Select a vapor cycle and two parameters for optimization. If a Rankine cycle is

Question

Select a vapor cycle and two parameters for optimization. If a Rankine cycle is used, it must include at least one reheat or regenerative cycle. Heat pumps or refrigerators should include at least one feature more complex than the basic cycle, such as a multistage (refrigerator-freezer) system. All mechanical devices must consider efficiency. Typical values are 80 to 95% for turbines and 70 to 85% for pumps and compressors. Heat exchanger efficiency and piping pressure losses can be neglected. Boiler efficiency information is difficult to locate, so assume a maximum boiler efficiency of 90% at 900ºF that decreases by 1% for every 10ºF increase in boiler temperature.

The objective for this project is to optimize the cycle efficiency(or COP) for two parameters. For a Rankine Cycle, parameters can include items such as boiler temperature and pressure, pressure point for reheat, percentage of fluid or pressure points for regeneration, or number or reheat taps. For refrigerators, consider options such as condenser pressure and choice or refrigerant. The first thing to do is choose a starting point. For example, a coal plant has a boiler temperature and pressure of 2,520 psi at 1,007°F and flow rate of 6,125,000 lbm / hr (1701.4 lbm / sec) to generate 1,150,000 hp. Start with a large range for the two parameters and create a grid of at least 16 points. From this create a surface, contour, or multiline plot to show the efficiency. Use the results to narrow the range of the two parameters and repeat the optimization over the new range.

Reheat section Low-pressure turbine 0in High- pressure turbine - Steam generator out Condenser Pump Ideal reheat cycle Combustor- Work CompressorGas turbine Air Heat exchanger By-products of 9 combustion Work Steam turbine Work o Combined eyele Pump

Explanation / Answer

The coal power plant works on the principal of rinkine cycle. This is used to predict the performence of steam power plant for electricity generation. It is also used for the prediction of performence in the reciprocting steam engine. It is an idealized cycle for the conversion of heat which is disorganised form of energy into work which is the organised form of work obtained at the shaft. Generally water is used as the working fluid and the heat is supplied to water while it runs in a closed loop.

The processes involved in the the operation of power plant are :-

Process1-2 = Reversible adeabatic process. In this the water is pumped to the boiler from low pressure in the condenser to the high pressure in the boiler using centerifugal pump.

Process 2-3 = This process is heat addition process where water is converted to the steam. In this process the temperature of the working fluid rises at constant pressure. So the working fluid attains high temperture and high pressure in the boiler.

Process 3-4 = This process is adeabatic expansion of high pressure and high temperature steam in the turbine where the work output is obtained in the form of shaft work.

Process 4-1 = It is the constant pressure heat rejection process where the expanded steam from the turbine is condensed to water for recirculation to the pump and continue the cycle.

The efficiency of this type of ower plant is very low as the most of the heat is lost at the turbine exit without giving useful work. So, in order to improve the effectiveness of cycle reheating and regenerative processes are implemented. These processes uses additional unit for expansion of the steam so that the heat energy from the boiler is utilized to the most.

The two parameter which can be used to optimize the effectiveness of the rankine cycle through reheat cycle are:-

(a) The effectiveness of the cyscle can be improved by increasing the temperature of the reheat grater to the temperature as obatained after the boiler (T3>T1). If thiis is the situation then the expansion of the steam in the low pressure turbine will be more which will consequently improve the output at the shaft. The studies have shown that if the temperature at the inlet of the turbine increases the power output perportionately increases.

(b) Another way of improving the performence is the reduction in the temperaure of water leaving the condenser which will create the negative back pressure so as to improve the flow rate thus improving the performence.

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