Chillers operate on the classic refrigeration cycle: a fluid/gas is compressed from gas to a liquid state, then heat is removed from that hot liquid through the condensor side of the chiller, the "cool" liquid is then sent to an expansion valve where it changes into a cold gas and then absorbs heat from the enviornment in the evaporator side, which in the case of an aquarium chiller is the heat exchanger. The now hot gas is then compressed into a liquid state and the cycle continues....
Chillers (except thermoelectric ones) do have refrigerant in them to transfer heat, but it's no longer Freon, which is a trademark name, (R11 or R22) but I think they now have R134, which is a more enviornmentally friendly refrigerant that doesn't destroy ozone tha way the old refrigerants do.
The evaporator in most aquarium chillers is a titanium heat exchanger due to the corrosive nature of saltwater and this is one reason why even small chillers, like the 1/15HP unit I have, are pretty expensive compared to other refrigeration units such as A/C's and refrigerators, etc. So, yes, they usually do have a refrigerant fluid/gas that is similar to Freaon, but it is just a different type of refrigerant.
Hope this helps!
