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Chapter 6

Q. 6.36

(a) Why is drum type construction preferred to disc type construction in reaction turbine ?
(b) Why is partial admission of steam adopted for H.P. impulse stages while full admission is essential for any stage of a reaction turbine ?
(c) In a 50% reaction turbine, the speed of rotation of a blade group is 3000 r.p.m. with mean blade velocity of 120 m/s. The velocity ratio is 0.8 and the exit angle of the blades is 20°. If the mean blade height is 30 mm, calculate the total steam flow rate through the turbine. Neglect the effect of blade edge thickness of the annular area but consider 10% of the total steam flow rate as the tip leakage loss. The mean condition of steam in that blade group is found to be 2.7 bar and 0.95 dry.
(d) What do you mean by once through boilder ?                                                                           (AMIE Summer, 1998)

Step-by-Step

Verified Solution

(a) The rotor of the turbine can be of drum type or disc type. Disc type construction is difficult (complicated) to make, but lighter in weight. Hence the centrifugal stresses are lower at a particular speed. On the other hand drum type construction is simple in construction, and it is easy to attach aerofoil shape blades. Further it is easier to design for tip leakage reduction which is a major problem in reaction turbines. Moreover due to small pressure drop per stage (larger number of stages) in reaction turbines, their rotational speeds are lower and so the centrifugal stresses are not very high (even the reaction blades are lighter). Therefore drum type construction is preferred to disc type in reaction turbines.
To accommodate increase in specific volume at lower pressures the drum diameter is stepped up which allows greater area without unduly increasing blade height. The increased drum diameter also increases the torque due to steam pressure.
(b) In impulse turbines there is no expansion of steam in moving blades, and the pressure of steam remains constant while flowing over the moving blades. The expansion takes place only in the nozzles at the inlet to the turbine in H.P. stages, or through the fixed blades in the subsequent stages. The nozzles need not occupy the complete circumference. Therefore partial admission of steam is feasible and adopted for H.P. impulse stages.
In reaction turbines, pressure drop is required in the moving blades also. This is not possible with partial admission. Hence full admission is essential for all stages of a reaction turbine.
(c) Refer Fig. 37.

Given :                                    N = 3000 r.p.m. ; φ = α = 20° ;   C_{bl}  =   120   m/s  ;      \frac{C_{bl}}{C_{1}}  = 0.8 ;

∴                                                    C_{1}  =   \frac{C_{bl}}{0.8}  =   \frac{120}{0.8}   = 150 m/s

Also                                                          C_{bl}  =  \frac{πDN}{60}

or                                                             120 =     \frac{πDN}{60}

∴                                                              D =   \frac{120    ×     60}{π    ×     3000} = 0.764 m.

From steam tables,   v_{g}   (at 2.7 bar) = 0.668 m³/kg
v = 0.95 × 0.668 = 0.6346 m³/kg
Flow area                                            A = πDh = π × 0.764 ×   \frac{30}{1000}   =  0.072  m²

Flow velocity                                    C_{f}  =   C_{1}   sin   α   =   150    sin    20°   =   51.3   m/s          (C_{f_1}    =  C_{f_0}    =   C_{f} )

Mass flow rate                                          \dot{m}  =  \frac{A  C_{f}}{v}  =  \frac{0.072    ×     51.3}{0.6346}      = 5.82 kg/s

Accounting for 10 per cent leakage (of total steam flow), the total steam flow rate is

\frac{5.82}{0.9}    = 6.467 kg/s. 

(d) Once through boiler is a boiler which does not require any water or steam drum. It is a monotube boiler using about 1.5 kg long tube arranged in the combustion chamber and the furnace. The economizer, boiler and superheater are in series with no fixed surfaces as separators between the steam and water.
Benson boiler is an example of once through boiler, operating at supercritical pressure. The tube length to diameter ratio of such a boiler is about 2500. Due to large frictional resistance the feed pressure should be about 1.4 times the boiler pressure.

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