A winding can contain cellulose and aramid in the same drying load. Cellulose drying models use material-specific moisture and transport inputs. [1] A Nomex study also found that humidity and prior thermal ageing changed its surface electrical properties. That evidence calls for an aramid-specific endpoint; it does not supply a hybrid-load drying cycle. [2]
Identify the exact material grades and forms, including paper, board, laminates and adhesives. Thickness and covered surfaces determine the moisture path. The generic labels “cellulose” and “aramid” are insufficient for quantitative model parameters.
| Model input | Cellulose evidence | Aramid evidence |
|---|---|---|
| Sorption behavior | Qualified grade/temperature data | Qualified grade/temperature data |
| Moisture transport | Thickness and boundary-dependent model | Material-specific model and geometry |
| Thermal exposure | Permitted process history | Permitted history including interfaces |
| Endpoint representation | Representative region or witness relationship | Separate justification, not inherited automatically |
Find the region that limits each material
The thickest cellulose component may control diffusion time while an adhesive or another interface controls temperature. A single “worst part” may not bound every constraint. Map the slow moisture region and the thermally sensitive region separately.
Use the temperature history experienced by each region rather than the oven wall setting. Avoid assigning one diffusivity or equilibrium curve to the entire mixed system merely to simplify the calculation.
Keep retained electrical and mechanical properties within each grade’s qualified process envelope as well as meeting its moisture endpoint.
Keep the two material water ledgers separate
Assumptions: the load contains 100 kg dry cellulose and 20 kg dry aramid; each has an independently validated final dry-basis water limit of 0.50%. Representative final water inventories are 0.30 kg and 0.16 kg. Cellulose is at 0.30%, but aramid is at 0.80% and exceeds its limit by 0.30 percentage points. The pooled result, 0.46/120 × 100 = 0.383%, would hide that failure. Hold mixed-load release and resolve the aramid endpoint without exceeding either material’s qualified thermal history.
References
- A. Betie, F. Meghnefi, I. Fofana and Z. Yeo, Modeling the Insulation Paper Drying Process from Thermogravimetric Analyses, Energies 11, 517, 2018, section 2. DOI: 10.3390/en11030517.
- L. Li et al., Effects of ambient humidity and thermal aging on properties of Nomex insulation in mining dry-type transformer, High Voltage 6, 71–81, 2021. DOI: 10.1049/hve.2019.0293.