ROBERT C. PLUMB
chemical principles exemplified
Worccrtcr Polylechnic lnrlitute Worceaer, Mosmchussla 01 609
The Convertible Effect Illustrating the first low of fhermodynomics ond odiobotic processes
Riding in a convertible on a hot summer day is a pleasure, not only for the fresh air and wind, but because it is cooler. And it's not just that it feels cooler due to enhanced evaporation of perspiration-the air temperature is really lower! A thermometer about one foot above the back seat of a VW convertible read an average of 25.1°C when the car was stopped and 24.6"C when traveling a t 60 mph. The sun was to the rear, to avoid cffects of radiation upon the seats, and the temperatures are thought to be reliable air temperatures. Why should the temperature be lower in a moving convertible? It appears likely that it is the same effect as that produced at airplane wing tips, causing vapor trails. If, due to the shape of a moving object, a partial vacuum is created in some regions of space, the air in that rcgion must have expanded. Since the expansion is rapid, the process must be adiabatic and by the first law of thermodynamics the internal energy must drop. The internal energy of air is almost solely composcd of kinetic energy of translation of the gas molecules, and if the internal energy drops the temperature drops, The flow of air around a moving convertible apparently produces a partial vacuum in the back scat-and thus i t is a somewhat cooler place to ride!
Auto Air Conditioning without Refrigerant
Purdue University. The novel feature is that it cools the air in the car without refrigerant. It does this by a simple compression, heat exchange, and expansion cycle. The device is shown schematically in the accompanying figure. Hot air from inside the car is
f ,
vane
compressor w
H e a l Exchanger hot en6 T2
c o l d end 13
drawn into a rotary vane compressor, \\.here, on compression, it gets even hotter. It is then passed through a heat exchanger whcre it is cooled to the outside temperature. From the heat exchanger it is decompressed through the other side of the vane compressor (helping drive thc rotor) and the temperature drops. The cold air is then returned to the interior of the car. A quantitative thermodynamic analysis of the process is readily carried out. The steps in the process are represented in the diagram below. a i intake ~
air ezhaust
lllustroting the First Low of Thermodynamics
Suggestion by Arra Nergararian Worcester Pol~itechnicInstitute The conventional automobile is a fruitful source of illust,rations for the chemistry classroom. Kow a neur automotive device has been invented which illustrates in a particularly simple and elegant way some elementary thermodynamic principles. The invention is of general interest to the public and can be understood by the novice student of thermodynamics or used for quantitative problems for advanced students. The device is an auto air conditioner invented by Thomas C. Edwards1 uvhile earning his doctorate a t 1 Present address: Florida Technological University, Orlando, Floridit.
Consider 1mole of air carried through the entire process. Assume the compression and expansion are reversible and adiabatic. Upon compression the temperature changes to a value T2given by the equation
and the compression requires an amount of work w = C,(T, - T,)
(2)
In the heat exchanger the temperature drops from T z to T3ivhilc the pressure is approximately constant (a good way to visualize it is as a long pip? at temperature T z at the hot end and Ta at t h cold ~ end-vith a small volume of gas moving from the hot to the cold Volume 49, Number 4, April 1972
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end and losing thermal energy along the way). Upon expansion the temperature drops to a final value T4 +en by the equation
This equation tells how the exhaust air temperature is related to the pressure and temperature of the heat exchanger. Assuming a pressure of 1.3 atmosphere in the heat exchanger and using the C, of air
and requires an amount of work
w = C(T,- T , )
(4)
a negative quantity. Combining eqns. (1) and (3) and noting that Pp = PI and Pa = Pa one obtains the ~articularlvs i m ~ l eresult
If the day is very hot a reasonable value of Tais 100°F (313°K) and the air discharged in the car will be at TI = 63'F(29OoK)
which says that the exhaust and inlet temperatures are in the same ratio as the temperatures at the two ends of the heat exchanger. We may calculate T2for a particular PZ,PI, and Tl from eqn. (1) and substituting this into eqn. (5) gives us
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Journal of Chemical Education
This temperature does not depend upon how hot the air in the car was initially; it depends only upon the temperature of the cold end of the heat exchanger and the decompression ratio. It is a simple matter to calculate the coefficient of performance (Q removed from air/W) of this r e frigerator assuming a particular value of Tl and verify that it appears to be a practical refrigeration system.
