Type 304 stainless steel is ideally suited for vacuum vessel construction because of its machining characteristics, excellent corrosion resistance and overall cost effectiveness. Unless otherwise specified all materials used in the fabrication of vacuum chambers will be type 304 stainless steel. MDC can quote the fabrication of chambers using other materials as required by a customer’s application. Other materials used include 304L, 316 and 316L stainless steels. For more exotic stainless steel material types, consult with MDC’s technical sales engineers. Low magnetic permeability materials such as mu-metal have also been used for the construction of magnetic shields used on conventional vacuum chambers. MDC specifies a magnetic permeability of less than or equal to 1.02µ when buying raw 304 stainless steel materials. Stresses induced during metal forming and welding operations will produce a change to this value in the proximity of an affected area, which may be significant to some applications. Distances between an application’s process and the affected areas should be maximized for applications that are highly sensitive to this increase in the material’s magnetic permeability.
MDC can provide water cooling features on most any vacuum chamber geometry. Even though various water cooling methods for vacuum vessels have been adopted by the industry, one geometry stands out as the most efficient, reliable and cost-effective, this being the double-wall cooling method. We’ve been designing and fabricating watercooled vacuum chambers for over twenty years and the knowledge acquired over this period of time has lead us to recommend double-wall construction as the solution of choice for most watercooled vacuum chamber applications.
Typically, standard double-wall construction is limited to a chamber’s main body and does not include any port extension tubes. Double wall cooling of port extension tubes is available on request, but significantly impacts a chamber’s sale price. Double-wall chambers are fitted with water directing baffles between the walls to provide water flow over most chamber surfaces and thus minimize hot spots and water flow dead zones.Watercooled chambers are typically fitted with 1/4 inch NPT (7/16 inch nominal inside diameter) fittings. As a general point of reference, the water flow rate through a 1/4 inch NPT fitting with a total linear path of 50 inches, inlet water pressure of 60 PSI and an unrestricted outlet, will be a nominal 8-10 gallons per minute. Practical and safe water flow rates must be determined by the user by taking into account variables such as coolant path, temperature, pressure and a coolant’s heat dissipating capacity.
Other less popular construction methods include tube-wrapping and half-channel geometries. Tube-wrap construction is a labor intensive hand-fitted process which provides reduced cooling capabilities. The half-channel construction comes closest to
the cooling capabilities of a double-wall system, but it too is extremely labor intensive and thus not cost-effective. Of the three construction methods discussed double-wall construction also offers the most aesthetic solution, which is critical to most commercial process applications.
Customers requiring water cooling for their chamber should keep in mind that the water cooling features must be incorporated in the early stages of a chamber’s design or construction.Water cooling of existing or partially constructed chambers is extremely difficult and may be cost prohibitive.
MDC employs advanced tungsten inert gas welding techniques (TIG) with high purity Argon as the inert gas medium. Manual as well as automated orbital welding machines are at the heart of all welding operations. MDC adheres to stringent UHV welding standards and practices including that of nonfiller metal, all internal fusion weld joints. External weld joints are not used unless required for structural reinforcement and then, limited to skip or span weld configurations. In the event that a weld joint can not be made internally, a one hundred percent full penetration external weld will be used. All weld joints are leak tested using a mass spectrometer leak detector with a minimum Helium sensitivity of 2x10-10 standard cc/sec.
Vacuum chamber finishes include internal and external glass bead blasting of all body and port tube surfaces. Flanges and machined components have a standard 63 micro-inch surface finish. Seal surfaces and groove bottoms are machined with a 32 micro-inch concentric finish suitable for standard elastomer sealing. Optional electrochemical polishing of chambers is available at an added cost. The electropolishing process is the reverse of electroplating where the work piece is made the anode and dissolves or gives up material in the electrolytic process. The electropolishing operating conditions are controlled so that atomic oxygen forms and reacts with the metal work piece surface. According to one theory, the high points of the metal surface are most readily oxidized and this oxidized material is thereupon dissolved in the electrolyte and otherwise removed. Selective solution of the high points of a surface tends to give a very smooth finish comparable or superior to a mechanically buffed surface. In contrast to mechanical polishing, electropolishing produces a strain free surface.
MDC is dedicated to providing standard as well as custom high and ultrahigh vacuum components of the highest quality and performance. To this end all vacuum rated components manufactured by MDC and its divisions are leak tested repeatedly, as required, throughout the manufacturing process to insure these goals are met. All weld joints, braze joints and seals are leak tested using a mass spectrometer leak detectors with a minimum Helium sensitivity of 2x10-10 standard cc/sec.
Upon final machining, which uses water soluble non-sulfurous cutting oils, all components are washed in a high temperature detergent bath followed by multiple tap and deionized water rinses. After the last rinse, parts are blown dry with dry nitrogen and packaged. Products are shipped clean and leak tested, ready for high vacuum installation and service.
Packaging includes the meticulous covering of all open ports with oil-free aluminum foil and the installation of protective plastic flange covers to prevent seal surface damage during transit. Where applicable components are heat-shrink skin packed, boxed or crated. For a fee MDC can ship chambers pumped and under vacuum with ports blanked-off and sealed. For this, chambers are evacuated to a rough vacuum level and sealed off by pinching a flange fitted copper roughing line. Note that this fee will include the purchase of all required port hardware including blank flanges, gaskets, fasteners and installation labor.
MDC vacuum chambers and components are constructed to maximize their high and ultrahigh vacuum capabilities. As such they are not designed for positive pressure applications with ratings above one atmosphere.