Freeze protection in cold climates
I am programming the controls for several air handling units that run 100% outside air. They are variable speed and use hot and chilled water with freeze stats for freeze protection. During the winter they will be seeing 10-20 below 0 for outside air temps. Supply air control is failing to properly regulate the hot water valve, and so I'm looking for alternative strategies for keeping these units up and running during sub 0 days.
Are you having problems with the running or tripping when the units are off @ night?
I use MAT to limit the dampers @ 55F while occupied. During the night I keep the HW valve active and keep the MAT @ 60F. If you don't have a MAT sensor, you can always bang the HW valve open 15-25% during the night.
Without a MAT sensor, you will have to play with your dampers and SA setpoint to keep from tripping.
kinda tough to control mixed air dampers on a 100% outside air unit. Truthfully, you need to find out what the problem is with your supply air temp control loop. You probably could get by using a fixed percentage on a sliding scale based upon outdor air temp for the short term, so that at design temp of say -20you are at 100% open and live with it until you can get the loop tuned properly. Just curious what type of application uses 100% outside air at that type of outdoor air temps?
Here's a little more background on the units.
The units are being used in a commercial kitchen space, both as temp conditioning and as make-up air for exhaust hoods. The supply fan in the unit as well as the kitchen exhaust fans are all VFD driven. No mixed air, 2-position OA damper.
I've tried using a minimum valve position scaled based on OA temp, but because of the variable speed fan, it's not working so well.
How are the coils setup?
Is it OA damper then cooling then heating?
Where are the freezestats located?
OA Damper - heating coil - freeze stat - cooling coil (currently drained) - supply air temp
Originally Posted by mechinc
I believe controls are the wrong approach to a freezing problem. I've repaired way too many coils to even consider this solution.
Antifreeze is a simple, solid state, solution that wont break at the worse time.
DowTherm and Dow Frost are two glycol products that are designed for this application. I've used a whole lot of it and that's all I would consider.
I know control guys tend to solve problems with controls but glycol is a better solution in this situation.
I should have played the g'tar on the MTV. MK
You can be anything you want......As long as you don't suck at it.
+100%....run the glycol and sleep well @ nite....
Originally Posted by hvacker
when in doubt swap it out....NOT!!
No freeze circ pump? Does valve fail open?
Expect nothing, yet expect the unexpected.
Press on Regardless, Endeavor to Persevere.
If possible, lower your preheat hot water temp to the AHU. You always want the hot water valve around 50% open. As long as you have good flow through the coil, you will avoid most freezstat trips. By doing this you will also have an easier time with loop tuning on your discharge air.
Does this unit have a face bypass? That would be silly if it didn't and the design temp is that low. I have had problems with similar units that there is a long delay from when the valve opens and the supply starts to warm. Most guys would then slow down the pid loop so it doesn't overshoot but then it suffers nuisance tripping on freeze protection. I would install a sensor after the heating coil and control off that.
Is the unit tripping out while running or on start-up?
This is a good question why in the world "if" there is a snow balls chance in hell of something bad happening that Glycol would not be considered in such an app??
Originally Posted by hvacker
And maybe explain a bit more,,,,,,,
When does it trip? What changes during Occ and UnOcc? What Changes between a demand for minimal ventilation and the need for make up air for the kitchen exhaust??
One PID driving the heating with no outside air and the same PID driving the train with 100% outside air?
Duel PID'S maybe a thought?