Figure 1.3: Enthalpy-Entropy Diagram for Steam Thermal Efficiency The thermal efficiency of a plant is the ratio: However, the ways in which the useful work output and the heat input are measured is important and thermal efficiency can be misleading if the bases used are not clearly defined. For example, in a generating station the useful work output is electrical energy but in other applications the mechanical shaft power is the useful output. Similarly, the heat input may be taken as the quantity of fuel consumed multiplied by its lower heating value89 or it may be taken as the actual heat transferred from the combustion products to the working fluid. The thermal efficiency may be determined at any running condition but it is normal to use full load conditions, i.e. at maximum inlet pressure, minimum exhaust pressure and with the turbine load and speed for maximum power output. The useful work output from a steam plant usually does not take into account the work input to feed pump because this value is small in comparison to turbine work. Heat to Power Ratio Condensing Turbine For a given steam inlet condition, a turbine has its greatest power output when the exhaust pressure is at a minimum. A very low exhaust pressure (about 0.1 bar) can be obtained when a steam turbine discharges into a condenser cooled by water from the sea, a river or a cooling tower. Where the production of shaft power is only the consideration (as in most electricity generating stations) this is the normal exhaust arrangement. Assuming this exhaust pressure, the will condense at about 40°C and if the condenser heat transfer surface is to be compact as possible, the cooling water temperature must not exceed 35°C. Water leaving a condenser at this temperature is not generally regarded as useful and is usually rejected to the environment. Back Pressure Turbine Most energy consumers have a demand for both electrical power (for motors, lighting, consumer goods, etc.) and heat (for space heating, process heating, hot water, etc.). The ratio of heat to power varies considerably from one consumer to another, from one climate to another and according to the time of day and year. Where suitable conditions exist a back pressure turbine
Figure 1.3: Enthalpy-Entropy Diagram for Steam Thermal Efficiency The thermal efficiency of a plant is the ratio: However, the ways in which the useful work output and the heat input are measured is important and thermal efficiency can be misleading if the bases used are not clearly defined. For example, in a generating station the useful work output is electrical energy but in other applications the mechanical shaft power is the useful output. Similarly, the heat input may be taken as the quantity of fuel consumed multiplied by its lower heating value89 or it may be taken as the actual heat transferred from the combustion products to the working fluid. The thermal efficiency may be determined at any running condition but it is normal to use full load conditions, i.e. at maximum inlet pressure, minimum exhaust pressure and with the turbine load and speed for maximum power output. The useful work output from a steam plant usually does not take into account the work input to feed pump because this value is small in comparison to turbine work. Heat to Power Ratio Condensing Turbine For a given steam inlet condition, a turbine has its greatest power output when the exhaust pressure is at a minimum. A very low exhaust pressure (about 0.1 bar) can be obtained when a steam turbine discharges into a condenser cooled by water from the sea, a river or a cooling tower. Where the production of shaft power is only the consideration (as in most electricity generating stations) this is the normal exhaust arrangement. Assuming this exhaust pressure, the will condense at about 40°C and if the condenser heat transfer surface is to be compact as possible, the cooling water temperature must not exceed 35°C. Water leaving a condenser at this temperature is not generally regarded as useful and is usually rejected to the environment. Back Pressure Turbine Most energy consumers have a demand for both electrical power (for motors, lighting, consumer goods, etc.) and heat (for space heating, process heating, hot water, etc.). The ratio of heat to power varies considerably from one consumer to another, from one climate to another and according to the time of day and year. Where suitable conditions exist a back pressure turbine
Thermodynamics%20II%20Lab%20Manual%20(Fall%202009%20Edition).docx