Chapter 2: Principles of Thermodynamics & Heat Transfer
🔹 2.1 What is Thermodynamics in HVAC?
Thermodynamics is the branch of science that deals with heat transfer and energy conversion. In HVAC, it helps in understanding how heat moves in and out of spaces and how machines like ACs and heaters work.
📌 Key Goal in HVAC:
Maintain desired indoor temperature by controlling heat flow.
🔹 2.2 Laws of Thermodynamics (Used in HVAC)
A. First Law of Thermodynamics (Law of Energy Conservation)
Energy can neither be created nor destroyed; it can only be transformed.
In HVAC:
Electrical energy → Mechanical energy (fan motor)Mechanical energy → Heat transfer (refrigerant)
📌 Example: When an AC runs, it does not destroy heat; it moves heat from inside to outside.
B. Second Law of Thermodynamics
Heat always flows from a hotter body to a colder body, not the reverse unless external work is done.
In HVAC:
Cooling is done by using energy (compressor) to move heat from cooler indoor space to hotter outdoors.📌 Example: A refrigerator removes heat from cold items and sends it to the external air using a compressor.
🔹 2.3 Key HVAC Thermodynamic Terms
| Term | Meaning |
|---|---|
| Sensible Heat | Heat that changes the temperature of air (can be measured by thermometer) |
| Latent Heat | Heat required for phase change (like water to vapor), no temp change |
| Enthalpy (h) | Total heat content (sensible + latent) of air |
| COP (Coefficient of Performance) | Ratio of output cooling to input energy in HVAC systems |
📌 Formula:
🔹 2.4 Heat Transfer Mechanisms
1. Conduction
Heat transfer through solids (without movement)
📌 Example: Heat transfer through metal parts of an AC
2. Convection
Heat transfer through liquids or gases due to fluid motion
📌 Example: Warm air rising and cool air falling in a room
3. Radiation
Heat transfer via electromagnetic waves (no medium needed)
📌 Example: Sunlight heating a room through a glass window
🔹 2.5 HVAC Heat Load Concepts
To design HVAC systems, we calculate heat load (amount of heat added or removed from a space):
| Heat Gain Sources (Cooling) | Heat Loss Sources (Heating) |
|---|---|
| Sunlight through windows | Cold air entering from outside |
| People inside the room | Walls and windows losing heat |
| Electrical appliances | Cold floors, uninsulated walls |
| Lighting fixtures | Air leaks, unsealed windows |
📌 Purpose of Load Calculation:
To select the right size of AC, chiller, or heating unit for efficiency.
🛠️ Practical Application Tips
Touch a metal handle in sunlight — feel conduction.Boil water — observe convection.
Sit under sunlight — feel radiant heat.
In HVAC service:
Technicians check sensible and latent heat to size units.Engineers use COP to evaluate system performance.
📄 Assignment Questions
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Define the first and second laws of thermodynamics with HVAC examples.
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What is the difference between sensible and latent heat?
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Write a real-life example of each mode of heat transfer.
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What is COP, and why is it important in HVAC?
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How is heat load calculated in building cooling systems?
💼 Interview Questions
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What is the importance of thermodynamics in HVAC?
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How does heat transfer occur inside an air conditioning unit?
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Define conduction, convection, and radiation with HVAC-related examples.
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What is the function of a refrigerant in the thermodynamic cycle?
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Why is COP important when selecting an AC or chiller?