I didn't get the original message but Karl, you can use R-408 as a drop in
replacment for R-502. The tech manuals will not agree but We have done it
many times, sometimes on a unit with a hermeticly sealed compressor where
there was not an easy way to change the oil. On an apple box I suspect you
have semi-hermetic compressors. The efficiency will be very close to
original with only slightly higher head pressures. Keep those condensers
clean! If you equipment is shot maybe it is time to upgrade. But if the
system has lost it's charge then use the R-408. I recommend if the system is
very short, remove the refrigerant change oil to alkybenzine (avoid poe if
possible it cleans the system and cloggs strainers) change filter, evacuate
and charge with R-408. If system is only slightly short of refrigerant top
it off . BTW R-502 is closer to the very popular R-404 today's choice than
R-12 in terms of pressures.
Good Luck Lyndell

wrote in message
ps.com...
On Aug 28, 6:29 pm, "Karl Townsend"
wrote:
After only 30 years it looks like one of my apple cooler's compressor and
evaporator unit needs to be replaced. It uses 502 refrigerant. This was
the
best choice for low temperature coolers with electric defrost when I got
it
new.

Not many 502 systems are installed today because 502 is so hard to get.
(Its
very similar to the now banned R12) But I own a 30 lb. jug plus about a
10
lb. charge in the old system.

A fella told me the 502 systems are inherently more efficient because
they
run at lower operating pressure. Efficiency is important, the electric is
north of $1K per month when it runs.

I'm searching all the commercial cooler units catalogs and I'm finding
nothing on efficiency. Most compressor/evaporators can be set up for more
than one refrigerant.

So, what's best?

Karl

I've several old engineering manuals that put ammonia gas as #1 for
overall efficiency. Nasty stuff, but still used for large warehouse
and ice-making operations. Also biodegradeable and ozone-friendly.
You probably won't find the information you need out of a catalog,
efficiency depends on a lot of things, one reason the books are so
thick...

Stan