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Power Eng 3rd Class practice questions

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Question 1Applied Thermodynamics and Heat Transfer

In a steady-flow process with negligible changes in kinetic and potential energy, the first law for a control volume with one inlet and one outlet reduces to which relationship?

  1. q - w = u2 - u1
  2. q - w = (h2 - h1) + (P2v2 - P1v1)
  3. q + w = h2 - h1
  4. q - w = h2 - h1 Correct answer.

Explanation

For steady flow with negligible KE and PE changes, the energy balance is q - w = h2 - h1, where w is work done by the system and h is specific enthalpy. Enthalpy already contains the flow work term, so it does not appear separately.

Question 2Applied Thermodynamics and Heat Transfer

A closed rigid tank contains a gas that is heated. Which statement about the process is true?

  1. The enthalpy change equals the heat added
  2. The internal energy stays constant because no work is done
  3. Boundary work is zero because the volume is constant Correct answer.
  4. The gas does positive boundary work as it is heated

Explanation

Boundary work is the integral of P dV, so with dV equal to zero no boundary work is performed. All heat added goes into raising internal energy.

Question 3Applied Thermodynamics and Heat Transfer

Which statement best describes the second law of thermodynamics as applied to a cyclic device?

  1. A cyclic device can have a thermal efficiency of 100 percent if it is frictionless
  2. Heat can flow spontaneously from a colder body to a hotter body without work input
  3. Entropy of a system always decreases in a real process
  4. No cyclic device can convert all of its heat input into work Correct answer.

Explanation

The Kelvin-Planck statement of the second law says no cyclic device can convert all heat received from a single reservoir into work. A finite heat rejection is always required.

Question 4Applied Thermodynamics and Heat Transfer

The thermal efficiency of a heat engine is defined as which ratio?

  1. Heat rejected divided by total heat input
  2. Total heat input divided by net work output
  3. Net work output divided by total heat input Correct answer.
  4. Net work output divided by heat rejected

Explanation

Thermal efficiency is the desired output (net work) divided by the required input (heat supplied). It is always less than one for a real cycle.

Question 5Applied Thermodynamics and Heat Transfer

Heat transfer by conduction through a plane wall is governed primarily by which relationship?

  1. q = σεA(T^4)
  2. q = hA(ΔT)
  3. q = kA(ΔT)/L Correct answer.
  4. q = mcΔT

Explanation

Fourier's law for a plane wall gives q = kA(ΔT)/L, where k is thermal conductivity, A is area, and L is thickness. The rate is proportional to the temperature gradient.

Question 6Applied Thermodynamics and Heat Transfer

In a Rankine cycle, the purpose of the condenser is to do which of the following?

  1. Extract work from the expanding steam
  2. Add heat to convert liquid to steam
  3. Reject heat so the exhaust steam condenses to liquid Correct answer.
  4. Raise the pressure of the working fluid

Explanation

The condenser removes heat from low-pressure exhaust steam at constant pressure, condensing it to saturated liquid before it enters the pump. This heat rejection is required by the second law.

Question 7Applied Thermodynamics and Heat Transfer

A gas undergoes a process in which 500 kJ of heat is added and 200 kJ of work is done by the gas. What is the change in internal energy?

  1. 700 kJ decrease
  2. 300 kJ decrease
  3. 300 kJ increase Correct answer.
  4. 700 kJ increase

Explanation

The first law for a closed system gives ΔU = Q - W. Substituting gives 500 - 200 = 300 kJ, a net increase in internal energy.

Question 8Applied Thermodynamics and Heat Transfer

Two plane walls of equal area are in series. Wall A has thermal resistance 0.5 K/W and wall B has 1.5 K/W. If the total temperature drop across both is 40 K, what is the heat transfer rate?

  1. 10 W
  2. 80 W
  3. 40 W
  4. 20 W Correct answer.

Explanation

Series resistances add, giving a total of 2.0 K/W. The heat rate is q = ΔT/R_total = 40/2.0 = 20 W.

Question 9Applied Thermodynamics and Heat Transfer

A Carnot engine operates between 600 K and 300 K. What is its maximum thermal efficiency?

  1. 33 percent
  2. 50 percent Correct answer.
  3. 67 percent
  4. 100 percent

Explanation

Carnot efficiency is 1 - Tc/Th using absolute temperatures. Here 1 - 300/600 = 0.50, or 50 percent.

Question 10Applied Thermodynamics and Heat Transfer

A pump raises the pressure of saturated liquid water. Which statement best describes the pump work per unit mass in the ideal Rankine cycle?

  1. It is approximately equal in magnitude to the turbine work
  2. It is approximately v times the pressure rise and is small compared with turbine work Correct answer.
  3. It is approximately h times the pressure rise
  4. It is zero because liquids are incompressible

Explanation

For an incompressible liquid the reversible pump work is w = v(P2 - P1). Because specific volume is small, pump work is typically only a few percent of turbine work.

Question 11Applied Thermodynamics and Heat Transfer

Which dimensionless number is used to determine whether flow is laminar or turbulent in a pipe?

  1. Prandtl number
  2. Biot number
  3. Nusselt number
  4. Reynolds number Correct answer.

Explanation

The Reynolds number compares inertial to viscous forces and indicates the flow regime. In pipe flow the transition occurs near a Reynolds number of about 2300.

Question 12Applied Thermodynamics and Heat Transfer

A heat exchanger is designed such that the two fluid streams flow in opposite directions. Compared with parallel flow, counterflow generally provides which advantage?

  1. A smaller heat transfer area for the same duty
  2. No change in the log mean temperature difference
  3. A higher pressure drop in both streams
  4. A larger mean temperature difference for the same terminal temperatures Correct answer.

Explanation

In counterflow the temperature difference between hot and cold streams stays more nearly uniform along the exchanger. This yields a larger log mean temperature difference than parallel flow for the same terminal temperatures, which generally reduces the required area.

Question 13Applied Thermodynamics and Heat Transfer

In a simple ideal Brayton cycle, compression and expansion are modeled as which processes?

  1. Constant volume
  2. Isentropic Correct answer.
  3. Constant pressure with heat transfer
  4. Isothermal

Explanation

The ideal Brayton cycle assumes isentropic compression in the compressor and isentropic expansion in the turbine. Heat addition and rejection occur at constant pressure.

Question 14Applied Thermodynamics and Heat Transfer

A wall separates a hot gas at 1000 K from a cooler surface. If the surface temperature rises, the net radiation heat transfer from the gas to the surface changes in which way?

  1. It decreases because the fourth-power temperature difference shrinks Correct answer.
  2. It stays the same because emissivity is unchanged
  3. It becomes zero only when the surface reaches absolute zero
  4. It increases because the surface emits more

Explanation

Net radiation exchange depends on the difference of the fourth powers of the absolute temperatures. As the surface warms toward the gas temperature, this difference and the net transfer both fall.

Question 15Applied Thermodynamics and Heat Transfer

Water at 100 kPa enters a boiler as saturated liquid and leaves as saturated vapor. If the enthalpy of saturated liquid is 417 kJ/kg and the enthalpy of vaporization is 2258 kJ/kg, what is the heat added per kilogram?

  1. 417 kJ/kg
  2. 1841 kJ/kg
  3. 2675 kJ/kg
  4. 2258 kJ/kg Correct answer.

Explanation

The heat added at constant pressure equals the change in enthalpy. From saturated liquid to saturated vapor this change is exactly the enthalpy of vaporization, 2258 kJ/kg.

Question 16Applied Thermodynamics and Heat Transfer

A throttling valve is used to reduce the pressure of a refrigerant. Which assumption is appropriate for an ideal throttling process?

  1. Temperature remains constant
  2. Enthalpy remains constant Correct answer.
  3. Velocity remains constant and dominant
  4. Entropy remains constant

Explanation

A throttling process is adiabatic with no work, and changes in kinetic and potential energy are negligible, so enthalpy is essentially constant. Entropy increases because the process is irreversible.

Question 17Applied Thermodynamics and Heat Transfer

A copper rod 0.5 m long with a cross section of 0.01 m^2 has a thermal conductivity of 400 W/(m·K). If its ends differ in temperature by 50 K, what is the steady conduction rate?

  1. 800 W
  2. 40 W
  3. 200 W
  4. 400 W Correct answer.

Explanation

Using q = kA(ΔT)/L gives 400 × 0.01 × 50 / 0.5 = 400 W. The rate scales with conductivity, area, and temperature gradient.

Question 18Applied Thermodynamics and Heat Transfer

A vapor-compression refrigeration cycle has a coefficient of performance of 4. If the compressor work input is 5 kW, what is the cooling capacity?

  1. 9 kW
  2. 20 kW Correct answer.
  3. 0.8 kW
  4. 1.25 kW

Explanation

The coefficient of performance is the cooling effect divided by the work input, so cooling capacity equals COP times work. Here 4 × 5 = 20 kW.

Question 19Applied Thermodynamics and Heat Transfer

In the ideal Rankine cycle, superheating the steam entering the turbine primarily achieves which benefit?

  1. It makes the compression process isothermal
  2. It raises the average temperature of heat addition and reduces turbine exhaust moisture Correct answer.
  3. It eliminates the need for the feed pump
  4. It lowers the condenser pressure below the saturation value

Explanation

Superheating increases the turbine inlet temperature, raising the mean temperature of heat addition and thus cycle efficiency. It also keeps the exhaust quality higher, protecting turbine blades from excessive moisture.

Question 20Applied Thermodynamics and Heat Transfer

A counterflow heat exchanger has a hot stream entering at 150 C and leaving at 90 C, and a cold stream entering at 30 C and leaving at 70 C. What is the log mean temperature difference?

  1. About 80.0 K
  2. About 66.6 K Correct answer.
  3. About 60.0 K
  4. About 70.0 K

Explanation

For counterflow the terminal differences are 150 - 70 = 80 K and 90 - 30 = 60 K. The LMTD is (80 - 60)/ln(80/60), which is approximately 66.6 K.

Question 21Boilers and Combustion Systems

In a fire-tube boiler, where do the hot combustion gases flow relative to the water?

  1. Around tubes that carry water
  2. Through tubes surrounded by water Correct answer.
  3. Inside a water-filled shell only
  4. Outside the boiler shell

Explanation

In a fire-tube boiler the products of combustion pass through tubes that are submerged in water contained in the shell. Heat transfers from the hot gases through the tube walls into the water.

Question 22Boilers and Combustion Systems

What is the primary purpose of the steam drum in a water-tube boiler?

  1. To separate steam from water and store water inventory Correct answer.
  2. To drive the forced draft fan
  3. To remove ash from flue gas
  4. To preheat incoming feedwater

Explanation

The steam drum provides space and separation for steam to disengage from the water and holds a reserve of water for the circulation loop. Mechanical separators inside the drum improve steam quality.

Question 23Boilers and Combustion Systems

Which condition produces the greatest heat release per unit of fuel in a combustion process?

  1. Combustion with saturated fuel
  2. Combustion with a rich fuel-air mixture
  3. Complete combustion with minimum excess air Correct answer.
  4. Incomplete combustion with high excess air

Explanation

Complete combustion converts all combustible constituents to their fully oxidized products, releasing maximum heat, and minimum excess air avoids losing heat up the stack.

Question 24Boilers and Combustion Systems

What does the term 'three-element control' refer to in boiler feedwater regulation?

  1. Drum level, steam flow, and feedwater flow Correct answer.
  2. Feedwater temperature, pH, and conductivity
  3. Drum pressure, fuel flow, and air flow
  4. Steam temperature, drum level, and flue gas oxygen

Explanation

Three-element feedwater control uses drum level plus steam flow and feedwater flow to anticipate load changes and maintain level. This reduces the cycling common with single-element level control.

Question 25Boilers and Combustion Systems

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

  1. To condense exhaust steam
  2. To cool the furnace refractory
  3. To increase combustion air pressure
  4. To heat feedwater using flue gas heat Correct answer.

Explanation

An economizer recovers sensible heat from the leaving flue gas to preheat feedwater before it enters the boiler. This lowers stack temperature and improves overall plant efficiency.

Question 26Boilers and Combustion Systems

What is the approximate stoichiometric air-fuel ratio requirement for burning natural gas (methane) on a mass basis?

  1. About 100 kg air per kg fuel
  2. About 17 kg air per kg fuel Correct answer.
  3. About 60 kg air per kg fuel
  4. About 2 kg air per kg fuel

Explanation

Methane requires roughly 17 kg of air per kg of fuel for complete combustion, reflecting its high hydrogen content and corresponding oxygen demand. This is the theoretical or stoichiometric ratio.

Question 27Boilers and Combustion Systems

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

  1. To drain water from the lowest point of the boiler
  2. To regulate feedwater flow
  3. To admit air during start-up
  4. To relieve pressure before it exceeds the maximum allowable working pressure Correct answer.

Explanation

A safety valve opens automatically at its set pressure to discharge steam and prevent the boiler from exceeding its maximum allowable working pressure. It is the final overpressure protection device.

Question 28Boilers and Combustion Systems

What is the main reason for using excess air in a boiler furnace?

  1. To increase the flame temperature
  2. To lower the dew point of the flue gas
  3. To ensure enough oxygen reaches every fuel particle Correct answer.
  4. To reduce the flue gas volume

Explanation

Excess air provides a margin of oxygen so that fuel and oxygen mix adequately throughout the furnace and complete combustion is achieved. Without it, localized oxygen shortages cause unburned fuel and smoke.

Question 29Boilers and Combustion Systems

A boiler burns fuel at 1000 kg/h and produces 13,000 kg/h of steam. What is the approximate steam-to-fuel ratio?

  1. 0.077:1
  2. 1.3:1
  3. 13:1 Correct answer.
  4. 130:1

Explanation

The steam-to-fuel ratio is steam mass divided by fuel mass: 13,000/1,000 = 13. This ratio is used as a simple indicator of boiler performance.

Question 30Boilers and Combustion Systems

A boiler's flue gas is measured at 3% oxygen dry basis. What does this indicate about the excess air level?

  1. Moderate excess air, roughly 15-20% Correct answer.
  2. Insufficient air with incomplete combustion
  3. Zero excess air at stoichiometric conditions
  4. Very high excess air, above 100%

Explanation

About 3% oxygen in dry flue gas corresponds to roughly 15-20% excess air, a typical efficient operating target for gas firing. Higher oxygen readings indicate more excess air.

Question 31Boilers and Combustion Systems

Why is a boiler blowdown performed?

  1. To raise the steam pressure
  2. To cool the superheater
  3. To add oxygen to the water
  4. To control dissolved and suspended solids in the boiler water Correct answer.

Explanation

Blowdown removes water containing concentrated dissolved solids and sludge so that boiler water chemistry stays within limits. Without it, carryover, scaling, and foaming can occur.

Question 32Boilers and Combustion Systems

What is the effect of a high furnace draft (excessive negative pressure)?

  1. Increased air infiltration and reduced efficiency Correct answer.
  2. Increased steam purity
  3. Higher flame temperature and better efficiency
  4. Reduced excess air and lower stack losses

Explanation

Excessive negative furnace pressure draws tramp air through openings, raising excess air and flue gas volume and lowering efficiency. Draft should be maintained at the manufacturer's recommended value.

Question 33Boilers and Combustion Systems

A boiler uses 500 kg/h of fuel with a lower heating value of 42,000 kJ/kg. What is the approximate heat input?

  1. 21,000,000 kJ/h Correct answer.
  2. 210,000 kJ/h
  3. 84,000 kJ/h
  4. 2,100,000 kJ/h

Explanation

Heat input equals fuel flow times heating value: 500 x 42,000 = 21,000,000 kJ/h. This value is the denominator when calculating boiler efficiency.

Question 34Boilers and Combustion Systems

What is the purpose of an air preheater in a boiler system?

  1. To remove moisture from the fuel
  2. To increase the steam drum level
  3. To recover heat from flue gas and warm combustion air Correct answer.
  4. To cool the flue gas below its acid dew point

Explanation

An air preheater transfers heat from the leaving flue gas to the incoming combustion air, improving combustion and reducing stack losses. This raises overall boiler efficiency.

Question 35Boilers and Combustion Systems

What does a rise in stack temperature at constant load most likely indicate?

  1. Increased feedwater temperature
  2. Improved combustion efficiency
  3. Fouled heat transfer surfaces Correct answer.
  4. Lower excess air

Explanation

Fouling or scaling on heat transfer surfaces reduces heat absorption, so more heat leaves with the flue gas and stack temperature rises. This is a common efficiency-loss indicator.

Question 36Boilers and Combustion Systems

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

  1. To raise steam pressure in the drum
  2. To filter suspended ash from flue gas
  3. To increase the dissolved solids content
  4. To remove dissolved oxygen and other gases from feedwater Correct answer.

Explanation

A deaerator heats feedwater and strips dissolved oxygen and other noncondensable gases to reduce corrosion in the boiler and condensate system. It also serves as a feedwater storage and heating vessel.

Question 37Boilers and Combustion Systems

A boiler has a heat output of 16,000,000 kJ/h and a heat input of 20,000,000 kJ/h. What is the efficiency?

  1. 125%
  2. 16%
  3. 20%
  4. 80% Correct answer.

Explanation

Efficiency equals output divided by input: 16,000,000/20,000,000 = 0.80, or 80%. This is the indirect or input-output method of efficiency calculation.

Question 38Boilers and Combustion Systems

What is the primary reason for maintaining the correct water level in a boiler drum?

  1. To minimize the use of safety valves
  2. To increase the fuel heating value
  3. To maximize the firing rate
  4. To prevent carryover and avoid uncovering of heating surfaces Correct answer.

Explanation

Too high a level causes priming and carryover of water into the steam lines, while too low a level can uncover tubes and cause overheating. Level control protects both steam quality and equipment.

Question 39Boilers and Combustion Systems

What is the purpose of a superheater in a boiler?

  1. To heat feedwater before the economizer
  2. To cool the flue gas before the stack
  3. To condense steam back to water
  4. To raise steam temperature above saturation for its pressure Correct answer.

Explanation

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

Question 40Boilers and Combustion Systems

What causes 'priming' in a boiler?

  1. Low firing rate and low drum pressure
  2. Excessive blowdown frequency
  3. High water level, high dissolved solids, or sudden steam demand Correct answer.
  4. Cold feedwater entering the economizer

Explanation

Priming is the carryover of boiler water with steam, caused by high water level, high total dissolved solids, foaming, or sudden load swings. Controlling level and water chemistry prevents it.

Question 41Boilers and Combustion Systems

A furnace is fired with fuel oil containing sulfur. What is the main corrosion risk if the flue gas is cooled below the acid dew point?

  1. Increased oxygen in the flue gas
  2. Loss of flame stability
  3. Sulfuric acid condensation on cold-end surfaces Correct answer.
  4. Formation of explosive gas mixtures

Explanation

Sulfur in the fuel oxidizes to SO3, which combines with moisture to form sulfuric acid that condenses on cold surfaces below the acid dew point. This corrodes air preheaters and ductwork.

Question 42Boilers and Combustion Systems

A boiler operates with 10% excess air, but the flue gas shows 2000 ppm CO. What is the most likely cause?

  1. Poor fuel-air mixing in the furnace Correct answer.
  2. Insufficient total air supply
  3. Too much air preheater leakage
  4. Excessively high furnace temperature

Explanation

Adequate total air is present, but elevated CO indicates localized oxygen-starved zones from poor mixing of fuel and air. Improving burner adjustment and turbulence corrects this condition.

Question 43Boilers and Combustion Systems

Why must a boiler be given a slow, controlled warm-up before being pressurized after a cold start?

  1. To prevent the safety valve from lifting
  2. To increase the water conductivity
  3. To limit thermal stresses in the pressure parts Correct answer.
  4. To conserve fuel during the start

Explanation

Rapid heating creates large temperature gradients through thick metal sections, causing differential expansion and high thermal stress that can crack or distort pressure parts. A controlled warm-up keeps gradients within safe limits.

Question 44Boilers and Combustion Systems

In a balanced draft boiler, what happens if the induced draft fan speed is reduced while the forced draft fan remains unchanged?

  1. Furnace pressure becomes more negative
  2. Flue gas oxygen decreases sharply
  3. Steam drum level rises immediately
  4. Furnace pressure becomes positive and gases may leak out Correct answer.

Explanation

Reducing induced draft while holding forced draft reduces the gas leaving the furnace, so furnace pressure rises toward positive. Positive pressure can force hot gases and flame out through openings.

Question 45Boilers and Combustion Systems

What is the primary reason for using staged combustion (air staging) in a boiler furnace?

  1. To eliminate the need for excess air
  2. To raise the acid dew point of the flue gas
  3. To increase the maximum steam output
  4. To reduce thermal NOx formation by lowering peak flame temperature Correct answer.

Explanation

Air staging creates a fuel-rich primary zone and adds the remaining air downstream, lowering peak flame temperature and suppressing thermal NOx formation. It is an emissions-control technique.

Question 46Boilers and Combustion Systems

Which condition describes the flame failure that a boiler's flame scanner is designed to detect and respond to?

  1. A rise in furnace temperature above the set point
  2. Loss of the flame signal, which trips the fuel safety shutoff valves Correct answer.
  3. An increase in drum water level
  4. A drop in feedwater pump discharge pressure

Explanation

Flame scanners monitor the flame in the furnace and, on loss of signal, the burner management system closes the fuel safety shutoff valves to prevent unburned fuel from accumulating. Furnace temperature, drum level, and feedwater pressure are handled by other controls and interlocks.

Question 47Boilers and Combustion Systems

What is the main function of the forced draft fan in a boiler with a balanced draft system?

  1. To maintain the furnace at a fixed negative pressure
  2. To circulate boiler water through the economizer
  3. To remove flue gas from the stack outlet
  4. To supply combustion air to the furnace under positive pressure Correct answer.

Explanation

The forced draft fan pushes combustion air into the furnace, while the induced draft fan pulls flue gas out and holds the furnace slightly negative. Together they balance the draft through the gas path.

Question 48Boilers and Combustion Systems

A boiler is rated at 20,000 kg/h of steam at 80% efficiency. If the steam output is increased to 25,000 kg/h while efficiency stays constant, what happens to the required fuel heat input?

  1. It rises by about 25% Correct answer.
  2. It rises by about 80%
  3. It stays the same
  4. It falls by about 20%

Explanation

At constant efficiency, fuel heat input is proportional to steam output, so increasing output from 20,000 to 25,000 kg/h is a 25% increase and requires about 25% more heat input. Efficiency only sets the ratio of useful output to input, not the scaling.

Question 49Boilers and Combustion Systems

Why is a boiler furnace purged with air before each light-off?

  1. To cool the refractory lining to ambient temperature
  2. To raise the furnace pressure above atmospheric
  3. To warm the superheater tubes before firing
  4. To clear any accumulated combustible gases before ignition Correct answer.

Explanation

A pre-ignition purge removes residual fuel and combustible gases from the furnace and gas passages so that ignition cannot cause an explosion. The purge volume and time are set by the burner management system.

Question 50Boilers and Combustion Systems

A boiler shows a steady rise in exit gas temperature at constant steam load while excess air and fuel flow are unchanged. Which fault best explains this trend?

  1. Progressive fouling of the heat transfer surfaces Correct answer.
  2. A small increase in combustion air temperature
  3. A slight reduction in fuel moisture content
  4. Normal variation in the fuel's heating value

Explanation

At constant load and firing rate, a rising stack temperature means less heat is being absorbed by the boiler surfaces, which is consistent with fouling or scale building up on the tubes. The other listed changes would not produce a steady upward drift of exit gas temperature.