City Basic
DesignMechanical Engineering
Dr. R. Vaira Vignesh
Assistant Professor (Selection Grade) – Research Track
Department of Mechanical Engineering
Amrita School of Engineering – Coimbatore
Amrita Vishwa Vidyapeetham (Coimbatore Campus)
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Refrigerants
▪ 1850’s – 1870’s: ammonia, ammonia/water, CO2
▪ Early 1900’s: SO2, methyl chloride used for domestic refrigerators
▪ 1930’s: halocarbon refrigerants (R-12, R-22, R-114, R-22)
▪ Halocarbon advantages – stable compounds, favourable thermodynamic properties
▪ Thermodynamic efficiency of a refrigeration system depends mainly on its operating temperatures.
▪ However, important practical issues such as the system design, size, initial and operating costs, safety,
reliability, and serviceability etc. depend very much on type of refrigerant selected for a given
application
▪ Due to several environmental issues such as ozone layer depletion and global warming and their
relation to various refrigerants used
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Refrigerants
▪ Working substances
▪ Primary and Secondary refrigerants
▪ Primary refrigerants are those fluids, which are used directly
as working fluids
▪ Fluids provide refrigeration by undergoing a phase change
process in the evaporator
▪ Secondary refrigerants – used for transporting thermal energy
from one location to another
▪ Secondary refrigerants are also known under name brines
or antifreezes
Primary Refrigerant
▪ Refrigerants which are directly used to obtain the cooling
effect in evaporator by undergoing a phase change process
▪ Halocarbon Refrigerants
▪ Azeotropes Refrigerants
▪ Inorganic Refrigerants
▪ Hydro-carbon Refrigerants
Halocarbon Refrigerants
▪ Synthetically produced (Derived from Methane , Ethane)
▪ Developed as the Freon family of refrigerants
▪ Commonly used in Domestic, Commercial and Industrial Purposes
▪ Wide range of boiling points at atmospheric pressure
▪ Presence of fluorine makes it non-toxic
▪ Ozone unfriendly refrigerants
Halocarbon Refrigerants
CFC’s
▪ First developed by General Motor’s researchers in the 1920’s and commercialized as Freon‘s
▪ Most stable – remain in atmosphere for many years, allowing them to diffuse to high altitudes
▪ Contains Chlorine, Fluorine, Carbon.
▪ CFC’s break down, and Cl combines with and consumes some ozone
▪ R11 (CCl3F) Trichloromonofluromethane
▪ R12 (CCl2F2) Dichlorodifluoromethane
▪ R40 (CH3Cl) Methyl Chloride
▪ R13, R21, R22,R113, R114, R115
Halocarbon Refrigerants
HCFC’s
▪ Hydrogenated
▪ Not as stable – most of it breaks down before reaching high altitudes
▪ Contains Hydrogen, Chlorine, Fluorine, Carbon
▪ Less damaging to ozone
▪ R22 (CHClF)2 Monochlorodifluromethane
▪ R123 (C2HCl2F3) Dichlorotrifluoroethane
Halocarbon Refrigerants
HFC’s
▪ Contains Hydrogen, Fluorine, Carbon.
▪ Contains no Cl (Chlorine)
▪ Causes no depletion of ozone
▪ R134a (CH2FCF3) - Tetrafluoroethane
▪ R404a, R407C, R410a - mixture of
Difluromethane (CH2F2, called R-32)
▪ Pentafluroethane (CHF2CF3, called R-125)
Azeotrope (Mixed) Refrigerants
▪ Stable mixture of two or several refrigerants whose
vapour-liquid phases retain identical compositions over a
wide range of temperatures.
▪ Thermodynamic properties remain fixed
▪ Code starts with digit 5
▪ R500 – mixture of 73.8% of R12 and 26.2% of R152
▪ R502 – Mixture of 49% of R22 and 51% of R115
Zeotropic Refrigerants
▪ Zeotropic mixture is one whose composition in liquid phase differs to that in vapour phase
▪ Zeotropic refrigerants therefore do not boil at constant temperatures unlike azeotropic refrigerants
Examples :
▪ R404a : R125/143a/134a (44%,52%,4%)
▪ R407c : R32/125/134a (23%, 25%, 52%)
▪ R410a : R32/125 (50%, 50%)
▪ R413a : R600a/218/134a (3%, 9%, 88%)
▪ Zeotropic refrigerants do not boil and condense at constant temperature – advantage
▪ Phenomenon – temperature glide – match the pressure drop in heat in heat exchangers thereby increasing
their efficiency => improved COP of the refrigeration cycle
Inorganic (Natural) Refrigerants
▪ Before Halocarbons, natural refrigerants
was extensively used
▪ Designated by R followed by Number.
▪ Number = 700 + M
▪ Example: Carbon Dioxide , Water ,
Ammonia , Air , Sulphur dioxide
▪ R717 - (NH3)Ammonia
▪ 717 = 700 + 17
Hydro-Carbon Refrigerants
▪ Satisfactory thermodynamic properties
▪ Extraordinary reliability - fewer compressor failures.
▪ Virtually no refrigerant losses
▪ Long term solutions to environmental problems
▪ Most of the hydrocarbons – highly flammable and require additional safety precaution
▪ Not used in Industry and commercial installations
▪ Dominant in domestic market like household refrigerators and freezers
▪ Growing use in very small commercial systems like car air-conditioning system.
▪ Examples: R170 – Ethane (C2H6), R600 – Butane (C4H10), R600a – Isobutane (C4H10)
Secondary Refrigerant
▪ Used for transporting thermal energy from one location
to other
▪ Does not undergo phase change process
▪ Used when refrigeration is required at sub-zero
temperatures
▪ Known as Brines or Antifreezes
▪ Used in large refrigeration units
▪ Commonly used secondary refrigerants are: Solution of
water & ethylene glycol, propylene glycol etc.
Secondary Refrigerant
▪ If working temperature is above 3℃, water is used as SR
▪ Brine is aqueous solution of NaCl and CaCl2 in water and used at
a temperature below freezing point of water 0 ℃
▪ Used in cooling of fish, meat & ice plant
▪ Ethylene glycol & Propylene glycol mixes with water and gives
colorless & odorless solutions
▪ Capacity to lower freezing temperatures and hence used as
antifreeze mixtures for I.C. engine cooling systems
▪ Solutions become corrosive after some use; hence, corrosive
treatment is necessary.
Advantages of Secondary Refrigerant
▪ Different rooms of building can be cooled up to different temperatures by adjusting the flow rates of
secondary refrigerants
▪ SR can be easily handled
▪ SR can be easily controlled
▪ Eliminates long refrigeration lines and thus reduces pressure drops
Desirable Properties
▪ Low freezing point, High heat transfer coefficients
▪ High specific heat, Low vapour pressure
▪ Good stability, Non-flammable and non-toxic
Current and Future Refrigerants
▪ R-134a has emerged as the primary
substitution for many CFC’s
▪ HCFC-22 and HCFC–123 are viable alternatives
for now but will eventually be phased out.
▪ In Europe, natural refrigerants such as
ammonia, CO2, propane, and water are being
used more.
▪ Legal system makes flammable refrigerants
questionable in the US
Environmental Effects of Refrigerants
Global warming ▪ CFC refrigerants leaked during manufacturing and
▪ Refrigerants directly contributing to global warming normal operation or at the time of servicing or
when released to the atmosphere. repair, mix with surrounding air and rise to
troposphere and then into stratosphere due to
▪ Indirect contribution based on the energy
normal wind or storm
consumption of among others the compressors
(CO produced by power stations ) ▪ Ultraviolet rays act on CFC releasing Cl atom, which
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retards the normal reaction:
▪ Ozone Depletion:
Retarded Reaction
Normal reaction
O2 = O + O O3 = O2 + O
O2 + O = O3
CCL2F2 = CCLF2 + CL
O3 + CL = CLO + O2
Refrigerant Properties
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
▪ Higher suction pressure is better as it leads to smaller compressor displacement
Discharge pressure
▪ At a given condenser temperature, the discharge pressure should be as small as possible to allow
lightweight construction of the compressor, condenser, etc.
Pressure ratio
▪ Should be as small as possible for high volumetric efficiency and low power consumption
Refrigerant Properties
Latent heat of vaporization
▪ Should be as large as possible so that the required mass flow rate per unit cooling capacity will be small.
Isentropic index of compression
▪ Should be as small as possible so that the temperature rise during compression will be small
Liquid specific heat
▪ Should be small so that degree of sub cooling will be large leading to smaller amount of flash gas at
evaporator inlet.
Vapour specific heat
▪ Should be large so that the degree of superheating will be small.
Refrigerant Properties
Thermal conductivity
▪ Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer
coefficients .
Viscosity
▪ Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops.
Thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature,
molecular weight and structure
Environmental and safety properties
▪ At present the environment friendliness of the refrigerant is a major factor in deciding the usefulness of a
particular refrigerant.
Ozone Depletion Potential (ODP)
▪ According to the Montreal protocol, the ODP of refrigerants should be zero, i.e., they should be non-ozone
depleting substances.
▪ 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).
▪ Since ODP depends mainly on the presence of chlorine or bromine in the molecules, refrigerants having
either chlorine (i.e., CFCs and HCFCs) or bromine cannot be used under the new regulations.
Environmental and safety properties
Toxicity
▪ Ideally, refrigerants should be non-toxic.
▪ Toxicity is a relative term, which becomes meaningful only when the degree of concentration and time of
exposure required to produce harmful effects are specified.
In general the degree of hazard depends on:
▪ Amount of refrigerant used vs total space
▪ Type of occupancy
▪ Presence of open flames
▪ Odor of refrigerant, and
▪ Maintenance condition
Environmental and safety properties
Global Warming Potential (GWP)
▪ Refrigerants should have as low a GWP value as possible to minimize the problem of global warming.
▪ Refrigerants with zero ODP but a high value of GWP (e.g. R134a) are likely to be regulated in future.
Total Equivalent Warming Index (TEWI)
▪ Considers both direct (due to release into atmosphere) and indirect (through energy consumption)
contributions of refrigerants to global warming.
▪ Naturally, refrigerants with as a low a value of TEWI are preferable from global warming point of view
Flammability
▪ Refrigerants should preferably be non-flammable and non-explosive.
▪ For flammable refrigerants special precautions should be taken to avoid accidents.
Environmental and safety properties
Chemical stability
▪ The refrigerants should be chemically stable as long as they are inside the refrigeration system.
▪ Compatibility with common materials of construction (both metals and non-metals)
Miscibility with lubricating oils
▪ Oil separators have to be used if the refrigerant is not miscible with lubricating oil (e.g. ammonia).
▪ Refrigerants that are completely miscible with oils are easier to handle(R12).
Ease of leak detection
▪ In the event of leakage of refrigerant from the system, it should be easy to detect the leaks.
▪ r.vairavignesh@gmail.com
Thank You ▪ r_vairavignesh@cb.amrita.edu
▪ sites.google.com/site/rvairavignesh
▪ www.amrita.edu/faculty/r-vairavignesh
The contents and images in the presentation are obtained from internet or articles or books that may be protected by copyright. The
presentation is distributed freely but only for classroom instruction or the use of students. The presentation may not be retained or
disseminated. Any further use of this material may be in violation of Exceptions To Infringement Under Copyright Act, 1957 (Government
of India)
Please do not remove the credit line on the title page and republish the file as your own, in whole or in part.
