30079_58.pdf

CHAPTER 58

STEAM TURBINES

William G. Steltz

Ttarboflow International Inc.

Orlando, Florida

58.1

HISTORICAL BACKGROUND

1765

58.4.2 Turbine Stage Designs

1782

58.4.3 Stage Performance

Characteristics

58.2

THE HEAT ENGINE AND

ENERGY CONVERSION

PROCESSES

1784

58.4.4 Low-Pressure Turbine

Design

1767

1788

58.4.5 Flow Field Solution

Techniques

1790

58.3

SELECTED STEAM

THERMODYNAMIC

PROPERTIES

58.4.6 Field Test Verification

of Flow Field Design

1791

1772

58.4.7 Blade-to-Blade Flow

Analysis

1796

58.4 BLADEPATHDESIGN

1775

58.4.8 Blade Aerodynamic

Considerations

58.4.1 Thermal to Mechanical

Energy Conversion

1796

1776

58.1 HISTORICAL BACKGROUND

The process of generating power depends on several energy-conversion processes, starting with the

chemical energy in fossil fuels or the nuclear energy within the atom. This energy is converted to

thermal energy, which is then transferred to the working fluid, in our case, steam. This thermal energy

is converted to mechanical energy with the help of a high-speed turbine rotor and a final conversion

to electrical energy is made by means of an electrical generator in the electrical power-generation

application. The presentation in this section focuses on the electrical power application, but is also

relevant to other applications, such as ship propulsion.

Throughout the world, the power-generation industry relies primarily on the steam turbine for the

production of electrical energy. In the United States, approximately 77% of installed power-generating

capacity is steam turbine-driven. Of the remaining 23%, hydroelectric installations contribute 13%,

gas turbines account for 9%, and the remaining 1% is split among geothermal, diesel, and solar power

sources. In effect, over 99% of electric power generated in the United States is developed by tur-

bomachinery of one design or another, with steam turbines carrying by far the greatest share of the

burden.

Steam turbines have had a long and eventful life since their initial practical development in the

late 19th century due primarily to efforts led by C. A. Parsons and G. deLaval. Significant devel-

opments came quite rapidly in those early days in the fields of ship propulsion and later in the power-

generation industry. Steam conditions at the throttle progressively climbed, contributing to increases

in power production and thermal efficiency. The recent advent of nuclear energy as a heat source for

power production had an opposite effect in the late 1950s. Steam conditions tumbled to accommodate

reactor designs, and unit heat rates underwent a step change increase. By this time, fossil unit throttle

steam conditions had essentially settled out at 2400 psi and 100O 0 F with single reheat to 100O 0 F.

Further advances in steam powerplants were achieved by the use of once-through boilers delivering

supercritical pressure steam at 3500-4500 psi. A unique steam plant utilizing advanced steam con-

This chapter was previously published in J. A. Schetz and A. E. Fuhs (eds.), Handbook of Fluid

Dynamics and Fluid Machinery, Vol. 3, Applications of Fluid Dynamics, New York, Wiley, 1996,

Chapter 27.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

ditions is Eddystone No. 1, designed to deliver steam at 5000 psi and 120O 0 F to the throttle, with

reheat to 105O 0 F and second reheat also to 105O 0 F.

Unit sizes increased rapidly in the period from 1950 to 1970; the maximum unit size increased

from 200 mW to 1200 mW (a sixfold increase) in this span of 20 years. In the 1970s, unit sizes

stabilized, with new units generally rated at substantially less than the maximum size. At the present

time, however, the expected size of new units is considerably less, appearing to be in the range of

350-500 mW.

In terms of heat rate (or thermal efficiency), the changes have not been so dramatic. A general

trend showing the reduction of power station heat rate over an 80-year period is presented in Fig.

58.1. The advent of regenerative feedwater heating in the 1920s brought about a step change reduction

in heat rate. A further reduction was brought about by the introduction of steam reheating. Gradual

improvements continued in steam systems and were recently supplemented by the technology of the

combined cycle, the gas turbine/steam turbine system (see Fig. 58.2). In the same period of time

that unit sizes changed by a factor of six (1950 to 1970), heat rate diminished by less than 20%, a

change that includes the combined cycle. In reality, the improvement is even less, as environmental

regulations and the energy required to satisfy them can consume up to 6% or so of a unit's generated

power.

The rate of improvement of turbine cycle heat rate is obviously decreasing. Powerplant and

machinery designers are working hard to achieve small improvements both in new designs and in

retrofit and repowering programs tailored to existing units. Considering the worth of energy, what,

then, are our options leading to thermal performance improvements and the management of our

energy and financial resources? Exotic energy-conversion processes are a possibility: MHD, solar

YEAR

Fig. 58.1 Steam cycle development.

Fig. 58.2 Fossil-fueled unit heat rate as a function of time.

power, the breeder reactor, and fusion are some of the longer-range possibilities. A more near-term

possibility is through the improvement (increase) of steam conditions. The effect of improved steam

conditions on turbine cycle heat rate is shown in Fig. 58.3, where heat rate is plotted as a function

of throttle pressure with parameters of steam temperature level. The plus mark indicates the placement

of the Eddystone unit previously mentioned.

58.2 THE HEAT ENGINE AND ENERGY CONVERSION PROCESSES

The mechanism for conversion of thermal energy is the heat engine, a thermodynamic concept,

defined and sketched out by Carnot and applied by many, the power generation industry in particular.

The heat engine is a device that accepts thermal energy (heat) as input and converts this energy to

useful work. In the process, it rejects a portion of this supplied heat as unusable by the work pro-

duction process. The efficiency of the ideal conversion process is known as the Carnot efficiency. It

serves as a guide to the practitioner and as a limit for which no practical process can exceed. The

Carnot efficiency is defined in terms of the absolute temperatures of the heat source T ho t and the heat

sink r col d as follows:

Carnot efficiency - ^ 0 1 ~ 7 ^

(58.1)

•Miot

Consider Fig. 58.4, which depicts a heat engine in fundamental terms consisting of a quantity of

heat supplied, heat added, a quantity of heat rejected, heat rejected, and an amount of useful work

done, work done. The thermal efficiency of this basic engine can be defined as

Fig. 58.3 Comparison of turbine cycle heat rate as a function of steam conditions.

Heat

added

Work

Heat

done

engine

Heat

rejected

Fig. 58.4 The basic heat engine.

r _ .

work done

efficiency =

——-

(58.2)

heat added

This thermal efficiency is fundamental to any heat engine and is, in effect, a measure of the heat

rate of any turbine-generator unit of interest. Figure 58.5 is the same basic heat engine redefined in

terms of turbine cycle terminology, that is, heat added is the heat input to the steam generator, heat

rejected is the heat removed by the condenser, and the difference is the work done (power) produced

by the turbine cycle. Figure 58.6 is a depiction of a simple turbine cycle showing the same parameters,

but described in conventional terms. Heat rate is now defined as the quantity of heat input required

to generate a unit of electrical power (kW).

heat added

heat rate =

(58.3)

work done

The units of heat rate are usually in terms of Btu/kW-hr.

Further definition of the turbine cycle is presented in Fig. 58.7, which shows the simple turbine

cycle with pumps and a feedwater heater included (of the open type). In this instance, two types of

heat rate are identified: (1) a gross heat rate, in which the turbine-generator set's natural output (i.e.,

gross electrical power) is the denominator of the heat rate expression, and (2) a net heat rate, in

which the gross power output has been debited by the power requirement of the boiler feed pump,

resulting in a larger numeric value of heat rate. This procedure is conventional in the power-generation

industry, as it accounts for the inner requirements of the cycle needed to make it operate. In other,

more complex cycles, the boiler feed pump power might be supplied by a steam turbine-driven feed

pump. These effects are then included in the heat balance describing the unit's performance.

The same accounting procedures are true for all cycles, regardless of their complexity. A typical

450-mW fossil unit turbine cycle heat balance is presented in Fig. 58.8. Steam conditions are 2415

Heat added in

steam generator

Electrical power

Turbine

generated

cycle

*•

Heat rejected

in condenser

Fig. 58.5 The basic heat engine described in today's terms.

Fig. 58.6 A simple turbine cycle.

psia/1000°F/1000°F/2.5 inHga, and the cycle features seven feedwater heaters and a motor-driven

boiler feed pump. Only pertinent flow and steam property parameters have been shown, in order to

avoid confusion and to support the conceptual simplicity of heat rate. As shown in the two heat rate

expressions, only two flow rates, four enthalpies, and two kW values are required to determine the

gross and net heat rates of 8044 and 8272 Btu/kW-hr, respectively.

To supplement the fossil unit of Fig. 58.8, Fig. 58.9 presents a typical nuclear unit of 1000 mW

capability. Again, only the pertinent parameters are included in this sketch for simplicity. Steam

conditions at the throttle are 690 psi with 1 A% moisture, and the condenser pressure is 3.0 inHga.

The cycle features six feedwater heaters, a steam turbine-driven feed pump, and a moisture separator

reheater (MSR). The reheater portion of the MSR takes throttle steam to heat the low-pressure (LP)

flow to 473 0 F from 369 0 F (saturation at 164 psia). In this cycle, the feed pump is turbine-driven by

steam taken from the MSR exit; hence, only one heat rate is shown, the net heat rate, 10,516 Btu/

kW-hr. This heat rate comprises only four numbers, the throttle mass flow rate, the throttle enthalpy,

the final feedwater enthalpy, and the net power output of the cycle.

Fig. 58.7 A simple turbine cycle with an open heater and a boiler feed pump.

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