Turbines are designed for a particular operating conditions like steam inlet pressure, steam inlet temperature and turbine exhaust pressure/ exhaust vacuum, which affects the performance of the turbines in a significant way. Variations in these parameters affect the steam consumption in the turbines and also the turbine efficiency. Theoretical turbine efficiency is calculated as work done by the turbine to the heat supplied to generate the steam.
Higher exhaust pressure/ lower vacuum, increases the steam consumption in the turbine, keeping all other operating parameters constant. Exhaust pressure lower than the specified will reduce the steam consumption and improves the turbine efficiency. Similarly exhaust vacuum lower than the specified, will lower the turbine efficiency and reduces the steam consumption.
The air extraction equipment must be capable of meeting two conditions; one met during normal operation, the other when raising vacuum on the turbo-generator unit.
When raising vacuum the air extraction equipment must deal rapidly with a large quantity of air and sufficient capacity must be installed to reduce pressure quickly in the condenser to a level which allows the turbine to be started. The last row blades of a 500 MW turbine will overheat if they run at speed and at low load in a poor vacuum. Thus, a vacuum of 20 in.Hg must be obtained before steam is admitted to the turbine and a vacuum of 26 in.Hg for full speed. It is important that the time taken to bring a turbo-generator on load shall not be increased by sufficient air extraction capacity.
Under normal operating conditions the quantity of air to be exhausted is lower. It consists of air leakage into the condenser via flanges and glands and also of incondensable gases that are present in the steam exhausting from the turbine. Air and incondensable must be removed from the condenser as their presence in any quantity impairs the heat transfer capability of the condenser and hence its performance. Conversely, excessive air extraction capacity should not be run, as this involves unnecessary running costs and also results in the extraction of an unwanted quantity of water vapor from the condenser steam space.