In an industrial tumble dryer an existing gland packing was to be replaced by a defined sealing system. The aims were better sealing performance, less maintenance effort and greater operational reliability.
The starting point
A gland packing seals by compressing packing rings axially. That works, but it demands attention: the packing beds down, loses preload and has to be readjusted in service. Its leakage behaviour changes over its life — and that is precisely the problem at an emission-relevant point.
| Machine | Industrial tumble dryer |
|---|---|
| Task | Replacement of a gland packing |
| Seal size | 235/275 mm |
| Temperature | 160–200 °C |
| Pressure | 3–3,5 bar |
| Speed | 6–12 min⁻¹ |
| Circumferential speed at Ø 235 mm | about 0.07–0.15 m/s |
| Regulatory framework | Sealing point subject to TA Luft |
TA Luft is the German Technical Instructions on Air Quality Control. It is not a statement about a medium or a material, but a regulatory framework setting out requirements for air quality control in industrial plant.
Technical assessment
The speed is low. The design challenge here therefore does not lie in circumferential speed or friction power, but in the combination of several boundary conditions:
- operating temperature up to 200 °C
- pressure load up to 3.5 bar
- large seal diameter
- possible gas or vapour phase
- requirement to reduce fugitive emissions
- replacement of an adjustable packing by a geometrically defined sealing system
For agitators working in the gas or vapour phase of pressurised vessels, TA Luft points to sealing systems with low leakage losses, to multi-chamber lip sealing systems or to equivalently technically tight systems. A tumble dryer is not a classical agitator — but the sealing task is comparable: a rotating shaft passage, pressure, temperature and an emission-relevant sealing point.
The sealing concept
A special seal of size 235/275 mm was designed for this application. Not a catalogue part — but engineered on the basis of the actual operating data and the existing installation geometry.
- multi-lip sealing system in place of the gland packing
- machined metal support body
- defined sealing gap to limit gap extrusion
- high-temperature elastomer matched to temperature and medium
- static sealing against the housing
- optional grease or barrier chamber, depending on machine design
The four design points
Temperature
At 160 to 200 °C standard NBR is ruled out. Depending on the medium and the chemical load, FKM, VMQ or FFKM come into question. The final choice of material is governed not by temperature alone, but equally by process vapours, oxygen exposure, cleaning agents, dry-running phases and the lubrication conditions.
Pressure
3.5 bar corresponds to about 0.35 MPa. This pressure level is manageable with a correctly designed radial shaft sealing system. What is decisive are the diameter of the support body and the sealing gap ds: under pressure the elastomer has to be supported so that gap extrusion stays limited. That is why a machined support body is preferable here to a simple pressed part.
Friction
Compared with conventional spring-loaded sealing elements, the VR sealing principle works with lower radial force. Less radial force means less friction, less heat generation and less wear on the shaft surface. When a gland packing is replaced, that counts for a great deal, because there the sealing action arises precisely from the axial compression of the packing rings — with corresponding friction on the shaft.
Maintenance
The packing has to be watched and readjusted in service. The special seal works with a defined lip geometry. The aim is a more stable operating condition with considerably less adjustment and maintenance effort.
The result
The gland packing was replaced by a seal engineered for these operating conditions. Technically that brings:
- a mechanically defined sealing system instead of a compressed packing
- lower frictional load on the shaft
- less maintenance effort
- more controllable leakage paths
- a design approach suited to sealing points subject to TA Luft
- adaptability to grease, flushing or barrier medium concepts
Transferable to
Tumble dryers, mixers, agitators and similar process apparatus. Wherever a gland packing sits today, the check is worth making — provided temperature, pressure and emission requirements are properly assessed. In this case the task was to design a special seal 235/275 mm for 160 to 200 °C, 3 to 3.5 bar and 6 to 12 min⁻¹.
Editorial note: the original English post gave “235/375 mm” in one place and “235/275 mm” in three others. The value attested three times, 235/275 mm, has been adopted. Please check against the manufacturing documents.