Freeze Stat Tripping
Freeze Stat Tripping
(OP)
My company has been having a problem with the freeze stat's tripping and shutting-off the air handlers occasionally under cold conditions. Originally I was led to believe that is was a condensate logging problem becuase Spirax-Sarco proposed replacing the existing F&T traps with trap-pumps. I didn't like this idea and starting digging to find out that the freeze stat's were tripping when the temperature sensor that controls the steam valves was reading 75 degrees or so. There are four coils in a 2X2 config. and four AHU's. Each coil has a freeze stat and the temp sensor runs diagonally across two coils (2 and 3). It was suggested that I use another temp. sensor across the other 2 coils (1 and 4). I also thought about using a sight glass and pressure gauges to check if the coils were backing up. One issue is that the problem happens occasionally on random AHU's and I can't figure out how to trace the problem. Also, there are vacuum breakers on the inlet side and the modulating valves are pneumatic. I would appreciate any help, thanks.





RE: Freeze Stat Tripping
First thing I would recommend is that face and bypass dampers be used if at all possible. Steam valves should be fully open below 35-40°F outside and the F&B dampers should modulate to maintain discharge temperature. Also, the freeze stats should not be placed at the immediate discharge of the steam coil, and if possible, a mixing baffle should be placed between the steam coil and the down stream coil to prevent air stratification. The freeze-stat should then be placed just before the downstream cooling coil; the device they are mainly protecting. I have yet to see a steam coil freeze, although maybe others have.
Last, use the discharge temperature (not the preheat temperature) to control the steam valves and F&B dampers. The controlling device should be in a location where air is fully mixed, such as just downstream of the supply fan.
Good luck! -CB
RE: Freeze Stat Tripping
RE: Freeze Stat Tripping
RE: Freeze Stat Tripping
RE: Freeze Stat Tripping
Usually we require freezestat to be wrapped around 1' on centers to cover @ SF of coil. See ChasBean1 above for placement.
RE: Freeze Stat Tripping
RE: Freeze Stat Tripping
RE: Freeze Stat Tripping