Peter,

> The 4 should have been 14 (typo). 14at is 200psi. The
> temperature/pressure
> charts I have here show:

Temperature/Pressure charts are just the start of our science - and it
is a science. We use "subcooling" and "superheating" in order to
perform refrigeration.

>                 R12     R22     R500    R502    R717
>
> 0 deg C         30.1    57.5    37.9    68.2    47.6    (psig)
> 32 deg F

> 17 deg C        60.1    105.4   73.5    119.6   96.9    (psig)
> 62 deg F

And what refrigerant are you referring to? Don't forget that R12 does
not exist any more. Unless of course, you live in a country that chose
not to sign the Montreal Protocol.

> The refrigeration manual says that the evaporator pressure is always
> positive (why).

This is not correct. Is that "Refrigeration for Dummies"? :-)

But, for *most* systems, this is correct. Let's say you run standard
refrigeration systems on a vacuum and you got a leak on the Low
(Suction) side. Instead of venting refrigerant to atmosphere (which
used to be OK), you'd suck in all sorts of nasty contaminants -
including moisture and non-condensables into your system.

>  System charging is always by mass not by pressure.

Ah! Again. "Refrigeration for Dummies"? :-) This is not correct. A good
refrigeration and/or airconditioning technician will always charge by
pressures, temperatures, superheat temperatures and experience. Mass is
ONLY possible when you start with an empty system and a reliable
charging weight from the equipment manufacturer. If the system has been
changed in any way, this is no longer possible.

Charging by mass or "Bomb Charging" as it is affectionately known is
usually reserved for production lines, domestic refrigerators and A/C
units (Window Rattlers) and the automotive industry.

> The pressure in the system may be highest on the compressor outlet as
> you
> say.

Not may be. Is.

> Strange, those split a/c units are circulating refrigerent at 15 at in
> and
> out of a house using flexible hoses ?! Ouch.

Flexible hoses? Not in this country. However it could be done. The A/C
lines in your car are flexible. They handle the pressures OK, but they
are porous. That's why they now try to run as much as possible in
aluminium tube. Reduce the amount we lose to the atmosphere. I can't
see why they can't use a lined hose.

What is your question? I will try to help.

Regards,

Sean

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