[ale] [WAY OT] WARNING - diy car ac recharge kits can be dangerous

[Phil Turmel]

On 08/16/2013 11:57 AM, Jim Kinney wrote:
> IF you KNOW:
> 
> 1. the total volume of the tubing on both high and low pressure sides
> 2. the pumping rate of the compressor
> 3. the ambient air temp
> 4. the cold-side coil temp
> 5. the hot-side coil temp
> 6. the specs of the particular refrigerant
> 7. the total volume of oil in the system
> 8. the somewhat messy thermodynamic equations
> 9. differential equations
> 
> you CAN calculate what the pressures should be and generate pressure/temp
> curves to extrapolate your data with.
> 
> There's a reason no one does it that way. The old pressure method was
> unreliable and will cause damage to modern, high-efficiency systems. About
> the best you can do is to use a high/low gauge and make sure that you are
> below the max on the high and between the min/max on the low. Get the low
> side too high and you'll freeze up the coils or crap out the compressor
> trying to compress a flow that's partially liquid.

Actually, it's "get the low side too *low* and you'll freeze the coil".
 But yes, the dual pressure gages don't give you enough information to
perform a complete thermo analysis.  But they *do* give you enough
information to roughly judge the operating margin.

The pressure/temperature table for r-134a tells you the possible
temperature the corresponding coil *can* reach (not *will* reach).  This
correlation is printed on my gage dials--no pocket chart needed.

Anyways, if the low-side pressure indicates that the cold coil can reach
below 32F, it's likely to freeze in extended operation.  (And also
indicates that the low-side cycling switch has failed.)

If the high side pressure corresponds to 100F, then you can be sure the
system *won't* cool when the ambient is 105F.

If there isn't enough refrigerant in the system, the low-side cycling
switch will kill the compressor before the high side has built enough
pressure to "push" heat out the radiator.

If the orifice is obstructed, one of the pressure switches will cut the
compressor after a very short "on" duty cycle.  Usually the cycling
switch unless the system has been overcharged.

If the orifice is eroded or blown out, the compressor will stay on--the
low side won't drop enough to trigger the cycling switch.  (This is also
true for most systems at engine idle--not enough flow to keep up.)

If all else appears Ok, with the engine RPM high enough to make the
compressor cycle on and off, you can compare the ideal coil temperatures
with actual.  Large discrepancies suggest contaminated refrigerant or a
failing compressor.

These basics are as true for modern high-efficiency systems as for older
pieces of junk.  The new stuff is just engineered with fancier fluids
that can achieve higher delta-T for a given heat/fluid flow rate.

Phil