steam cycle

Steam cycle and steam power plant

http://www.ohio.edu/mechanical/thermo/property_tables/H2O/

H2O_Super3.html

steam table

Elements in steam cycle

• Saturation temperature• Subcooled liquid• Compressed liquid• Saturated liquid• Vapor• Saturated vapor• Superheated vapor• Degree superheat• Degree subcooled

• Wet vapor• Quality• Moisture• Latent heat of

vaporization• Critical point• Sensible heat• Latent heat

steam cyclets

pvsteam

rankine

Specify whether the steam is wet, dry or superheated for the following conditions:

• t = 200⁰C, p = 1.44 MPa• t = 220⁰C, p = 2.318 MPa• p = 1.0 MPa, s = 6.672 kJ/kg•K• p = 3.0 MPa, t = 234⁰C• t = 250⁰C, v = 54.2x10⁻3 m3/kg• P = 11.0 MPa, h = 2805 kJ/kg• P = 4.0 MPa, s = 5.897 kJ/kg•K• p = 15.0 MPa, t = 310⁰C

Processes of vapors

• Constant pressure• Constant volume• Constant temperature• Constant entropy• Constant enthalpy

cp

cv

ct

cs

ch

Rankine cycle

pumps

boiler

turbine

condenser

Ideal Rankine cycle processes (SPSP)

• Adiabatic pumping (1 to 2)

• Constant pressure heat addition of heat in the steam generator (2 to 3)

• Isentropic expansion in the engine (3 to 4)

• Constant pressure heat rejection in the condenser (4 to 1)

Ideal versus actual rankine cycle

Improving rankine cycle efficiecy

• Lowering the condenser pressure in rankine cycle

• Increasing the boiler pressure in rankine cycle• Superheating the steam to a higher

temperature in rankine cycle• Reheat cycle• Regenerative cycle• Reheat – regenerative cycle

Rankine cycle statements• Rankine cycle is the ideal cycle for vapor power plant• In a rankine cycle, water enters the pump as saturated liquid and is

compressed isentropically to the operating pressure of the boiler• In the pump, the water pressure and temperature increases

somewhat during the isentropic compression process due to a slight decrease in specific volume of water

• The superheated vapor enters the turbine and expands isentropically and produces work by the rotating shaft. The temperature and pressure may drop during the process.

• Steam is condensed at constant pressure in the condenser.• The boiler and condenser do not involve any work, and the pump

and turbine are assumed to be isentropic.

Rankine cycle statements

• Rankine cycle power plant converts 26% of the heat it recieves in the boiler to net work.

• The lesser the back work ratio, the better is the cycle.• Only .4% of the turbine work output is required to operate the

pump.• In actual condensers, the liquid is usually subcooled to prevent

cavitation.• Fluid friction causes pressure drops in the boiler, the condenser,

and piping between various compunents.• The pressure in the condenser is usually very small.• The major source of irreversibility is the heat loss from the

steam to the surroundings.

Rankine cycle statements

• To compensate pressure drops in Rankine cycle, the water must be pumped to a sufficient higher pressure than the ideal cycle.

• To increase the thermal efficiency of rankine cycle, increase the average temperature at which heat is transferred to the working fluid in the boiler.

• To increase the thermal efficiency of rankine cycle, decrease the average temperature at which heat is rejected from the working fluid in the condenser.

• Lowering the operating pressure of the condenser automatically lowers the temperature of the steam, and thus the temperature at which heat is rejected.

Rankine cycle statements

• The overall effect of lowering the condenser pressure is an increase in the efficiency of rankine cycle.

• To take advantage of the increased efficiencies at low pressure, the condenser of steam power plants usually operate well below the atmospheric pressure.

• The average temperature at which heat is added to the steam can be increased without increasing the boiler pressure by superheating the steam to high temperature.

• Superheating the steam to higher temperatures decrease the moisture content of the steam at the turbine exit.

Rankine cycle statements

• Presently the highest steam temperature allowed at the turbine inlet is about 620⁰C.

• Raises the average temperature at which heat is added to the steam raises the thermal efficiency of the cycle.

• The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages.

• As the number of stages is increased, the expansion and reheat process approached an isothermal process at the maximum temperature.

• In a reheat cycle, the optimum reheat pressure is about ¼ of the maximum cycle.

Rankine cycle statements

• The main purpose of reheating is to reduce the moisture content of the steam at the final stage expansion.

• Regeneration also provides a convenient means of dearating the feedwater to prevent corrosion in the boiler.

• The cycle efficiency increases further as the number of feedwater heaters is increased.

• A trap allows the liquid to be throttled to a lower pressure region but traps the vapor.

• A closed feedwater heater is more expansive than the open feedwater heater.

Rankine cycle statements

• Cogeneration is the production of more than one useful form of energy from the same energy source.

• The overall thermal efficiency of a power plant can be increased by binary cycles or combined cycles.

• A binary cycles is composed of two separate cycle, one at high temperatures and the other at relative low temperatures.

Problem solving

steam is generated at 4.1 MPa and 440⁰C and condensation occurs at .105 MPa.(a) For a Rankine cycle operating between these

limits, compute the thermal efficiency and the heat rate.

(b) Considering that a Rankine cycle occurs between the same limits, determine Q

Steam power plant design