[Diy_efi] Intercooled question

[Brian Dessent]

elcamino73 wrote:

> Maybe if I give you some details you might make things clearer (easier) for
> me to understand.
> I want to inject freon into an intercooler to bring air which will be at
> approximately 120 degrees down (F) to 40 degrees(F). The max pressure is 15
> psi, the max cfm is 1500. The freon will be pre-pressurized in a 5 to 10
> pound container which will be recovered when off boost. I would be happy to
> be able to do this for thirty seconds but 60 seconds would be preferable. I

The way I would approach this is to think in terms of the mass flow rate
-- kg/s (I like to work in SI units.)  So the air entering the
intercooler has T = 322K, P = 205kPa, and volumetric flow rate =
0.708m^3/s.  From the ideal gas equation (air can be treated as ideal),
PV = mRT, or m = PV/RT.  R for air is 0.287 kJ/(kg*K).  

So the mass flow rate (or "m dot", the dot meaning first derivative
w.r.t time) = 205 * 0.708 / 0.287 / 322 = 1.57kg/s.

Since the air is free to contract as heat is removed, we want to use
C_p, the specific heat at constant pressure.  C_p is about 1000
J/(kg*K).  You desire to lower the temperature by 44.4K.  Therefore
m-dot * C_p * delta-T = Q-dot, which is the rate that energy (heat) must
be removed.  In this example it comes to 67.7 kJ/s.  1kJ = 0.948 BTU, so
that means 64.2 BTU/s.  

If one pound of R12 can remove 70 BTU, then you're looking at about 11
seconds worth of cooling for 10 lbs of R12.  Now, this is worst-case, in
that the pressure and flow rate that you mentioned are the maximums, so
if in reality they are less, then the R12 would last longer.  

(You said that you were planning on recovering all the R12, and since
it's pretty expensive these days and requires certification to acquire,
I would assume you know what you're doing.  It sure seems that R134a
would be a much easier and cheaper alternative though, and safer for the
environment in case something goes wrong...)

Brian

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