Re: nitrous in your car?

[Robert Myers <rmyers@inetone.net>]

At 07:39 PM 09/29/1999 -0500, you wrote:
>There is a significant decrease in pressure between a full tank and a near
>empty tank at the same temperature (which thereby leads to a significant
>decrease in available HP).  Thus, the bottle heaters are designed to
>compensate for the loss of volume as the tank gets empty, which keeps the
>pressure closer to that of a full tank.
>
>Said another way:
>
>pv=nRT

This is the applicable equation only after nearly all the liquid N2O has
evaporated.  Before then the liquid's presence maintains nearly uniform
pressure in the tank *assuming constant liquid temperature*.  Upon release
of gaseous N2O more liquid will evaporate to replace the gas in the tank
thus keeping pressure up.  It takes energy to convert the liquid into gas.
That energy comes at the expense, initially, of the liquid (and tank) which
will get colder as gas is released.  This cooling effect will reduce the
equilibrium vapor pressure of the (now cooler) liquid and the pressure in
the tank will tend to fall.  When there is much less liquid and gas is
consumed at approximately the same rate the heat needed to drive the
evaporative process is derived from a smaller and smaller mass of liquid
and the chilling effect is incresed since the same amount of energy is
being crawn from a decreasing mass of liquid in the same time period.
Apparently these effects are compensated for by use of a heater to maintain
the temperature at a more or less constant value.

Once all the liquid has been evaporated then the PV=nRT equation applies.

Note the assumption that the volume of the gas does not change is just a
first approximation.  The volume of the liquid does slowly decrease and
therefore then volume of the gas does slowly increase but as a first
approximation it can be regarded as more or less constant since the tank
doesn't change size appreciably.


___
   Bob
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