The ejectors which are distinguished from other types of compressors as having no moving parts, works on the principle of converting the pressure energy of a motivating fluid to velocity energy in order to entrain the suction fluid. Vacuum is created, air or gas is entrained and mixture of gas and steam enters the venturi diffuser where its velocity energy is converted into pressure, sufficient to discharge against a predetermined back pressure.

Advantages

The principal advantages of steam-jet ejectors over other types of vacuum producing units are:

• Low Capital Costs
• No moving parts hence maintenance free
• Compact construction hence easy installation
• Reliable operation and long working life
• Corrosion / erosion resistant as it can be constructed in a wide range of materials
• High vacuum performance at high suction loads
• Can be installed at remote or inaccessible locations

Apart from single stage ejectors, multi stage vacuum systems are used to create vacuum for process applications.

Water vapour is generally used as a motive agent, but in special cases process vapours may be used too. When using multi stage steam jet vacuum systems it is possible to achieve absolute suction pressures down to 0.01 mbar.

Chem steam-jet ejectors can be staged from single through five stages. A detailed analysis of initial costs and steam consumption should be made before making a decision as to the stages and type required to meet the specific requirements.

Chem engineers should be consulted for their recommendations, which are based on their extensive experience in various applications. Chem offers a size ideally suited for individual requirements. Newer designs provide far greater capacities than available before and the smallest unit covers a range, which previously required two or more ejectors.

The usual method adopted for staging is to use a vapour condenser between the stages. The size and types of condenser used is a function of the air-vapour ratios, cooling water temperatures and utility costs.

Non-Condensing Multi-stage Vacuum Systems

Non-condensing type has the first stage ejector discharging directly into the suction of the second stage ejector without employing condensers. The non-condensing design has the advantage of lower purchase and installation costs. In corrosive environments non-condensing ejectors are significantly more reliable than the condensing systems. Motive steam requirements for non-condensible ejectors are much higher than motive steam requirements for multi stage ejectors.

Condensing Multi-stage Vacuum Systems

Multi stage steam jet vacuum systems employs intercondensers between ejector stages to condense steam and vapour of preceeding stages, and cooling air and non-condensible vapour to reduce load to succeeding stages. The intercondensers may be direct contact type or surface type arranged barometrically or at a low level. Ejector systems with direct contact type condenser is considerably cheaper than a system with surface condenser. Vacuum system with surface condenser is suitable where the cooling water is not to be contaminated with process vapour.

There are two basic types of condensers - surface and direct-contact.

Direct Contact Condensers / Mixing Condensers

Advantages

• Lower capital costs
• Lower installation costs
• Smaller terminal difference allows operating at lower absolute pressure, hence less water required for a given vacuum condition
• Less floor area required
• No fouling, hence little or no maintenance required
• Trouble-free operation
• Can be fabricated with corrosion resistant materials, rubber or any other suitable lining
• Open barometric discharge provides safe operation without an atmospheric relief valve
• Separation of process and cooling water flows possible

Disadvantages

• Mixing of process and cooling water flows

Options Available

• Controllable arrangement
• Can be combined with a liquid ring vacuum pump, resulting in a hybrid vacuum system
• Non-barometric installation possible

Surface Condensers

• Steam and vapour condensate is recovered
• Process product may be recovered as condensate
• No contamination of cooling water eliminates water treatment
• Vacuum surges will be less likely to carry water back to the process
• Less head room required
• Can be constructed with either fixed or removable tube bundles. Condensers with fixed tube bundles have the limitation of being cleaned on the tube side only whereas condensers with removable bundles may also be cleaned on the shell side
• Makes more efficient use of cooling water and requires lesser cooling water pressure

Disadvantages

• Pollution may occur on the process and cooling water side

Options Available

• Controllable arrangement
• Combination with liquid ring vacuum pumps
• Vertical installation