The steam-jet ejector is a pumping device with no moving parts. It offers a simple and reliable means for producing vacuum, and is expected to provide many years of trouble free operation.

An ejector has no moving parts.

The figure shows the main components of a steam-jet ejector.

The steam chest is the connection thru which the high pressure motive supply is introduced into the steam nozzle.

The steam nozzle is the heart of an ejector. It converts motive pressure to velocity energy which is used to entrain vapors in the suction head.

The suction head connects the steam chest to the diffuser, which is made up of an inlet diffuser, throat and outlet diffuser.

Why Wear Occurs
The main problem an ejector is likely to experience during operation is reduced capacity and/or loss of vacuum because of worn parts. The sources of wear are the moving fluids including:

• Motive fluid which enters the nozzle
  
• Suction fluid which is entrained in the suction head
  
• Discharge fluid which is a mixture of the motive fluid and the suction fluids.

Steam is the fluid of choice for most vacuum applications. Any moisture in the motive steam will eventually affect the performance of the jet. The high velocity wet steam will erode the metal in a process known as wiredrawing. For this reason a steam separator with a trap is always recommended.

Recommended Spare Parts Inventory

• One spare steam nozzle for every ejector stage size
  
• One spare diffuser for each of the final two stages
  
• One complete last stage (Z-stage) ejector (for critical services involving multistage ejectors)
  
• One complete spare ejector (for critical services when unit is made of a special-purpose material like Ceramic, Haveg, Carbon, Teflon or Fiberglass.)

Forgiving Operation
Even with worn parts, a steam-jet ejector has the ability to continue performing. As a result, operating personnel sometimes delay the inconvenience and expense of replacing worn parts by increasing steam pressure to the ejector.

This is poor economy! The cost of the higher pressure steam quickly exceeds the cost of a replacement part.

The best solution: replace worn parts immediately and return operating to optimum performance.

Internal Inspection
Critical dimensions, such as the diameter of the nozzle orifice and venturi bore can be quickly obtained from Croll-Reynolds. Usually, however, a visual check tells the story. When inspecting an ejector, assume that any pitting or etching obvious to the eye or touch will affect the system's performance. Velocity of steam -- particularly wet steam -- will show as wiredrawing lines etched up and down the inside of the steam nozzle. The point along the diameter where the steam contacts the venturi is another location that may be gouged.

When significant wear is found on internal inspection, replace the part immediately. Performance will be restored to optimum. The ejector will only be dismantled and reassembled a single time.

Keep An Inventory
A modest inventory of recommended spare parts saves time when internal inspection is performed. it is also insurance against prolonged outage as most large ejectors are custom-designed, and delivery of parts, even on a rush basis, will take at least a few weeks.

Recommended Spare Parts
Cast-iron ejectors have four basic parts: steam chest, nozzle, suction head and diffuser. in a fabricated-steel ejector these parts are combined into three: nozzle, mixing chamber and venturi throat.

In most services, spare parts consisting of one spare steam nozzle for every ejector stage size, and a spare diffuser for each of the final two stages should be kept on hand. In critical services involving multistage ejectors, a duplicate of the entire last stage (Z-stage) ejector (which is typically the smallest ejector stage) may be recommended.

Some small chemical handling ejectors are made of special-purpose materials like Ceramic, Haveg, Carbon, Teflon or Fiberglass. These are more difficult to replace.

To maintain continuity of service, it is wise to keep an entire spare ejector unit in stock.

Each new replacement part is identified with the stage designation, and serial number of the system. When installing spare parts, be sure to check that all stages are together, i.e. all Y's, all Z's, etc. Many ejector parts and even complete stages are physically -- but not operationally -- interchangeable and care must be taken not to mix them.

Operating and Troubleshooting Support
Croll-Reynolds engineers are always available to advise on troubleshooting as well as system operation. In many cases a customer's problem can be analyzed over the phone, and the repair made by the plant's own maintenance staff.

In summation, of all vacuum-producing devices, the steam-jet ejector is the most forgiving. Adherence to design conditions, occasional inspection, and replacement of worn parts will keep your Croll-Reynolds steam-jet ejector on-line for many years of reliable service.