On Sun, 29 Apr 2007 11:20:49 -0500, "Leon"
wrote:


Tom Veatch wrote in message
.. .
On Sun, 29 Apr 2007 05:11:22 GMT, Lew Hodgett
wrote:


If you dry the air going into the compressor, you'll have dry air in
the storage tank. The compressor doesn't create water.

Consider this. Given the proper sized compressor the amount of air being
dried on the "exhaust side" is much less than the air on the "in take" side.
The dryer would have to dry much faster on the intake side. Typically with
the correct sized compressor for the task at hand the exhaust side releases
air at a smaller CFM than the intake side. Additionally, the compressor
tank catches a majority of the water and lessens the work on the dryer.


Not to mention, as I previously stated, the state change physics of
water is such that higher pressure at a given temperature in a closed
container forces the equilibrium point toward more condensation.

Assuming the temperature of the air entering the dryer is the same in
either case, the pressure upstream of the compressor is less than it
is downstream of the tank. Therefore, a given drying process located
downstream of the tank operates at higher pressure and condenses more
water from a given mass of air than it would if that same process were
operating upstream of the compressor. Repeat, that's assuming the
process occurs at the same temperature in both cases.

My comment "... The compressor doesn't create water." was directed at
a previous post that stated "That's where the water is." It is
patently obvious that a given mass of air contains the same amount of
water entering the compressor as it does exiting the compressor and
entering the storage tank. The compressor might add oil vapor to the
air but it doesn't add water.

Technical discussion follows, skip it if you wish.

Atmospheric air is a mixture of gasses which includes some amount of
water vapor. The partial pressure of each gas in the mixture is
related to the total pressure in the same ratio as the number of
molecules of each gas is related to the total number of molecules in
the sample. For example, and using round numbers for convenience, if
the air contains 1% water vapor and is compressed to 100psi, the
partial pressure of the water is 1psi. At 200psi, the partial pressure
of the water is 2psi.

As the sample of gas is compressed, the partial pressure of the water
vapor increases in proportion to the increase in total pressure. At
any given temperature, water has a specific vapor pressure which
increases as the temperature increases. If the partial pressure of the
vapor is greater than the vapor pressure of the liquid at that
temperature, the rate of condensation will exceed the rate of
evaporation and the amount of vapor will decrease as the amount of
liquid increases. Net condensation will occur until the partial
pressure of the remaining vapor becomes equal to the vapor pressure of
the liquid. That is the equilibrium point where the rates of
condensation and evaporation are equal to each other.

From that point, isothermal compression causes net condensation and an
increase in the amount of liquid in the container. The reverse is true
for Isothermal expansion. Isobaric temperature increase drives net
evaporation. The amount of liquid reduces and the amount of vapor
increases until the partial pressure of the vapor again equals the
vapor pressure of the liquid. Likewise, isobaric temperature decrease
drives net condensation.

Everything that is correct in this thread follows from those basic
facts of physics.