Piston compressors such as in some refrig don't pump liquid. If you
transport a fridge on the side, there is a chance some of the lubricating
oil will get into the piston.
Upright for 24 hours gives it a chance to drain out of the piston. I have
personally seen fridges which were ruined by the "on the side"
Doesn't seem to be a problem with AC, or dehumidifiers, or other equipment.
Wonder why fridge compressors are so persnikety?
Christopher A. Young
Do good work.
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