PE8491 CET - Refrigeration and Liquefaction
PE8491 CET - Refrigeration and Liquefaction
PE8491 CET - Refrigeration and Liquefaction
Thermodynamics
Unit –IV
December 2019-April 2020
Dr. R.Malini,
Scientist,
Electro-inorganic chemicals Division
CSIR-CECRI
Unit IV
Refrigeration and liquefaction process,
Thermodynamic Potentials,
thermodynamic correlation,
Maxwell relations,
criteria for Equilibria and stability.
Clapeyron equation
REFRIGERATION
Refrigeration is the process of producing and maintaining a temperature below that of the
surrounding atmosphere.
Examples:
air conditioning of buildings, transportation, and preservation of foods and beverages, the
manufacture of ice and the dehydration of gases, lubricating-oil purification, low-temperature
reactions, and separation of volatile hydrocarbons.
Since heat cannot flow from a body at low temperature to one at a higher temperature
spontaneously, external work is required to achieve refrigeration. Thus, refrigeration is
essentially an operation involving the pumping of heat from one temperature to a higher
temperature.
In mechanical refrigeration, which is the most commonly used method for commercial
applications, the low temperature is produced by the evaporation of a liquid whose properties
are, such that, at the pressure of evaporation, the saturation temperature is low. The evaporated
liquid is then returned to its original state for continuous operation.
The complete series of processes that the working fluid— the refrigerant—undergoes,
constitute a refrigeration cycle.
A typical refrigeration cycle includes evaporation of the liquid refrigerant, compression of the
refrigerant vapour, condensation of the vapour into liquid, and finally expansion of the liquid.
THE CARNOT
REFRIGERATOR
The refrigeration cycle is a reversed heat-engine cycle.
It consists of two isothermal steps in which heat |Qc| is absorbed at the lower
temperature Tc and heat |QH | is rejected at the higher temperature TH, and two
adiabatic steps.
The cycle requires the addition of net work W to the system. Since ΔU of the
working fluid is zero for the cycle, the first law is written as
A liquid evaporating at constant pressure (line 1 →2) provides a means for heat
absorption at a low constant temperature.
The vapor produced is compressed to a higher pressure, and is then cooled and
condensed with rejection of heat at a higher temperature level.
Liquid from the condenser returns to its original pressure by an expansion process.
The pressure drop in this irreversible process results from fluid friction in the valve.
line 4 → 1 represents this throttling process.
The dashed line 2 → 3' is the path of isentropic compression
Line 2 → 3, representing the actual compression process, slopes in the direction of
increasing entropy, reflecting inherent irreversibilities.
To design the evaporator, compressor, condenser, and auxiliary equipment one must
know
the rate of circulation of refrigerant m. This is determined from the rate of heat
absorption in
the evaporator by the equation: