Rankine cycle is a cycle that converse heat energy to power or motion energy. Almost all of steam engine and power plant use Rankine cycle for producing electricity.
figure 1. Rankine Cycle
Water as working fluid at the Rankine cycle is close loop cycle. It's mean water constantly at the end of process will goes back to initial process. At this cycle, This water undergoes four processes in accordance with the picture below :
figure 2. Temperature - Enthalpy ( T-H ) diagram
The process that happen are:
- C-D Process, working fluid or water is pumped from low pressure to high pressure, and water is in liquid phase so pump no need high enough input power. This process is namely Isentropic-compression because there is no entropy changing ideally when pumping process.
- D-F Process, high pressure water entry into boiler for isobaric heating (constant pressure). Heating source is from coal burning, HSD (High Speed Diesel/Oil) or Nuclear Reaction. In Boiler, water is phase changing from liquid to saturated steam (liquid and steam mixing), steam and 100 % superheated steam.
- F-G Process, this process is happen at steam turbine. Saturated steam from boiler enter to Turbine and undergoes isentropic expansion. Inherent energy from steam is converted to motion energy in Turbine.
- G-C Process, steam from turbine enter to condenser and happen condensation isobaric. Steam is converted to liquid again so can be used at the cycle.
The description of the cycle through the T-H diagram above is the most basic and simple Rankine cycle. In the aplication there are some modification in order to reach higher thermal efficiency. For example is using preheater ( low pressure heater and high pressure heater) before entring the boiler. The other modification is using Reheater at the outlet steam leaving the turbine, then use reheated steam enter second turbine (intermediate pressure turbine). For easy understanding, kindly please to see the figure below:
figure 3. steam water cycle overview
At the above figure, condensate water is pumped by Condensate Pump from Condenser to Deaeretor or Feed Water Tank which happen preheating process. Water is also through preheater (LP H and HP H) before entering boiler. The heating source is from extraction steam that taken from certain stages in the steam turbine.
Thermal Efficiency
Thermal efficiency of Rankine cycle is comparison between power that resulting from steam turbine which have been reduced by pump power, with heat power entering the boiler. Kindly please to see T-H Diagram in figure. 2 above.
The heating value absorbed by steam can be calculated by following formula :
Qin = m (Hf - Hd)
Steam has decreased enthalpy when conversion process from heat power to motion power in turbine, showed in F-G line in the figure 2 above. The number of decreased enthalpy can be used to calculated how much motion energy resulted from turbine, as following formula:
The next process is condensation in Condenser. Steam that leave from steam turbine is going to Condenser. Here can be seen that there is a heat energy not converted completely to motion energy. Steam that condensation is decreasing enthalpy ( G - C line ), this can be used to calculate the heat out with following formula :
Qout=m(Hg-Hc)
The pressure of condensate water is increased by pump before entering boiler, showed in C-D line. The line show not more enthalpy increasing. It mean the energy that given to water is not significant. The entering energy can be calculated by:
Win=m( Hd-Hc)
The Thermal Efficiency formula is:
ηtermal = (Wout – Win) / Qin
No comments:
Post a Comment