Saturday, 26 January 2013

What is a Refrigerant | Desired properties of a Refrigerant | Important Refrigerant.

What is a Refrigerant ?

1) A refrigerent is a substance that carries heat from the refrigerated space or body to the outside surrounding.

2) Therefore a refrigerent may be defined as any substance that
absorbs heat either by expansion or vapourisation and rejects it through condensation.

3) In Mechanical refrigeration systems refrigerant is subjected to
cyclic thermodynamic processes, so that it is forced to vapourise in the evaporator by absorbing heat from refrigerated space and it is forced to condense and convert into liquid in the condenser by rejecting heat to surrounding.

4) The blood of any Refrigerating system is the refrigerant
circulating inside the system.

Desirable Properties of a Good Refrigerant

1) Thermodynamic Properties :-

a) Boiling Point :- It should have Low Boiling Point.

b) Freezing Point :- It should be below the Evaporator temperature.

c) Evaporative Pressure :- It should be above Atmospheric pressure.

d) Condensing Pressure :- 
It should have LOW Condensing pressure.

e) Latent Heat of Vapourisation :- It should have HIGH Latent heat of Vaporisation.

f) Critical Temperature & Pressure :- It should be above the condensing Temperature & Pressure.

2) Chemical Properties :-

a) Toxicity :- It should not be Poisonous or injurious . It should not be non-irritating to eyes.

b) Corrosiveness :- It should not be corrosive & should not have any effect on materials used in equipment.

c) Leak Detection :- It should have less tendency to leak & if it is leaking it should be easily detectable.

d) Flammability :- It should not be Inflammable.

e) Miscibility with Oil :- It should be immiscible with oil & should not have any effect on the properties of Oil used for Lubrication.

f) Effect on Foodstuff :- It should not affect on food articles Or make them poisonous or unportable.

g) Stability :- It should be capable of withstanding high pressure & temperature.

3) Physical Properties :-

a) Specific Volume :- It should be LOW in Vapour state.

b) Viscosity :- It should have LOW viscosity.

c) Thermal Conductivity :- It should have HIGH Thermal Conductivity.

d) Di-Electric Strength :- It should have High strength.

4) Other Properties :-

a) Handling & Maintainance :- It should be easy & safe to handle.

b) Cost & Availability :- It should be readily available at LOW cost.

c) Performance of the System :- It should have high COP & LOW power requirement.

Important Refrigerants

1) Water :-
i) Working temp. :- above 3 degree celcius.
ii) It is the cheapest, less corrosive & have high specific heat.

2) Brines :-
i) They are the solutions of Nacl & Cacl2 .
ii) Used below the freezing point of water.

3) Ethylene glycol & Propylene glycol :-
i) Used as anti-freeze mixtures.
ii) Mixture with water makes colourless & odourless.
iii) Capacity to lower freezing point.

4) Halocarbon compounds :-
i) These contains one or more of 3 halogens i.e., chlorine, bromine or fluorine. These are sold under names Freon, Genetron, Isotron, Aretron etc.

ii) Have low freezing points.

Thursday, 24 January 2013

What are Ceramics | Their Classification | Structure | Properties


The word "ceramics" is derived from an ancient Greek word "keramos",
which refers to potter's clay and the objects made from it.

What are ceramics ???? 

Defination :-
1) Ceramics are Inorganic, Non-Metallic compounds for which Interatomic Bonds may be either of the following two types :-
a) Ionic, but with some Covalent character or
b) Covalent in some cases.

2) Ceramics are prepared by heating inorganic , nonmetallic solids and
then cooling them to retain the given shape.

Classification of Ceramics :-
Ceramics can be classified on the basis of their Industrial applications and structure as follows :-

i) Functional Classification :- Abrasives, Fireclay products, Cementing products, Glass, Rocks, Minerals, Clay, Refractories etc.

ii) Structural Classification :- Crystalline ceramics & Non-Crystalline ceramics.

Structure :-
1) Ceramics have crystal structure which is similar to a metal structure.
2) Atoms of ceramics are ionically bonded or covalently bonded.
3) Ionically bonded ceramics are hard ,rigid and posses high chemical stability. Example:- Nacl.
4) Covalently bonded ceramics are also hard,rigid and posses high electrical resistance. Example:- SiO
5) Combinedly bonded ceramics are similar in properties and show better characteristics than both. Example :- MgO.

Properties :- The following properties of ceramic materials make them useful for Engineering Applications :-

1) Mechanical Properties :-

i) Ceramic materials have low tensile strength.
ii) They generally fail due to stress concentration on cracks, pores etc.
iii) Ceramics posses great hardness and resistance to wear and can be used for grinding.
iv) Ceramics posses high compressive strength.
v) Most ceramics posses low fracture strength and fail in a brittle manner.
vi) Values of Modulus of Elasticity for ceramics ranges from 7 x 1010
N/m2 to 40 x 1010 N/m2 .

2) Electrical Properties :-

i) Ceramics are often used for electric insulation.
ii) Some ceramics conduct electrically well and are used as Semiconductors i.e N.T.C & P.T.C resistors.
iii) Many ceramics have a dielectric constant value upto 100 and very low dielectric losses.
iv) Some ceramics also exhibit piezoelectric properties and can transfer mechanical deformations in to voltage changes.

3) Chemical Properties :-

i) Majority of ceramics are highly resistant to all chemicals and
organic solvents.
ii) Ceramics are completely resistant to oxidation even at high temperature.
iii) Glazed porcelain ceramics is used for chemical vessels.

4) Optical Properties :-

i) Many types of glasses are used for windows and optical lenses.
ii) They also find use in selective transmission or absorption of certain wavelengths.

5) Thermal Properties :-

i) Ceramic materials do not have enough electrons for bringing about
electronic thermal conductivity.