Effect on Efficiency of a Steam Turbine of Variation of Size of a Two-Row Velocity Stage Wheel

134 Pages·04.001 MB·English

by  Ahmad, Mukhtar

EFFECT ON EFFICIENCY OF A STEAM TURBINE OF
VARIATION OF SIZE OF A TWO-RON VELOCITY
STAGE WHEEL
THESIS
Submitted in partial fulfilment
of the requirement for the
degree of
MASTER OF MECHANICAL ENGINEERING
at the
/POLYTECHNIC -INSTITUTE OF BROOKLYN
MUKHTAR AHMAD
June 1950
Approved;-
Thesi
ead of Depa/Went
ProQuest Number: 27591586
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DEDICATED
to
iny
/\FATHER
ACKNOWLEDGEMENT
The author expresses his deep appreciation and sincere
thanks to Prof. Edwin F, Church Jr. of the Mechanical Engi­
neering department. Polytechnic Institute of Brooklyn , for
his encouragement and guidance in the completion of the
present thesis.
VITA
The author , î-iuklitar Ahmad , was born on October 1st. 1927
in the town of Azamgarh , U.P, j,India.) ■ . He graduated from
Muslim University High School in the year 1941. He joined the
Engineering Colle^g , hhislim University , Aligarh in 1945 after
completing two years in the University., He took his Bachlor's
Degree in Mechanical Engineering in June 1949, During his stty
in the College the author compiled a comprehensive design report
on "Design of Thermo-electric Power Station".
After graduation the author joined the Tata Iron and Steel
Works , Jamshedpore, Tatanagçr (India) , as an apprentice in
December 1346 and was with them till June 1347, The author aws
selected by Government of Pakistan for studies in this country.
The author |iad an opportunity to be a lecturer in Mechanical
Engineering Department of the N.E.D. Engineering College, Karachi
(Pakistan) from February 1948 till July 1948,
He came to this country in August 1948 and joined the Poly-
tchnic Institute of Brooklyn as a full-time graduate student in
the Mechanical Engineering Department, The present investigation
■statted in May 1949 and was completed in January 1350,
TABLE OF CONTENT;
Page
(l) Introduction
(2) Object and Working.
procedure 4
(3) Discussions 7
(4) Results and Conclusions---------------- 15
(b) Sample Calculations ------ — 19
USED.
A, Ap Cross- sectional area of nozzle at exit and throat
Drv^ Diameter of rotor wheel at mean blade height
h Blade height in inches
h ^ Nozzle exit height at ex
Ah Enthalpy difference a^on actual or irreversible path
in BTU per lb
Cumulative enthalpy drop in BTU per lb
Hpj_ Hors© Power loss due to disc friction
Velocity coefficient for flow through a nozzle
kh Velocity coefficient for flow through blade passage
m Thickness coefficient for blades or nozzle edges
N Revolution per minute
P Absolute pressure in psia
Mean c ircumfrencial pitch
R Reheat Factor
Reheat Factor for infinite no. of stages
r = p/p Fressure Ratio
2. '
T-fR Ratio of blade width to radius of blade face
s Entropy
V^ Absolute velocity of steam issuing from nozzle
V3 Absolute velocity of steam at exit from blades
Vgy Relative velocity of steam at inlet to blades
V^y Relative velocity .of steam at out let from blades
Vb Mean peripheral velocity of blades
Velocity of whirl
V^ Velocity of axial flow
V (Specific Volume of steam in cu ft per Ih
¥ Weight flow in lb per sec
X Quality of steam
o( Nozzle angle
|3 Blade enterence angle
Y Blade exit angle
X Absolute direction of steam leaving blades
Nozzle efficiency
I
p Ratio of blade speed to steam speed
0- Percentage of circumfrenece occupied by active nozzles
Combined nozzle and blading efficiency
n b
Blading or Diagram efficiency
Stage efficiency
1
Overall efficiency
INTRODUCTION
With increasing demand from Central Poorer Stations for high­
er efficiency , reliability and comparatively cheap turbines , high­
er rotative speeds were introduced to meet this demand. For a 50-
cycle generator a two - pole alternator will require a speed of 500u
RPM and for 6u - cycle a speed of 5,000 RPH,
•Mr» O.B, Warren of the General Electric Company ( reference #
has pointed out that the turbine efficiency is increased §.s the des­
igned speed is increased j and that the increase is small in the last
stages of the turbine due to high specific volume of steam. But when
the volume of steam is small -, which is generally the case when hi^
pressure and high temperature steam is used , the g- in in efficiency
is greater with increasing speed. This fact made it possible to use
high pressures and temperatures in the turbines. Another important
feetor in using higher pressures and temperatures in turbines is
that it is easier to build reliable turbines if the dimensions of t
the shell and rotor etc are kept small , and higher pressures and
temperatures provide these conditions. Go it becomes a very signifi­
cant point in the design of a turbina to design the e-arly stages
of this high pressure,and high temperature turbine with great care.
In this country two- row velocity wheels are nearly uni­
versally used as the first stage in large and medium size turbines,
The choice of a t'wo-ro^v velocity wheel in the early sU-.ges is usual­
ly the best design for reasons to be stated presently.
.Suppose we were to use pressure stages instead of a two-row
velocity wheel. The number of pressure stages required for replacing
(2)
a two-row velocity wheel will be about to 6 stages, ( in this
case , depending upon the size of the wheels) in order to absorb
the energy utilizes in a two - row velocity wheel. This increase
in the number of stgaes of the turbine might increase the.overall
efficiency of the turbine. But the fact remains that there will be
y.
.
considerable amount of loss in partial admission in all these
early -pres sure .-.stages as compared to losses in the two-rox velocity
wheel. These losses are due to the. fact that the specific volume-is
small at high pressures and the flow area required is such that
partial admission has to be introduced. If 'we want to avoid .partial
admission and cut down the disc friction losses , with the same area
available for flow , the nozzle heights and the cor ‘esponding blade
heights will be shorter. But using shorter blades ther is a definite
decrease in the nozzle efficiency , and the stage efficiency shows
a greater trend towards low values • This may be due to t^;fact
; . ..
that tfie turbulance and other disturbances in the flow passages are
q-. ite Hight. Not only this but by using two-row/wiieel the pressures
within the shell will be considerably lowered and hence- the ^pressure
' ' A,
on high pressure packings and on diaphgram packings will be reduced
to a graeter extent. These losses are major items to be taken-care
of in low volume flow. The rotational losses in the early stages of
the turbine are also reduced considerably^
• r.'. ' ;
There is no doubt that a t%m-row: velocity stage is not as
efficient as a single pressui'e stage , but the efficiency . ith a
two-row velocity stage rems.ins relatively constant over a wide r
range of operating loads. We find that use of a t o-row velocity
stage is desirable for reasons of efficiency , less cost and greater,
reliability in operation.