MadSci Network: Engineering

Re: Sound Waves used as a refrigerant

Date: Sun Jan 17 09:55:41 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Engineering
ID: 916420719.Eg


Not having seen the same show, I am sure I canot give a perfect answer, 
but since you are studying air conditioning, I will assume that you 
understand a few of the basic principles.

First, ordinary air conditioning requires a refrigerant to work.  
Refrigerants are materials that are chosen for many properties (non-
corrosive, easily pressurized, non-toxic, etc.), but primarily, they must 
be able to work between the temperatures of the environment being cooled 
and the environment that is to be heated. Many different Freons(tm) are 
made to work within many different environments.  All refrigerants will 
cool when their pressure is let down (often through a small restriction in 
a tube) and will heat when compressed. Engineers have compiled diagrams 
that show the properties of many refrigerants and other gases, generally 
referred to as Mollier diagrams, that indicate easily just how much a gas 
will cool when being dropped from a high pressure to a low pressure at a 
specific temperature.  I am sure you have seen one of these, or if not, a 
table constructed from one.  The same diagram will indicate how hot the 
gas will become when compressed from a low to high pressure at a given 
temperature.  Incidently, not all gases will heat when compressed.  
Hydrogen, especially at high pressures, will cool when compressed and will 
heat when its pressure is relieved, but hydrogen is an unusual gas.

Air, however, is a normal gas.  When compressed, it heats.  When 
depressurized, it cools.  Mollier diagrams have been compiled for air.  To 
get the type of effect you saw, the experimenter might have chosen his 
conditions carefully.  To form frost, the metal tube would have to be 
cooled, but if the temperature in the tube was already near freezing, only 
a little bit of cooling would be required.  After choosing the right 
conditions, so that only a little bit of cooling is needed to form frost, 
he needs to create a pump in an open tube.  This may sound impossible, but 
is actually quite easy.  Choosing a pipe of the right diameter and length, 
then stimulating one end at the right frequency, the sound waves will be 
amplified.  Pipe organs work on this principle.  Inside the pipe standing 
waves are created.  The waves consist of high pressure and low pressure 
regions that are stable and may be only a few inches or less from one 
another.  The differences in pressure can be very large.  That could be 
the source of the pump.  In a closed tube, the gas does not move much 
between the high and low pressure zones, but the pressure differences are 
there if one chose to exploit them.  Therefore, the trick would be to put 
a hole in the tube where there is low pressure. Air rushing in from the 
environment would be cooled and would lower the temperature of the inside 
of the tube.  The higher the pressure or the more intense the sound 
stimulation, the greater the cooling effect will be.

Practicality?  Will this ever be used?  Probably not, because the process 
would be very inefficient, and few people would like to live inside a 
organ pipe.  The existing cooling and heating methods are reliable, 
widespread, and accepted, so replacing them in our lifetimes by even a 
highly superior system is not likely.  That is not to say that there could 
not be very unique and special applications where this would be an 
important technique, but wide spread application is unlikely in my opinion.

Thnka you for your question. I look forward to seeing the experiment you 
talk about on TV soon.

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