Free PE Mechanical Engineering: Thermal and Fluid Systems Practice Questions

Sample Practice Questions

Try these 10 sample questions from the PE Mechanical Engineering: Thermal and Fluid Systems practice bank. Each includes the answer and explanation. Continue with the interactive practice above to review the full question set.

1. 1An engineer estimates steady one-dimensional conduction through a flat insulation panel. Its area is 10 m², thickness is 0.12 m, and constant thermal conductivity is 0.04 W/(m·K). The two surface temperatures differ by 60 K. Neglect edge effects. What heat-transfer rate passes through the panel?

   1. A. 20 W
   2. B. 200 W
   3. C. 2,000 W
   4. D. 24 W

   Answer: B

   Explanation: DOE describes "The use of Equations 2-4 and 2-5 in determining the amount of heat transferred by conduction" and gives the rectangular-wall form in Eq. 2-4. For the stated uniform wall, Q̇ = kAΔT/L = 0.04(10)(60)/0.12 = 200 W. The other results omit area, use one-tenth the thickness, or omit division by thickness.

   Reference: DOE Fundamentals Handbook: Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2 — Module 2, Conduction Heat Transfer, pp. 6–7, Equation 2-4

2. 2A valve leaks externally where its stem passes through the bonnet. Which component is specifically intended to seal that leakage path?

   1. A. The seat ring
   2. B. The packing
   3. C. The handwheel
   4. D. The valve disk

   Answer: B

   Explanation: The handbook states, "Most valves use some form of packing to prevent leakage from the space between the stem and the bonnet." Packing seals the stem penetration. Seat and disk close the internal flow passage; the handwheel supplies actuation.

   Reference: DOE Fundamentals Handbook: Mechanical Science, Volume 2 — Module 4, Valve Functions and Basic Parts, p. 5, Valve Packing

3. 3A steam turbine receives 10 kg/s at 3,200 kJ/kg specific enthalpy and exhausts at 2,800 kJ/kg. Neglect heat, kinetic-energy, and elevation changes. What shaft power does the turbine deliver?

   1. A. 0.4 MW
   2. B. 2.0 MW
   3. C. 4.0 MW
   4. D. 40 MW

   Answer: C

   Explanation: The DOE handbook states that "the decrease in the enthalpy of the working fluid Hin - Hout equals the work done by the working fluid in the turbine" under the stated simplifications. Thus 10(3,200 − 2,800) = 4,000 kW = 4.0 MW. The other values omit, halve, or apply mass flow twice.

   Reference: DOE Fundamentals Handbook: Thermodynamics, Heat Transfer, and Fluid Flow, Volume 1 — Thermodynamics, Second Law, p. 79, turbine first-law balance

4. 4A process designer specifies a crossflow exchanger in which one stream moves across the other rather than in parallel or countercurrent channels. What nominal angle separates the two mean stream directions?

   1. A. 0°
   2. B. 45°
   3. C. 90°
   4. D. 180°

   Answer: C

   Explanation: DOE defines crossflow: "one fluid flows through tubes and the second fluid passes around the tubes at 90° angle." Thus the two mean stream directions intersect at 90°. Zero degrees is parallel flow, 180° counterflow, and 45° oblique rather than nominal crossflow.

   Reference: DOE Fundamentals Handbook: Mechanical Science, Volume 1 — Module 2, Types of Heat Exchangers, p. 6, Cross Flow

5. 5Two pump-system alternatives have already been discounted to a common present date. Alternative A has present-value benefits of $150,000 and present-value costs of $100,000. What is its net present value?

   1. A. $25,000
   2. B. $50,000
   3. C. $100,000
   4. D. $250,000

   Answer: B

   Explanation: NIST defines net present value as "the difference between the present value of all cash inflows and the present value of all cash outflows". Here $150,000 − $100,000 = $50,000. The other values halve, retain only costs, or add the two present values.

   Reference: Capital Investment Analysis — National Institute of Standards and Technology — Methods for Investment Analysis, Net present value

6. 6A heater transfers 24 kW uniformly through a 6 m² surface during a thermal test. What average heat flux should the engineer report?

   1. A. 144 kW/m²
   2. B. 24 kW/m²
   3. C. 4 kW/m²
   4. D. 0.25 kW/m²

   Answer: C

   Explanation: The handbook states, "The heat flux can be determined by dividing the heat transfer rate by the area through which the heat is being transferred." Therefore q″ = 24/6 = 4 kW/m². Multiplying gives 144, omitting area gives 24, and reversing the division gives 0.25.

   Reference: DOE Fundamentals Handbook: Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2 — Module 2, Heat Transfer Terminology, p. 3, Heat Flux

7. 7In a pressure-boundary review of a bolted-bonnet valve, how should the engineer classify the bolts attaching the bonnet to the body?

   1. A. As pressure-retaining parts
   2. B. As flow-guiding trim parts
   3. C. As external actuator parts
   4. D. As stem-positioning parts

   Answer: A

   Explanation: The handbook states, "the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts." They secure a pressure-boundary closure, rather than guiding the fluid, driving the actuator, or positioning the stem.

   Reference: DOE Fundamentals Handbook: Mechanical Science, Volume 2 — Module 4, Valve Functions and Basic Parts, p. 3, Valve Bonnet

8. 8For the same inlet state and exhaust pressure, a steam turbine produces 400 kJ/kg of actual shaft work while an ideal isentropic expansion would produce 500 kJ/kg. What is its isentropic turbine efficiency?

   1. A. 0.40
   2. B. 0.80
   3. C. 1.00
   4. D. 1.25

   Answer: B

   Explanation: The handbook defines turbine efficiency as "the ratio of the actual work done by the turbine Wt, actual to the work that would be done by the turbine if it were an ideal turbine Wt,ideal." Here 400/500 = 0.80. The other values halve the ratio, ignore losses, or invert it.

   Reference: DOE Fundamentals Handbook: Thermodynamics, Heat Transfer, and Fluid Flow, Volume 1 — Thermodynamics, Second Law, p. 80, turbine isentropic efficiency

9. 9Inside a shell-and-tube exchanger, the shell-side fluid must repeatedly cross the tube bundle to improve heat transfer. Which internal component redirects that flow?

   1. A. Tube sheet
   2. B. Baffles
   3. C. Tube plugs
   4. D. Expansion joint

   Answer: B

   Explanation: The handbook states that "baffles" direct shell-side fluid back and forth across the tubes. The other named exchanger components have distinct mechanical functions and do not provide that flow path.

   Reference: DOE Fundamentals Handbook: Mechanical Science, Volume 1 — Module 2, Types of Heat Exchangers, p. 2, Tube and Shell

10. 10A fabrication line for thermal equipment has $60,000 fixed cost. Each unit sells for $500 and incurs $300 variable cost. How many units must be sold to break even?

    1. A. 120 units
    2. B. 200 units
    3. C. 300 units
    4. D. 500 units

    Answer: C

    Explanation: The DOE break-even module gives "Q = FC/(SP-VC)". The contribution margin is $200 per unit, so Q = 60,000/200 = 300 units. The alternatives divide fixed cost by sale price or variable cost, or use the wrong net margin.

    Reference: DOE Module 3C: Break-Even Analysis — Break-Even Analysis, p. 51, break-even equation