Autoclaves are generally erected in tall, well-lighted sheds provided with a travelling crane. Plate VII shows in section the interior of a colour shed with the adjoining autoclave shed. It may be seen from this how the materials are brought to the vessel, and how the finished product is blown over directly into the intermediate shed.

Laboratory Autoclaves are constructed on precisely similar lines to those used in the works. The diagrams on Plates I and X show two vessels made from cast steel with and without stirring gear. Plate XIII also shows all the details of a properly constructed stirring-autoclave. As already mentioned, the liner must be very carefully fixed in by means of solder. If a solder-bath be used instead of oil, an iron bath must be provided, as copper is attacked by the lead. In the laboratory the stuffing-box is not usually cooled, as the loss by possible blowing off is very slight; it is only advisable to cool where high pressures and temperatures are encountered, but for such cases it is preferable to use the rotating autoclave already described. Attention may be called to the fact that the stirrer should always work clockwise, to avoid unscrewing the nut of the stuffing-box. The nuts which close the autoclave are tightened carefully and regularly in the laboratory, just as is done on the large plant, with the difference, however, that it is inadvisable to tighten up the nuts with a hammer, as they may be broken off. It suffices to tighten up by means of a long spanner, the autoclave being placed in a stand which will prevent it rotating.

The cover may be either dome-shaped or flat, as may be seen from the sectional diagram of Fig. 34 and from Plates I, XIII, and XIV, Fig. 35. The flat top is to be preferred as it is easier to screw the agitator bracket down to it firmly, and the flange-pipes can more easily be rendered air-tight. The vertical autoclave shown on Plate XIV, Fig. 35, has a domed cover with flange-pieces for affixing the various fittings.

The heating is done by means of a Fletcher burner, and, later, it may be done with a good Bunsen burner directly under the middle, but not by several burners on different sides. The autoclave must be protected from draughts and be insulated by a tin cover, about 70 % of gas being saved in this way. In order to cool, the whole apparatus is removed from the bath and is stood on an iron triangle, so that the oil may run back into the bath. Heating and cooling will occupy only about an hour. In the event of anything unforeseen occurring the same rules apply as on the large scale, making due allowances for the different circumstances. The screws must not be loosened so long as there is any pressure, but that on the stuffing-box may be moved without danger. For the rest, attention is called to the general rules given below."

Instead of using an enamelled liner, it is also possible to have the cover and the inside of a laboratory autoclave enamelled directly, but there are few factories which do this satisfactorily. The cost of the enamelling is calculated according to the weight of the apparatus.

Fig. 34. - Section through a laboratory autoclave.

A. Stuffing-box. B. Packing. C. Oil-bath. D. Cast-steel vessel. E. Lead.

F. Liner.

In those cases where it is necessary to carry out an operation under pressure in the laboratory with stirring, considerable difficulties are found when pressures of about 20 atmos. are reached, as it is necessary to tighten up the stuffing-box and also to cool it. For this reason I have made use for many years of a piece of apparatus which is similar in construction to the known form, but has also certain novel features. Plate XV, Fig. 37, shows such a rotating autoclave in use, and Fig. 38 shows it in section.

The opening is contracted so that as few bolts as possible may be required, the whole vessel being turned in one piece from an old wrought-iron printing roller. In order that the pressure and the temperature may be measured, the apparatus is arranged diagonally, and the angle of inclination may be altered as desired. The top opening is utilized for the manometer, and the bottom for the thermometer, which is fixed in by means of asbestos paper. The weight of the autoclave does not rest upon the axle of the worm-drive, but is taken up by a bronze stuffing-box which is attached to the supporting columns. For this reason surprisingly little power is required to drive it. For a content of 400 c.cs. the apparatus weighs 11 kilos., and is constructed to stand 100 atmos. pressure; the stand weighs as much again. Whilst it is being heated, the cylinder is covered with a tin cover, so that less than 20 % of the gas is needed which would be required by any other form of autoclave. Experiments made with a view to replacing the expensive bolts and nuts by a simple screw fastening have met with no success, as at about 180o the screw packing always blows out. The packing simply sticks to the cover on screwing up, and it is impossible to make the apparatus tight.

General Rules for the Use of Autoclaves.

1. The packing ring must always be clean.

2. Tightening up must always be done at diametrically opposite points by first screwing up the bolts gently, and then tightening up by working round in a circle.

3. If neutral or similar liquids are heated, which evolve no ammonia, manometers fitted with bronze tubes may be used. If, however, vapours are given off which attack copper or bronze, a steel tube manometer must be used, as copper and bronze are soon destroyed.

4. A liner fixed in position by means of solder must always be

Fig. 38. - Section through rotating autoclave.

Fig. 38A. - Details of rotating autoclave used; any solder which is squeezed out being replaced. Only under quite special conditions can the use of a liner be dispensed with.

A. frame supporting autoclave. B. Hinge. C. Bronze bushing for axle. D. Worm shaft.

E. Collar. F. Oil-hole. G. Bronze pulley-wheel.

5. The temperature must be measured both inside and in the oil- or metal-bath, the latter temperature being about 25° higher than the former.

Pressure in Atmospheres.

Temperature-pressure curve for aqueous caustic soda.

6. The autoclave must be protected from draughts, for which purpose it should be insulated and, on the large scale, provided with a cover.

7. If the vessel is found to leak, the experiments must be stopped. The screws must not be tightened so long as there is any pressure; the stuffing-box, however, may safely be tightened up during the course of the process.

8. An autoclave may only be opened after the pressure has been blown off, as the manometer often fails to indicate a pressure, although it may be present.

9. Neither the vessel nor the oil-bath may ever be completely filled, as both water and oil expand very considerably on heating. If the vessel is completely full it is certain to burst.

10. Every autoclave should be officially examined annually, tested, and a report made on its condition. The date of examination should be stamped on the vessel. Both the capacity and the maximum pressure allowable should also be marked.

11. Works autoclaves should be thoroughly cleaned out, cooled, and provided with a ventilating tube before the examination.

12. The stonework for a works autoclave should first be erected on firm foundations, and then the complete autoclave should be lowered into it. After it has been mounted the apparatus should at once be ready for use.