Steam Jet Ejector Systems

Twin Element System

The most common steam jet ejector air removal system layout is a twin element two stage ejector system with an inter/after condenser and separate hogging ejector. This configuration consists of two trains of ejectors, each comprised of one first stage ejector and one second stage ejector. Each train is designed for 100% capacity, with one train being operated and the other train on standby as an installed spare. The inter/after condenser is typically designed for 200% capacity to allow both trains to operate while switching from one train to another, but is occasionally supplied with 100% capacity to lower the cost.

Single Element System

A simpler layout is a single element two stage system with an inter/after condenser. This layout consists of one first stage ejector and one second stage ejector, each designed for 100% capacity, with a inter/after condenser also designed for 100% capacity. This system offers no redundancy but comes with the lowest capital cost.

Enhanced Venting / Low O2 Design

A special air venting design is the enhanced venting system, which is a three stage air removal system. This system also utilizes a vacuum de-aeration feedwater system and special condenser design features to achieve a low O2 content nearing 5 ppm without the need for oxygen scavengers. While not as common as single and twin element configurations, these systems are often used when steam needs to be exported from the system to a secondary user requiring a large boiler water make-up stream.

Single Stage Design

The last air removal system configuration is for systems that do not run at a deeper vacuum, normally at a design pressure poorer than 6 inHgA. This system layout consists of a single stage ejector with an after condenser and can be supplied as a single or twin element design. While effective these systems are not as common and are normally found on smaller units.

Inter/After Condenser Design

The inter and after condensers can be designed and laid out in different ways. The most common configuration we supply today is a design referred to as an “inline” inter/after condenser. This configuration combines the inter/after condenser into one shell with a vertical division plate between the inter/after condensers, as shown in the picture below. Another inter/after condenser design is referred to as a “side by side” inter/after condenser. While this older design also has the inter and after condensers sharing the same shell, the division plate does not run vertical in the condenser but is instead run horizontally the length of the shell. These older side by side designs are effective but tend to eventually wear over time at the longitudinal division plate due to corrosion. The inter and after condensers can also be supplied as separate condensers with separate shells and sized for 100% capacity or 200% capacity. They can also be supplied as two separate sets of inter and after condensers, which is the equivalent of installing parallel single element systems.

Hogging Ejectors

Many steam ejector air removal systems are supplied with an additional ejector that is commonly referred to as the hogging ejector. Hogging ejectors are designed to aid in the startup of a system and used to evacuate air from the condenser, turbine, and duct work during start up. They are sized to have a lot of capacity but also use a lot of steam. Hogging ejectors are normally sized to pull the main condenser down to a pressure of 10 inHgA, after which the system would be transferred over to the main air removal package. Hogging ejectors can be used as a back up to the air removal package in case of an air leak or mechanical issues but are not designed to run continuously. If run continuously, the hogging ejector will greatly increase the steam consumption of the air removal system.

Question of the Month