chapter 8site.iugaza.edu.ps/mhaiba/files/2012/01/ch8-vapor-power-systems.pdf · chapter 8 vapor...

Chapter 8

Vapor Power

Systems

5/7/2012 8:47 AM

Dr. Mohammad Abuhaiba, PE 1

chapter objective

Study vapor power plants in which the

working fluid is alternately vaporized

and condensed.

5/7/2012 8:47 AM

Dr. Mohammad Abuhaiba, PE

2

Modeling Vapor Power Systems

To facilitate thermodynamic analysis, overall

plant can be broken down into four major

subsystems identified by letters A through D on

the diagram.

The focus in this chapter is subsystem A, where

important energy conversion from heat to work

occurs.

5/7/2012 8:47 AM


3

5/7/2012 8:47 AM


4

Modeling Vapor Power Systems

Modeling Vapor Power Systems Subsystem B supplies energy required to vaporize water

passing through boiler. In fossil-fuel plants, this is accomplished by heat transfer to working fluid passing through tubes and drums in boiler from hot gases produced by combustion of a fossil fuel.

Regardless of energy source, vapor produced in boiler passes through a turbine, where it expands to a lower pressure.

The shaft of turbine is connected to an electric generator (subsystem D).

Vapor leaving turbine passes through condenser, where it condenses on outside of tubes carrying cooling water.

Cooling water circuit comprises subsystem C. Cooling water is sent to a cooling tower, where energy taken up in condenser is rejected to atmosphere. Cooling water is then recirculated through the condenser.

5/7/2012 8:47 AM


5

Analyzing Vapor Power

Systems - Rankine Cycle Evaluating Principal Work and Heat Transfers

Rankine cycle is a

thermodynamic cycle that

models subsystem A on Fig.

8.1

5/7/2012 8:47 AM


6



TURBINE

CONDENSER

5/7/2012 8:47 AM


7



Pump

Boiler

Thermal Efficiency

5/7/2012 8:47 AM


8



back work ratio: bwr, ratio of pump work input to work

developed by the turbine

5/7/2012 8:47 AM


9


Systems - Rankine Cycle Ideal Rankine Cycle

5/7/2012 8:47 AM


10

EXAMPLE 8.1

Ideal Rankine Cycle

Steam is the working fluid in an ideal Rankine cycle. Saturated

vapor enters the turbine at 8.0 MPa and saturated liquid exits the

condenser at a pressure of 0.008 MPa. The net power output of the

cycle is 100 MW. Determine for the cycle

a. The thermal efficiency

b. back work ratio

c. mass flow rate of the steam, in kg/h

d. rate of heat transfer into the working fluid as it passes

through the boiler, in MW

e. rate of heat transfer, from the condensing steam as it passes

through the condenser, in MW

f. mass flow rate of the condenser cooling water, in kg/ h, if cooling

water enters the condenser at 15°C and exits at 35°C.

5/7/2012 8:47 AM


11

EXAMPLE 8.1

Ideal Rankine Cycle

5/7/2012 8:47 AM


12


Systems - Rankine Cycle Effects of Boiler and Condenser Pressures

Thermal efficiency of power cycles tends to increase as average temperature at which energy is added by heat transfer increases and/or average temperature at which energy is rejected decreases.

Increasing boiler pressure of ideal Rankine cycle tends to increase thermal efficiency.

5/7/2012 8:47 AM


13



Decreasing condenser pressure tends to increase

thermal efficiency.

5/7/2012 8:47 AM


14



Lowest feasible condenser pressure is saturation pressure corresponding to ambient temperature, lowest possible temperature for heat rejection to surroundings.

The goal of maintaining lowest practical turbine exhaust (condenser) pressure is a primary reason for including the condenser in a power plant. Liquid water at atmospheric pressure could be drawn into boiler by a pump, and steam could be discharged directly to atmosphere at turbine exit.

By including a condenser in which steam side is operated at a pressure below atmospheric, turbine has a lower-pressure region in which to discharge, resulting in a significant increase in net work and thermal efficiency.

The addition of a condenser also allows working fluid to flow in a closed loop.

5/7/2012 8:47 AM


15


Systems - Rankine Cycle Comparison with Carnot Cycle

Carnot cycle has two shortcomings as a model for simple vapor power

cycle:

1. Heat passing to working fluid of a vapor power plant is usually

obtained from hot products of combustion cooling at approximately

constant pressure. To exploit fully the energy released on

combustion, hot products should be cooled as much as possible.

First portion of heating process of Rankine cycle is achieved by

cooling combustion products below max TH. With Carnot cycle,

combustion products would be cooled at the most to TH. Thus, a

smaller portion of energy released on combustion would be used.

2. State 3 is a two-phase liquid–vapor mixture. Significant practical

problems are encountered in developing pumps that handle two-

phase mixtures, as would be required by Carnot cycle. It is far easier

to condense vapor completely and handle only liquid in pump, as is

done in Rankine cycle. Pumping from 3 to 4 and constant-pressure

heating without work from 4 to 4’ are processes that can be closely

achieved in practice.

5/7/2012 8:47 AM


16

Ideal Rankine cycle has a lower thermal efficiency than Carnot cycle having

same max TH and min TC because average temperature between 4 and 4’ is

less than TH.


Systems - Rankine Cycle Principal Irreversibilities and Losses - TURBINE

An actual adiabatic

expansion through turbine

is accompanied by an

increase in entropy.

Work developed is less

than for corresponding

isentropic expansion.

Isentropic turbine

efficiency is

5/7/2012 8:47 AM


17


Systems - Rankine Cycle Principal Irreversibilities and Losses - PUMP

Work input to pump required to overcome frictional effects reduces net power output of plant.

In absence of heat transfer to surroundings, there would be an increase in entropy across the pump.

Work input for this process is greater than for corresponding isentropic process.

Isentropic pump efficiency is

5/7/2012 8:47 AM


18

EXAMPLE 8.2 Rankine Cycle with Irreversibilities

Reconsider the vapor power cycle of Example 8.1, but include in the analysis that the turbine and the pump each have an isentropic efficiency of 85%. Determine for the modified cycle

a. thermal efficiency

b. mass flow rate of steam, in kg/h for a net power output of 100 MW

c. rate of heat transfer into the working fluid as it passes through the boiler, in MW

d. rate of heat transfer from the condensing steam as it passes through the condenser, in MW

e. mass flow rate of the condenser cooling water, in kg/h, if cooling water enters the condenser at 15°C and exits as 35°C. Discuss the effects on the vapor cycle of irreversibilities within the turbine and pump.

5/7/2012 8:47 AM


19

EXAMPLE 8.2 Rankine Cycle with Irreversibilities

5/7/2012 8:47 AM


20

Improving Performance - Superheat and Reheat

An increase in boiler pressure or a decrease in condenser pressure may result in a reduction of steam quality at exit of turbine.

If quality of mixture passing through turbine becomes too low, impact of liquid droplets in flowing liquid–vapor mixture can erode turbine blades, causing a decrease in turbine efficiency and an increased need for maintenance.

Common practice: maintain at least 90% quality at turbine exit.

Cycle modifications known as superheat and reheat permit advantageous operating pressures in boiler and condenser and yet offset problem of low quality of turbine exhaust.

5/7/2012 8:47 AM


21

Improving Performance - Superheat

Further energy can be added by heat

transfer to steam, bringing it to a

superheated vapor condition at

turbine inlet.

Steam generator: combination of

boiler and superheater.

The cycle with superheat has a higher

average temperature of heat addition

than cycle without superheating, so

thermal efficiency is higher.

Quality at turbine exhaust is greater.

Superheating tends to alleviate

problem of low steam quality at

turbine exhaust.

With sufficient superheating, turbine

exhaust state may even fall in the

superheated vapor region.

5/7/2012 8:47 AM


22

Improving Performance - Reheat

Steam does not expand to condenser pressure in a

single stage. The steam expands through a first-stage

turbine (Process 1–2) to some pressure between

steam generator and condenser pressures.

Steam is then reheated in steam generator (Process

2–3).

After reheating, steam expands in a second-stage

turbine to condenser pressure (Process 3–4).

Principal advantage of reheat: increase quality of

steam at turbine exhaust.

5/7/2012 8:47 AM


23

Improving Performance - Reheat

5/7/2012 8:47 AM


24

Improving Performance - Supercritical Cycle

Temperature of steam entering turbine is

restricted by metallurgical limitations imposed

by materials used to fabricate superheater,

reheater, and turbine.

High pressure in steam generator requires

piping that can withstand great stresses at

elevated temperatures.

Although these factors limit gains that can be

realized through superheating and reheating,

improved materials and methods of fabrication

have permitted significant increases over years

in max allowed cycle temperatures and steam

generator pressures, with corresponding

increases in thermal efficiency.

5/7/2012 8:47 AM


25

EXAMPLE 8.3 Ideal Reheat Cycle

Steam is the working fluid in an ideal Rankine cycle with superheat and reheat. Steam enters the first-stage turbine at 8.0 MPa, 480°C, and expands to 0.7 MPa. It is then reheated to 440°C before entering the second-stage turbine, where it expands to the condenser pressure of 0.008 MPa. The net power output is 100 MW. Determine

a. thermal efficiency of the cycle

b. mass flow rate of steam, in kg/h

c. rate of heat transfer from the condensing steam as it passes through the condenser, in MW. Discuss the effects of reheat on the vapor power cycle.

5/7/2012 8:47 AM


26

EXAMPLE 8.3 Ideal Reheat Cycle

5/7/2012 8:47 AM


27

EXAMPLE 8.4 Reheat Cycle with Turbine Irreversibility

Reconsider the reheat cycle of Example 8.3, but include in the analysis that each turbine stage has the same isentropic efficiency.

a. If ht = 85%, determine the thermal efficiency.

b. Plot the thermal efficiency versus turbine stage efficiency ranging from 85 to 100%.

5/7/2012 8:47 AM


28

Open Feed Water Heaters Open feedwater heater, a direct contact-type heat exchanger in which streams at

different temperatures mix to form a stream at an intermediate temperature.

For this cycle, working fluid passes isentropically through turbine stages and pumps,

and the flow through steam generator, condenser, and feed water heater takes

place with no pressure drop in any of these components.

Steam enters first-stage turbine at state 1 and expands to state 2, where a fraction

of total flow is extracted, or bled, into an open feedwater heater operating at

extraction pressure, p2. The rest of steam expands through second-stage turbine to

state 3. This portion of total flow is condensed to saturated liquid, state 4, and then

pumped to extraction pressure and introduced into feedwater heater at state 5.

A single mixed stream exits feed water heater at state 6.

Mass flow rates of streams entering feed water heater are chosen so that stream

exiting feed water heater is a saturated liquid at the extraction pressure.

Liquid at state 6 is then pumped to steam generator pressure and enters steam

generator at state 7. Finally, working fluid is heated from state 7 to state 1 in steam

generator.

5/7/2012 8:47 AM


29

Open Feed Water Heaters

5/7/2012 8:47 AM


30

Open Feed Water Heaters Heat addition would take place from state 7 to state 1,

rather than from state a to state 1, as would be the case without regeneration.

Accordingly, amount of energy that must be supplied from combustion of a fossil fuel, to vaporize and superheat the steam would be reduced.

Only a portion of total flow expands through 2nd stage turbine (Process 2–3), however, so less work would be developed as well.

In practice, operating conditions are chosen so that reduction in heat added more than offsets the decrease in net work developed, resulting in an increased thermal efficiency in regenerative power plants.

5/7/2012 8:47 AM


31


Taking a single control volume enclosing both turbine stages, mass rate balance reduces at steady state to

Assuming no heat transfer between feedwater heater and its surroundings and ignoring kinetic and potential energy effects, mass and energy rate balances reduce at steady state to give

5/7/2012 8:47 AM


32


5/7/2012 8:47 AM


33

EXAMPLE 8.5 Regenerative Cycle with Open Feedwater Heater

Consider a regenerative vapor power cycle with one open feedwater heater. Steam enters the turbine at 8.0 MPa, 480°C and expands to 0.7 MPa, where some of the steam is extracted and diverted to the open feedwater heater operating at 0.7 MPa. The remaining steam expands through the second-stage turbine to the condenser pressure of 0.008 MPa. Saturated liquid exits the open feedwater heater at 0.7 MPa. The isentropic efficiency of each turbine stage is 85% and each pump operates isentropically. If the net power output of the cycle is 100 MW, determine


b. mass flow rate of steam entering first turbine stage, in kg/h.

5/7/2012 8:47 AM


34

EXAMPLE 8.5 Regenerative Cycle with Open Feedwater Heater

5/7/2012 8:47 AM


35

Improving Performance Closed Feedwater Heaters

Closed heaters: shell-and-tube-type recuperators in which feedwater

temperature increases as extracted steam condenses on outside of tubes

carrying feedwater.

Since the two streams do not mix, they can be at different pressures.

5/7/2012 8:47 AM


36

Improving Performance Closed Feedwater Heaters

5/7/2012 8:47 AM


37

Improving Performance Multiple Feedwater Heaters

5/7/2012 8:47 AM


38

EXAMPLE 8.6 Reheat–Regenerative Cycle with Two

Feedwater Heaters

Consider a reheat–regenerative vapor power cycle with two feedwater heaters, a

closed feedwater heater and an open feedwater heater. Steam enters the first

turbine at 8.0 MPa, 480°C and expands to 0.7 MPa. The steam is reheated to

440°C before entering the second turbine, where it expands to the condenser

pressure of 0.008 MPa. Steam is extracted from the first turbine at 2 MPa and

fed to the closed feedwater heater. Feedwater leaves the closed heater at

205°C and 8.0 MPa, and condensate exits as saturated liquid at 2 MPa. The

condensate is trapped into the open feedwater heater. Steam extracted from

the second turbine at 0.3 MPa is also fed into the open feedwater heater, which

operates at 0.3 MPa. The stream exiting the open feedwater heater is saturated

liquid at 0.3 MPa. The net power output of the cycle is 100 MW. There is no

stray heat transfer from any component to its surroundings. If the working fluid

experiences no irreversibilities as it passes through the turbines, pumps, steam

generator, reheater, and condenser, determine


b. mass flow rate of the steam entering the first turbine, in kg/h

5/7/2012 8:47 AM


39

EXAMPLE 8.6 Reheat–Regenerative

Cycle with Two

Feedwater Heaters

5/7/2012 8:47 AM


40

Home Work Assignment CH8

1, 9, 16, 22, 33, 41, 47

Due Wednesday

16/5/2012

5/7/2012 8:47 AM


41

chapter 8site.iugaza.edu.ps/mhaiba/files/2012/01/ch8-vapor-power-systems.pdf · chapter 8 vapor...

Documents