Vacuduct vacuum-formed ceramic-fiber insulation can be incorporated into a wide variety of different component shapes and sizes.

High process gas temperatures, corrosive constituents in exhaust gases, and thermal shock from wide extremes in operating temperatures put a severe strain on ductwork and other industrial plant components exposed to the harsh process environments. While castable insulating materials provide protection for these components, they have some drawbacks, such as high density and susceptibility to failure due to thermal shock.

A high-temperature insulation consisting of a low-density (~17 lb/ft3, or 0.272 g/cm3) ceramic-fiber sleeve or custom fitting cured inside a metal skin, offers an alternative to castables. Danser Inc.'s VacuductR has a maximum continuous use temperature of 2800F (1540C); provides instant-on/off thermal protection to minimize thermal shock; and at just 20% the weight of castable insulation, requires less support structure and less manpower and time to install than does castable ductwork.

The vacuum-formed ceramic-fiber product is not limited to ductwork. It can be manufactured in a wide range of shapes and sizes to meet customer needs, including bolt-together pipes, manifolds, elbows, transitions and custom fittings, as well as nonmetal-jacketed board shapes, burner blocks, peep-sight windows, and sleeves for any unique system design. It also is possible to embed heating elements in the hot face of the insulation.

Vacuduct hot-air recirculation system on the roof of brick-plant tunnel kiln

The Vacuum-Forming Process

The ceramic-fiber product is manufactured in a state-of-the-art vacuum-forming and finishing facility, which includes two 14-ft (4.2 m) deep by 8 and 10 ft (2.4 and 3 m) in diameter draw tanks, and a 12 ft by 13 ft by 14 ft (3.6 by 3.9 by 4.2 m) microwave oven.

In the forming process, ceramic fiber and binders are suspended in a water solution, and the solution is drawn onto the product-shaped filter die by means of a vacuum system. Water is drawn through the filter, depositing the ceramic fiber on the die forming the product, after which the die is removed from the slurry and inserted in a sheet-steel sleeve or fitting. At this stage in processing, the product is about two thirds water, which is removed by curing in a drying oven. Conventional ovens are used to dry thinner parts and production parts. Large components are dried using the microwave oven.

After drying, products are finished by cutting, sanding, and adding required accessories. For example, anchors, hangers and heating elements are added to components that are intended for use with furnaces used to cure other products.

Vacuduct exhaust flues on a shuttle kiln

Solving industrial problems

The versatile, vacuum-formed insulation is used in industrial applications to manage extreme process temperatures, particularly components operating in a temperature range of 1600 to 2800F (870 to 1540C), such as flues, stacks and ductwork on incinerators and brick and ceramics kilns. For example, retrofitting an older brick kiln could involve the following:

  • Replacing a corroded, crumbling brick stack with flanged sections of steel-jacketed ceramic fiber sleeves
  • Fabricating a heat recirculation system using the patented ceramic-fiber insulation, which eliminates not only the structural fatigue associated with stainless steel, but also eliminates the need for expansion joints
  • Bringing all high-temperature kiln ductwork that is within the reach of employees up to cold-face compliance
  • Replacing fractured and cracked burner blocks made of castable refractory with instant-on/off ceramic-fiber blocks to eliminate thermal shock; these usually are just vacuum-formed ceramic fiber but Danser can make them metal-jacketed if necessary


Slide damper made of 2-in. thick ceramic-fiber board
Kiln applications

In an application at a plant in Georgia, a company wanted a safeguard built into its operation in case there was a power outage when its two kilns were in operation. Danser replaced stainless steel ductwork with vacuum-formed ceramic-fiber insulation in the first recirculating system on each kiln, where most of the heat would migrate if the company lost power to the system. Stainless steel ductwork would not be able to withstand the kiln operating temperature of 2100F (1150C) or higher, even if the kilns were only down for a few minutes; the ductwork would collapse and the dampers would become stuck resulting in a very high replacement cost.

For another company in Georgia, Danser supplied the sheet-metal installation with vacuum-formed ceramic-fiber insulation and all the combustion piping for a new Swindell Dressler kiln. Each 5-ft (1.5 m) flanged section of Vacuduct weighed about 1800 lb (820 kg)-only about 20% the weight of castable duct of the same size. With the exhaust ducts about 35 ft (10.6 m) off the floor, the reduced weight not only made installation much easier, but also greatly reduced the support structure required. In this case, most of the duct was supported on the kiln, but in many instances, the engineered load rating of the customer's building allows supporting the duct from roof columns.

In this particular application, sulfur, steam, unburned hydrocarbons and other products of combustion are exhausted from the kiln. Although the ceramic-fiber lining of the duct easily withstands the heat and chemicals, corrosive off-gases migrate through the fiber, so prior to inserting the insulation, a corrosion-resistant coating was applied to the inside diameter of the steel casing. In addition, even though the process combustion-heat curve is spiked, the vacuum-formed insulation virtually eliminates thermal shock.

Four-way "T" fitting for a fresh-air bleed

The same type of steel-jacketed ceramic-fiber insulation is used on three tunnel kilns in a plant in Virginia. It was tied into an existing ceramic fiber-module system on the oldest kiln, but in the newest kilns, it was installed in all exhaust and recirculation ductwork. The insulation also is used on the POC (products of combustion) system, which pulls heat off at the beginning of the firing zone operating at a temperature of about 2200F (1200C). The ductwork also includes slide dampers made of 2-in. (50 mm) thick ceramic-fiber board, operating at a temperature of 2600F (1430C). Earlier dampers were made of 2 in. thick board, which fit into the ID of the duct's ceramic-fiber lining and had metal slides mounted to the outside of the duct for alignment. In a new damper design, a shelf configuration is incorporated inside the duct lining for the damper to fit into, which provides a better seal. Also, the outside alignment slides have been upgraded to a ceramic fiber insulation-lined box configuration, which reduces heat loss.

In a brick kiln in Nebraska, recirculating ductwork handles smoke from the kiln. Polluted air is pulled off the kiln, then reburned through the kiln to clean it and prevent pollution. Castable was replaced with vacuum-formed ceramic-fiber insulation in the original ductwork. Some of the ductwork was installed while the kiln was hot. To accomplish this, all pieces are made to fit, which makes the refitting job much faster. The kiln is idled back, backdrafted, and the air is pulled back down to another fan while the old ductwork is pulled off, and the new ductwork is set on and bolted together, saving a lot of time and money. If the refit is not handled in this manner, it takes longer to insulate and lag the ductwork than it does to put the ductwork in.

In a similar hot installation of a hot air off-take line at a brick plant in Maryland, after back drafting, the old off-take covering three separate waste heat sleeves was removed and replaced with separate vacuum-formed ceramic-fiber insulation sleeves to a header without having to shut the kiln down.

In a tunnel kiln application in Virginia, because the kiln couldn't be taken out of production, the company had to redesign the exhaust ductwork in one of its tunnel kilns "on the fly." Based on the customer's requirements, Danser designed an exhaust system that fit into the limited space available, with a connection providing access to the interior part of the kiln and use of the exhaust system while the kiln was hot. Ducts were about 2 ft (0.6 m) in diameter in each leg, with damper adjustments, and were tied into an existing exhaust system. Total installation was completed in four days. Later, an exhaust system about 100 ft (30.5 m) long was installed in another kiln at the same location, which enabled exhausting different parts of the kiln.

High-temperature trunk line "T" fitting with end cap
Other applications

Incinerator stacks are good candidates for retrofitting using vacuum-formed ceramic-fiber insulation to withstand potentially corrosive exhaust gases. For a company in North Carolina that converts recyclable oil into Number 2 heating oil, Danser relined an incinerator 8 ft (2.4 m) in diameter by 21 ft (6.4 m) long, and added an additional 12 ft (3.6 m) in length to minimize condensation by increasing dwell time. To this, Danser mounted a 37-ft (11.3 m) long exhaust duct (40 in. ID by 48 in. OD, or 1 by 1.2 m), and applied a corrosion-resistant coating to the ID of the steel.

Replacing fractured and cracked burner blocks made of castable refractory with vacuum-formed ceramic-fiber blocks offers a number of advantages including resistance to thermal shock, which most likely damaged the blocks in the first place. The ceramic-fiber blocks also can be easily adjusted; the size of the blocks is readily altered using a rasp to shave the material.

In plants where burner blocks have to be moved quite a distance, the relatively light weight of the vacuum-formed burner blocks is an advantage. They can easily be carried by two maintenance personnel to the kiln or furnace for installation and then manually lifted into place, instead of having to use a fork truck.

Vacuduct fittings incorporating stainless steel damper

In an entirely different application, Danser's steel-jacketed ceramic-fiber insulation was successfully used in a glass-artists' co-op in Brooklyn, N.Y. The facility has 11 furnaces (also called glory holes), all of which are connected with the ceramic fiber-lined duct to one main trunk line in the building. Each glory hole can be run individually at a temperature to 2800F (1540C), and then shut down immediately. The instant-on/instant-off capability of the ductwork insulation is particularly useful in this application.

For more information: Dick Croy is director, Marketing, Danser Inc., PO Box 4098, Parkersburg, WV 26104; tel: 304-679-3666; fax: 304-679-3354; e-mail: danser@vacuduct.com.