Q: Does a freeze stat always go after preheat coil and before chilled water coil or is it OK putting it on the leaving side of chilled water coil?
A: You should use a freeze stat to protect your water coils from freezing. Rooftop units with water coils should always have freeze stats, even if using a water/glycol mixture.
Normally the freeze stat goes on the leaving face of the heating coil set at 38F. Sometimes with a hot water coil there will be a small freeze protection pump as part of the piping system to rat race water around at the coil to keep it from freezing in the event of a heating plant failure. That is another option.
In cases with limited access I have seen a serpentine freeze stat installed on a metal frame that slides in a track similar to a side access filter track in between the hot water (HW) and chilled water (CW) coils (this assumes that the coils aren’t actually touching).
Additionally, I’ve seen on units with HW/CW coils together that the freeze stat is on leaving side of CW coil. Int that case it’s basically the same as leaving air of the HW coil just 12” or so away. My experience is that freeze stat is never on the incoming side of the HW coil – always after the HW coil.