In vacuum furnaces the heat-treating process takes place inside an airtight vessel, where a vacuum is created. This helps to alleviate surface reactions. Furnaces are built of several kinds of high temperature (refractory) materials to hold the process material and hold in the heat without breaking down during the several months that they usually run.

Specifications

Important specifications to consider when searching for vacuum furnaces include process temperature, height, width or tube outer diameter, length, and vacuum pressure range.  The process temperature is the maximum temperature at which the unit may operate and still maintain rated performances. The height refers to the internal height dimension of the unit. The width refers to the internal width dimension of the unit, or for tube furnaces, the outer diameter of the tube. The length refers to the length dimension of the unit, or on tube furnaces, the heated length. The vacuum or pressure range can be rough or low (< 760, > 1 torr), medium (< 1, >10^-3 torr), high vacuum (< 10^-3, >10^-8 torr), ultra-high vacuum (< 10^-8 torr), and elevated pressures (> 760 torr).

Configurations for Vacuum Furnaces

Common configurations for vacuum furnaces include ashing, box or muffle, bottom loading, top loading, tube, bench or cabinet, continuous or conveying, and walk-in or truck-in.  Ashing is also referred to as charring; this is the step in a graphite furnace AA program that is designed to remove matrix constituents that might interference with the measurement of the analyte. Box furnaces are convenient furnaces to use. As the name implies, the furnace has a box shape and a box-shaped interior.  Bottom loading furnaces load the sample or product to be treated through the bottom of the chamber via a platform elevator.  In a top loading furnace the product to be processed is loaded from the top.  Cabinet or bench furnaces describe small batch equipment typically mounted on integral stands. Batch furnaces and ovens are typically suited for processing quantities of product in a single batch.  A belt furnace has a continuous belt, carrying the unprocessed substrates through the furnace.  In general continuous or conveying conveyor units tend to be oriented toward automated production of greater quantities of small-to-medium-sized product.  A pusher furnace is a type of continuous furnace in which parts to be heated are periodically charged into the furnace in containers, which are pushed along the hearth against a line of previously charged containers thus advancing the containers toward the discharge end of the furnace, where they are used.  Walk-in or truck-in furnaces describe larger size batch equipment, typically with double doors and integral carts, shelves, etc.

Atmospheres for Vacuum Furnaces

Common atmospheres for vacuum furnaces include air or oxidizing, inert, reducing, salt bath, and vacuum.  Furnaces may have either a single set point or a programmable controller to adjust temperature and temperature stability.  Choices for heat source include arc, combustion, electrical or resistance, indirect or contact or conduction, induction, infrared or radiant, natural gas, propane, oil, other fuel, RF or microwave or dielectric, and steam.  Common applications for vacuum ovens include aging, annealing, baking, brazing or soldering, burn-off, curing, drying, firing or sintering, foundry or melting, heat treating, hot pressing, industrial, laboratory, preheating, and quenching.  Other features include overtemperature protection, three-zone or multi-zone, computer interface, application software, view ports, and service or entry holes.