Refrigerant is the substance employed as the heat absorber or cooling agent in any refrigerating process. Cooling process may be classified as either sensible or latent according to the effect the absorbed heat has upon refrigerant. When the absorbed heat causes an increase in the temperature of the refrigerant, the cooling process is said to be sensible heat. When the absorbed heat causes a change in the physical state of the refrigerant (either melting or vapourizing), the cooling process is said to be latent heat.
Refrigerants are predominantly used to produce refrigeration effect. Refrigeration relates to the cooling of air or liquids, thus providing lower temperature to preserve food, cool beverages, make ice and for many others.
History of refrigerants
Ancient people of India and Egypt cooled liquids in porous earthen jars.
All coastal cities Viz Madras, Bombay and Calcutta used to have ICE house where the Britishers used to store ICE harvested from western part of the globe to be used during summer season for keeping cool.
Most evidence indicates that the Chinese were the first to store natural ice and snow to cool wine and other delicacies. In 1834, Jacob Perkins, an American, developed a closed refrigeration system using liquid expansion and then compression to produce cooling.
Principle of refrigeration
Refrigeration and Air-conditioning equipment is predominantly vapour compression refrigeration system built upon the thermodynamic principles of the reverse Carnot cycle. Refrigerant changes phase during cooling and used again and again.
What is a refrigerant?
A refrigerant is the substance employed as the heat absorber or cooling agent in any refrigerating process. Fluids suitable for refrigeration purposes can be classified into primary and secondary refrigerants.
Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems.
These fluids provide refrigeration by undergoing a phase change process in the evaporator (Chiller). Secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other. Secondary refrigerants are also known under the name brines or antifreeze solution.
Desired properties of refrigerant
The desired thermodynamic properties are a boiling point somewhat below the target temperature, a high heat of vapourization.
The ideal refrigerant would have favourable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including free from toxicity and flammability.
It would not cause ozone depletion or climate. Since different fluids have the desired traits in different degree, choice is a matter of trade-off, a moderate density in liquid form, a relatively high density in gaseous form, and a high critical temperature.
Since boiling point and gas density are affected by pressure, refrigerants may be made more suitable for a particular application by choice of operating pressures.
What is ozone layer?
Ozone is an isotope of oxygen with three atoms instead of normal two. It is a naturally occurring gas which is created by high energy radiation from the Sun.
The greatest concentrations of ozone are found from 12 km to 50 km above the earth forming a layer in the stratosphere, which is called the ozone layer.
This layer, which forms a semi-permeable blanket, protects the earth by reducing the intensity of harmful Ultra-Violet (UV) radiation from the sun.
Ozone layer depletion
In the early 70s, scientists Sherwood Roland and Mario Molina at the University of California at Irvine were the first to discover the loss of ozone in stratosphere while investigating the ozone layer from highflying aircraft and spacecraft.
They postulated the theory that exceptionally stable chlorine containing fluorocarbons could, overtime, migrate to the upper reaches of the atmosphere and be broken by the intense radiation and release chlorine atoms responsible for catalytic ozone depletion.
Harmful effects of ozone layer depletion
For humans – Increase in skin cancer slow blindness, cataracts
Less immunity to – infectious diseases, malaria, herpes
For plants – smaller size, lower yield, increased toxicity, and altered form
For marine life – Reduced, plankton, juvenile fish, larval crabs and shrimps
Montreal protocol
INDIA RATIFIED ON SEPT 17, 1992, ONE OF MOST SUCCESSFUL EXAMPLE OF INTERNATIONAL COOPERATION IN UN HISTORY
Montreal Protocol control schedule
Vapour compression refrigeration system components…
- Compressor
cold vapour from the evaporator is compressed, raising its temperature and boiling point adiabatic compression T, b.p.
~ P work done on the gas
- Condenser
hot vapour from the compressor condenses outside the cold box, releasing latent heat isothermal, isobaric condensation (horizontal line on PV diagram given below) high temperature T (hot) latent heat of vaporization Q (hot)
- Expansion valve (throttling valve) hot liquid from the condenser is depressurized, lowering its temperature and boiling point adiabatic, isochoric expansion (vertical line on PV diagram) T, b.p. ~ P no work done W = 0
- Evaporator
cold liquid from the expansion valve boils inside the cold box, absorbing latent heat isothermal, isobaric boiling (horizontal line on PV diagram) low temperature T (cold) latent heat of vaporization Q (cold)
Importance of refrigerant
- The 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 the 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 the various refrigerants used, the selection of suitable refrigerant has become one of the most important issues in recent times.
Refrigerant selection criteria
- Selection of refrigerant for a particular application is based on the following requirements:
– Thermodynamic and thermo-physical properties
– Environmental and safety properties
– Economics
Thermodynamic and thermo-physical properties
- The requirements are: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. Higher suction pressure is better as it leads to smaller compressor displacement
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 etc.
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.
In addition to the above properties; the following properties are also important:
e) Isentropic index of compression: Should be as small as possible so that the temperature rise during compression will be small
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) Vapour specific heat: Should be large so that the degree of superheating will be small
h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients
i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops
- The thermodynamic properties are interrelated and mainly depend on normal boiling point, critical temperature, molecular weight and structure.
Environmental effects of refrigerants
Global warming: Refrigerants directly contributing to global warming when released to the atmosphere
Indirect contribution based on the energy consumption of among others the compressors ( CO2 produced by power stations )
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):
The factor 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.
Toxicity: Ideally, refrigerants used in a refrigeration system 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. Some fluids are toxic even in small concentrations. Some fluids are mildly toxic, i.e., they are dangerous only when the concentration is large and duration of exposure is long. 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
Flammability: The refrigerants should preferably be non-flammable and nonexplosive. For flammable refrigerants special precautions should be taken to avoid accidents.
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.
Economic properties The refrigerant used should preferably be inexpensive and easily available.
ECO Friendly Refrigerants
Halocarbon Refrigerant is all synthetically produced and was developed as the Freon family of refrigerants.
Examples: – CFC’s : R11, R12, R113, R114, R115
– HCFC’s : R22, R123
– HFC’s : R134a, R404a, R407C, R410a
HFC
Remain a popular choice -especially for R22 phase out
Good efforts at improving leakage performance – e.g., Real Zero project
Interest in R407A to replace R404A – 50% reduction in GWP
Inorganic Refrigerants
- Carbon Dioxide, Water, Ammonia, Air, Sulphur dioxide
Conclusions: In the aftermath of the Montreal protocol, HFCs have predominantly replaced CFCs and HCFCs in Refrigerating and air conditioning equipment. Due to their high GWP, HFC’s are not a long term replacement solution.
The solutions are the natural refrigerants: Ammonia, Hydrocarbons and Carbon dioxide System need to have low TEWI (Total Equivalent Warming Index) factor. High efficiency with ammonia and lower power consumption with hydrocarbons.
Even among the natural refrigerants ammonia and hydrocarbons are popular and carbon dioxide is not yet taken off in India as the working pressure is 80 Kg/cm2(80 Bar).
