A roller kiln is a tunnel kiln with a narrow and elongated cross-section. Unlike car tunnel kilns, it operates not with a car loaded with products but with a series of parallel rollers that span the cross-section of the kiln's working channel, forming the "roller track." Products are placed on the roller track and 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, in the ceramic industry, roller kilns are classified by combining fuel and heating methods.
Mingyan Roller hearth kiln – The flame enters the space above and below the roller track, contacting and directly heating the products.
(1) Gas-fired radiant roller kiln. Common gas fuels include natural gas, blast furnace gas, and liquefied petroleum gas, with a requirement for clean gas.
(2) Open-flame roller hearth kilns fueled by light diesel oil. Due to the simpler fuel supply system and lower investment compared to the gas supply system, most of the open-flame roller hearth kilns constructed domestically in recent years have been fueled by light diesel oil.
2. Flame Retarding Roller Kiln - The flame typically enters only the muffle channel separated from the kiln chamber, where it is radiated to the products through flame-retarding plates for heating.
(1) Coal-burning flame-separation roller kilns burn coal in a firebox, with the flames entering the flame-separation channel (muffle channel) beneath the roller to indirectly heat the products. Some domestic coal-burning roller kilns, to stabilize the kiln temperature and reduce the temperature difference between the upper and lower sections, install several electric heating elements (silicon carbide rods) on the roller to compensate for the heating of the products, which has a certain effect on improving product quality. Such roller kilns can be called coal-electric mixed-firing roller kilns, but they still fall under the category of coal-burning flame-separation roller kilns.
(2) Oil-burning flame-separation roller kilns use heavy oil or residue oil as fuel, with the flame typically entering the muffle channel beneath the kiln chamber for indirect heating of the products. The oil-burning flame-separation roller kilns built in China in the early 1980s, in addition to the muffle channels below the rollers, also had muffle channels above the rollers, but these were generally removed later. In the mid-late 1980s, most heavy-oil roller kilns were upgraded to oil-burning semi-flame-separation roller kilns, which involved leaving a fire outlet at appropriate locations to allow some combustion products to enter the working channel. Since the semi-flame-separation roller kilns, except for the fire outlets, have similar structures to the full flame-separation roller kilns, they can be categorized together.
3. Electric heated roller kiln - Utilizes electric heating elements (silicon carbide rods or electric heating wires) installed above and below the rollers as the heat source for radiant heating of products. Suitable for manufacturers with abundant electricity resources or small-scale roller kilns. Among the aforementioned types of roller kilns, the flameless roller kiln is beneficial for improving heat transfer efficiency, achieving uniform temperature distribution within the kiln cross-section, and energy saving, as the 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 classified based on the number of working channels: single-layer roller kiln, double-layer roller kiln, triple-layer roller kiln, etc. Multi-layer roller kilns can save fuel, shorten the kiln length, reduce land use, and lower investment costs. However, with an increase in the number of layers, it complicates the transportation lines for entering and exiting the kiln, the interlocking control system, and the kiln structure itself, making it more difficult to remove brick fragment debris.
Mostly in China, single-layer roller kilns are used, with some employing double channels, one for firing products and the other for drying green bodies. The drying heat source utilizes the residual heat from the firing layer. Generally speaking, multi-layer kilns are preferred when the kiln width is narrow, the operating temperature is not too high, and the land area is limited, but it's usually not advisable to exceed three layers.
