Thermal sleeve and expansion seal at a rotary kiln end: carrying the seal surface off a hot shell and absorbing axial growth and centreline rise

01

How far a kiln end moves once it is hot: 41 mm of axial travel and a 2.5 mm rise of the centreline at a DOE pilot kiln

Drexler's 1990 report on the Process Experimental Pilot Plant (PREPP) kiln at the Idaho National Engineering Laboratory is a measured case of end seals designed for a cold kiln. The incinerator kiln was about 8.5 ft (2.6 m) in diameter and 25 ft (7.6 m) long, ran about 0.5 in water (1 mm Hg) below room pressure, and was to reach 980 °C. [1]

Its original primary and secondary seals were rotating drums attached to the kiln shell through a cylindrical thermal sleeve, running inside stuffing boxes with five rings of packing 19 mm square in section and 10 m long; cooling air was ducted into the annulus between shell and sleeve to keep the seals below the 232 °C shell temperature, and metal bellows sealed between the end structures and the seal support rings. [1] On heat-up the kiln could not reach its operating temperature because of air leakage through the end seals, and drive motor current rose. [1]

The engineering evaluation found that the drum seal could take the 41 mm (1.6 in) axial motion from the kiln's float on its rollers and the shell's thermal expansion, but not a vertical shift of the centre of rotation. The support rollers bear on the tyre about 1.25 m below the kiln centreline, and thermal growth lifted the centreline about 2.5 mm above its cold position; with the stuffing box rigidly fixed at the centreline, the rising drum crushed the packing at the top and opened a gap at the bottom. [1] The report also flagged the sleeve joint itself: differential expansion between the hot shell and the much cooler sleeve, seal drag torque, drum weight and cooling-air thermal cycles made fatigue failure of the sleeve-to-shell joint very probable. [1]

A temporary fix let the stuffing boxes float on brass shims and slotted bolts and added torque rods that resisted drag without restraining vertical or axial movement; it cut kiln drive amperage by 20 % and allowed a year of test operation. [1]

Ask the kiln supplier for the axial movement at each end (float plus thermal growth), the rise of the centreline between cold and hot, the shell temperature at the seal location, and the eccentricity of the rotating seal surface.

02

Sleeve or stand-off between shell and seal: holding a 116 °C seal disc on a 232 °C shell, with 25 mm radial and 38 mm axial allowance

The replacement design in the same report moves the sealing surface onto a smooth-faced rotating disc fixed to a steel stand-off structure welded to the kiln shell, bedded in high-temperature silicone RTV. The stand-off and bedding provide support, thermal insulation, accommodation of differential expansion and seal continuity; with the shell measured at up to 232 °C, the disc temperature was calculated at 116 °C. [1]

The report is not consistent on axial travel: it describes the original seal as taking 41 mm of axial motion, while the replacement membrane gives a total axial motion capability of 38 mm at each seal assembly, and its abstract speaks of a 1.5 in axial shift. [1] A buyer working from these figures should confirm which value applies to the kiln in hand. Editorial calculation: the 2.5 mm centreline rise that destroyed the original packing is one tenth of the 25 mm radial allowance designed into the replacement, and the calculated disc temperature is half the measured shell temperature. [1]

Flexible membranes of fibreglass fabric impregnated with a high-temperature material join the two seal housings and connect the primary housing to the firing hood; the outer one takes kiln float, shell expansion between inlet and outlet seals and wobble of the disc. The housings hang from overhead rods that let them swing axially with the shell but keep their elevation at the disc's centre of rotation, and horizontal tie rods take the drag torque. [1]

For the sleeve or stand-off, ask how it is attached to the shell, what temperature the seal surface will run at, how differential expansion between shell and sleeve is relieved, and how the joint is protected from cyclic and drag loads.

03

Leaf, double-leaf, bellows and labyrinth seals: leakage classes and how much expansion each takes

FEECO, a US maker of rotary dryers and kilns, groups its seals by relative leakage, and points to the bellows type where longitudinal expansion is large.

FEECO lists what it needs to specify a replacement seal, including shell temperature at the seal area, maximum material temperature, drum speed and outside diameter, axial expansion of the shell at the seal location, shell material, which end, breeching type and opening, exposed length available for the sealing surface, and suction or pressure. It gives lead times of 1–4 weeks for new leaf seal plates and 4–10 weeks for a newly designed seal system. [3] Its signs of a failing seal are missing, worn or drooping leaves, material spilling back through the seal, chattering or vibration near the breechings and unexplained rises in fuel use. [3]

Guide rollers matter as much as the seal faces. On a 45 in (1.14 m) test replica of a John Zink double-faced kiln seal at the Savannah River Site, the seal plates rode on cam followers spaced 60° apart on the rim of a rotor turning with the kiln. Followers were found up to 3.5° off perpendicular because their mounting holes were poorly machined; high contact forces flaked metal off the rotor and weight-bearing followers backed out of their threads. After re-fitting with machined spacers and nuts the deviations were 0.0–0.3°, inside the follower maker's recommended 0.5°. [2] The follower maker's notes in the same report give 1.5° misalignment tolerance for a crowned follower with a 24 in crown radius, a sealed operating limit of 250 °F (275 °F intermittent) and a surface hardness of Rockwell C 58 to 60. [2]

State the seal type and leakage class wanted, whether the kiln runs under suction or pressure, the axial expansion at the seal, and how guide rollers or followers will be aligned and kept within their temperature limit.

04

Data to send with a kiln-end sleeve and seal enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Kiln: [type; outside diameter; length; speed; direct or indirect firing]
End: [feed or discharge; breeching type and opening]
Temperatures: [shell at seal location; maximum material; hood gas]
Movement: [axial expansion at seal; axial float; centreline rise cold to hot; runout]
Pressure: [suction or positive; any purge or seal air]
Seal: [labyrinth, leaf, double leaf, bellows, graphite face; leakage target]
Sleeve: [attachment to shell; cooling of annulus; exposed length available]
Atmosphere: [corrosive or hazardous gases; dust]

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