Lecture 6 2013

Refrigeration and Air Conditioning

Refrigeration is a process of maintaining low temperature in comparison to surrounding temperature.

“Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature.”

It is usually done with the aid of a mechanical device (e.g. pump/compressor) using a substance (called a refrigerant) which absorbs heat from low temperature (objects/space) and releases heat to elsewhere at high temperature.

A refrigerant usually works in two-phase conditions, i.e., liquid and gas, e.g., vapor compression refrigeration system.

In some cases, refrigerants remain in single-phase, e.g. air cycle refrigeration.

Reversed Carnot Cycle:

Vapor Compression Refrigeration System:

Unit of Refrigeration:

Refrigerants:

Refrigerants are heat carrying medium which absorb heat from a low temperature medium (refrigerated space) and discharge heat so absorbed to a higher temperature medium (environment) during their cycle in the refrigeration system.

Types of refrigerants:

The refrigerants that are most widely used in compressors are in a chemical group called either fluorinated hydrocarbons or halocarbons. 

 All of the halocarbon refrigerants can be divided into three subgroups, according their constituents.

Chlorofluorocarbons(CFCs): These are composed of chlorine, fluorine, and carbon atoms. Some in this group are CFC-11, CFC-12, and CFC-14. (The more familiar identification is R-11, R-12, and R-14)

Hydrochlorofluorocarbons(HCFCs): These are composed of hydrogen, chlorine, fluorine, and carbon atoms. Some in this group are HCFC-22 (R-22), and HCFC-123 (R-123).

Hydrofluorocarbons(HFCs): These are composed of hydrogen, fluorine, and carbon atoms. Some in this group are HFC-134a (R-134a), and HFC-125 (R-125).

Ozone Depletion

Despite their excellent properties for use as refrigerants, an extremely serious environmental problem exists with those halocarbons that contain chlorine, especially the CFCs. It has been found that they cause depletion of the ozone layer in the stratosphere.

Ozone(O3) is an oxygen(O2) molecule with an added oxygen

atom. The ozone layer blocks out much of the harmful ultraviolet (UV) radiation from the sun. The ozone layer has been progressively depleting. One chlorine atom can destroy 100, 000 ozone molecules.

Effects of a decreased ozone layer over earth include An increase in skin cancer Reduction in immunity against disease Harmful effects on corps, timber, and marine life

The relative ability of a substance to deplete the ozone layer is called its ozone depletion potential(ODP). A list of some of the refrigerants and their ODP values is given below:

Psychrometric Processes:

Apparatus dew point:

For cooling and dehumidification, the temperature of the cooling surface or spray water must be below the dew point temperature of the entering air. If the cooling surface or the spray shower is of large magnitude, the air may come at the saturated condition, known as apparatus dew point (adp) or effective surface temperature (EST).