CPT CPT Heat Transfer & Thermodynamics 2 — Questions and Answers
Question 1: What is 'latent heat' in the context of process thermodynamics?
- Heat stored in insulation materials
- Heat absorbed or released during a phase change at constant temperature (Correct answer)
- The minimum heat required to start a reaction
- Residual heat in a vessel after shutdown
Correct answer: Heat absorbed or released during a phase change at constant temperature
Latent heat is the energy absorbed or released when a substance changes phase (e.g., liquid to vapor) without a change in temperature.
Question 2: A steam trap in a process plant is used to:
- Prevent steam from entering condensate return lines by discharging condensate while retaining steam (Correct answer)
- Boost steam pressure at low-load conditions
- Measure steam flow rate in process lines
- Filter particulates from steam supply
Correct answer: Prevent steam from entering condensate return lines by discharging condensate while retaining steam
A steam trap automatically discharges liquid condensate and non-condensable gases from steam lines while preventing live steam from escaping, maintaining system efficiency.
Question 3: Fouling in a heat exchanger reduces its performance primarily by:
- Increasing turbulent flow through tubes
- Adding thermal resistance that reduces the overall heat transfer coefficient (Correct answer)
- Increasing the temperature driving force between streams
- Reducing the pressure drop across the exchanger
Correct answer: Adding thermal resistance that reduces the overall heat transfer coefficient
Scale, biological growth, or deposits on heat transfer surfaces act as insulating layers, increasing thermal resistance and lowering the effective heat transfer rate.
Question 4: What is the relationship between pressure and boiling point for water in a closed system?
- Higher pressure lowers the boiling point
- Higher pressure raises the boiling point (Correct answer)
- Pressure has no effect on boiling point
- Boiling point is fixed at 212°F regardless of pressure
Correct answer: Higher pressure raises the boiling point
In a closed system, increasing pressure requires more energy to convert liquid to vapor, raising the boiling point — this principle is used in pressure vessels and steam systems.
Question 5: Which instrument directly measures the enthalpy of steam entering a process unit?
- Thermocouple alone
- Flow meter alone
- Combined temperature and pressure measurement used with steam tables (Correct answer)
- A colorimeter
Correct answer: Combined temperature and pressure measurement used with steam tables
Steam enthalpy is determined from steam tables using both measured temperature and pressure; neither reading alone is sufficient to determine the thermodynamic state.
Question 6: In a counterflow heat exchanger, fluids flow in opposite directions. What is the main advantage over parallel flow?
- Lower initial equipment cost
- More uniform temperature driving force along the exchanger, enabling greater heat recovery (Correct answer)
- Reduced risk of tube-side fouling
- Easier mechanical cleaning access
Correct answer: More uniform temperature driving force along the exchanger, enabling greater heat recovery
Counterflow maintains a more consistent temperature difference between streams throughout the exchanger, allowing the cold fluid to be heated closer to the hot fluid inlet temperature.
CPT CPT Heat Transfer & Thermodynamics 2
What is 'latent heat' in the context of process thermodynamics?