Steam Ejectors: The Basics

Steam ejectors are the simplest of all vacuum systems; they have no moving parts. This article, the first of five, looks at their technology. Future articles will discus the technology in more depth and offer tips on troubleshooting.

Steam ejectors' low installed cost and high longevity offer a reliable, economical means of producing vacuum. New materials and nozzle configurations and other innovations improve their performance and expand applications.

The conventional steam-jet ejector has four basic elements: Steam chest Nozzle(s) Mixing chamber Diffuser Ejectors are specified in either single-stage or multi-stage configurations depending on the required suction pressure. A practical compression ratio for a single-stage unit is approximately 6:1, while six-stage units can reach a system ratio of 100,000:1. Single-stage ejectors are the most common type. We typically recommend them for pressures ranging from 30 to 3 in. Hg abs. The typical discharge is at or near atmospheric pressure.

BASIC EJECTOR OPERATION: A high pressure motivating fluid enters at 1 and expands through a converging-diverging nozzle to 2; The suction fluid enters at 3 and mixes with the motivating fluid in the mixing chamber at 4; Both are then recompressed through the diffuser to 5

Multistage ejectors are classified as non-condensing or condensing. Non-condensing ejectors produce suction pressures below 4 in. Hg abs. Condensing ejectors are equipped with either direct-contact or surface or surface-type interstage condensers and can achieve suction pressures as low as 5µm Hg abs.

Although most ejectors are steam-driven, certain applications require the use of other fluids, which must be compatible with the product to maintain purity. For example, ejectors are designed to handle a variety of gases, both organic and inorganic, including air, water, HCl, butane, SO2 and ethylene glycol.

In ejector design and specification, it is convenient to divide subatmospheric pressure into four regions:

• Rough vacuum - 760 to 1 torr
• Medium vacuum - 1 to 10^-3 torr
• High vacuum - 10^-3 to 10^-7 torr
• Ultra-high vacuum - below 10^-7 torr

The rough-vacuum region is of the greatest interest to chemical engineers. This is the normal range of vacuum distillation, evaporation, drying and filtration. Medium vacuum is used for molten-metal degreasing, molecular distillation and freeze-drying. High and ultra-high vacuums are used in space simulation, low-temperature research, mass spectrometry, surface physics and the production of thin films.

For more information about steam ejectors and other Croll Reynolds vacuum systems,click here to visit our library. Our next article will discuss condensers.