From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Varnish impregnation methods for coils, addressing moisture, solvent recovery, and temperature control.
| Identifier | ExFiles\Box 24\2\ Scan294 | |
| Date | 26th March 1928 guessed | |
| -7- I am in a position to state that more than one manufacturer has found that in the absence of special air-drying systems it is best to eliminate the compressed-air process, as the moisture introduced into the chamber and varnish more than off-sets the better penetration of the varnish into the coils. It is common fallacy to imagine that a coil impregnated with varnish, but still "wet", is in a state to resist the entrance of moisture. Another of your correspondents outlines a system whereby the evaporation and the oxidisation of the varnish are in two separate operations, and for this purpose a centrifugal air blower is introduced in the pipe line, so that warm air may be circulated over the articles impregnated. Here again the idea is excellent in theory, but I can assure your reads that it will only be successful in practice provided that the condenser for the extraction and/or recovery of the solvent is of very large dimensions and of the highest efficiency, whereas in general the condenser supplied with impregnating plants is calculated to deal with the volume of air and solvent vapour at a rate based on the speed and capacity of the vacuum pump, which is, of course, only a small fraction of the rate at which the air and solvent vapour will be passed through the condenser when impelled by the centrifugal air blower necessary to carry this dual process of evaporation and oxidisation of the varnish. In the absence of a condenser of suitable design and dimensions to quickly remove the solvent new vapour circulated at a high rate by the centrifugal blower, the coils undergoing impregnation will not only take much longer to dry as compared with oven drying, but they will be so thoroughly washed with the solvent in vapour form that practically all the varnish will be removed from the outer layers. Incidentally, recent experience seems to show that a varnish of relatively high gravity is preferable, although the consistency of the material would be uncomfortably thick for dipping or brushing work. A further point which I would wish to see discussed is concerned with correct methods for measuring the temperature in the vacuum chamber. It has been noted that with high vacuum it is difficult to measure temperatures accurately, as in such a case the transfer of heat in the chamber is mainly by radiation, and therefore the kind of thermometer may be of importance. It is customary to impregnate at temperatures ranging from 90 deg. to 100 deg. C., or in other words, close to the limits of temperature to which cotton and silk insulation may be carried with safety, and it is therefore clear that a 10-20 deg. error in temperature may give rise to unfortunate results. | ||