Steam Consumption

Steam is a major energy consumer. Optimizing process operating conditions can considerably improve turbine water rate, which in turn will significantly reduce energy requirement. Various operating parameters affect condensing and back pressure turbine steam consumption and efficiency.


Effect of operating conditions on steam turbines
Turbine exhaust operating below atmosphere is condensed in a shell and tube exchanger called surface condenser. Condensate flows in the shell side of the condenser and steam is condensed by the cooling water. Vacuum in the surface condenser i.e. turbine exhaust vacuum is controlled/ maintained by vacuum ejector system of the surface condenser.

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. Efforts are made to show the impact of various operating conditions by considering the following steam conditions as illustration.

In the above referred turbines, 1 % reduction in steam consumption saves around $ 47000 annually for condensing turbines and around $ 84000 annually in back pressure turbine. LHV of the fuel for generating steam is considered as10,500 kcal/kg and boiler efficiency is taken as 87 %. Effect of various operating parameters is illustrated in the succeeding paragraphs.


Effect of Steam inlet pressure
Steam inlet pressure of the turbine also affects the turbine performance. All the turbines are designed for a specified steam inlet pressure. For obtaining the design efficiency, team inlet pressure shall be maintained at design level.

Lowering the steam inlet pressure will hampers the turbine efficiency and steam consumption in the turbine will increase. Similarly at higher steam inlet pressure energy available to run the turbine will be high, which in turn will reduce the steam consumption in the turbine.

In case of back pressure type turbine increase in steam inlet pressure by 1 kg/cm2 reduces the steam consumption in the turbine by about 0.7 % and improves the turbine efficiency by about 0.16 %. Improvement in back pressure type turbine is more than the condensing type turbine.


Effect of Steam inlet temperature
Enthalpy of steam is a function of temperature and pressure. At lower temperature, enthalpy will be low, work done by the turbine will be low, turbine efficiency will be low, hence steam consumption for the required output will be higher. In other words, at higher steam inlet temperature, heat extraction by the turbine will be higher and hence for the required output, steam consumption will reduce.

Increase in steam inlet temperature by 10 deg C in condensing type turbine reduces the steam consumption in the turbine by about 1.1 % and improves the turbine efficiency by about 0.12 % respectively.

In case of back pressure type turbine increase in steam inlet temperature by 10 deg C reduces the steam consumption in the turbine by about 1.5 % and improves the turbine efficiency by about 0.12 %.

Effect of exhaust pressure/ vacuum
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.

Improvement in exhaust vacuum by 10 mm Hg, reduces the steam consumption in the turbine by about 1.1 %. Improvement in turbine efficiency varies significantly from 0.24% to 0.4 %.

In case of back pressure type turbine reduction in exhaust pressure by 1.0 kg/cm2, reduces the steam consumption in the turbine by about 0.8 % and improves the turbine efficiency by about 0.14 %.