Concept Notes (Deep Explanation + Examples)
🔹 Introduction
In Refrigeration and Air Conditioning (RAC), the refrigerant is the working fluid that absorbs heat from the low-temperature region (evaporator) and rejects it at a high-temperature region (condenser).
Simply put — a refrigerant is the heart of the refrigeration cycle.
Without it, cooling or air-conditioning cannot happen.
🔹 What is a Refrigerant?
A refrigerant is a substance (usually a fluid) that circulates through the refrigeration system, undergoing phase changes (liquid ↔ vapor) while transferring heat from one region to another.
It absorbs heat during evaporation (cooling) and releases heat during condensation (heating).
Example:
- In your refrigerator, refrigerant R-134a absorbs heat from food items (inside evaporator) and releases it behind the fridge (at condenser coils).
🔹 Classification of Refrigerants
- Primary Refrigerants
- Directly take part in the refrigeration cycle.
- Examples: Ammonia (R-717), Freon-12 (R-12), R-22, R-134a.
- Secondary Refrigerants
- Used to carry heat from one place to another after being cooled by primary refrigerants.
- Examples: Brine, chilled water, glycol mixtures.
🔹 Desirable Properties of a Good Refrigerant
1. Thermodynamic Properties
- Low boiling point → So it can vaporize easily at low temperatures.
- High latent heat of vaporization → More heat absorption per kg improves efficiency.
- Low specific heat of liquid → Less energy required for cooling liquid refrigerant.
- Low specific volume of vapor → Smaller compressor size and less power.
2. Physical Properties
- Low viscosity → Reduces friction and pressure drops.
- Non-corrosive → Should not damage metal components.
- Leak detection → Should be easily detectable by odor or instruments.
3. Chemical Properties
- Stable chemically under all working conditions.
- Non-toxic and non-flammable for safety.
- Non-reactive with lubricating oil or system materials.
4. Environmental Properties
- Zero Ozone Depletion Potential (ODP)
- Low Global Warming Potential (GWP)
Example: R-134a (ODP = 0), R-1234yf (next-gen eco-friendly refrigerant).
🔹 Commonly Used Refrigerants
| Refrigerant | Chemical Name | ODP | GWP | Remarks |
|---|---|---|---|---|
| R-12 | Dichlorodifluoromethane | 1.0 | 8500 | Banned due to high ODP |
| R-22 | Chlorodifluoromethane | 0.05 | 1700 | Used in old ACs |
| R-134a | Tetrafluoroethane | 0 | 1430 | Common in cars & fridges |
| R-290 | Propane | 0 | 3 | Eco-friendly, flammable |
| R-600a | Isobutane | 0 | 3 | Domestic refrigerators |
| R-717 | Ammonia | 0 | 0 | High efficiency, toxic |
| R-744 | Carbon dioxide | 0 | 1 | Used in transcritical systems |
🔹 Environmental Impact
Old refrigerants (CFCs & HCFCs) caused ozone layer depletion.
Hence, modern systems use HFCs, HFOs, and natural refrigerants (CO₂, NH₃, hydrocarbons).
🔹 Real-Life Example (Workshop Logic)
In a workshop refrigeration test rig, if the compressor runs but no cooling occurs, it’s often due to low refrigerant charge or leakage.
Technicians detect leaks with soap-bubble tests or electronic detectors, recharge refrigerant, and monitor pressure gauges to ensure correct properties.
🔹 Key Practical Understanding
- Too high evaporator pressure = inefficient cooling.
- Too low condenser pressure = undercharged system.
- Always maintain subcooling and superheating range within limits for best performance.
3️⃣ ⚙️ Formulas (Plain LaTeX, NO boxes)
4️⃣ 🔟 10 MCQs (GATE + ECET Mix)
- The main function of a refrigerant in a refrigeration system is to:
A) Lubricate the compressor
B) Absorb and reject heat
C) Increase pressure in the condenser
D) Cool the compressor - Which of the following refrigerants has zero ODP and low GWP?
A) R-22
B) R-134a
C) R-12
D) R-717 - The COP of a refrigerator depends on:
A) Type of compressor
B) Temperature difference between condenser and evaporator
C) Type of expansion valve
D) Type of evaporator - Which refrigerant is also known as ammonia?
A) R-22
B) R-12
C) R-717
D) R-134a - Which property should be high for a good refrigerant?
A) Toxicity
B) Latent heat of vaporization
C) Flammability
D) Specific volume of vapor - The refrigerant R-600a is:
A) Propane
B) Isobutane
C) Carbon dioxide
D) Ammonia - Which of the following refrigerants is environmentally friendly and used in green AC systems?
A) R-12
B) R-22
C) R-290
D) R-11 - The performance of a refrigeration system improves when:
A) Condenser pressure increases
B) Evaporator temperature decreases
C) Condenser temperature decreases
D) Compressor work increases - Which of the following has the highest GWP?
A) R-12
B) R-717
C) R-290
D) R-744 - Which of these is a secondary refrigerant?
A) R-22
B) Brine
C) R-134a
D) R-717
5️⃣ ✅ Answer Key (WordPress Table Format)
Q.No Answer
1 B
2 B
3 B
4 C
5 B
6 B
7 C
8 C
9 A
10 B
6️⃣ 🧠 MCQ Explanations
1️⃣ (B) — Refrigerant absorbs heat in evaporator and rejects it in condenser; it’s the working fluid of the cycle.
2️⃣ (B) — R-134a has ODP = 0 and moderate GWP; eco-friendly compared to others.
3️⃣ (B) — COP = Tₑ / (T꜀ – Tₑ); smaller temperature difference gives higher COP.
4️⃣ (C) — R-717 = Ammonia (NH₃). Common in ice plants and industrial refrigeration.
5️⃣ (B) — High latent heat means more cooling per kg, thus higher efficiency.
6️⃣ (B) — R-600a = Isobutane; used in domestic refrigerators, very eco-friendly.
7️⃣ (C) — R-290 (Propane) has ODP = 0, GWP = 3, hence environmentally safe.
8️⃣ (C) — Lower condenser temperature reduces compressor work → higher COP.
9️⃣ (A) — R-12 (Freon-12) has very high GWP (~8500).
10️⃣ (B) — Brine (secondary refrigerant) carries heat between system and load area.
7️⃣ 🎯 Motivation (ECET 2026 Specific)
Refrigerants are one of the most repeated topics in ECET Mechanical RAC Unit — 2–3 questions almost every year!
Understanding properties like COP, ODP, and GWP helps in both MCQs and numerical questions.
Stay consistent, revise every day, and solve at least 10 numericals per topic.
Remember — small daily practice turns into big exam success!
8️⃣ 📲 CTA (Always Fixed)
Join our ECET 2026 Mechanical WhatsApp Group for daily quizzes & study notes:
https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
