George....I'll share my experience with these. Usually, when I find this type of unit, I have an excavator handy and just grab it off the roof and toss it in a truck, too easy no? I did have a demolition job that had one of these bad boys on a building that wasn't going down yet. I couldn't get the excavator in position to get it AND my dad wanted it for his office, so I had to devise a new plan. My helper and I got two 20 foot 2" pipes and chained them together with something (can't remember, a board maybe) holding them about 2 feet apart.. We then levered the unit to the edge and up onto the pipes. After tying a rope to the unit and going around a railing, we shoved it off and it slid easily and gently with only a little pressure on the rope to make it safely onto the trailer. It was a little scary at first, but when it happened, it seemed easy. The trick is to get the pipes tied together so they don't spread out too far and drop it, plus getting the end of your pipe ramp on the trailer and (important) looping the rope around something so you have a friction brake.

Good luck and be careful. As a second method, if you're not keeping the unit, I'd definitely break it down on the roof before removal. I've done this before with some I didn't have to keep whole. It's much easier handling the parts if you pull out the actual compressor and the coil first.

Just thinking about your post. If you mean not the whole unit, but just those big piston compressors. Those can be a bear to move too and you can't break them down much. You may still be able to slide them down something, just make sure they are tied and can't get away. Also, if you're more than one story up.......my methods won't work.....helicopter maybe???