Small Decentralized Hydropower Program National ... - Cd3wd.com

sistema metric0 y metros en el sistema metrko. La
conversi6n es la siguiente:
n, = 43.5 Ns (Unidades inglesas)
n, = 193.1 N s (Unidades m&ricas utiiizando caballos de
fuena de1 sistma mCtrico)
n, = 166 Ns (Unidades m&-icas utilizando kW para fuerza)
Asi, una turbina fabricada en el Japdn y ensayada
en Europa tendria una velocidad especifica 4.4 veces
rnas tilta que la misma turbina ensayada en 10s
EE.UU. Nos pregunatamos cual seria la velocidad
especffica en la luna!
Las ecuaciones (19), (22) y (23) describen la escala
de la tasa de flujo, veiocidad, y potencia con ei
tamario y la &da para una maquina determinada,
suponlendo una veiocidad especgica con&ante. Cada
diseno de una maquina tiene una eficiencia de punta
a un valor de velocidad especifica. Este es el valor de
la velocidad especifica citada por el diseriador o
fabricante. La forma en que la eficiencia varia con la
velocidad especifica es una funcidn de la clase de
maquina.
La Tecnologia de las Turbinas
Descripcih de las Turbinas
Hay dos clases basicas de turbinas, llamedas a im-
pulsos y de reacci6n. En una turbina de impulsos, la
&da disponible se convierte a energia cinetica a una
presi6n atmosferica antes de entrar en el rotor, ex-
trayendose la energia disponible de1 flujo a presi6n
atmosferica. En una turbina de reacci6n, el rotor
est.& completamente sumergido y tanto la presi6n
coma la velocidad disminuyen desde la entrada hasta
la salida. En cualquiera de las maquinas, la torsi6n es
igual a la tasa de cambio de1 impulso angular a
traves de la m&quina tal coma lo expresa la ecuaci6n
Euler.
Las turbinas modernas de impulsos consisten en
ruedas Pelton, turbinas Turgo y turbinas de Flujo
Transversal. Las .lurbinas de reaccidn son o bien tur-
binas radiales, turbinas Francis de flujo mixto, o tur-
binas a helice de flujo axiai. Hay muchas variaciones
de las turbinas de flujo axial. La turbina Kaplan tiene
helices de Bngulo ajustables y compuertas de
mariposa, tal corn0 se ilustra en la figura 4.
Las variaciones modernas de Ia turbina de flujo
axial comprenden 10s tipos de tubo, bombilla y
Straflow ilustrados en la figura 5. El tipo de tuba
puede ser una helice fija, semi-Kaplan o totalmente
Equations (i9), (22), and (23) describe the scal-
ing of flow rate, speed, and power with size and
head for a given machine assuming constant
specific speed. Each machine design has peak
efficiency at one value of specific speed. It is
this value of specific speed that is quoted by the
designer or manufacturer. The manner in which
efficiency varies with specific speed is a function
of the type of machine.
Turbine Technology
Description of Turbines
There are two basic types of turbines, denoted
as impulse and reaction. In an impulse turbine,
the avialabje head is converted to kinetic
energy at atmospheric pressure before entering
the runner, the power available being extracted
from the fiow at atmospheric pressure. In a
reaction turbine, the runner is completely
submerged and both the pressure and the
velocity decreases from inlet to outlet.
In either machine the torque is equal to
the rate of change of angular momentum through
the machine as expressed by the Euler equation.
Modern impulse turbines consist of Pelton
wheels, Turgo and Cross Flow turbines. Reaction
turbines are either radial or mixed flow Francis
turbines or axial flow propeller turbines. There
are many variations of the axial flow turbine. The
Kaplan turbine has adjustable blades and wicket
gates, as shown in Fig. 4.
Modern variations of the axial flow turbine in-
clude the tube, bulb and Staflow types as i!-
lustrated in Fig. 5. The tube type can be fixed
propeller, semi-Kaplan or fully adjustable. The
externally-mounted generator is driven by a shaft.
The flow enters and exits with relatively minor
changes in direction. The bulb turbine features a
generator and turbine housed within a pod or
bulb and is generally used in larger applications.
The Straflow turbine is an elegant solution to the
generator problem and was first proposed by the
founder of the Harza Engineering Company,
Leroy F. Harza. In this variation, the generator is
mounted at the periphery of the turbine runner
thereby providing minimum obstruction to the
flow. Two examples of Francis turbines are
shown in Fig. 6. The similarity to a cen,irifugal
pump should not go unnoticed. In fact, pumps
can be operated as turbines. The use of small
pumps as small turbines is currently under
study.5 A typical Pelton wheel arrangement is
shown in Fig. 7a. The lower portion of the figure
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