[Diy_efi] Intercooled question

[Brian Dessent]

elcamino73 wrote:
> 
> Any one know how many btu's it takes to change one cubic foot of air one
> degree if the air is under 15 psi of pressure?

That would depend on a couple of things: the temperature of the air and
whether you are adding the heat with volume constant or pressure
constant.  I assume you meant 1 degree Fahrenheit.  The specific heats
of air at 15C are:

c_p =  1004 J/(kg*K)
c_v = 717.4 J/(kg*K)

The density of air at 15C and 1 atm is 1.23 kg/m^3.  So a cubic foot of
air at 15C and 15psi gague would be 70.4g.  To raise this mass of air by
(5/9)K would be 39.3 J or 28.1 J depending on whether you do it under
constant pressure or constant volume.  1 BTU = 1055 Joules.

For other temperatures, you could probably use the same c_p and c_v --
they are a function of temperature but I assume we're talking about
temps relatively close to room temp.  For example, at 350K they are 1008
and 721, not much different.  You would, of course, have to compensate
for a different temperature in the mass calculation as well.

Brian

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