A roller kiln is a tunnel kiln with a narrow and elongated cross-section. Unlike car tunnel kilns, it operates not with cars loaded with products but with a series of parallel rollers that cross the kiln's working channel, forming a "roller track." Products are placed on the roller track and are conveyed into the kiln as the rollers turn, completing the firing process within the kiln, hence the name roller kiln. Roller kilns can be classified by the combustion structure used, the heating method, or the number of channels. Generally, roller kilns in the building ceramics industry are classified by combining fuel and heating methods.
Bright Flame Roller Kiln - The flame enters the upper and lower spaces of the roller track, contacting and directly heating the products.
(1) Gas-fired flame roller kiln. Common gas fuels include natural gas, blast furnace gas, liquefied petroleum gas, etc., with a requirement for clean gas.
(2) Direct flame roller kilns burning light diesel oil. Due to the simpler fuel supply system and lower investment compared to the gas supply system, most of the direct flame roller kilns built domestically in recent years have been designed to burn light diesel oil.
2. Flame Retardant Roller Kiln - The flame typically enters only the muffle channel isolated from the kiln chamber, transferring heat to the products via flame-retardant plates for heating.
(1) The coal-fired flame-separation roller hearth kiln burns coal in the firebox, with the flame entering the flame-separation channel (muffle channel) beneath the roller to indirectly heat the products. Some domestic coal-fired roller hearth kilns, in order to stabilize kiln temperature and reduce the temperature difference between the upper and lower parts, install several electric heating elements (silicon carbide rods) on the rollers to compensate for heating the products, which has a certain effect on improving product quality. Such roller hearth kilns can be called coal-electric mixed-firing roller hearth kilns, but they also fall under the category of coal-fired flame-separation roller hearth kilns.
(2) Oil-burning flameless roller hearth kilns use heavy oil or residue oil as fuel, and the flame typically enters the furnace through the muffler channel below, indirectly heating the products. The oil-burning flameless roller hearth kilns constructed in China in the early 1980s, in addition to having a muffler channel below the roller, also had one above it, but the upper muffler channel was generally removed later. By the mid to late 1980s, most roller hearth kilns burning heavy oil were improved to oil-burning semi-flameless roller hearth kilns, where a fire outlet was left at an appropriate location to allow some combustion products into the working channel. Since the oil-burning semi-flameless roller hearth kilns, apart from the fire outlet, have the same structure as the oil-burning full flameless roller hearth kilns, they can be categorized together.
3. Electric Heating Roller Kiln - Heated by electric heating elements (silicon carbide rods or electric heating wires) installed above and below the rollers, which radiate heat to the products. Suitable for manufacturers with abundant electric power resources or small-scale roller kilns. Among the aforementioned types of roller kilns, the flameless roller kiln is advantageous for improving heat transfer efficiency, evenly distributing the temperature field within the kiln, and energy conservation, as its combustion products directly contact the products, representing the mainstream of roller kilns. Of course, each region has its own resource characteristics, and other types of roller kilns are also widely used in our country.
The roller kiln can also be categorized by the number of working channels: single-layer, double-layer, triple-layer roller kilns, and so on. Multi-layer roller kilns can save fuel, shorten the kiln length, reduce land usage, and lower investment costs. However, the increased number of layers complicates the transportation lines for loading and unloading, the interlocking control system, and the kiln structure itself, which also poses considerable difficulties in removing brick fragment debris.
Mostly, China currently uses single-layer roller kilns, with some employing two-layer channels, one for firing products and the other for drying green bodies. The drying heat source utilizes the waste heat from the firing layer. Generally speaking, it is advisable to use multi-layer kilns when the kiln width is narrow, the working temperature is not too high, and the land area is limited, but it is generally not recommended to exceed three layers.
