- Turbine Bypass Desuperheating Systems
General: The attached figures illustrate the two types of Steam Jet Air Ejector (SJAE) systems used in the industry to remove non-condensables and water vapor from main condensers. These power systems have found wide acceptance in modern plants because of the small space requirements, simplicity, reliability, low maintenance and quick starting characteristics.
Fig. 1 shows the intercondenser and aftercondenser combined in a single shell. The intercondenser compartment is separated from the aftercondenser compartment by means of a common tubesheet. Special care is taken in the design and fabrication to prevent leakage between the intercondenser and aftercondenser compartments.
This design has two first stage and two second stage ejectors. The two first stages as well as the two second stages operate in parallel. The advantage of flexibility is apparent as 100 pct or 200 pct air handling capacities can be obtained, provided the condensers are properly designed.
In operation, it is usually necessary to operate only the first and second stage; the others jets are held in reserve. Block or Isolation valves between the second stages and the intercondenser are necessary to prevent short circuiting of air and vapor from the aftercondenser to the intercondenser through the standby ejectors. Also a block or isolation valve is also necessary for the suction side of each jet.
Fig. 2 shows a stacked design. Here the intercondenser and aftercondenser are built separately. Either design will provide the same performance. The difference lies in the purchase cost and installation requirements.
Operation: Air and non-condensible vapors from the main condenser, enter the first stage ejector suction connection. High pressure steam enters the steam chest and expands to the suction pressure through a properly designed steam nozzle. Steam exits from the nozzle at high velocity, and entrains the air and non-condensible vapors compressing them to a higher pressure by means of a converging-diverging throat. The mixture then enters an intercondenser where the steam is condensed. The intercondenser compartments are designed with special internal baffles (not shown) and arranged to allow the air to be cooled to the lowest possible temperature thereby reducing the volume of the air/vapor mixture to a minimum before it enters the second stage ejector. The second stage ejector handles the air/vapor mixture in the same manner as the first, and the steam/air mixture from the second stage enters an aftercondenser where the action is similar to that in the intercondenser. Air exits the discharge vent opening at a low temperature to reduce the heat loss. The air flow can be measured by means of a rotameter.
To rapidly reduce the main condenser pressure during start up, priming ejectors (a.k.a. hoggers) are commonly installed in parallel with the unit. These ejectors are designed to handle large capacities for the rapid initial evacuation. Noise levels sometime exceed OSHA maximum levels, and silencers are often installed to bring noise to acceptable levels.