Sunday, September 26, 2004

A Contest - Fiasco

Well, it was a fiasco. Just one tried to answer. Kudos to mr. Symeonidis, who gave a correct, though quite incomplete answer. Here is my submition to the matter. It is still a first draft, so I am open to suggestions. It contains the usuall spelling and grammatical errors.

4 comments:

Lipis said...

den mporo na po oti den katalava.. katalava kai to katalava mia xara.. alla to thema einai pos einai ligo diskolo na to katanoiseis afto to prama.. giati les ok zesti einai efkolo.. vazeis kati stin priza kai zestenontai oi antistaseis kai zestenete to meros.. me to krio omos einai ligo pio diskolo... DEN GINETE NA VGAZEI KRIO TO AIR CONDITION... :D

Anonymous said...

Egw eixa diabasma kai den mporousa na asxoli8w me tin sugrafi apantisis sto blog ... sorry. 8a summetasxw energa sto epomeno. Chris.

Superhappyjen said...

What did he win?

Zafiris said...

Your explanation about "how a cooling machine works" is quite correct and satisfactory. The only point that is awkwardly described is the one concerning the condition that "the expanded hot gas is left to exchange heatwith the cold surroundings and attain ambient temperature". I believe that it is not necessary (and practically cannot be done) for some time to lapse to have the hot gas attain ambient temperature. The cycle compression-decompression) is continuous and the piston of the compressor makes its up and down movements continuously.
What is necessary is that "heat must be removed from the expanded hot gas". This is done by the circulating air(around the coils) of the balcony or the kitchen (in the case of the home refrigerator). The more efficiently the heat is rejected the better the performance of the machine (lower temperature achieved). This is the reason that the back of the refrigerator must not be in a tight fit with
the kitchen wall but in some distance from it for the free circulation ofair and the effective dissipation of heat from the hot expanded gas, before the stage of compression begins. Also, this explains why during summer (hotter air) the performance of a cooling machine is inferior relative to winter, and in the old days or in a big scale factories, the coils of the
expanded gas were cooled by circulating cold water.

If you are thirsty for a more techical and short description (and in terms of Thermodynamics) of how a cooling machine works,e.g. a refrigerator, read the following:(of course, some knowledge of Thermodynamics is needed). A refrigerator may be considered to be a "heat engine" (for example, an automobile's combustion engine) operated in reverse. That is, a heat engine takes in heat from a hot reservoir (burning gasoline), converts a part of the heat into mechanical work output (moves the pistons), and rejects the difference as heat to a cold reservoir (exhaust fumes in the environment).

A refrigerator, however, takes in heat from a cold reservoir (ice compartment and food in the refrigerator), the compressor (electric motor)supplies mechanical work input, and heat is rejected to a hot reservoir (the air in the kitchen).

Have fun.
V. Keramidas (vkeramid@agro.auth.gr)