The Kiln Hood: Inlet and Outlet Hood Configurations

The kiln hood is the stationary, refractory-lined steel enclosure at each end of a rotating kiln. It channels gas flow, contains burning-zone radiation, houses the main burner at the discharge end, and forms the mechanical interface between the rotating shell and the fixed plant ductwork. Every rotary kiln has two: an inlet hood at the feed (preheater) side and an outlet hood at the discharge (cooler) side.

This piece covers the function of each hood, the configurations seen in practice, and why the gap between hood and shell is one of the largest false-air entry points in a cement plant. "Kiln hood" here refers to the 5-15 m diameter industrial steel structures used in cement, lime, DRI, and calcination plants, not the studio-pottery component of the same name.

Kiln hood: the stationary, refractory-lined enclosure at the feed end (inlet hood) and discharge end (outlet hood) of a rotary kiln. It channels gas flow, contains burning-zone radiation, houses the main burner, and forms the interface to fixed plant ductwork via a hood seal.

Inlet hood vs outlet hood: what's the difference?

The inlet hood sits at the feed end at gas temperatures of roughly 1,000-1,100 °C and connects to the riser duct or preheater. The outlet hood sits at the discharge end, exposed to 1,300-1,500 °C radiation from the burning zone, connects to the clinker cooler, and houses the main burner. The temperature delta drives different refractory and sealing choices: the inlet hood handles upstream draft and gas-flow direction; the outlet hood handles burner stability, clinker discharge, and (in precalciner plants) the tertiary-air takeoff to the calciner.

Attribute	Inlet hood	Outlet hood
End of kiln	Feed end (upstream)	Discharge end (downstream)
Typical gas / radiation temperature	1,000-1,100 °C (Holderbank Cement Course, Vol 2)	1,300-1,500 °C (Holderbank Cement Course, Vol 2)
Connects to	Riser duct / preheater	Clinker cooler
Houses the burner?	No	Yes (in cement and most calcination kilns)
Tertiary air takeoff (precalciner plants)	No	Yes
Refractory	High-alumina or basic brick	Abrasion-resistant + high-temperature refractory at burner flame zone
Main false-air drivers	Inlet hood seal, inspection ports, riser duct expansion joints	Outlet hood seal, cooler-to-hood transition, burner-pipe penetration

Inlet-hood sealing and outlet-hood sealing are not interchangeable problems. An outlet-hood seal sees higher continuous temperature and heavier dust load; an inlet-hood seal sees lower temperature but more aggressive thermal cycling on shutdowns.

Kiln hood configurations

Kiln hood geometry is dictated by burner arrangement, tertiary-air takeoff, and kiln-to-preheater alignment. Five configurations cover almost everything seen in practice.

Outlet hood:

• Straight single-burner hood. Burner aligned with the kiln axis. The default for dry-process cement and most lime kilns.
• Angled hood. Burner offset to manage flame impingement or make space for alternative-fuel injection lances.
• Multi-burner hood. Main fuel burner plus alternative-fuel and ignition burners, common in co-processing plants; the hood is widened to accept the burner array.

Inlet hood:

• Straight inlet hood. Kiln axis aligned with the riser duct. Cleanest gas flow, lowest pressure drop.
• Offset inlet hood. Kiln axis laterally displaced from the riser; a curved transition redirects the gas. Pressure drop and dust drop-out are higher, and the offset has to be engineered to avoid dust accumulation.

In precalciner cement plants, the outlet hood also houses the tertiary-air takeoff to the calciner, so any burner-arrangement retrofit has to revisit the tertiary-air ductwork as a coupled change.

Where the kiln hood meets the shell: the sealing problem

The gap between the rotating kiln shell and the stationary hood is one of the largest false-air entry points in a cement plant. Untreated, hood-area leakage typically accounts for 30-50% of total kiln-side false air; a well-sealed hood holds total combustion-air leakage under 5%, while a damaged seal can drive it past 20% [2][3].

The gap exists because the kiln moves and the hood does not. Three modes of relative motion have to be absorbed by the seal:

• Axial thermal expansion. Cement kiln shells are designed for roughly 0.25-0.3% longitudinal expansion between cold start and steady-state operation; on a 60-70 m kiln that is approximately 100-200 mm of axial growth (Phillips Kiln Services / cementkilns.co.uk design references) [1].
• Radial wobble and shell ovality. Shell ovality of 0.5-1% of diameter is normal; on a 5 m shell that is a 25-50 mm radial deviation per revolution.
• Vibration. Drive-side and load-shift vibration overlays a high-frequency component on the slow thermal and ovality movement.

The seal also tolerates dust ingress, refractory spalling at the hood edge, and thermal cycling on every shutdown. The Oswal product literature puts it directly: "Conventional sealing systems are designed for static conditions, while rotary kilns operate under continuous axial movement, thermal expansion, shell ovality, and mechanical distortion" [4]. For plants that have not measured hood-area false air recently, how false air is measured in a cement kiln covers the O₂-balance method, and understanding false air in cement kilns explains why the percentage matters for fuel cost.

Sealing technologies for kiln hoods

Three sealing technologies dominate kiln-hood applications: lamella seals for mechanical movement, graphite seals for thermal contact, and hybrid duplex systems that combine the two. Most installations also separate radial and axial compensation into dedicated elements within a single assembly.

Sealing type	Temperature limit	Movement tolerance	Dust handling	Thermal cycling tolerance	Typical retrofit complexity
Lamella	~600 °C continuous	High (axial + radial)	Good	Excellent	Low to moderate
Graphite	~1,000 °C continuous	Moderate	Moderate	Sensitive to rapid cycling	Moderate
Duplex hybrid (lamella + graphite)	~1,000 °C continuous	High	Good	Good	Moderate to high

Ranges per Oswal product specifications [4][5]. Specific limits depend on seal design and kiln operating envelope.

Lamella seals. Multi-blade overlapping steel segments spring-loaded against the rotating shell. Each blade flexes independently, so axial and radial movement is absorbed well. The lamella-based sealing elements line is the typical choice for inlet hoods and for outlet hoods where gas-side temperature stays below the lamella material's continuous limit.

Graphite seals. Segmented graphite blocks held against the shell by counter-pressure. Graphite handles continuous temperature better than steel but is sensitive to rapid thermal cycling; the blocks can crack if shock-loaded. The graphite-based sealing elements line fits outlet-hood radiation environments beyond what a lamella seal sustains.

Duplex hybrid. The Oswal Duplex Kiln Sealing System combines a lamella primary interface (absorbing axial and radial movement) with a graphite secondary interface (maintaining thermal sealing continuity) in one assembly [4]. It is engineered for kilns with high ovality or frequent thermal cycling, conditions under which a single-technology seal loses contact intermittently and leaks.

At the system level, the kiln inlet sealing system, kiln outlet sealing system, and integrated false air control product lines bundle hood-area sealing with monitoring and retrofit scope. The Oswal Duplex catalogue cites a 6-18 month payback range on integrated retrofits from lower fuel consumption and reduced ID-fan power draw [4]; the figure on any given plant depends on baseline false-air percentage and local fuel cost.