Autoclave heat and pressure is applied to the workload placed inside of it. Typically, there are two classes. Those pressurized with steam process workloads which can withstand exposure to water, and those which circulate heated gas to provide greater flexibility and control of the heating atmosphere.
Autoclave processing is far more costly than oven heating and is therefore generally used only when isostatic pressure must be applied to a workload of comparatively complex shape. For smaller flat parts, heated presses offer much shorter cycle times. In other applications, the pressure is not required by the process but is integral with the use of steam, since steam temperature is directly related to steam pressure.
The selection of the materials used to make the machine turns entirely upon the application. For steam applications, carbon steel is used, but a corrosion allowance is added to the calculated thickness. This accommodates the rusting that occurs with repeated cycles of exposure to steam, water, and air. Implicit in this is the need to monitor the loss of metal and decommission the vessel when excessive thickness loss has occurred.
Of the entire machine, the costliest and most important single piece of hardware is the fast-opening door. It must be of full diameter to allow access to the working space, seal tightly against rated pressure at the highest shell temperature, operate readily and quickly, and comply with the same safety code that governs the rest of the pressure vessel. Of all safety-related concerns, the most critical are those which relate to the door's operation.
The internal layout varies from one machine to another. Some have an air duct at the six-o'clock position which also carries the cart rails, while others have a wide floor with the mechanical components beneath it. Others have the air duct at the top. Typically, they use an annular air duct running across the full circumference of the interior.
Introducing heat into the working chamber can be done in a variety of ways. For most, and particularly those used to process composite parts or perform adhesive bonding of metal structures, the easiest and least costly initially is electric heat. Resistance heaters are compact and reliable and can be placed conveniently in the circulating air duct. Since the thermal mass of these heaters is small, control of chamber temperature is precise and additional heaters can usually be installed at a later date without excessive bother. These heaters are essentially 100% efficient and can be fitted for any voltage, single or three phase.
Cool-down at the end of the process cycle requires a means of extracting heat from the autoclave. The necessity of controlled cool-down will itself depend upon the work being processed. With some composite materials in thick lay-ups, slow cooling prevents internal microcracking of the resin matrix resulting from thermally induced stresses.
The purpose of circulating the air or inert gas through is to assure effective heat transfer and temperature uniformity. Vigorous circulation and careful attention to where the airflow actually goes are the best ways of accomplishing this. As a rough rule of thumb, do not consider less than 300 feet per minute average air speed through the empty workspace of the machine. More than this will make heat transfer more effective.
The substantial mass of the pressure vessel provides assurance of pressure containment, but it represents an equally massive heat sink which must be heated and cooled cyclically as the autoclave runs. Steam autoclaves are necessarily insulated on the exterior, making this heat loss unavoidable. Autoclaves using air or another gas employ thermal insulation on the interior, and this incurs a one-time penalty in the cost of the pressure vessel and a slight operating cost resulting from the somewhat greater internal volume to be pressurized.
Parts processed in an autoclave are often vacuum bagged to enable the pressure to operate isostatically on the workpieces. In simplest form, the workload is fully contained inside a loosely fitting bag made of resilient plastic capable of withstanding the temperatures involved. When vacuum is drawn, the bag is compressed by atmospheric pressure and compacts the components inside. Between the parts and the bag, an absorbent material provides a channel for the evacuation of the air and wicks up the excess resin squeezed out during curing.
Controls & Instrumentation
While much of the operation of a simple autoclave can remain manual, temperature control is virtually always automated, as this is easily done at low cost. The value of the products processed justifies a high degree of automation. The hardware and software available for industrial process automation makes fully automatic operation affordable and reliable. It is realistic to design and implement such automation without the services of an outside vendor in many cases.