DESIGNING STEAM JET VACUUM SYSTEMS
For cost-effective vacuum pumping, the proper placement of condensers and pipe supports are critical
DAVID B. BIRGENHEIER AND THOMAS L. BUTZBACH, Graham Manufacturing Company, Inc.
DONALD E. BOLT, Foster Wheeler Energy Corp.
RAJENDER K. BHATNAGAR, Nash-Kenema, Inc.
RUSSELL E. OJALA, Croll-Reynolds, Co.
JOHN AGLITZ, Nitech, Inc.
Steam-jet vacuum systems combine ejectors, condensers and interconnecting piping to provide relatively low-cost and low-maintenance vacuum pumping. These systems operate on the ejector-venturi principle, which relies on the momentum of a high-velocity jet of steam to move air and other gases from a connecting pipe or vessel.
During system design, critical decisions must be made regarding process conditions, component orientation and layout. A reliable source of steam and cooling water must be available, and provisions must be made to carry out condensate removal under vacuum. Finally, the appropriate monitoring and control instrumentation must be specified. Specific guidelines should be followed during equipment layout and installation, to optimize system performance.
EQUIPMENT ARRANGEMENT
Ejectors. An ejector is a type of vacuum pump or compressor. Since an ejector has no valves, rotors, pistons or other moving parts, it is a relatively low-cost component is easy to operate and requires relatively little maintenance. In a steam-jet ejector, the suction chamber is connected to the vessel or pipeline that is to be evacuated under vacuum. The gas that is to be induced into the suction chamber can be any fluid that is compatible with the steam and the components’ materials of construction.
The steam nozzle discharges a high-velocity jet across the suction chamber. This steam jet creates a vacuum which extracts air or gas from the adjoining vessel. As these gases are entrained in the steam, the mixture travels through the ejector into a venturishaped diffuser. In the diffuser, its velocity energy is converted into pressure energy, which helps to discharge the mixture against a predetermined back pressure, either to atmosphere or to a condenser (Figure 1).
