the black stuff you see is pieces of the compressor more then likely. The
low side pressure to my knowledge should be just that low side pressure not
a vacuum. You only get vacuum when you suck the system down empty. Are you
adding oil to the system when you install a new compressor? sounds like you
might be running low on oil and causing the compressor to seize up thus the
black material in the filter. It;s important to make sure you have the right
amount of refrigeration oil to lubricate the internal workings of the
compressor. Do you have a low pressure cutoff switch in the system? this
will protect the compressor by not allowing it to turn on if the system is
low on freon.
If you have the proper amount of freon in the system I would think the sight
glass would show it almost imediately.
You need to vacum the system and replace the drier the expansion valve along
with the compressor and add oil to it when you replace the compressor.
I'm not a certified ac tech but I've worked on many an ac system in my own
auto's and home to know about them but don't take my word for sure I'm just
offering you some idea's that might be useful knowledge to You.
"Akka" wrote in message
oups.com...
We continue to have problems with our engine-driven refrigeration
system, and wondered if anybody knows what we might be doing wrong. We
keep on blowing up compressors -- either they start to leak after a
couple of months use, or they fail (i.e., no longer bring down the
low-side pressure to a vacuum). We know it's not from 'slugging' ice,
because we routinely monitor the state of the return line about 4 feet
from the compressor, and it's never frozen. When the compressors leak,
we naturally don't know it until the fridge doesn't cool anymore, when
we look at the sight glass and see it's empty. This entails running
the system for 5 minutes or more, empty, because that's how long it
takes for liquid to appear in the sight glass when the system is full.
Is that a long enough time to ruin the compressor? The last time we
opened the system up, we found some black residue blocking one of the
filters at the expansion valve. Any idea how dirt could have gotten
into the system? Maybe from a compressor going bad? How do you clean
it all out?

I forgot to mention you need to flush out the system when you vacuum it
before you install the new components. Shops usually have a pump they flush
oil through the system with and then suck it out this gets all the metal
particles that came from the compressor out so they don't foul anything new.
"Akka" wrote in message
oups.com...
We continue to have problems with our engine-driven refrigeration
system, and wondered if anybody knows what we might be doing wrong. We
keep on blowing up compressors -- either they start to leak after a
couple of months use, or they fail (i.e., no longer bring down the
low-side pressure to a vacuum). We know it's not from 'slugging' ice,
because we routinely monitor the state of the return line about 4 feet
from the compressor, and it's never frozen. When the compressors leak,
we naturally don't know it until the fridge doesn't cool anymore, when
we look at the sight glass and see it's empty. This entails running
the system for 5 minutes or more, empty, because that's how long it
takes for liquid to appear in the sight glass when the system is full.
Is that a long enough time to ruin the compressor? The last time we
opened the system up, we found some black residue blocking one of the
filters at the expansion valve. Any idea how dirt could have gotten
into the system? Maybe from a compressor going bad? How do you clean
it all out?