Test: Boiler & Steam Turbine - 2 - Mechanical Engineering MCQ

A steam boiler is designed to absorb the maximum amount of heat released from the process of combustion.

Heat transfer within a steam boiler is accomplished by three methods: radiation, convection, and conduction.

The heating surface in the furnace area receives heat primarily by radiation. The remaining heating surface in the steam boiler receives heat by convection from the hot flue gases. Heat received by the heating surface travels through the metal by conduction, heat is then transferred from the metal to the water by convection.

Locomotive boiler:

• A fire-tube boiler is sometimes called a “smoke-tube boiler” or “shell boiler”.
• This type of boiler was used on virtually all steam locomotives in the horizontal “locomotive” form.
• It has a cylindrical barrel containing the fire tubes but also has an extension at one end to house the firebox.
• The locomotive boiler is mainly used in the train engine which was running on coal.
• A locomotive boiler is a multi-tubular, horizontal, internally fired, and mobile boiler. It consists of a shell or barrel having 1.5 m diameters and 4 m in length. 

Features and characteristics of Locomotive boiler:

• Internally fired,
• Horizontal,
• Multi-tubular,
• Fire tube,
• Natural circulation,
• Artificial draft,
• Portable boiler

• A. Lancashire Boiler: This is a type of fire-tube boiler known for having two internal horizontal fire tubes. It would best match with "Horizontal straight tube, fire-tube boiler."
• B. Benson Boiler: The Benson boiler is known for its ability to operate at critical pressures, making it a high-pressure boiler. Hence, it matches "High pressure boiler."
• C. Babcock and Wilcox Boiler: This is a water-tube boiler with inclined tubes, which best fits "Inclined straight tube, water-tube boiler."
• D. Stirling Boiler: A Stirling boiler features bent tubes instead of straight ones and is a water-tube boiler, making it align with "Bent tube, water-tube boiler."

Using the information above, the correct matches from List I to List II would be:

• A -> 1
• B -> 4
• C -> 2
• D -> 3

The correct answer is therefore (b): A: 1, B: 4, C: 2, D: 3.

• A: It has a very small drum compared to a conventional boiler: This statement, as you highlighted, is not true because supercritical steam generators do not have a drum at all. The operation of supercritical boilers is based on the principle that water at supercritical pressure does not undergo a distinct phase change, and thus, no steam drum is needed to separate steam from water.
• B: A supercritical pressure plant has higher efficiency than a subcritical pressure plant: This statement remains true. The efficiency of supercritical plants is higher due to their ability to operate at higher temperatures and pressures, which increases the thermal efficiency of the steam cycle.
• C: The feed pressure required is very high, almost 1.2 to 1.4 times the boiler pressure: While this statement might sound misleading or exaggerated, the critical aspect here is understanding the pressure comparison. In practical terms, the feedwater pump pressure must exceed the critical pressure to ensure it remains in a fluid state; however, it does not necessarily have to be 1.2 to 1.4 times the boiler's operational pressure. This factor depends on the specific boiler design and operational parameters.
• D: As it requires absolutely pure feed water, preparation of feed water is more important than in a subcritical pressure boiler: True. The need for extremely pure feedwater in supercritical boilers is crucial to avoid the detrimental effects of scaling and corrosion at high temperatures and pressures.

An attemperator is a device used in steam boilers to control the degree of superheat of the steam. By controlling the temperature of the steam after it leaves the superheater and before it enters the turbine, the attemperator ensures the steam is within the materials' tolerance limits, preventing damage and enhancing efficiency.
Other Options:

• A: Ahead of super heater for initial superheating: Incorrect, as the attemperator does not function ahead of the superheater for initial superheating. It is placed between stages of the superheater or after the final superheater.
• B: For optimizing steam output from the generator: Incorrect, while controlling superheat indirectly affects efficiency and performance, the primary function of the attemperator is not to optimize steam output but to control steam temperature.
• C: To regulate steam pressure: Incorrect, the primary role of the attemperator is not to regulate pressure but to control the temperature of the steam.

Thus, D is the correct answer as it directly relates to the primary function of an attemperator in utility boilers.

Assertion (A): An 'air-to-close’ valve should be used to control the fuel supply to the furnace.
This assertion is true because an 'air-to-close' valve, which closes when the air pressure fails or is removed, ensures that in the event of a pneumatic system failure, the valve will close automatically. This type of valve is critical for safety in systems where uninterrupted fuel supply in the absence of control can lead to dangerous situations such as overheating or fire.

Reason (R): In the event of air failure, the valve would be closed and the fuels cut off to prevent overheating.
This reason is a true statement and correctly explains why an 'air-to-close' valve is used. The closing of the valve in the event of an air failure is a fail-safe feature designed to immediately stop the fuel supply, thereby preventing potential overheating and ensuring the safety of the system.

The reason (R) correctly and directly explains the assertion (A), affirming that the choice of an 'air-to-close' valve is primarily for safety to automatically cut off fuel supply in the event of air failure, making A the correct answer.

The Velox boiler is notable for its use of supersonic airflow to enhance combustion efficiency. This feature involves forcing air through the combustion chamber at speeds exceeding the speed of sound, significantly improving heat transfer and operational efficiency. Other options like being drumless or operating under supercritical pressures are not distinctive characteristics of the Velox boiler. Thus, the unique attribute of the Velox boiler is its use of supersonic flow.

Impulse Turbine:

• In Impulse Turbine, the available hydraulic energy is first converted into kinetic energy by means of an efficient nozzle
• The high-velocity jet issuing from the nozzle then strikes a series of suitably shaped buckets fixed around the rim of a wheel
• The buckets change the direction of the jet without changing its pressure
• The resulting change in momentum sets buckets and wheel into rotary motion and thus mechanical energy is made available at the turbine shaft
• There is no drop of pressure in the moving blade after coming out of the nozzle and is equal to atmospheric pressure generally.
• The absolute velocity of the steam decreases as it proceeds further in the moving blade.
• In an impulse turbine, the relative velocity at the inlet is the same as the relative velocity at the outlet if friction is neglected.
• Some resistance is always offered by the blade surface to the idling steam, whose effect is to reduce the velocity of the jet.
• Due to the friction velocity of steam jet decreases. which causes the reduction of momentum transfer between the steam jet and blade. Hence work done by the steam jet on blades decreases.
• The blade friction in the impulse turbine reduces the velocity of steam 10 to 15%, as the steam passes over the blade.

Velocity diagram for impulse turbine considering blade friction:

u = Tangential velocity of blades

v_a1,v_a2 ⇒  the absolute velocity of steam at inlet and outlet of a moving blade.

v_w1, v_w2 ⇒ the velocity of whirl at inlet and outlet of a moving blade. 

v_f1, v_f2 ⇒ the velocity of flow at the inlet and outlet of a moving blade. 

v_r1, v_r2 ⇒ the relative velocity of steam at inlet and outlet of a moving blade. 

α₁, α₂  ⇒ outlet, and inlet angle of a fixed blade. 

β₁, β₂ ⇒ inlet and outlet angle of a moving blade. 

When friction is neglected at the blade surfaces, v_r1 = v_r2

In the given the question V_r1 = C₁ , V_r2 = C₂ and V_a1 = W₁ , V_a2 = W₂ have been used

So, The Velocity diagram for impulse turbine without blade friction as per the question notation is given by,

Given the ambient air conditions with a dry-bulb temperature of 45°C and a wet-bulb temperature of 27°C, the lowest possible condensing temperature for an evaporative cooled condenser cannot be below the wet-bulb temperature. This is because the wet-bulb temperature represents the lowest temperature achievable through evaporative cooling.
Given the wet-bulb temperature of 27°C:

• A: 25°C: This is below the wet-bulb temperature, and thus not feasible since evaporative cooling cannot cool below the wet-bulb temperature.
• B: 30°C: This is slightly above the wet-bulb temperature and is a plausible achievable temperature for the condenser, representing a realistic operational efficiency of the cooling system.
• C: 42°C and D: 48°C: Both of these temperatures are significantly higher than the wet-bulb temperature, making them less efficient and not the lowest possible temperatures.

Therefore, the correct answer is B: 30°C, as it is the lowest realistic temperature that is above the wet-bulb temperature, thus feasible for an evaporative cooled condenser under these conditions.

Diaphragm in steam turbines is a separating wall between rotors carrying nozzles. Additionally, as there is a significant pressure drop across a reaction turbine stage, the diaphragm also acts as a partition between the pressure stages.

Figure at (a) is for impulse turbine blade. As no pressure change from inlet to outlet no area change is allowed.

The correct answer is A: 1, 2, and 3. Here’s why each of the statements about impulse steam turbines is correct:

1. Blade passages are of constant cross-sectional area: In impulse steam turbines, the steam expands completely in the nozzle before hitting the turbine blades. This type of turbine uses blades that have passages of constant cross-sectional area. The energy transfer occurs primarily through kinetic energy changes as the steam strikes the blades, which contrasts with reaction turbines where the steam continues to expand as it passes through varying cross-sectional areas in the blade passages.
2. Partial admission of steam is permissible: Impulse turbines can operate with partial admission, where steam is admitted to only a portion of the circumference of the rotor. This feature is beneficial in controlling the flow rate and the power output, especially in turbines that do not operate at full capacity continuously. It allows for better adaptation to varying load demands without sacrificing efficiency.
3. Axial thrust is only due to change in flow velocity of steam at inlet and outlet of moving blade: In impulse turbines, the axial thrust is primarily due to the change in flow velocity of steam as it enters and exits the blades. Unlike reaction turbines, which experience additional thrust due to pressure differences across the blades, the axial thrust in impulse turbines comes only from the momentum changes of the steam flow, making this statement accurate.

Thus, all three statements accurately describe characteristics of impulse steam turbines, making Option A the correct choice.

The efficiency of an impulse turbine is maximum when the blade speed (V_b​) is equal to half the velocity of steam (V) entering the blade, adjusted for the angle (α\alphaα) of the steam at the nozzle. This optimal velocity condition is expressed as:

V_b = 0.5 × V × cos⁡(α)

Therefore, the correct answer is:

A: V_b = 0.5V cos⁡α

Compounding is done to decrease the high rotational speed of simple impulse turbines to bring it down to the practical limit required for electricity generation. There are two ways of compounding i.e. pressure compounding and velocity compounding.

Compounding in steam turbines is primarily utilized to reduce the rotor speed to practical limits. In an impulse steam turbine, the steam's high velocity can cause the rotor to spin at excessively high speeds, which are not suitable for most mechanical systems or power-generating equipment. By implementing compounding, where the turbine's total power output is distributed across multiple stages or sets of blades, the rotational speed at each stage is significantly reduced. This makes it feasible to connect the turbine to standard electrical generators and other machinery, which require lower operational speeds to function effectively and safely.

• A. Relative velocity of steam at inlet tip of blade corresponds to the nozzle angle (1), as the steam enters the blade passage at this angle.
• B. Absolute velocity of steam at inlet tip of blade corresponds to the moving blade leading edge angle (2), since this is where the steam strikes first on the moving blade.
• C. Relative velocity of steam at outlet tip of blade corresponds to the moving blade trailing edge angle (3), as this is where the steam exits the moving blade.
• D. Absolute velocity of steam at outlet tip of blade corresponds to the fixed blade leading edge angle (4), which influences the direction of the steam leaving the turbine blades.

Correct Matching: Based on this, the correct matching is: (b): A - 2, B - 1, C - 3, D - 4
Thus, the correct answer is Option B: b.

Assertion (A): Impulse staging is commonly employed in the high-pressure part of the steam turbine, and reaction staging is used in the intermediate and low-pressure parts. This is true because impulse stages are better suited for handling the higher velocities and pressures at the turbine's inlet. As the steam expands and pressure decreases, reaction stages become more efficient in the intermediate and low-pressure sections.

Reason (R): In impulse staging, the pressure drop occurs primarily across the fixed blades or nozzles, which reduces the pressure across the moving blades. This small pressure drop across the moving blades results in less tip leakage compared to reaction turbines, where the pressure drop is shared between both fixed and moving blades. Therefore, the reason is true and correctly explains the assertion.

Thus, both A and R are true, and R provides the correct explanation for A.

Compounding of steam turbines is done to reduce the excessively high rotational speed of the turbine. In impulse turbines, when steam expands in a single stage, it results in very high rotational speeds, which are impractical for mechanical systems. By using compounding, where the pressure drop is divided across multiple stages, the rotational speed of the turbine is reduced to a more practical level, making it suitable for real-world applications such as driving generators or machinery.

The most effective way to bring down the running speeds of a steam turbine to practical limits is compounding. Compounding reduces the high rotational speed by dividing the energy extraction process into multiple stages, which lowers the turbine’s overall speed without sacrificing performance.

1. Using a heavy flywheel: This helps in smoothing out speed fluctuations but doesn't reduce the high running speed of the turbine.
2. Using a quick response governor: This helps control speed but is more about regulating fluctuations rather than directly reducing high turbine speeds.
3. Compounding: The primary method for reducing turbine speed. It divides the energy conversion process into multiple stages, effectively reducing the rotor speed to a practical level.
4. Reducing fuel feed to the furnace: This would reduce power output but isn't an effective way to control the running speed of the turbine.

Thus, compounding is the main method for reducing turbine speed, making Option A the correct choice.

Pressure compounding (Rateau staging) involves dividing the total pressure drop of the steam across multiple stages of nozzles and blades. This results in lower steam velocities at each stage, which helps in achieving high efficiency at relatively low fluid velocities. By controlling the velocity and reducing pressure in stages, the turbine can extract energy more efficiently from the steam without the need for excessively high steam velocities, which would otherwise lead to energy losses and inefficiencies.

Thus, high efficiency with low fluid velocities is the key feature of pressure compounding in Rateau staging.

3 is wrong because blade velocity coefficient for an impulse turbine is of the order less than

• Modern turbines have velocity compounding at the initial stages and pressure compounding in subsequent stages.
• This is due to fact that there is a high enthalpy drop in velocity compounding turbine (one 2 row Curtis stage is equivalent to eight 50% reaction), two single-stage impulse turbines and also there is higher density of steam. Hence statement I is true.
• Now, statement II says excessive tip leakage occurs in the high-pressure region of reaction blading i.e. at the Curtis stage, which is also true but along with it there is another cause also which is having all pressure stages or reaction stages, the length and size of the turbine become very large which eventually increases the cost of the turbine.

Since both the assertion and reason are individually correct but the reason is not the only or main cause, so the correct option is option (A).

Pressure-velocity compounding of steam turbines results is less number of stages

work done in different stages in velocity compounding is

In Parson's reaction turbines, the velocity diagrams at the inlet and outlet are congruent. This is because in a reaction turbine, the steam experiences an equal pressure drop in both the fixed and moving blades, resulting in symmetrical flow conditions at the inlet and outlet of each stage. Consequently, the velocity triangles at the inlet and outlet are identical or congruent in shape, reflecting equal velocity conditions as the steam enters and exits the blades.

Concept:

In steam turbines, the steam jet flows over the moving blades, imparting kinetic energy to the blades and causing the turbine to rotate. Frictional resistance plays a significant role in this process, affecting the efficiency and performance of the turbine.

Analysis of Statements:

1. There is no effect of frictional resistance to the flow of the steam jet over the blade: Incorrect. Frictional resistance affects the flow of steam over the blades, causing energy losses and a reduction in velocity.
2. The velocity of flow at the outlet to the moving blade is less than that of flow at the inlet to the moving blade: Correct. Due to frictional resistance, the steam jet loses kinetic energy as it flows over the blades. This loss of energy results in a decrease in velocity at the outlet compared to the inlet.
3. The velocity of flow at the outlet to the moving blade is greater than that of flow at the inlet to the moving blade: Incorrect. Frictional losses would cause a reduction in velocity, not an increase.
4. The velocity of flow at the outlet to the moving blade is equal to that of flow at the inlet to the moving blade: Incorrect. Frictional resistance causes a loss in kinetic energy, leading to a lower velocity at the outlet compared to the inlet.

Conclusion

The correct statement regarding the effect of frictional resistance to the flow of the steam jet over the blade in steam turbines is:
2) The velocity of flow at the outlet to the moving blade is less than that of flow at the inlet to the moving blade.

Degree of reaction is the ratio of enthalpy drop in the rotor to the enthalpy drop in a stage.

In the case of impulse turbine, there is no change in the enthalpy in the rotor, so the degree of reaction is zero.

R = 0.

And for Parson’s turbine or 50% reaction turbine R = 0.5 and for Hero’s turbine R = 1.