lecture 4 refrigerants 6th week

Mechanical Engineering Dept. CEME NUST 1

Lecture 4: Refrigerants

Books:

Refrigeration and Air Conditioning Technology by William C.

Whitman, William M. Johnson, John A. Tomczyk, Eugene

Silberstein Unit 9

Refrigeration & Air-Conditioning by R. C. Arora Chapter 4

Refrigeration & Air-Conditioning by Stoecker and Jones

Chapter 15


Course Contents

S.No. Description

1. Introduction to Refrigeration and Review of Fundamentals

2. Vapor Compression Cycle

3. Multi-pressure Systems

4. Air Cycle Refrigeration System

5. Absorption Refrigeration Systems and Chillers

6. Refrigerants

7. Psychrometry

8. Air Conditioning Systems

9. Heating and Cooling Load Calculations

10. Fluid Flow in Piping and Ducts

11. Fans and Air Distribution


Introduction Classification

Heat Exchange Chemical composition

Refrigerant Selection Criteria Desirable thermo-physical properties Environmental and Safety properties

Common Refrigerants

Refrigerants


Refrigerants

Refrigerant is a substance, which acts as a cooling agent by absorbing heat from another body or substance.

oIn case of vapor compression cycle, the refrigerant is the working fluid of

the cycle which alternately vaporizes and condenses as it absorbs and

gives off heat.

Selection of suitable refrigerant has become one of the most important issues in recent times due to Ozone Layer depletion and global warming concerns.

Introduction

In practice, the choice is limited by factors such as oSystem Design and Size

oInitial And Operating Costs

oSafety and Reliability

oServiceability etc.


Classification of Refrigerants

Based on Heat Exchange

Primary Refrigerants are those fluids, which are used directly as working fluids

Secondary Refrigerants are those liquids, which are used for transporting thermal energy from one location to other

If the operating temperatures are above 0oC, then Pure Water can also be used as secondary

refrigerant, e.g. in large Air Conditioning

Systems

Antifreezes or Brines are used when refrigeration is required at sub-zero temperatures

Secondary Refrigerants are usually liquids, and are used to transfer heat from the substance

being cooled to a heat exchanger where the heat

is absorbed by a primary refrigerant


Classification of Refrigerants

Based on Chemical Composition

Refrigerants

Pure substances

Synthetic

CFC HFC

HCFC

Natural

Hydro Carbons

Inorganic

Mixtures

Azeotropic Zeotropic


Refrigerants

Refrigerant Selection Criteria

Thermodynamic and Thermo-physical Properties

a) Suction Pressure: At a given evaporator temperature, the saturation pressure should be above atmospheric for prevention of air or moisture ingress into

the system and ease of leak detection

b) Discharge Pressure: At a given condenser temperature, the discharge pressure should be as small as possible to allow light-weight construction of

compressor, condenser

c) Pressure Ratio: Should be as small as possible for high volumetric efficiency and low power consumption

d) Latent heat of vaporization: Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small


e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small

Refrigerants


Thermodynamic and Thermo-physical Properties

f) Liquid specific heat: Should be small so that degree of subcooling will be large leading to smaller amount of flash gas at evaporator inlet

g) Vapor specific heat: Should be large so that the degree of superheating will be small

h) Thermal conductivity: Should be high for higher heat transfer coefficients for both liquid as well as vapor phase

i) Viscosity: Viscosity should be small in both liquid and vapor phases for smaller frictional pressure drops

Thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature, molecular weight and structure


Refrigerants


Environmental and safety properties

a) Ozone Depletion Potential (ODP): ODP of refrigerants should be zero. Refrigerants having non-zero ODP have either already been phased-out (e.g. R 11, R 12) or will be

phased-out in near-future(e.g. R22)

b) Global Warming Potential (GWP): Refrigerants should have as low a GWP value as possible to minimize the problem of global warming

c) Total Equivalent Warming Index (TEWI): The factor TEWI considers both direct (due to release into atmosphere) and indirect (through energy consumption) contributions of

refrigerants to global warming.

d) Toxicity: Ideally, refrigerants used in a refrigeration system should be non-toxic. All fluids other than air can be called as toxic as they will cause suffocation when their

concentration is large enough.

e) Flammability: Refrigerants should preferably be non-flammable and non-explosive


Refrigerants

Inorganic Refrigerants

These refrigerants are inorganic in nature. These are designated by number 7 followed by the molecular

weight of the refrigerant (rounded-off).

For Example: Ammonia: Molecular weight is 17, the designation is R 717 Carbon dioxide: Molecular weight is 44, the designation is

R 744

Water: Molecular weight is 18, the designation is R 718


Refrigerants

Hydro Carbons

Most refrigerants are made from two molecules, methane and ethane

Pure hydrocarbons were at one time considered good refrigerants, but because of their flammability were not used after the 1930s in any large scale

Refrigerants have no fluorine or chlorine in their molecule; 0 ODP

Contribute to global warming


These refrigerants are designated by R XYZ, where:

o X+1 indicates the number of Carbon (C) atoms

o Y-1 indicates number of Hydrogen (H) atoms, and

o Z indicates number of Fluorine (F) atoms

o If there are only two digits after R then it means that X = 0.

Refrigerants

Designation of refrigerants

Fully saturated, halogenated compounds:

These refrigerants are derivatives of alkanes (CnH2n+2) such as methane (CH4), ethane (C2H6).

Example: R 22

oX = 0 No. of Carbon atoms = 0+1 = 1 derivative of methane (CH4)

oY = 2 No. of Hydrogen atoms = 2-1 = 1

oZ = 2 No. of Fluorine atoms = 2

oThe balance = 4 no. of (H+F) atoms = 4-1-2 = 1 No. of Chlorine atoms = 1

The chemical formula of R 22 = CHClF2


Refrigerants

Synthetic or Derivates of Hydro Carbons

Some of the hydrogen atoms are removed from either the methane or ethane molecule and replaced with either

chlorine or fluorine

Chlorofluorocarbons (CFCs) Hydrochlorofluorocarbons (HCFCs) Hydrofluorocarbons (HFCs)


Refrigerants

Synthetic or Derivates of Hydro Carbons

CFC Refrigerants

CFCs contain chlorine, fluorine, and carbon and are considered the most damaging because their molecules are not destroyed as they reach the

stratosphere

As of July 1, 1992, it became illegal to intentionally vent CFC refrigerants into the atmosphere


Refrigerants

Important Refrigerants

HCFC Refrigerants

These have much less potential for ozone depletion because they tend to break down in the atmosphere, releasing the chlorine before it reaches and reacts with

the ozone in the stratosphere

However, the HCFC group is scheduled for a total phase out by the year 2030


Refrigerants


HFC Refrigerants

HFC molecules contain no chlorine atoms and will not deplete the earths protective ozone layer

HFCs do have small global-warming potentials


Refrigerants


Refrigerant Blends/Mixtures

Ozone depletion and global warming issues have caused a need for much research on refrigerant blends

Blends can have as many as four refrigerants mixed together to give properties and efficiencies similar to the refrigerants they will replace

Azeotropic blends Consists of two or more substances that can not be separated by

distillation.

An Azeotrope evaporates and condenses as a single substance with properties that are different from those of either constituent.

It is designated by 500 series For example R-502 is a mixture of 48.8 % R 22 and 51.2 % R 115


Refrigerants


Refrigerant Blends/Mixtures

Zeotropic blends Consists of two or more substances that have different boiling temperatures It is designated by 400 series For example

R404A : Mixture of R 125 (44%), R 143a (52%) and R 134a (4%) R407A : Mixture of R 32 (20%), R 125 (40%) and R 134a (40%) R407B : Mixture of R 32 (10%), R 125 (70%) and R 134a (20%) R410A : Mixture of R 32 (50%) and R 125 (50%)


Assignment 1

Refrigerants

Discuss Ozone Depletion Potential of different types of

refrigerants by explaining it as a phenomenon and what steps

are being taken to counter this problem.

Assignment Submission:

13th April, 2015

Note: Late submission will be awarded with 20% reduction of maximum

marks per day

lecture 4 refrigerants 6th week

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