REFRIGERANTS CLASS NOTES

FOR MECHANICAL

ENGINEERING

 

Refrigerant causes cooling in a refrigeration

system. It flows in the refrigeration system like

blood circulates in the human body. There are

different refrigerants for different applications.

For making their study and use simple, there is a

nomenclature and numbering of refrigerants. It

has to have desirable properties for to be suitable

for a particular application. Many refrigerants cause

Ozone depletion and Global warming. Selections

considers the thermodynamic, transport, chemical

and economic properties. In the event of leakage,

it should not make the products unfit for use.

Definition

The working substance in a refrigeration unit is a refrigerant.

IDEAL REFRIGERANT

A refrigerant having all the desirable properties and is usable in every refrigeration unit is an ideal refrigerant. But there is no such refrigerant in actual practice.

 TYPES OF REFRIGERANTS ON THE BASIS OF THEIR APPLICATION

Four types

a. PRIMARY REFRIGERANT

 A primary refrigerant circulates in a refrigeration cycle.  For example: F-22, F-134a, Ammonia etc.

b. SECONDARY REFRIGERANT

  Primary refrigerant cools it and  it cools the products/space. It does not undergo the refrigeration cycle. For example: air in a fridge, air in a window air conditioner, brine solution in an ice plant.

c. TERTIARY REFRIGERANT

 A  secondary refrigerant cools it and in turn cools the products/space. It also does not under go the refrigeration cycle. For example: air in a central air conditioning plant.

Here primary refrigerant normally R-22 which chills the water (secondary refrigerant). Chilled water cools air (Tertiary refrigerant). Air cools the space .

Thus air is primary refrigerant (in aircraft refrigeration). Air is secondary refrigerant (in a fridge).  air is a tertiary refrigerant in a central air conditioning system.

d. CRYOGENIC REFRIGERANT

 A refrigerant which produces a temperature lower than –150⁰C . It is normally permanent gases, like air, hydrogen and helium

TYPES OF REFRIGERANTS ON THE BASIS OF THEIR COMPOSITION

a. ORGANIC REFRIGERANTS–  further two types.

(i)  Saturated hydrocarbons and their halogenated derivatives

Methane, ethane, propane, CH₃Cl, CH₂Cl₂, CH₃Cl, CH₂ClF, CHCl₂F etc.

(ii)  Unsaturated hydrocarbons and their halogenated derivatives

 C₂H₄, C₃H₆, C₄H₈, C₂H₃Cl

b. Inorganic refrigerants: Air, water, carbon dioxide, ammonia

c. Mixed refrigerants

 A mixture of two or more components in certain proportion, 50 % of R-22 and 50 % of R-12

d. AZEOTROPES REFRIGERANT

Definition

Azeotrope is a refrigerant. It is a mixture of two or more components in a fixed proportion by mass. It behaves as a pure refrigerant by evaporating and condensing at fixed temperatures. For example R500, R-501, R-502, R-503.

Types of Azeotropes refrigerant

The azeotropes are of two types.

Minimum Boiling azeotrope

Minimum Boiling azeotrope posses boiling point lower than the boiling point of the either component in the mixture. It has positive deviation from Raoult’s Law. Obtain these from refrigerants which are endothermic on mixing. The liquid mixture enthalpy increases but its  latent heat decreases as compared to latent heat of its components. Latent heat of such azeotropes is lower than the either components. All the Azeotropes of R-500 series are minimum boiling azeotropes.

EXAMPLES

Sr. No.	Name of Azeotrope	Components	Normal Boiling Point of components	% by mass	Normal Boiling Point of the azeotrope
1.	R-500	R-12/R-152	-29 & -24.7	73.8/26.2	–33.2
2.	R-501	R-22/R-12	-41 & -29	75.0/25.0	-45.0
3.	R-502	R-22/R-115	-41 & -39	48.8/51.2	–46.0

Maximum boiling azeotrope

Maximum boiling azeotrope is that which has boiling point higher than boiling points of either component. These have negative deviation from the Raoult’s Law.  These are from refrigerants which on mixing are exothermic. Therefore the liquid mixture enthalpy decreases and latent heat increases. Latent heat of such azeotrope is higher than the either components.

EXAMPLE  MAXIMUM BOILING AZEOTROPES          

An azeotrope of R-22 and di-methyl ether (50/50 by mass) has a boiling point of -20.3⁰C.  The boiling points of R-22 and di-methyl ether are -40.8 and -23.7⁰C respectively.

NECESSARY CONDITION FOR THE POSSIBLE FORMATION OF AN AZEOTROPE

Refrigerants on mixing must be endothermic or exothermic in nature. This is non ideality in the liquid phase. This non-ideality should be large for

(i) Large heat of mixing in the liquid phase

(ii) Large deviations from Raoult’s Law

NUMBERING OF REFRIGERANTS

(i)For saturated hydrocarbons and their derivatives– three digit number

First digit                         = C – 1   = one less than the ‘C’ atoms in the compound

Second digit                    = H + 1=one more than the ‘H’ atoms in the compound

Third digit                       = Number of fluorine atoms

FOR EXAMPLE:

C₂H₂Cl₂F₂       = R-132

CH₄                     = R-50

(ii)For unsaturated hydrocarbons and their derivatives– four digit number

First digit                         = 1    (number of double or triple bonds)

Second digit                    = C-1   = one less than the ‘C’ atoms in the compound

Third digit                       = H + 1=one more than the ‘H’ atoms in the compound

Fourth digit                     = Number of fluorine atoms

FOR EXAMPLE:

C₂Cl₂F₂           = R-1112

C₂H₂                 = R-1130

(iii) Inorganic compounds—700 + molecular weight

FOR EXAMPLE:

Water              = R-718

Air                    = R-729

NH₃                  = R-717

(iv)Azeotropes

 Numbering starts from R-500

Next known azeotrope as R-501 and so on.

                             Azeotrope          B.P.       Composition by MASS

                              R-500              -33.3      R-12/R-152, 73.8/26.2

                               R-501               -45          R-22/R-12, 75/25

                                R-502               -46          R-22/R-115, 48.8/51.2

(v)Blends/Mixed refrigerants

For example:       R-401A is HCFC blend of R-32 + R-152a + R-124

 R-404A is HCFC blend of R-143a + R-125 + R-134a

By mass         (52% + 44 % + 4 %)

NOMENCLATURE OF REFRIGERANTS

(i) IUPAC NOMENCLATURE

Write halogen as prefix. For example, mono-chloro-methane, di-chloro-fluoro-methane

(ii) ANSI/ASHRAE NOMENCLATURE

Write R as prefix.               For example R-22, R-717

(iii) MOST RECENT NOMENCLATURE

Written as CFC, HCFC, HFC, FC, PFC

Where PFC stands for per-fluorocarbons such as R-218—C₃F₈.

(iv) DECODING SYSTEM OF CFC

R for refrigerant

First digit       = Number of double bonds (omitted if zero)

Second digit  = carbon atoms –1            (omitted if zero)

Third digit     = Hydrogen atoms + 1

Fourth digit  =Fluorine atoms

A letter as a suffix identifies an isomers. There is a “ normal” isomer without any suffix. A normal isomer has the smallest mass difference of atoms attached on each carbon atom. Use suffixes a, b, c  as the mass difference increases from normal. For example C₂H₂F₄ has two isomers

CHF ₂ CHF ₂ =R-134    Normal without a suffix

CH₃ CH₂F = R-134 a    Isomer with a suffix ‘a’

History & Next Generation Refrigerants

Fig. History & Next Generation Refrigerants

Fig. Progressive Refrigerants

REFRIGERANT SELECTION

A single refrigerant is not useable in all the applications. Because a single substance does not posses all the desirable properties. Therefore, select on the best combination of properties possessed by a refrigerant for a particular application. It is a sort of compromise.

DESIRABLE PROPERTIES OF A REFRIGERANT 

THERMODYNAMIC PROPERTIES

Lower boiling point than the application temperature

Working pressure higher than atmospheric pressure

Lesser specific volume

PHYSICAL PROPERTIES

Lower boiling point

Lower freezing point

High latent heat

Less work input or minimum running charges

High COP

CHEMICAL PROPERTIES

Non-toxic

Non- inflammable

Non corrosive

Non-decomposable

Inert

Chemically inert with the material of construction

Inert with materials cooled

Inert with the lubricant

ECONOMIC PROPERTIES

Cheap

Easily available in abundance

Available everywhere

 Less leaking tendency

Leakage easily detectable

Does not spoil the products in the event of leakage

FINAL SELECTION

Make the FINAL SELECTION on the basis of

(i)            Suitable Vapor-Temperature behavior

(ii)           Zero Ozone Potential

(iii)          Low GWP

(iv)         Non toxic

(v)          Non inflammable

(vi)          Stability in the refrigeration cycle

(vii)        Compatibility with materials of construction and lubricant

(viii)       High COP

Final selection is a compromise of properties.

 Industry uses ammonia in spite of toxic and inflammable. It is cheap, abundance availability, high latent heat and low boiling point.

COMPARISON OF REFRIGERANTS

———————————————————————————————————————-

Refrigerant  Evap       Cond      Pressure      N       W              COP    Specific vol   Vol dis.         kW/TR

                      Press     press       Ratio         kJ/kg   kJ/kg         ——      m³/kg      m³/min

————————————————————————————————————————–

R-11               0.21     1.26        6.1             157     31                 5.05        0.79         1.08           0.69

R-12              1.83        7.44      4.07          118.5   26                 4.57        0.92          0.16          0.76

R-13              13.25    —-           —-         —       —-           —                   0.012         —-            —–

R-22               2.96       11.92      4.02         161     35                  4.63         0.08          0.10          0.76

R-113              0.07      0.544      7.83         121     25                   4.85         1.64           2.86         0.72

R-114              0.465    2.52        5.41         95       22                   4.42         0.263        0.58          0.79

R-123              0.016    0.11        4.70        142     29                     4.93          0.902        1.331         —

R-134 a          1.64       7.70        6.72         148     32                    4.61        0.121         0.171         0.76

R-500          2.143        8.79        4.1           141     31                    4.59       0.094         0.140          0.76

R-717           2.37        11 .67      4.94         1106.5 230                 4.89      0.51           0.097          0.72

R-744          22.91      72.11       3.148         131.7   45                   2.90     0.016          0.026         1.21

FIXED PROPERTIES OF REFRIGERANTS

Sr.     Refrigerant          Boiling   Freezing   Critical       Critical

No    Formula         Point             point        temp           pressure, bar 

1. R-11  C Cl ₃ F      +23.7    -111        198              44

2. R-12  C Cl ₂ F ₂     -29.8    -157         112              41.25

3. R-13   C CL F ₃     -81.5     -181         28.85          38.7

4. R-22   C H Cl F ₂  -40.8     -160         96.14          49.9

5. R-113 CCl₂FCCl₂F +45.9    -35       214.4             34.11

6. R 123 C₂HCl₂F₃                               183.6             36.7

7. R-134a CF₃CH₂F   -26.0     -101      101                40

8. Iso-butane            -11.7

9. R-702  H₂               -253

10. R-704  He                -269

11. R-717  NH₄              -33.5    -77.8     132              113.33

12. R-718  H₂O              +100

13. R-728  N₂                  -196

14. R-729  Air                  -191

15. R-732  O₂                   -183

16. R-744  CO₂                 -78.5                 30.8              73.75

17. R-764  SO₂                  -10.1

18. R-500                           -33.3                 105               44.26

        R-12/R-152, 73.8/26.2  (-29/-24.7 ⁰ C)

19. R-501                            -45      

        R-22/R-12, 75/25 (-41/-29⁰ C)     

20. R-502                              -46      

        R-22/R-115, 48.8/51.2  (-41/-39⁰ C)

21. R-503                              –  88      

          R-23/R-13, 40.1/59.9    (-82.1 /-81.5⁰)

22.  R-504                                 -57      

          R-32/R-115, 48.3/51.7   (-51.7 /-39⁰)

23.  R-505                                   -30      

        R-12/R-31       78/22      (-29/-9.1 ⁰ C)

REFRIGERANT CHARTS AND TABLES

There are pressure temperature, pressure enthalpy and temperature entropy charts. Use these in analysis of refrigeration systems.

Refrigerant table gives only specific properties of saturated liquid and saturated vapors.

 LEAKAGE AND LEAK DETECTION

TYPES OF LEAKAGES IN REFRIGERATION

There are two types of leakages.

(i)  External leakage

(ii) Internal leakage

External leakage: This leakage takes place when internal pressure is greater than atmospheric pressure which is quiet common. In this leakage, refrigerant goes out to atmosphere. If the leakage is not detectable (with most Freon’s) by smell then it is direct loss of money. Global warming and ozone depleting affect atmosphere.

Internal Leakage: This leakage takes place when working pressures are vacuum inside the plant. It is in case of R-11 and R-114. Due this type of leakage, dust, water and air will enter the system.

Dust

 Decreases the heat transfer in the evaporator and condenser. It increases the wear and tear of the compressor.

Water

(i)  Water will form acid or alkali with the refrigerant. It becomes corrosive to the materials of construction.

(ii)  It affects the lubrication.

(iii)  It freezes when reaches the expansion valve and choke the system. The unit is working physically but absolutely no cooling taking place.

Air

Air entering the system increases the working pressures in the evaporator and condenser as it is does not condense. It also increases the temperatures in the evaporator and condenser. Air reduces heat transfer in the evaporator and condenser as being poor conductor of heat. Work input increases and required temperatures are not achievable.

METHODS OF LEAK DETECTION FOR REFRIGERANTS

Fig. Halide torch leak detector

Hold the torch flame near to points of leakage, color of flame changes indicating leakage. It is dangerous. Poisonous gases produce when refrigerant and flame come across each other.

(iv) Electronic leak detector

Move the probe near the suspected points of leakage.

It gives sound when detects leakage.

It is a simple, quick and technical method of leak detection.

Testing for outward leakage

First evacuate the refrigerant from a system. Then fill the the system with Nitrogen at 20 atm. Keep the system under this pressure for at least 24 hours. If the pressure remains constant, then there is no outward leakage. If the pressure falls then check it for outward leakage with  soap solution.

Testing for inward leakage

First evacuate the refrigerant from the system. Create a perfect vacuum using a vacuum pump. Keep the system under this vacuum for at least 24 hours. If the vacuum remains constant,  then there is no inward leakage. If the vacuum brakes then there is an inward leakage. Find the point of leakage under high pressure using soap solution.

Dust will decrease the heat transfer in the evaporator and condenser.  It increases the wear and tear of the compressor.

Water

(i)  Water forms acid or alkali with the refrigerant.

(ii) It becomes more corrosive to the materials of construction.

(iii)  It will also affect the lubrication.

(iv)  It will freeze when reaches the expansion valve and choke the system. The unit will be working physically but absolutely no cooling taking place.

Air

Air entering the system increases the working pressures in the evaporator and condenser. As a result condensation becomes difficult. It also increases the temperatures in the evaporator and condenser. It reduces heat transfer in the evaporator and condenser as being poor conductor of heat. Work input increases and difficulty in achieving required temperatures.

CONCLUSIONS 

1. There is no ideal refrigerant in the universe.
2. Refrigerant circulates in a refrigeration system like blood circulates in the human body.
3. Thermodynamic, transport, chemical, physical and economic properties help in the selection of a refrigerant for a particular application.
4. Classification, nomenclature and numbering of refrigerants makes their study and application simple.
5. Selection dictates compromise on properties.
6. Halogenated hydrocarbon refrigerants cause ozone depletion and global warming.
7. Use ecofriendly substitutes.
8. Ozone depletion causes cancer.
9. Global warming causes skin problems.
10. Use a cheap, ample available, detectable leakage refrigerant.
11. Cheapest refrigerant ammonia having high latent heat finds its use in industry, cold storage and ice plants.
12. Refrigerant R-22 finds its limited use in air conditioning.
13. Refrigerant R-134a finds its use in domestic applications.
14. Earlier most commonly used refrigerants R-11 and R-12. Now these finds no use because of having high ozone depletion and global warming potentials.

 

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https://www.mesubjects.net/wp-admin/post.php?post=3109&action=edit      Q. ANS Refrigerants

https://mesubjects.net/wp-admin/post.php?post=3109&action=edit               Specific refrigerants