(SEM VI) THEORY EXAMINATION 2018-19 REFRIGERATION AIR CONDITIONING

Refrigeration & Air Conditioning (RME-061)

Complete Section-Wise Explanation (B.Tech Sem-VI)

Introduction to the Paper

The RME-061 Refrigeration & Air Conditioning paper is designed to test how well a student understands practical cooling systems, thermodynamic cycles, air conditioning concepts, and numerical problem-solving.
 

This subject is extremely important from an industry and application point of view, as it connects theory with real-world systems such as cold storages, air conditioners, aircraft cooling, and industrial refrigeration plants.
 

The paper is divided into three sections:

Section A → Concept clarity

Section B → Theory + Numericals

Section C → Advanced numericals & applications

SECTION A – Conceptual Foundation

Pattern:
Attempt all questions
7 questions × 2 marks = 14 marks

Nature of Section A

This section checks whether you understand the basic terminology of refrigeration and air conditioning. Answers should be short, direct, and correct, but the concept behind them must be clear in your mind.

Explanation of Topics Asked

One Tonne of Refrigeration (TR):
One tonne of refrigeration is defined as the rate of heat removal required to freeze one tonne of water at 0°C into ice at 0°C in 24 hours. Numerically, it is equal to 3.517 kW.

Coefficient of Performance (COP):
COP is the ratio of refrigeration effect produced to the work input required. It indicates how efficient a refrigeration or air conditioning system is.

Classification of Refrigerants:
Refrigerants are classified based on chemical composition, safety, application, and phase change characteristics. Examples include primary and secondary refrigerants.

GSHF and ADP:
GSHF (Grand Sensible Heat Factor) represents the ratio of sensible heat to total heat load.
ADP (Apparatus Dew Point) is the effective surface temperature of a cooling coil.

Function of Expansion Device:
The expansion device reduces pressure and temperature of refrigerant and controls the flow rate into the evaporator.

Bypass Factor:
Bypass factor indicates the fraction of air that does not come in contact with the cooling coil.

Cascade System:
A cascade system uses two or more refrigeration cycles operating at different temperature levels to achieve very low temperatures efficiently.

SECTION B – Applied Theory & Numerical Problems

Pattern:
Attempt any three questions
3 × 7 marks = 21 marks

Nature of Section B

This section requires clear explanation with calculations, diagrams (where needed), and step-by-step logic. This is where most students either score very well or lose marks due to improper presentation.

Explanation of Major Questions

Question 2(a): Cold Storage Numerical

This problem is based on COP comparison between ideal and actual refrigeration systems. The heat leakage is given, and actual COP is a fraction of ideal COP. The goal is to calculate power required to 
drive the plant.

This tests:        Understanding of COP

Relation between work input and refrigeration load

Practical efficiency losses

Question 2(b): Multistage Vapour Compression System

Multistage compression is used when the pressure difference between evaporator and condenser is large. It improves COP, reduces compressor work, and prevents excessive discharge temperature.

The liquid intercooler cools refrigerant between stages, improving efficiency and protecting the compressor.

Question 2(c): Lithium Bromide Water Absorption System

This system uses heat energy instead of mechanical work. Water acts as refrigerant, and lithium bromide acts as absorbent.

It is widely used in:                                     Large air conditioning plants

Hotels                                                         Industrial cooling

This question tests understanding of absorption refrigeration and its advantages over compression systems.

Question 2(d): Psychrometric Numerical

This is a high-scoring question if you know how to use the psychrometric chart.

From dry bulb temperature, wet bulb temperature, and pressure, students calculate:

Dew point temperature                             Relative humidity

Specific humidity                                       Degree of saturation

Vapour density                                          Enthalpy of air mixture

Question 2(e): Cold Storage Working

Cold storage maintains low temperature to preserve food items. It involves insulation, evaporator cooling, controlled humidity, and circulation of cold air.

A neat sketch and clear explanation are expected.

SECTION C – Advanced Numericals & System Analysis

Pattern:
Attempt one part from each question
5 questions × 7 marks = 35 marks

This section determines rank and distinction.

Question 3: Aircraft Refrigeration System

(a) Jet Aircraft Cooling Numerical

This problem is based on air refrigeration system used in aircrafts. It involves:

Ram effect

Compressor and turbine efficiencies               Cooling load in TR                        COP calculation

It tests deep understanding of gas cycle refrigeration.

(b) Boot-Strap Air Refrigeration System

This is a theoretical explanation with T-S diagram. The boot-strap system improves cooling efficiency by using an additional heat exchanger.

Question 4: Vapour Compression Analysis

(a) Effect of Suction and Discharge Pressure

This question checks conceptual understanding of how pressure changes affect:

Refrigeration effect                              Compressor work                                     COP

System performance

(b) Two-Stage Ammonia Plant Numerical

A detailed numerical involving:         Flash intercooling                    Water intercooling

Subcooling                                        p-h chart                                  Compressor power

Swept volume

This is one of the most difficult but most rewarding questions.

Question 5: Absorption System & Refrigerants

(a) Absorption Refrigerator Numerical

This question links thermodynamics and refrigeration, involving:

Maximum COP                      Actual COP                          Steam consumption      Heat source analysis

(b) Primary vs Secondary Refrigerants

Primary refrigerants undergo phase change directly, while secondary refrigerants transfer cooling effect indirectly.

Refrigerant numbering system is also tested.

Question 6: Air Conditioning & Human Comfort

(a) Comfort vs Industrial AC

Comfort AC focuses on human comfort, while industrial AC focuses on process requirements.

(b) Thermal Analysis of Human Body

This involves heat balance of the human body considering metabolism, evaporation, radiation, and convection.

Question 7: Cold Storage & Ice Plant

(a) Importance of Cold Storage

Cold storages are essential for food preservation, agriculture, and supply chain management. Numerical calculation of refrigeration load is required.

(b) Ice Manufacturing Plant

This explains how ice is produced using brine and refrigeration systems.