Refrigeration & Air Conditioning (NME-604)
Complete Section-Wise Explanation – B.Tech Semester VI
Introduction to the Subject
Refrigeration and Air Conditioning is a core mechanical engineering subject that deals with the production of low temperatures, comfort conditioning of air, and preservation of products. The subject combines thermodynamics, heat transfer, fluid flow, and psychrometry.
It helps engineers understand: Refrigeration cycles and systems
Refrigerants and their properties Air conditioning processes and comfort conditions
Psychrometric analysis Aircraft refrigeration systems
Duct design and air distribution
The question paper is divided into three sections: A, B, and C, and all sections must be attempted as instructed.
SECTION A – Short Answer Questions
(2 × 10 = 20 marks)
Section A checks basic definitions and fundamental understanding. Answers are brief but must be accurate and technically correct.
Explanation of Section A Questions
Refrigeration is the process of removing heat from a space or substance and maintaining its temperature below that of the surroundings.
A refrigerator removes heat from a low-temperature space for cooling purposes, whereas a heat pump supplies heat to a high-temperature space for heating.
The vapour compression refrigeration system offers high COP, compact size, reliable operation, and is suitable for a wide range of capacities.
A cascade refrigeration system uses multiple refrigerants in stages, enabling very low temperatures with improved efficiency and reduced compressor work.
A simple vapour absorption refrigeration system uses heat energy instead of mechanical work and consists of generator, condenser, absorber, pump, and evaporator.
An azeotrope is a mixture of refrigerants that evaporates and condenses at constant temperature like a pure substance.
The chemical name of R-112 is Dichlorotetrafluoroethane.
BPF (Bypass Factor) represents the fraction of air that bypasses a cooling coil without coming in contact with it, while SHF (Sensible Heat Factor) is the ratio of sensible heat to total heat.
A duct is a passage used to convey conditioned air from the air-conditioning unit to occupied spaces.
Dew point temperature is the temperature at which air becomes saturated and condensation begins when cooled at constant pressure.
SECTION B – Numerical & Descriptive Problems
(Attempt any three parts, 10 × 3 = 30 marks)
Section B focuses on thermodynamic cycles, refrigeration system analysis, and psychrometric calculations. Proper diagrams and step-by-step working are essential.
Bell-Coleman (Air Refrigeration) Cycle
This problem analyzes an air refrigeration system used in aircraft. Air undergoes compression, cooling at constant pressure, expansion, and heat absorption in the cold chamber.
The p-v and T-s diagrams explain each process clearly. Refrigeration effect, work input, and COP are calculated per unit mass of air.
Two-Stage Ammonia Refrigeration System with Intercooler
This numerical problem uses a p-h chart to determine compressor work and COP. Two-stage compression reduces work requirement and improves efficiency. The intercooler removes superheat and reduces compressor load.
Practical Vapour Absorption Refrigeration System
This theory question explains the working of a practical absorption system, highlighting rectifier, analyzer, absorber, and solution heat exchanger. It emphasizes reduced electrical power consumption.
Cooling Coil Psychrometric Analysis
This problem uses psychrometric charts to find outlet air temperature, cooling capacity, moisture removed, and sensible heat factor. Concepts like coil dew point temperature and bypass factor are applied.
Expansion Devices & Thermostatic Expansion Valve
Different expansion devices such as capillary tubes, thermostatic expansion valves, and float valves are explained.
The thermostatic expansion valve controls refrigerant flow based on evaporator superheat.
SECTION C – Advanced Applications & Systems
(5 × 10 = 50 marks)
Section C carries the maximum weightage and tests deep conceptual understanding and numerical skill.
Question 3 – Aircraft Refrigeration Systems
Aircraft refrigeration systems use air as refrigerant.
The numerical problem involves ramming effect, heat exchangers, turbine expansion, and cabin
conditions.
Power required and COP are calculated considering pressure rise, temperature drop, and turbine efficiency.
The Boot-Strap cooling system improves performance by using an auxiliary compressor and multiple heat exchangers, achieving lower cabin temperatures.
Question 4 – Vapour Compression System
This problem analyzes a vapour compression refrigeration system using refrigerant R-40.
COP is calculated using enthalpy values from refrigerant tables considering dry saturated suction and superheated discharge.
Question 5 – Vapour Absorption & Psychrometric Processes
Advantages of vapour absorption systems include low electrical energy consumption and use of waste heat.
The maximum refrigeration effect is calculated using Carnot efficiency between given temperatures.
Psychrometric processes explained include:
Cooling with adiabatic humidification
Adiabatic mixing of two air streams
Question 6 – Air Conditioning Load Calculations
This question focuses on comfort air conditioning.
Room sensible and latent heat loads, fresh air load, apparatus dew point temperature, and inlet air conditions are determined using psychrometric relations.
Alternatively, sensible heating, cooling, and dehumidification processes are explained with the help of psychrometric charts.
Refrigerants are classified based on chemical composition, and desirable properties such as safety, stability, and high latent heat are discussed.
Question 7 – Cold Storage & Duct Design
Cold storage design considers temperature control, insulation, product type, loading pattern, and humidity control.
The duct design problem involves calculating equivalent diameter, air velocity, and pressure loss using friction factor and airflow rate.
