Steam Turbines

Power Generation Through Steam Turbines

The basic process behind steam power generation is the “Rankine Cycle”. Water is heated until it is a saturated liquid. From there, it is compressed into steam. The steam is transferred to a turbine where the pressure of the steam is reduced (usually to sub atmospheric pressures) by expansion over the turbine blades. This process produces mechanical work which is latter converted into electricity through generator. The low pressure steam is condensed back to a liquid. The water, now referred to as return water, is mixed with new water, referred to as “feedwater”, and pumped back to the boiler. The figure below shows a common diagram used to describe the Rankine Cycle.

Steam Turbines have different classification criteria. These criterion are;

A-Steam Flow Direction in Steam Turbine

            i) Axial Steam Turbine

            ii) Radial Steam Turbine

B-Working Principle of Steam Turbine

            i) Impulse Steam Turbine

            ii) Reaction Steam Turbine

C-Based on Exit Steam and Application Based

            i) Condensing Steam Turbine

            ii) Non Condensing Back Pressure Steam Turbine

            iii) Extraction Steam Turbine

D-Based on Steam Pressure 

  

i)       Low pressure steam turbine, the turbine with pressure up to 2 ata. 

ii)      Middle pressure steam turbine, the turbine with pressure up to 40 ata. 

iii)     High pressure steam turbine, the turbine with pressure 40 – 170 ata. 

iv)     Very high steam turbine, the turbine with pressure exceeds 170 ata. 

 V)    Super critical pressure steam turbine, the turbine with pressure exceeds 225 ata.

A-Steam Flow Direction in Steam Turbine

i) Axial Steam Turbine 

ii) Radial Steam Turbine

In a radial-flow turbine, steam flows outward from the shaft to the casing.These steam turbines has direction of steam flow perpendicular to the axis of shaft. The unit is usually a reaction unit, having both fixed and moving blades. They are used for special jobs and are more common to European manufacturers, such as Sta-Laval (now ABB).

C-Three Classes of Steam Turbines on the Basis of Exit Steam and Application Based

1-Condensing Steam Turbines

2-Non Condensing (Back Pressure) Steam Turbines

3-Extraction Steam Turbines

1-Condensing Steam Turbine

These turbines Operate with an exhaust pressure less than atmospheric (vacuum pressure).

They experiences the maximum pressure drop through the turbine which results in greater energy extracted from each lbm or kg of steam input.ƒ Turbine efficiency ranges are  approx. 30-40%.ƒ The condenser can be either air or water cooled. Condenser cooling water can be utilized for processes or space heating loads. Condensing tubines areƒ Usually more expensive than Non-Condensing Back pressure turbines. These turbines are not used for Combined Heat and Power Applications. 

2-Non-Condensing Back Pressure Steam Turbine

Steam is expanded over turbine and exhaust low pressure steam is used to meat the thermal heating requirements. Steam is expanded to the extent providing required lower pressure of steam which facility can use. 

These turbines operate with an exhaust equal to or in excess of atmospheric pressure. Exhaust steam is used for lower pressure steam process loads. These turbines are available in smaller sizes and pass large amounts of steam per MW of output (low efficiencies). These turbines ƒ produce less useful work than a condensing turbine, but since the unused steam from the turbine is passed on to process loads, the lower turbine power generation efficiencies (15% to 35%) are not a concern. These turbines areƒ very cost effective when paralleled with pressure reduction valves (PRV), providing an efficient use of the pressure reduction requirements.ƒ Usually less costly than condensing turbines.

3-Extraction Steam Turbine

The other type of steam turbine used in CHP applications is called an extraction turbine. Basically these turbines are hybrid of Condensing and Non-Condensing turbines. In these turbines, steam in extracted from the turbine at some intermediate pressure. This steam can be used to meet the facilities steam need. The remaining steam is expanded further and condensed. Extraction turbines can also act as admission turbines. In admission turbines, additional steam is added to the turbine at some intermediate point. The figure below schematically shows the process of an extraction steam turbine.