HVAC Thermodynamics 4 — Questions and Answers
Question 1: What does the Zeroth Law of Thermodynamics establish?
- Energy cannot be created or destroyed
- Entropy always increases in an isolated system
- If two bodies are in thermal equilibrium with a third, they are in equilibrium with each other (Correct answer)
- Absolute zero cannot be reached
Correct answer: If two bodies are in thermal equilibrium with a third, they are in equilibrium with each other
The Zeroth Law is the basis for temperature measurement — it defines thermal equilibrium between bodies.
Question 2: In heat transfer, which mode involves the movement of a fluid (liquid or gas) carrying thermal energy from one place to another?
- Conduction
- Radiation
- Convection (Correct answer)
- Evaporation
Correct answer: Convection
Convection transfers heat through fluid movement, which is the primary mechanism in forced-air HVAC systems.
Question 3: What is the specific heat of water (BTU/lb·°F), a value commonly used in hydronic HVAC calculations?
- 0.24
- 0.50
- 1.0 (Correct answer)
- 4.18
Correct answer: 1.0
Water has a specific heat of 1.0 BTU/lb·°F, meaning 1 BTU raises 1 pound of water by 1°F.
Question 4: An HVAC technician measures 10°F of superheat at the evaporator outlet. This reading indicates:
- The evaporator is flooded with liquid
- There is a refrigerant overcharge
- The refrigerant is fully vaporized with 10°F above saturation (Correct answer)
- The compressor is short-cycling
Correct answer: The refrigerant is fully vaporized with 10°F above saturation
10°F superheat means the vapor exiting the evaporator is 10°F warmer than the saturation temperature at that pressure.
Question 5: The First Law of Thermodynamics, applied to a refrigeration system, means that:
- Heat naturally flows from cold to hot
- The total energy entering equals energy leaving plus any change in stored energy (Correct answer)
- Efficiency can exceed 100%
- Temperature is constant during phase changes
Correct answer: The total energy entering equals energy leaving plus any change in stored energy
The First Law is energy conservation: energy in equals energy out plus changes in stored energy, governing all HVAC energy balances.
Question 6: What property describes how much a substance resists temperature change when heat is added?
- Thermal conductivity
- Latent heat
- Specific heat capacity (Correct answer)
- Enthalpy
Correct answer: Specific heat capacity
Specific heat capacity (c) defines how much energy is needed to raise 1 unit mass by 1 degree of temperature.
Question 7: Which refrigerant state is found between the expansion valve and the evaporator inlet in a properly operating system?
- Superheated vapor
- Subcooled liquid
- Saturated mixture (liquid-vapor) (Correct answer)
- Compressed gas
Correct answer: Saturated mixture (liquid-vapor)
After expansion, the refrigerant is a low-pressure liquid-vapor mixture entering the evaporator to absorb heat.
HVAC Thermodynamics 4
What does the Zeroth Law of Thermodynamics establish?