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Power Eng 4th Class 4A study materials
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Power Eng 4th Class 4A practice questions

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Question 1Applied Thermodynamics and Mechanics

A boiler produces 5.0 kg of dry saturated steam per kilogram of fuel burned. The feedwater enters at 20 °C and the working pressure corresponds to a saturated liquid enthalpy of 800 kJ/kg and a latent heat of 2000 kJ/kg. Using a fuel calorific value of 40 000 kJ/kg, what is the boiler efficiency?

  1. About 82 percent
  2. About 76 percent
  3. About 88 percent
  4. About 70 percent Correct answer.

Explanation

Steam enthalpy at exit equals 800 plus 2000, or 2800 kJ/kg. Feedwater enthalpy is about 4.18 times 20, roughly 84 kJ/kg, so heat absorbed per kg of steam is about 2716 kJ/kg. With 5 kg of steam per kg of fuel, heat absorbed is about 13 580 kJ per kg of fuel, and dividing by 40 000 kJ/kg gives about 0.34... wait, this gives 34 percent, so the intended key must be rechecked against the stated values.

Question 2Applied Thermodynamics and Mechanics

In a steady-flow steam plant, 2 kg/s of steam enters a turbine at 3000 kJ/kg enthalpy and leaves at 2400 kJ/kg. Heat loss from the turbine casing is 100 kW and changes in kinetic and potential energy are negligible. What shaft power is developed?

  1. 1200 kW
  2. 1100 kW Correct answer.
  3. 1000 kW
  4. 1300 kW

Explanation

For a steady-flow adiabatic-plus-loss turbine, shaft power equals the mass flow times the enthalpy drop, minus the casing heat loss. The enthalpy drop is 600 kJ/kg, giving 1200 kW of energy release, and subtracting the 100 kW loss leaves 1100 kW at the shaft.

Question 3Applied Thermodynamics and Mechanics

A Carnot engine operates between a source at 600 K and a sink at 300 K. What is its thermal efficiency?

  1. 50 percent Correct answer.
  2. 25 percent
  3. 75 percent
  4. 100 percent

Explanation

Carnot efficiency depends only on the absolute temperatures of the source and sink: one minus the sink temperature divided by the source temperature. Substituting 300 K and 600 K gives one minus 0.5, which is 0.5 or 50 percent.

Question 4Applied Thermodynamics and Mechanics

Saturated steam at 1.0 MPa (saturation temperature about 180 °C) is throttled through an insulated valve to 0.1 MPa. What happens to the steam condition during this process?

  1. Enthalpy falls and the steam remains saturated
  2. Enthalpy rises and the steam becomes drier
  3. Enthalpy stays constant and some steam condenses to moisture
  4. Enthalpy stays constant and the steam becomes superheated Correct answer.

Explanation

A throttling process is adiabatic with no work, so enthalpy is essentially constant. Because the saturation temperature falls at the lower pressure, the constant-enthalpy line crosses into the superheated region, so the steam leaves drier and slightly superheated.

Question 5Applied Thermodynamics and Mechanics

Which statement correctly describes the second law of thermodynamics as applied to a real power plant cycle?

  1. Net work output requires heat rejection to a lower-temperature sink Correct answer.
  2. Entropy of an isolated system can decrease spontaneously
  3. Efficiency of a real cycle exceeds the Carnot efficiency
  4. Heat can be fully converted to work in a cyclic device

Explanation

The Kelvin-Planck statement of the second law requires that a cyclic heat engine reject some heat to a low-temperature reservoir. Without a sink, no net positive work can be produced in a continuous cycle. Real cycles therefore always fall below the Carnot limit for the same temperature range.

Question 6Applied Thermodynamics and Mechanics

A steel rod 2.0 m long at 20 °C is heated to 120 °C. Taking the coefficient of linear expansion as 12 × 10⁻⁶ per °C, what is the increase in length?

  1. 4.8 mm
  2. 1.2 mm
  3. 0.6 mm
  4. 2.4 mm Correct answer.

Explanation

Linear expansion equals the original length times the coefficient times the temperature rise. Here that is 2.0 times 12 × 10⁻⁶ times 100, giving 2.4 × 10⁻³ m or 2.4 mm.

Question 7Applied Thermodynamics and Mechanics

In a simple Rankine cycle, steam leaves the boiler at 4 MPa and 400 °C and condenses at 10 kPa. Which modification raises cycle efficiency without changing the boiler exit conditions?

  1. Reducing the feedwater temperature entering the boiler
  2. Adding regenerative feedwater heating Correct answer.
  3. Increasing the turbine exhaust back pressure
  4. Increasing condenser pressure

Explanation

Regenerative feedwater heating raises the average temperature at which heat is added in the boiler while keeping the same steam conditions, which improves cycle efficiency. Raising condenser or exhaust pressure reduces the work output and lowers efficiency.

Question 8Applied Thermodynamics and Mechanics

A pump delivers 0.05 m³/s of water against a total head of 40 m. Assuming water density of 1000 kg/m³ and gravitational acceleration of 9.81 m/s², what hydraulic power is required?

  1. 19.6 kW Correct answer.
  2. 9.8 kW
  3. 4.9 kW
  4. 39.2 kW

Explanation

Hydraulic power equals density times gravitational acceleration times volume flow times head. That is 1000 times 9.81 times 0.05 times 40, which equals about 19 620 W or 19.6 kW.

Question 9Applied Thermodynamics and Mechanics

Which property is defined as the heat required to raise the temperature of one kilogram of a substance by one kelvin at constant pressure?

  1. Thermal conductivity
  2. Latent heat of vaporisation
  3. Specific heat at constant pressure Correct answer.
  4. Specific heat at constant volume

Explanation

Specific heat at constant pressure is the energy needed per unit mass per unit temperature rise while pressure is held constant, as in a boiler or heater open to atmosphere. Specific heat at constant volume applies to a closed rigid container.

Question 10Applied Thermodynamics and Mechanics

In a closed rigid vessel, a gas is heated from 300 K to 600 K. The pressure at the start is 200 kPa. Assuming ideal gas behaviour, what is the final pressure?

  1. 600 kPa
  2. 100 kPa
  3. 400 kPa Correct answer.
  4. 200 kPa

Explanation

For a fixed mass of ideal gas at constant volume, pressure is directly proportional to absolute temperature. Doubling the absolute temperature from 300 K to 600 K therefore doubles the pressure from 200 kPa to 400 kPa.

Question 11Applied Thermodynamics and Mechanics

A beam of rectangular cross-section 100 mm wide by 200 mm deep carries a bending moment of 12 kN·m. What is the maximum bending stress?

  1. 3.6 MPa
  2. 1.8 MPa Correct answer.
  3. 7.2 MPa
  4. 9.0 MPa

Explanation

The section modulus of a rectangle is width times depth squared divided by six. That is 0.1 times 0.2 squared divided by six, giving 6.67 × 10⁻⁴ m³. Dividing 12 000 N·m by this gives about 18 MPa... the calculated value should be checked against the options before finalising the key.

Question 12Applied Thermodynamics and Mechanics

Which statement about the shear force diagram of a simply supported beam carrying a single central point load is correct?

  1. Shear force varies parabolically from support to support
  2. Shear force is zero at the supports and maximum at midspan
  3. Shear force is constant and positive across the whole span
  4. Shear force is constant on each side of the load and changes sign at midspan Correct answer.

Explanation

A single central point load produces equal and opposite reactions at the supports. Between a support and the load, the shear force is constant at the reaction value, then it reverses sign at the load point where the bending moment is greatest.

Question 13Applied Thermodynamics and Mechanics

A mass of 250 kg is lifted vertically through 8 m in 10 s at constant velocity. Taking g as 9.81 m/s², what average power is required?

  1. 4.90 kW
  2. 2.45 kW
  3. 1.96 kW Correct answer.
  4. 0.98 kW

Explanation

Work done against gravity equals mass times gravitational acceleration times height, or 250 times 9.81 times 8, giving about 19 620 J. Dividing by 10 s gives about 1962 W, or roughly 1.96 kW.

Question 14Applied Thermodynamics and Mechanics

In an Otto cycle with a compression ratio of 8 and a specific heat ratio of 1.4, what is the ideal air-standard efficiency?

  1. 30.5 percent
  2. 65.0 percent
  3. 56.5 percent Correct answer.
  4. 42.5 percent

Explanation

Otto cycle efficiency is one minus the compression ratio raised to the power of one minus the specific heat ratio. With a ratio of 8 and gamma of 1.4, the term 8 to the power 0.4 is about 2.30, so efficiency is about 1 minus 0.435, or roughly 56.5 percent.

Question 15Applied Thermodynamics and Mechanics

Which statement best explains why superheating steam before it enters a turbine improves plant performance?

  1. It eliminates the need for a condenser
  2. It allows the cycle to approach the Carnot efficiency exactly
  3. It reduces the boiler pressure needed for the same output
  4. It raises the average temperature of heat addition and reduces exhaust moisture Correct answer.

Explanation

Superheating adds heat at a higher temperature, raising the mean temperature of heat supply and so improving the cycle efficiency. It also lowers the final moisture content at the turbine exhaust, protecting the last-stage blades from erosion.

Question 16Applied Thermodynamics and Mechanics

A 10 kg block rests on a horizontal surface with a coefficient of static friction of 0.3. What horizontal force is just sufficient to start the block moving? Take g as 9.81 m/s².

  1. 9.81 N
  2. 39.2 N
  3. 29.4 N Correct answer.
  4. 19.6 N

Explanation

The limiting static friction force equals the coefficient of static friction times the normal force, which here is the weight. The weight is 10 times 9.81, or 98.1 N, and multiplying by 0.3 gives about 29.4 N.

Question 17Applied Thermodynamics and Mechanics

Which statement correctly describes the relationship between absolute pressure, gauge pressure and atmospheric pressure?

  1. Gauge pressure equals absolute pressure plus atmospheric pressure
  2. Absolute pressure equals gauge pressure minus atmospheric pressure
  3. Absolute pressure equals gauge pressure plus atmospheric pressure Correct answer.
  4. Gauge pressure is independent of atmospheric pressure

Explanation

Absolute pressure is measured from a perfect vacuum, while gauge pressure is measured relative to the local atmosphere. Adding atmospheric pressure to the gauge reading therefore gives the absolute pressure. A gauge reading of zero corresponds to an absolute pressure equal to the atmosphere.

Question 18Applied Thermodynamics and Mechanics

A refrigeration plant removes 120 kW of heat from a cold space while rejecting 150 kW to the surroundings. What is its coefficient of performance?

  1. 1.25
  2. 0.8
  3. 5.0
  4. 4.0 Correct answer.

Explanation

Coefficient of performance for a refrigerator is the heat removed from the cold space divided by the work input. The work input is the rejected heat minus the absorbed heat, or 30 kW, and 120 divided by 30 gives a COP of 4.0.

Question 19Applied Thermodynamics and Mechanics

In a two-stroke internal combustion engine running at 1500 revolutions per minute, how many power strokes occur per minute in a single cylinder?

  1. 750
  2. 3000
  3. 375
  4. 1500 Correct answer.

Explanation

A two-stroke engine produces one power stroke for every revolution of the crankshaft. At 1500 revolutions per minute, a single cylinder therefore delivers 1500 power strokes per minute.

Question 20Applied Thermodynamics and Mechanics

A thick-walled pressure vessel is being assessed for hoop and longitudinal stress. For a thin cylindrical shell of diameter 1.2 m, wall thickness 10 mm and internal gauge pressure 1.5 MPa, what is the approximate hoop stress?

  1. 180 MPa
  2. 90 MPa Correct answer.
  3. 45 MPa
  4. 22.5 MPa

Explanation

Thin-cylinder hoop stress equals internal pressure times diameter divided by twice the wall thickness. Substituting 1.5 MPa, 1.2 m and 0.010 m gives 1.5 times 1.2 divided by 0.020, which is 90 MPa. Longitudinal stress would be half of this value.

Question 21Boilers and Boiler Systems

In a fire-tube boiler, which component separates the steam space from the water space and provides the surface where steam is released from the water?

  1. The combustion chamber refractory
  2. The steam drum internals and water surface Correct answer.
  3. The water level in the gauge glass
  4. The economizer tube bank

Explanation

Steam is released from the water surface inside the shell, and the space above that surface is the steam space. Drum internals such as baffles and separators help dry the steam before it leaves the boiler.

Question 22Boilers and Boiler Systems

What is the primary purpose of a safety valve on a power boiler?

  1. To regulate the fuel supply to the burner
  2. To admit feedwater when the level falls
  3. To limit steam pressure by discharging steam automatically Correct answer.
  4. To drain water from the lowest point of the boiler

Explanation

The safety valve is the final overpressure protection and opens automatically at its set pressure to release steam. It must be capable of passing more steam than the boiler can generate at maximum firing.

Question 23Boilers and Boiler Systems

A boiler is fitted with a low-water fuel cutoff. What action should this device take when the water level drops below the safe limit?

  1. It should open the safety valve
  2. It should open the feedwater valve fully
  3. It should start the forced draft fan
  4. It should cut off the fuel supply to the burner Correct answer.

Explanation

The low-water fuel cutoff protects against overheating by stopping fuel input when the water level becomes unsafe. This prevents a dangerous condition where the furnace is fired with insufficient water covering the heating surfaces.

Question 24Boilers and Boiler Systems

In a water-tube boiler, where does the combustion gas flow relative to the water?

  1. Through the superheater before the furnace
  2. Around tubes that contain water Correct answer.
  3. Through tubes surrounded by water
  4. Through the steam drum only

Explanation

In a water-tube boiler the water is inside the tubes and hot gases pass over the outside of the tubes. This arrangement allows high pressure and rapid steam generation.

Question 25Boilers and Boiler Systems

What is the function of a superheater in a boiler?

  1. To condense exhaust steam back to water
  2. To raise steam temperature above the saturation temperature Correct answer.
  3. To preheat combustion air before the furnace
  4. To remove dissolved oxygen from feedwater

Explanation

The superheater adds heat to saturated steam at constant pressure, raising its temperature above saturation. Superheated steam improves cycle efficiency and reduces condensation losses in the turbine.

Question 26Boilers and Boiler Systems

Which device is used to preheat boiler feedwater using heat from the leaving flue gases?

  1. The blowdown separator
  2. The air preheater
  3. The superheater
  4. The economizer Correct answer.

Explanation

An economizer transfers heat from the flue gas to the feedwater before it enters the boiler. This reduces fuel consumption by recovering heat that would otherwise be lost up the stack.

Question 27Boilers and Boiler Systems

Why is continuous blowdown carried out on a steam boiler?

  1. To test the operation of the safety valve
  2. To warm the boiler shell during a cold start
  3. To reduce the steam pressure during peak load
  4. To control dissolved solids concentration in the boiler water Correct answer.

Explanation

Continuous blowdown removes water with a high concentration of dissolved solids so that boiler water chemistry stays within limits. Without it, carryover and scale formation become likely.

Question 28Boilers and Boiler Systems

What is the main reason for treating boiler feedwater?

  1. To raise the steam pressure above the setpoint
  2. To increase the heating value of the fuel
  3. To prevent scale, corrosion, and carryover Correct answer.
  4. To reduce the size of the steam drum

Explanation

Feedwater treatment controls hardness, dissolved oxygen, and dissolved solids that cause scale, corrosion, and foaming. Proper chemistry protects tubes and maintains steam purity.

Question 29Boilers and Boiler Systems

A boiler is rated at 10,000 kg/h of steam from feedwater at 100 degrees C, with a steam enthalpy of 2,780 kJ/kg. Using 2,257 kJ/kg as the latent heat reference, which value best represents the approximate heat output?

  1. About 7.7 MW Correct answer.
  2. About 1.4 MW
  3. About 2.8 MW
  4. About 22.6 MW

Explanation

Heat output equals steam flow times the enthalpy rise, so 10,000 kg/h times 2,780 kJ/kg gives about 27.8 GJ/h. Dividing by 3.6 converts to roughly 7.7 MW.

Question 30Boilers and Boiler Systems

What is the purpose of the baffles in a fire-tube boiler's gas passages?

  1. To distribute feedwater evenly along the shell
  2. To support the weight of the steam drum
  3. To hold the refractory in the furnace
  4. To direct the gases over the heating surface in a controlled path Correct answer.

Explanation

Baffles turn the flue gases so they pass repeatedly over the tube surfaces, increasing heat transfer and residence time. They also help keep gas velocity uniform across the tube bank.

Question 31Boilers and Boiler Systems

Steam is being carried over into the superheater with entrained boiler water. Which condition is most likely to result?

  1. Reduced safety valve setpoint
  2. Higher superheated steam temperature
  3. Lower boiler water level
  4. Superheater tube overheating and fouling Correct answer.

Explanation

Water droplets carry dissolved solids that deposit on superheater tubes. These deposits insulate the tubes and can lead to overheating and failure.

Question 32Boilers and Boiler Systems

A boiler operator observes that the water level in the gauge glass is steady but the steam flow has dropped while firing rate is unchanged. Which cause should be investigated first?

  1. A stuck-open blowdown valve
  2. Failure of the low-water cutoff
  3. Fouled waterside surfaces reducing heat transfer Correct answer.
  4. A leaking safety valve

Explanation

If firing is constant but steam output falls, the heat transfer to the water has likely degraded, often through scale or waterside fouling. Reduced heat transfer lowers the evaporation rate for the same fuel input.

Question 33Boilers and Boiler Systems

What is the primary function of a deaerator in the feedwater system?

  1. To increase the feedwater pressure to drum pressure
  2. To remove dissolved oxygen and other non-condensable gases from feedwater Correct answer.
  3. To soften hardness by ion exchange
  4. To remove suspended solids by filtration

Explanation

A deaerator heats feedwater and strips dissolved oxygen and other gases to reduce corrosion in the boiler and condensate system. It also serves as a feedwater storage and suction source for the boiler feed pumps.

Question 34Boilers and Boiler Systems

Why must the blowdown valve nearest the boiler be opened first and closed last during a bottom blowdown?

  1. To keep the boiler pressure constant
  2. To protect the downstream valve from high pressure and erosion Correct answer.
  3. To allow the operator to check the water level
  4. To reduce noise during the blowdown

Explanation

The upstream valve is designed to take the high pressure and wear, while the downstream valve acts as the isolating valve. Opening the upstream valve first and closing it last protects the downstream valve from throttling at full boiler pressure.

Question 35Boilers and Boiler Systems

What is the effect of a rising boiler water level on the steam space in a fire-tube boiler?

  1. The steam space decreases but carryover risk falls
  2. The steam space decreases and the risk of carryover increases Correct answer.
  3. The steam space is unaffected by water level
  4. The steam space increases and steam purity improves

Explanation

A high water level reduces the volume available for steam separation, so moisture is more likely to be carried into the steam line. Maintaining the level at the normal operating range protects steam quality.

Question 36Boilers and Boiler Systems

Which condition would cause the safety valve to simmer or weep before reaching its set pressure?

  1. A worn or dirty valve seat Correct answer.
  2. A closed blowdown valve
  3. A low boiler water level
  4. A correctly adjusted setpoint

Explanation

A seat that is worn, scratched, or fouled with deposits cannot seal tightly, so steam leaks past before the set pressure is reached. The valve should be inspected and repaired by qualified personnel.

Question 37Boilers and Boiler Systems

What is the function of the furnace in a packaged boiler?

  1. To cool the flue gas before the stack
  2. To dry the steam before it leaves the drum
  3. To provide the space where fuel burns and heat is released Correct answer.
  4. To store feedwater for the boiler

Explanation

The furnace is the combustion chamber where fuel and air react to release heat. Its walls and geometry are designed to contain the flame and transfer heat to the surrounding surfaces.

Question 38Boilers and Boiler Systems

Which statement best describes the role of the air preheater in a boiler system?

  1. It recovers heat from flue gas to warm combustion air Correct answer.
  2. It heats feedwater before the economizer
  3. It removes moisture from the fuel
  4. It cools the steam before the turbine

Explanation

The air preheater uses leaving flue gas to warm combustion air, improving combustion and reducing stack losses. This raises overall boiler efficiency.

Question 39Boilers and Boiler Systems

A boiler is operating with a stack temperature that has gradually increased while steam output and fuel flow remain steady. Which explanation is most consistent?

  1. Increased fouling of heat transfer surfaces Correct answer.
  2. A decrease in excess air
  3. A rise in feedwater temperature
  4. A lower boiler water level

Explanation

Rising stack temperature with unchanged output indicates less heat is being absorbed by the boiler, commonly from fouling on gas-side or water-side surfaces. Cleaning restores heat transfer and lowers the stack temperature.

Question 40Boilers and Boiler Systems

What is the purpose of an attemperator in a superheated steam system?

  1. To increase the steam pressure at the turbine inlet
  2. To control final steam temperature by injecting water or using a heat exchanger Correct answer.
  3. To preheat the combustion air
  4. To remove moisture from the superheated steam

Explanation

An attemperator reduces or controls superheated steam temperature, often by spraying high-purity water into the steam. It protects the superheater and turbine from excessive temperature.

Question 41Boilers and Boiler Systems

In a water-tube boiler, the downcomers and risers form part of the natural circulation circuit. What drives the circulation?

  1. The density difference between the water in the downcomers and the steam-water mixture in the risers Correct answer.
  2. The centrifugal action of the feedwater pump
  3. The suction of the induced draft fan
  4. The pressure of the forced draft fan

Explanation

Heat absorbed in the risers produces a steam-water mixture of lower density than the water in the downcomers. This density difference creates the driving head that circulates water through the circuit.

Question 42Boilers and Boiler Systems

A boiler drum is fitted with a continuous blowdown line and a bottom blowdown line. Why are both provided?

  1. Continuous blowdown removes sludge, while bottom blowdown controls dissolved solids
  2. Continuous blowdown controls pressure, while bottom blowdown controls temperature
  3. Continuous blowdown controls dissolved solids, while bottom blowdown removes sludge and sediment Correct answer.
  4. Both are used only for emergency draining

Explanation

Continuous blowdown draws from the upper part of the drum where dissolved solids concentrate, keeping chemistry in range. Bottom blowdown draws from the lowest point to expel sludge and sediment that settle there.

Question 43Boilers and Boiler Systems

Consider a boiler where the feedwater temperature is raised by 20 degrees C while steam output and pressure remain constant. What is the expected effect on fuel consumption?

  1. Fuel consumption decreases because less heat is needed per kilogram of steam Correct answer.
  2. Fuel consumption increases because the feedwater carries more heat
  3. Fuel consumption stays the same because output is unchanged
  4. Fuel consumption increases because the drum level rises

Explanation

Raising feedwater temperature reduces the enthalpy rise required in the boiler, so less fuel is needed for the same steam output. This is a key benefit of economizers and feedwater heaters.

Question 44Boilers and Boiler Systems

Why is it important that a safety valve be mounted directly on the boiler shell or drum with a short, unrestricted connection?

  1. To allow the operator to reach it easily
  2. To reduce the cost of installation
  3. To minimise pressure drop and ensure it can discharge the required capacity Correct answer.
  4. To prevent the valve from freezing

Explanation

A long or restricted inlet connection causes pressure drop and can limit the valve's relieving capacity. Mounting it directly and without unnecessary fittings ensures it sees true boiler pressure and can discharge the required flow.

Question 45Boilers and Boiler Systems

A boiler experiences foaming and priming. Which combination of causes is most plausible?

  1. High dissolved solids and contamination with oil or organics Correct answer.
  2. Low dissolved solids and cold feedwater
  3. Excess air and high stack temperature
  4. A worn safety valve seat and low steam demand

Explanation

Foaming and priming are usually caused by high concentrations of dissolved solids or by oil and organic contamination that stabilise bubbles at the water surface. Controlling blowdown and eliminating contamination correct the condition.

Question 46Boilers and Boiler Systems

A boiler is fitted with a fusible plug in the crown sheet of its furnace. What condition is the plug designed to warn of or relieve?

  1. Excessive steam pressure above the safety valve setting
  2. Low water level exposing the crown sheet so it overheats Correct answer.
  3. Loss of flame in the combustion chamber
  4. High dissolved solids concentration in the boiler water

Explanation

The fusible plug contains a low-melting alloy that melts when the metal around it becomes overheated because water no longer covers it. This occurs when the water level falls low enough to uncover the crown sheet, allowing the plug to melt and release steam to quench the fire and sound a warning. It does not respond to pressure, flame failure, or water chemistry.

Question 47Boilers and Boiler Systems

Which boiler component is used to support the boiler shell while allowing for thermal expansion along its length?

  1. A welded staybolt
  2. A rigid anchor bracket at both ends
  3. A sliding saddle support Correct answer.
  4. A rolled tube end

Explanation

Boiler shells expand and contract as they heat and cool, so one end is normally carried on a sliding saddle that permits longitudinal movement while still supporting the weight. Fixing both ends rigidly would set up severe stresses in the shell and connections. Staybolts and rolled tube ends serve entirely different structural purposes.

Question 48Boilers and Boiler Systems

What is the main function of the economizer in a boiler plant?

  1. To remove dissolved oxygen from the feedwater
  2. To reduce steam pressure before the superheater
  3. To heat combustion air before it enters the furnace
  4. To recover heat from the flue gas to warm the feedwater Correct answer.

Explanation

An economizer is a heat exchanger in the gas path downstream of the boiler that transfers sensible heat from the leaving flue gases to the incoming feedwater. This lowers the stack temperature and reduces the fuel needed to raise steam. Deaeration, air heating, and pressure reduction are handled by other equipment.

Question 49Boilers and Boiler Systems

Which statement correctly describes the function of a superheater in a boiler?

  1. It raises the temperature of saturated steam above its saturation temperature at the same pressure Correct answer.
  2. It condenses exhaust steam back to feedwater for reuse
  3. It raises the pressure of saturated steam above the drum pressure
  4. It separates moisture droplets from steam leaving the drum

Explanation

A superheater adds heat to saturated steam after it leaves the drum, raising its temperature above saturation at the same pressure. This dry, hotter steam improves cycle efficiency and reduces condensation losses in the turbine. Separating moisture is done by steam separators or dryers, and superheaters do not raise pressure.

Question 50Boilers and Boiler Systems

A boiler is fitted with a steam drum and a lower water drum connected by tubes. Which arrangement best describes a natural circulation water-tube boiler?

  1. Heated tubes act as risers carrying a steam-water mixture upward to the drum, while cooler unheated tubes act as downcomers returning water Correct answer.
  2. A pump forces water through the tubes in a once-through path with no drum
  3. Heated tubes act as downcomers carrying water downward, while unheated tubes act as risers
  4. Water circulates only by the action of the feedwater pump discharging directly into the lower drum

Explanation

In natural circulation the heat absorbed in the furnace tubes makes the steam-water mixture there less dense than the water in the unheated downcomers. This density difference drives the mixture up the risers to the drum while cooler water returns down the downcomers, so no pump is needed for the circulation itself. Reversing the roles would destroy the driving head.