Refrigeration & Air-Conditioning

Module 2. Refrigerants

Lesson 8
DEFINITION AND CLASSIFICATION OF REFRIGERANTS

8.1 Introduction

1. Any substance that absorbs heat through evaporation is known as refrigerant. It is a medium of heat transfer which absorbs heat by evaporation at low temperature and gives up heat by condensing at high temperature and pressure.
2. Broader definition includes secondary cooling mediums like cold water, brine solutions
3. Widest applications at present are ammonia or halocarbons.

8.2 Desirable Characteristics of Refrigerants

1. Non poisonous
2. Non corrosive
3. Non inflammable
4. Leaks easily detectable
5. Low boiling point
6. Stable gas
7. Suitable latent heat
8. Low specific volume
9. Minimum difference between evaporating & condensing pressure, one further classification of derivable properties; they can be categorized as thermodynamic, physical and chemical properties.

8.3Thermodynamics

1. Normal boiling point : should be low at atmospheric pressure

2. Freezing point : should be low

3. Evaporations & condensing pr : should be +ve and as near to atmospheric pressure

4. Critical temperature & pressure : should be high (ex.co2p n it is just 30°c)

5. Latent heat: should be high

6. Specific heat : low for liquid, high for vapour

7. Specific volume: low for vapour

8.4 Physical

1. Dielectric strength : high for use in hermetic compression
2. Thermal conductivity : should be high
3. Viscosity : should be low
4. Leak tendency : detection of leak should be easy
5. Flammability: should not be flammable within range of conditions of operation. Nh3 flammable between 16 to 27% in air

8.5 Chemical

1. Toxicity : should not be toxic
   
2. Corrosion properties: should not react in the commonly used metals and materials
   

8.6 Classification of Refrigerants(NRSC, USA)

Group-1: safest: R-11, R-12, R-22, R-744(co₂), R-500, R-502

Group-2: toxic& slightly inflammable: R-717, R-764(so₂), R-113

Group-3: inflammable: R-600(butane),R-170(ethane),R-290(propane)

(natural refrigerants: h₂o, air, nh₃, propane, butane)

8.7 Color Code for the Cylinders & Composition

R-717: (SHINY) Silver NH₃

R-11: ORANGE: (CCL₃F) Trichloromonofluoro Methane

R-12: WHITE: (CCL₂F₂) Dichlorodifluoromethane

R-22: GREEN : (CHCL F₂) Monochlorodi Fluoromethane

8.8 Numbering System by Ashrae

1) For Saturated Hydro-Carbon

R (M-1)(N+1)(P)

Where

M= No Of Carbon Atoms

N= No Of Hydrogen Atoms

P= No Of Fluorine Atoms

(2) For Saturated Compounds

Digit 1 kept before (M-1)

EG: ETHYLENE R-1150

(3) Inorganic Refrigerants

Add molecular weight To 700

EG: NH₃ (17+700)

CO₂ (44+700)

(4) Azeotropes: They are mixtures of refrigerants arbitrary nos