ECET 2026 Mechanical – Complete Guide to Air Conditioning Systems

Concept Notes (Deep Explanation + Examples)

🌬️ Introduction to Air Conditioning (A/C) Systems

Air conditioning (A/C) is the process of maintaining the desired conditions of air — temperature, humidity, purity, and air motion — in an enclosed space.

The term air conditioning doesn’t just mean cooling; it involves heating, cooling, humidification, dehumidification, filtration, and circulation of air to achieve thermal comfort and air quality suitable for humans or processes.

In ECET, this topic connects directly with RAC (Refrigeration and Air Conditioning) fundamentals.

🧊 Basic Functions of an Air Conditioning System

Temperature Control:
Maintains the air temperature at a comfortable level using heating or cooling coils.
Example: Cooling coil (evaporator) lowers the air temperature; heating coil raises it.
Humidity Control:
- Humidification: Adding moisture using a steam spray or evaporative pad.
- Dehumidification: Removing moisture through condensation on cooling coils.
Air Purification:
Filters trap dust, smoke, pollen, and odors.
Air Circulation and Distribution:
Fans and ducts distribute conditioned air uniformly in the space.

💡 Classification of Air Conditioning Systems

Air conditioning systems can be classified based on purpose, equipment arrangement, and seasonal use.

(A) According to Purpose:

Comfort Air Conditioning:
For humans (homes, offices, theaters).
Industrial Air Conditioning:
For process control (textile, printing, electronic manufacturing).

(B) According to Season:

Summer Air Conditioning System:
Removes heat and humidity using refrigeration and dehumidification.
→ Uses evaporator for cooling + dehumidifying the air.
Winter Air Conditioning System:
Adds heat and humidity.
→ Uses heating coil + humidifier.
Year-Round Air Conditioning System:
Uses both heating and cooling coils, automatic control system to maintain comfort all year.

(C) According to Equipment Arrangement:

Unitary System:
- Compact, used for small rooms.
- Example: Window A/C, Split A/C.
- Components: compressor, condenser, expansion valve, and evaporator in one package.
Central System:
- For large buildings.
- Central plant produces conditioned air which is distributed through ducts.

🧰 Main Components of an Air Conditioning System

Compressor:
Compresses refrigerant and circulates it in the system.
Condenser:
Rejects heat to outside air or water, converting refrigerant vapor to liquid.
Expansion Device:
Reduces pressure and temperature of the refrigerant (throttling).
Evaporator (Cooling Coil):
Absorbs heat from air → refrigerant evaporates → air gets cooled and dehumidified.
Air Handling Unit (AHU):
Contains filters, fans, coils, and ducts for distribution.
Fans and Ducts:
Supply and return air to maintain uniform cooling.

🔄 Psychrometric Processes in Air Conditioning

Air conditioning involves several psychrometric processes (changes in air properties):

Sensible Cooling:
Temperature ↓, humidity constant.
Sensible Heating:
Temperature ↑, humidity constant.
Cooling and Dehumidification:
Temperature ↓ and moisture ↓.
Heating and Humidification:
Temperature ↑ and moisture ↑.

🏠 Types of Air Conditioning Systems (Detailed)

Type	Description	Application
Window A/C	Single unit fitted in window	Homes, offices
Split A/C	Separate indoor (evaporator) and outdoor (condenser) units	Homes, small offices
Packaged A/C	Used for medium load (theaters, restaurants)	Commercial use
Central A/C	Large system with ducts	Malls, hospitals, complexes
Evaporative Cooler	Uses water evaporation (no compressor)	Dry climates

⚡ Performance of an Air Conditioning System

Coefficient of Performance (COP):

$COP = \frac{\text{Refrigerating Effect}}{\text{Work Input}}$

Higher COP → More efficient A/C system.

🧪 Example (Numerical)

Example:
An air conditioner removes 6000 kJ/hr of heat while consuming 2 kW power.
Find COP.

Solution:
Refrigerating effect = 6000 kJ/hr = 6000 / 3600 = 1.667 kJ/s = 1.667 kW
Power input = 2 kW

$COP = \frac{1.667}{2} = 0.833$

3️⃣ ⚙️ Formulas

$COP = \frac{Q_L}{W}$
$Q_H = Q_L + W$
$h = c_p (T_2 - T_1)$
$W = Q_H - Q_L$
$\text{Relative Humidity} = \frac{\text{Actual Vapour Pressure}}{\text{Saturation Vapour Pressure}} \times 100$
$\text{Sensible Heat Factor (SHF)} = \frac{\text{Sensible Heat}}{\text{Total Heat}}$
$Q = m \times c_p \times \Delta T$

$\text{Tonnage of Refrigeration} = \frac{\text{Heat Removed per Hour}}{210 \times 60}$

4️⃣ 🔟 10 MCQs

Which of the following is not a function of air conditioning?
A) Cooling
B) Heating
C) Power generation
D) Dehumidification
In summer air conditioning, the main purpose is to:
A) Increase temperature
B) Increase humidity
C) Reduce temperature and humidity
D) Increase pressure
The device used to remove moisture from air is:
A) Humidifier
B) Dehumidifier
C) Condenser
D) Compressor
COP of an air conditioning system is given by:
A) $\frac{W}{Q_L}$
B) $\frac{Q_L}{W}$
C) $\frac{Q_H}{Q_L}$
D) $\frac{W}{Q_H}$
Which of the following A/C types is best suited for a single room?
A) Central A/C
B) Split A/C
C) Packaged A/C
D) Evaporative cooler
The process of lowering air temperature at constant humidity is called:
A) Sensible cooling
B) Dehumidification
C) Evaporative cooling
D) Adiabatic heating
Which component of A/C circulates refrigerant?
A) Evaporator
B) Condenser
C) Compressor
D) Expansion valve
In central air conditioning, air is distributed using:
A) Pipes
B) Ducts
C) Cylinders
D) Coils
Relative humidity is the ratio of:
A) Dry air to wet air
B) Actual vapour pressure to saturation vapour pressure
C) Sensible heat to latent heat
D) Total heat to sensible heat
The refrigerant absorbs heat in which component?
A) Condenser
B) Evaporator
C) Compressor
D) Throttling valve

5️⃣ ✅ Answer Key

Q.No Answer
1 C
2 C
3 B
4 B
5 B
6 A
7 C
8 B
9 B
10 B

6️⃣ 🧠 MCQ Explanations

1️⃣ C – Power generation is not part of A/C; it manages temperature, humidity, and air quality.

2️⃣ C – Reduce temperature and humidity: Summer A/C removes both heat and moisture.

3️⃣ B – Dehumidifier removes moisture; humidifier adds it.

4️⃣ B – $\frac{Q_L}{W}$ : Definition of COP in refrigeration systems.

5️⃣ B – Split A/C: Used for single rooms and small spaces.

6️⃣ A – Sensible cooling: Only temperature drops; humidity remains same.

7️⃣ C – Compressor: Circulates refrigerant by compressing and sending it through condenser.

8️⃣ B – Ducts: Carry conditioned air throughout building.

9️⃣ B – Actual vapour pressure / Saturation vapour pressure: Formula for relative humidity.

10️⃣ B – Evaporator: The cooling coil where refrigerant absorbs heat from air.

7️⃣ 🎯 Motivation (ECET 2026 Specific)

Air Conditioning is a repeatedly asked concept in ECET because it blends thermodynamics, refrigeration, and fluid flow — all core areas of Mechanical Engineering.
Mastering this topic strengthens your understanding of RAC cycles, COP, and psychrometrics, which help you score 2–3 direct marks.
Consistency in daily revision and solving MCQs will push your rank upward — every small topic counts!

8️⃣ 📲 CTA

Join our ECET 2026 Mechanical WhatsApp Group for daily quizzes & study notes:
https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9

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ECET 2026 Mechanical Engineering – Complete Guide to Air Conditioning Systems (RAC Unit)

Concept Notes (Deep Explanation + Examples)

🌬️ Introduction to Air Conditioning (A/C) Systems

🧊 Basic Functions of an Air Conditioning System