Dry systems work by pumping in dry air with a compressor or air tank into piping which holds down the wet valve holding water back. When a head melts and releases the compressor or tank can not maintain the pressure and water is released. This way only the area where water comes up out of ground needs to be kept warm instead of heating entire building.
However a critical piece to the whole system is the drum drip pictured below.
Yes even thou dry air is pumped into piping it still contains moisture which must be removed and this is what the drip drum does.
While some more sophisticated dry systems use nitrogen in them to keep down condensate and corrosion they are more expensive to maintain so air from a compressor is the norm. The big problem with these drip drums there is usually more than one and they are hiding in corners and get stuff shoved in front of them. they should require a plaque on dry systems showing where all drip drums are located so system can be properly maintained because its usually the building owner or maintenance man doing it because some systems require being drained monthly.
I take care of several dry systems for customers because they just do not want to deal with it or worry there people trusted to do it will forget to do it.