Freeze Stat Tripping 1

[joepegas]

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.

 

[ChasBean1]

Joe, this is a big-time problem with steam coils that don't have face & bypass dampers. All the necessary heating is done at the top where the steam enters. Air that passes through lower regions of the coil barely picks up any heat. Usually if any one-foot section of freeze-stat element senses below setpoint, the unit trips.

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

 

[imok2]

Silly question but have you checked the set points on the freeze stats and verifiied the cutout temps or am I missing something?

 

[joepegas]

In the case where adding a bypass is not possible, would it be a good idea to use a W-shaped temperature sensor to average over a larger area that would sense the cold spots on the coil? If not, what type of cheap alternative may be an option. Also, the AC mechanics have already lowered the freeze stat trigger point as low as possible.

 

[ChasBean1]

Joe, for the averaging/controlling sensor, it would be a better idea to move it downstream so it feels less of a stratification effect. The freezestat should also be just upstream of the next coil, not at the immediate outlet of the heating coil. The freezestat should cover as much of the next coil area as possible.

 

[lilliput1]

It could be faulty. Replace a suspected one with one you know works. You may also be able to test one by dipping it in & out of a bucket of ice water & connecting the terminals to an ohmmeter. Arching at the switch contacts may have occured causing erratic contacts.
Usually we require freezestat to be wrapped around 1' on centers to cover @ SF of coil. See ChasBean1 above for placement.

 

[joepegas]

I was given another opinion on this situation due to two facts that may change the senario: 1) this is a 100% outdoor air system and 2) It uses VAV boxes to control temperature. The original assumption was that a condensate backup was the root cause of the problem. Could this be caused if the VAV boxes close,decreasing the demand for air, which increases the rate at which the condensate is produced? In effect when the less air moves over the coils the coils are basically oversized and more condensate is produced than the system is capable of handling. This seems to be a complicated problem and I was wondering if there is any way to test for a condensate logging problem...maybe a sight glass or a meter? I appreciate your help.

 

[lilliput1]

If condensate was backing up, you will get water hammer each time the trap discharges with the condensate line fully blocked. I thing your problem is faulty freezestat.

 

[ChasBean1]

Joe, no. With less air moving across the coil, less condensate will be produced. Steam condensing is a function of heat exchange between the moving airstream and the coil. With no air motion, very little steam will condense. It should be easier for the traps and drainage system to handle low load.