26-06-2012, 02:45 PM
Vapor Compression Refrigeration System
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INTRODUCTION
Vapor compression Refrigeration system is an improved type of air. The ability of certain liquids to absorb enormous quantities of heat as they vaporize is the basis of this system. Compared to melting solids (say ice) to obtain refrigeration effect, vaporizing liquid refrigerant has more advantages. To mention a few, the refrigerating effect can be started or stopped at will, the rate of cooling can be predetermined, the vaporizing temperatures can be governed by controlling the pressure at which the liquid vaporizes. Moreover, the vapor can be readily collected and condensed back into liquid state so that same liquid can be recirculated over and over again to obtain refrigeration effect. Thusthevapor compression system employs a liquid refrigerant which evaporates and condenses readily. The System is a closed one since the refrigerant never leaves the system.
The coefficientof performance of a refrigeration system is the ratio of refrigerating effect to the compression work; therefore the coefficient of performance can be increased by increasing the refrigerating effect or by decreasing the compression work.
P- H Diagram
The most convenient chart for studying the behavior of a refrigerant is the p-h chart, in which the vertical ordinates represent pressure and horizontal ordinates represent enthalpy. A typical chart is shown Fig.1.2, in which a few important lines of the complete chart are drawn. The saturated liquid line and saturated vapor line merge into one another at the critical point. A saturated liquid is one which has a temperature equal to the saturation temperature corresponding to its pressure. The spaces to the left of the saturated liquid line will, therefore, be sub cooled liquid region. The space between the liquid and the vapor lines is called wet vapor region and to the right of the saturated vapor line is a superheated vapor region.
DEFINITION
“Refrigeration is the science of providing and maintaining temperature below that of surrounding (ambient) temperature”.
The term ‘maintain’ implies, the continuous extraction or removal of heat from a body which is already at lower temperature than its surroundings.
Removal of heat from a body at lower temperature is possible only with the aid of external agency according to the Second Law of Thermodynamics.
Refrigerating Machine
A refrigerating machine® which is either cool or maintain a body at a temperature below that of surroundings. Fig.1.3. Represents a refrigerating machine R which will either cool or maintain a body at a temperature below that of surroundings.
The machine R, abstracts heat Q1 from a cold body at temperature To.And with the aid of work input Win form external agency, rejects heat Q2to the surroundings at higher temperature T2.
Methods of Refrigeration
The science of refrigeration utilizes several methods of providing temperature differential facilitating heat transfer between bodies. Following are the various methods of refrigeration.
• Ice refrigeration
• Dry ice refrigeration
• Air expansion refrigeration
• Evaporative refrigeration
• Gas-throttling refrigeration
• Steam jet refrigeration
• Liquid gas refrigeration
• Vapor compression refrigeration
• Vapor absorption refrigeration
• Thermo electric refrigeration