Vacuum furnaces are heat-treating furnaces that use a low atmospheric pressure instead of a protective gas atmosphere. It is used to minimize cracking; furnace outlet temperature is usually limited to around 750° F depending on the crude. Also to minimize and/or prevent coking, the furnace is designed with an average heat flux not exceeding 12000 Btu/hr/ft. High heat flux results in excessive temperatures contributing to high pressure fluid film and tube metal temperatures. Typical pipe diameters for vacuum furnaces are 4″ and 6″. To prevent coking and meet the sonic velocity limitations, the last pass of the radiant section is normally specified one standard pipe diameter larger. The amount of steam injected into the furnace tubes depends on its pressure and the thermodynamic effects on the system. A typical steam rate is about ½ of 1% of the volumetric flow rate.
Pressures below atmospheric are determined by means of a vacuum gauge. These gauges come in a wide variety of types and styles. The gauge may be of the dial type actuated by a bourdon tube similar to the conventional pressure gauge. Or the gauge may consist of a mercury tube, with the height of the mercury column indicating the difference in pressure between the condenser and the atmosphere.