fiber refractory as a replacement option. When changing the refractory material in a floor, wall, flue or roof, however, furnace owners should ensure that the furnace heat retention and loss characteristics do not change so much that the furnace performance is negatively altered (Fig. 2). Options for replacement of refractory lining include pinned ceramic-fiber blanket, ceramic-fiber modules, gunned refractory, monolithic studgun-applied modules or insulating firebrick.
Pinned Ceramic-Fiber Linings
Often, due to economic or other plant considerations, a furnace has been relined with pinned ceramic fiber (Fig. 3). Since only off-the-shelf components are required (stainless steel pins, clips, ceramic-fiber blanket), these linings are cost-effective and relatively easy to install. In furnaces where airstream velocities are low, corrosive conditions do not present problems for the stainless steel pins and washers, and temperatures are low enough to keep blanket shrinkage from creating hot spots, these linings perform sufficiently well.
A common failure with pinned ceramic-fiber linings is pillowing, fraying or complete tear-out of the blanket, especially at a joint (Fig. 3). This often occurs in a roof. The combined effects of gravity and highest convection heating in the uppermost portion of a furnace combine to create problems
for the joints and pins in the lining. Furnace owners may blame the failure on poor installation, but even the best installation techniques may not prevent this type of failure.
A better solution for the car-bottom furnace lining shown in Figure 3 is a hybrid system. Folded, dense ceramic-fiber modules in the roof, attached with side-impaling stainless anchors or threaded studs and nuts buried deep in the module, are recommended. The Z-Blok2 system built by Distribution International (DI) is a decades-old design that holds up extremely well in these instances. An example of a new Z-Blok2 roof installed in a large car-bottom furnace is shown in Figure 4. At the center of the roof is a series of modules that protect a water-cooling jacket surrounding a fan shaft.
Especially in a roof, special design attention to batten joints and support weight by an anchor must be exercised. The pinned ceramic-fiber blanket walls may remain if they continue to hold up well without creating excess cold-face temperatures on the exterior furnace walls.
Odd Shapes Previously Lined with Cast or Hard Refractory
A sudden failure of cast refractory, resulting in the loss of the furnace for production use, leaves few options for the owner. In this case, the “moldability and foldability” of refractory fiber may be the best quick-turnaround option. A DI customer lost a retort, or muffle, furnace ring made with cast refractory and needed a quick repair option that could be fired immediately after installation. A series of trapezoidal modules was designed to match the inside and outside diameter of the ring, each anchored individually. The installation took less than a day, and the furnace was brought to temperature immediately with no need for a drying regime (Fig. 5).
Duct/Flue Repair in Round or Square Segments
Many exhaust ducts, afterburner ducts, flues or other connecting “tunnels” for hot gases are lined with cast refractory or vacuum-formed sleeves. Failure of the lining in segments such as this leaves owners with few speedy options for repair. Again, ceramic-fiber module designs may be the answer if well-engineered.
A customer with a tee in an oxidizer that required a dense lining with no exposed attachment hardware chose a custom-designed series of modules with special anchoring hardware so that no field cutting was required during installation. The coping of each module was performed in the shop such that the lining covered all of the internal duct steel (Fig. 6). Where high velocities exist in ducts such as this, treatment of the hot face with colloidal silica rigidizer helps to ensure that fiber erosion is minimized.
Importance of Heat-Flow Analysis with Refractory Redesign
Refractory density, chemistry, heat capacity and thickness all impact the performance of the furnace lining and, if changed from an original design, should be studied with a fresh heat-flow analysis. New furnaces fresh from the manufacturing center have been heavily engineered. But what options are available for a furnace owner wanting to improve an older furnace with new components, especially a revised refractory lining?
A necessary tool in this case is the heat-flow analysis. Readers of Industrial Heating are generally no strangers to heat-flow studies. A capable refractory replacement contractor or manufacturer has the ability to perform its own in-house heat-flow studies to demonstrate that its proposed replacement is suitable for the intended use and furnace limits (Fig. 7).
For more information: Contact Monty Phillips, Z-Blok division manager, Distribution International, 2301 W. Commerce Street, Dallas, TX 75212; tel: 469-499-0901; fax: 713-428-3882; e-mail: email@example.com; web: www.distributioninternational.com