Last month's newsletter dealt with the operation of power plants. This month we are discussing the engineering analysis of individual components. If you will recall, the main components are the boiler, turbine, condenser and pump. These are the same components that are connected in series and through which the steam has to travel. There are variations of this cycle but, for our purposes, we will keep a simplified approach.
First, consider the boiler. The boiler has water entering and steam leaving. The boiler also has fuel and air entering and a combination of gases leaving. The amount of heat produced by the combustion process and used to change the water into steam can be determined by measuring the water pressure entering the boiler and the temperature of the water entering and the steam leaving the boiler. These measurements enable engineers to determine the value of a property known as enthalpy. Enthalpy is a property that is defined mathematically as follows: h = u + pv. The previous equation says that enthalpy is equal to the sum of the internal energy of a fluid and its flow work. The term pv is the product of the pressure and specific volume of the fluid at the entering or leaving point. Using steam tables that contain the tabulated values of enthalpy, the amount of heat transferred to the water can be determined using the following equation: Q = m(hl - he). Where Q is the heat transferred in BTU/hr, m is the mass of the water/steam circulated in pounds/hr, and l and e represent the leaving and entering conditions respectively.
A similar analysis is used on the turbine in order to determine the leaving condition of the steam. That is, Wt = m(he - hl). The previous equation says that the work extracted by the turbine is equal to the mass of steam multiplied by the change in enthalpy across the turbine. It should be noted that with the boiler, the change in enthalpy reflects the heat transferred IN to the steam whereas in the turbine, the change reflects the work taken OUT of the steam.
In order to determine the amount of heat that has to be transferred from the condenser in order to condense any remaining steam, the same equation used with the boiler, is applied. Once the steam is condensed, it has to be pumped back into the boiler. The same equation used with the turbine is also used with the pump. However, instead of using the "h" form of the equation that has been used, the equation is written as a function of the specific volume of the water and the change in pressure across the pump. In mathematical form, Wp = v(Pl - Pe).
