Thermal Engineering Interview Questions: SFEE & LMTD

Last updated: Jan 2026

Thermal engineering explains how heat, work, and fluid flow produce power or cooling in machines. This interview Q&A targets: Which law fits correctly, reading steam tables, writing SFEE, spotting entropy and exergy losses, using LMTD or NTU, and understanding boilers, combustion, nozzles, cycles, and psychrometrics.

Thermal engineering focuses on energy conversion and heat management in equipment like boilers, turbines, engines, compressors, chillers, and HVAC plants.

Engineers in this space work in power, process, automotive, refrigeration, and building services, where efficiency and reliability are measured daily.

Why does a simple control-volume question suddenly turn into property lookup, sign conventions, and heat-exchanger sizing?

In this blog, you will practice the exact interview queries that interviewers ask.

Thermo Basics

1. What does the zeroth law let you do in practice?

It justifies temperature measurement. If A and B are each in thermal equilibrium with C, then A and B have the same temperature.

2. Property vs path function: what is the difference?

Properties depend only on state (T, P, h). Path functions depend on process history (Q, W). That is why Q and W cannot be stored in a system.

3. Why is enthalpy the “flow energy” in open systems?

Enthalpy bundles internal energy with flow work. For steady devices, it makes the energy equation compact for turbines, compressors, nozzles, boilers, and condensers.

4. What does “reversible” mean in engineering terms?

It is an ideal limit with no friction, no mixing, and heat transfer across an infinitesimal temperature difference. Real processes are irreversible, so reversibility is a benchmark, not a claim.

Steam Tables

5. How do you identify compressed liquid vs saturated vs superheated steam?

Compare T to Tsat at the given pressure. Below Tsat is compressed liquid, equal is saturated, and above Tsat is superheated.

6. What is steam quality (x), and where is it valid?

Quality is the vapor mass fraction in the saturated two-phase region only. Outside the dome, x has no physical meaning.

7. How do you pull h and s from tables without getting the wrong region?

Confirm the region first, then read from the matching table. Interpolate only in one direction, and only when the table grid demands it.

8. When is PV = mRT acceptable for a gas?

Use it when the gas is far from saturation, and the pressure is modest. A quick sanity check is that the compressibility factor is close to 1 for that range.

9. What is the fastest way to catch a bad property pick in an interview?

Check “direction.” Compression should raise the temperature for most gases, throttling should keep h about constant, and turbine expansion should drop h if it is producing work.

First-Law-to-Exergy

10. Write the first law for a closed system in one line.

ΔE = Q − W, with E including internal, kinetic, and potential energies. Add KE and PE only when velocity or elevation changes matter.

11. What sign convention keeps answers consistent?

Heat into the system is +Q, work done by the system is +W. Then the first law stays ΔE = Q − W without sign flips.

12. What is the steady-flow energy equation “turbine form”?

For an adiabatic turbine with small KE and PE changes: Ẇ ≈ ṁ(h1 − h2). A positive result means the shaft power is out.

13. When can KE and PE be neglected in SFEE?

When ΔV²/2 and gΔz are small compared to Δh. If KE is a few kJ/kg while the enthalpy change is tens or hundreds, drop KE safely.

14. Why is throttling modeled as h1 ≈ h2?

A valve has no shaft work and is usually adiabatic. Energy balance then gives nearly constant enthalpy, while entropy rises due to irreversibility.

15. What does entropy generation tell you during troubleshooting?

It points to lost work potential. Friction, mixing, throttling, and large ΔT heat transfer raise Sgen, so they are common targets in performance recovery.

16. How do you estimate exergy destroyed quickly?

Use Ėdestroyed = T0 Ṡgen. With T0 = 300 K and Ṡgen = 0.2 kW/K, exergy destroyed is about 60 kW.

Heat Transfer and Exchangers

17. What does a thermal resistance network buy you?

It turns heat flow into Q = ΔT/Rtotal. That makes the bottleneck obvious, especially with wall conduction, film convection, contact resistance, and fouling.

18. One-dimensional conduction through a plane wall: what is the core equation?

Q = kA(ΔT/L). It is valid when the heat flow is steady, the temperature varies mainly across the thickness, not along the surface.

19. What does the Biot number decide in transient heating?

Bi = hL/k tells whether internal conduction is fast. If Bi < 0.1, lumped capacitance is usually acceptable.

20. Forced vs natural convection: what is the clean distinction?

Forced convection is driven by fans or pumps. Natural convection is driven by buoyancy. The difference changes the correlation, so it can shift strongly.

21. When does radiation become non-negligible in real equipment?

Hot surfaces, large view factors, or low convection conditions make radiation matter. Radiative heat exchange scales with temperature to the fourth power.

22. LMTD vs ε-NTU: when do you pick each?

Use LMTD when both outlet temperatures are known. Use ε-NTU when outlet temperatures are unknown. Effectiveness then links UA and capacity rates to heat duty.

23. Why do pressure drop and fouling dominate exchanger decisions?

Fouling cuts U and increases the required area. Pressure drop raises pumping power and can limit flow. Together, they decide whether “more heat transfer” is worth the operating cost.

Boilers and Combustion

24. Boiler mountings vs boiler accessories: what is the difference?

Mountings are safety and control fittings essential for operation. Accessories improve performance and efficiency, but the boiler can still run without them.

25. Fire-tube vs water-tube boiler: what is the fastest differentiator?

Fire-tube sends hot gas through tubes with water outside. Water-tube sends water through tubes with hot gas outside, enabling higher pressures and faster steam generation.

26. What is the quick identity of a Lancashire boiler in interviews?

It is a fire-tube boiler with large internal flue tubes and a bulky shell. It is associated with lower pressures and older, simpler steam generation layouts.

27. Natural vs forced draught: why does it matter?

Natural draught uses chimney buoyancy, so airflow is limited and sensitive to conditions. Forced draught uses fans, enabling tighter combustion control and higher heat release rates.

28. What does “excess air” mean, and what is the penalty?

Excess air is air above stoichiometric needs. It reduces CO and soot risk, but too much increases stack losses because more hot gas leaves the boiler.

29. Why is the air-fuel ratio a decision variable, not just a number?

It sets flame temperature, completeness of combustion, and emissions. A/F too low raises CO and soot, while too high raises flue-gas loss and lowers efficiency.

30. What is the simplest flue-gas loss logic you can say in an interview?

Higher stack temperature and higher excess air both increase sensible heat leaving the system. Reducing them through heat recovery and control usually improves boiler efficiency.

Nozzles and Cycles

31. What is nozzle choking and “critical pressure ratio”?

Choking happens when the mass flow stops increasing despite lowering the downstream pressure. At the critical pressure ratio, sonic conditions form at the throat, limiting further flow increase.

32. Rankine vs Brayton vs Otto/Diesel: what mainly drives ideal efficiency?

Rankine improves with higher boiler temperature and lower condenser pressure. Brayton depends strongly on pressure ratio and component efficiencies. Otto and Diesel are governed mainly by compression ratio and heat-addition characteristics.

33. Indicated power vs brake power: what is the clean definition?

Indicated power is developed in the cylinder from the pressure diagram. Brake power is delivered at the shaft. Brake power is lower because friction and pumping consume power.

34. What is mechanical efficiency in IC engines?

Mechanical efficiency is brake power divided by indicated power. It quantifies how much cylinder power survives after friction and mechanical losses.

35. What does effective pressure (MEP) represent?

MEP is the constant hypothetical pressure that would produce the same net work per cycle over the displacement volume. It is a clean way to compare engines independent of size.

36. COP sanity check with superheat and subcooling: what is the interview logic?

COP rises when the compressor work drops for the same cooling effect. Moderate superheat protects the compressor, and subcooling reduces flash gas, often improving capacity without a big work penalty.

HVAC and Psychrometrics

37. Humidity ratio vs relative humidity: Why do HVAC engineers prefer humidity ratio?

Humidity ratio is the mass of water vapor per mass of dry air, so it tracks moisture directly. Relative humidity swings with temperature, even when the moisture content is unchanged.

38. What does the dew point tell you during coil cooling?

Dew point is the temperature at which condensation starts at the current moisture content. Cooling below the dew point adds latent removal and changes coil load behavior.

39. What is the apparatus dew point (ADP) in one line?

ADP is the effective coil surface temperature that the air would approach with perfect contact. It anchors the coil process line during cooling and dehumidification.

40. What is the bypass factor, and why does it show up in interviews?

Bypass factor is the fraction of air that effectively “escapes” full contact with the coil. Lower bypass factor means better coil effectiveness and a closer approach to ADP.

Worked Example 

Given steady, adiabatic turbine flow.
ṁ = 2 kg/s
h1 = 3400 kJ/kg
h2 = 3000 kJ/kg
ΔKE ≈ 0, ΔPE ≈ 0
Ẇout ≈ ṁ(h1 − h2)
Ẇout ≈ 2(3400 − 3000) kW
Ẇout ≈ 800 kW
A positive sign means shaft power output.
State assumptions: steady, adiabatic, negligible KE and PE.

Conclusion

Strong thermal interviews are won by clean modeling, not long speeches. Pick the right control volume, say your assumptions first, then use one equation with the correct sign and units. That is how you stay fast, precise, and believable.

FAQ

1. What is the difference between LMTD and ε-NTU?

LMTD sizes an exchanger when outlet temperatures are known. ε-NTU predicts performance when outlets are unknown, using UA and capacity rates.

2. What is the most common steam-table mistake in interviews?

Skipping the region check. Picking saturated values for a superheated state can look “reasonable” numerically and still be wrong.

3. Why does throttling increase entropy if enthalpy stays constant?

A valve destroys exergy through irreversibility. Energy is conserved as h stays near constant, but entropy rises because the process cannot be reversed.

4. How do you decide whether to neglect kinetic energy in SFEE?

Compare ΔV²/2 to Δh. If the KE change is only a few kJ/kg while Δh is tens or hundreds, dropping KE is justified.

5. Why do HVAC interviews ask ADP and bypass factor?

They reveal whether you understand real coil behavior. Both terms connect coil surface temperature, approach, latent removal, and why the leaving air state rarely equals coil temperature.