(SEM VI) THEORY EXAMINATION 2017-18 REFRIGERATION & AIR CONDITIONING

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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.