Refrigerants | Thermodynamics - Mechanical Engineering PDF Download

REFRIGERANTS

Compressors for Different Refrigerants

Applications of Refrigerants

DESIGNATION OF REFRIGERANTS
Saturated Hydrocarbons

•  If the compound is C_mH_nF_pCl_q
  n + p + q = 2m + 2

then,

designation is R(m - 1) (n + 1) P

Eg. CCI₃ is R-11

•  The brominated refrigerants are denoted by and additional B and a member to denote as to how many chlorine atoms are replaced by bromine atoms

Eg. CF₃ Br is R13 B1

Unsaturated Compounds - C_mH_nF_pCl_q
where n + p + q = 2m
they are designated by R (m-1) (n+1)P
Eg. C₂H₄ is R1150

Inorganic Refrigerants
Designation is R (700 + molecular weight)
Eg. NH₃ is R 717, CO₂ is R744

Basic properties of Some important Refrigerants

CFC 12 or R-12-or CCl₂F₂

•  Normal boiling point (N.B.P.) = 29.8ºC
•  It is used in domestic refrigerators, car air conditioners, water coolers etc.
•  It is used with reciprocating compressor
•  Because of low value of g. discharge temperature is low. so it is very suitable for hermetically sealed units

CFC-11 or R-11 or CCl₃F

•  N.B.P. -23.7ºC
•  Having more boiling point, It has vacuum in the evaporator.
•  It is used with centrifugal compressor.
•  Its application is in central air conditioning.

HCFC-22 or R-22 or CHClF₂

•  N. B. P. – 40.8ºC
•  It is used in window air conditioning.
•  It is used with reciprocating compressor.
•  It has high g value, so discharge temperature is high. Thus it is not very suitable for hermetically sealed unit for more than one year.
•  N.B.P. of commercially used refrigerants vary from -50ºc to + 50ºC.

PROPERTIES OF GOOD REFRIGERANTS

•  Low boiling point
•  Evaporating pressure should be low and positive and near to atmospheric pressure
•  High critical temperature
•  Low critical pressure
•  Low freezing point
•  Non flammable, non explosive
•  Either completely miscible or completely immiscible with oil
•  Non corrosive
•  High dielectric strength
•  High thermal conductivity
•  Low viscosity
•  Low specific heat
•  Low surface tension
•  Low cost

SECONDARY REFRIGERANTS

•  They are used to reduce the quantity of main refrigerant and to reduce pressure losses. They are cheap in cost.

Eg. brines such as : CaCl₂ + Water & NaCl + water, Ethylene glycol + water and propylene glycol + water.

•  CaCl₂ and NaCl is mixed with water to reduce the freezing point of water.

OZONE DEPLETION 

•  Ozone layer is in stratosphere.
•  It protects humans from UV rays which can cause skin cancer.
•  Chlorine atoms in CFC causes ozone depletion.
•  Hydro-Chloro-flouro-carbons (HCFCs) have low ozone depletion tendency because of less chlorine.

But they have Global warming tendency.

• Pure hydro carbon is highly inflammable due to hydrogen atoms.
• Azerotrope: These are non ideal mixtures of two or more refrigerants behaving as pure substance having close N.B.P. Their bubble point temperature is equal to dew point temperature.
• Bubble temperature is the temperature at which liquid mixture starts boiling.
• Dew point temperature is the temperature at which vapour mixture starts condensing.
• CO₂ refrigeration system has very low COP because critical temperature of CO₂ is very low and very close to condensing temperature.
• Because of flammability & toxicity, ammonia is not used in domestic refrigerator, comfort air conditioning.
• If water content in refrigerant is more than that can be dissolved by the refrigerant, there is a danger of ice formation and consequently choking of capillary tube used in throttling system. This is moisture chocking.
• Silica gel is used in liquid line for dehydration of refrigerant.
• Miscibility of refrigerants with oil

•  In evaporator, oil separates from refrigerant vapour. Accumulation of oil in evaporator called oil choking.
•  In immiscible oil refrigerant mixture, oil separator is used after compressor.
•  Ammonia corrodes copper. So iron and steel are used with its system.
•  Refrigerants in decreasing order of normal boiling point.

R-11(–23.7ºC) > R-12(-29⁰C) > NH₃(–33.3⁰C) > R-22(–41⁰C) > CO₂(– 73.6⁰C)

• Refrigerants in increasing order of freezing point 

CO₂ < NH₃ < R-11 < R-12 < R-22

• Refrigerants in decreasing order of critical temperature

R-11 < NH₃ < R-12 < R-22 < CO₂

• Refrigerants in decreasing order of Critical Pressure

NH₃(113.86) > CO₂(73.8) > R-22 > R-11 > R-12(41.2)

• Refrigerants in decreasing of COP

R -11 > NH₃> R-12> R-22> CO₂

Leakage Detection of Refrigerants