HVAC-Novice
Mechanical
- Jul 29, 2022
- 325
I'm in southern Wisconsin. We have a wide variety of direct-fired make-up air units for garages. Many of them have the issue that snow gets sucked in. that clogs up the filters, collapses them and then the MUA basically is filter-less. Sometimes the unit also shuts off due to lack of airflow (if filter doesn't' collapse. Maintenance then remove filters, or disable the unit. both obviously not great solutions. Most MUA have a lover intake and are located inside. Garages typically heated to 50-60F. Some garages have some occasional hosing down of trucks or equipment of the floor, so water content can be higher than outside air. MUA run intermittently, but can run for some hours at a time.
I'm designing an MUA replacement and so far my idea is to use an as large as possible rain-tight louver. Free air velocity is a bit under 400 fpm (face velocity under 300 fpm). There is a physical limit how large intake -louver I can install. In addition the MUA will be a bit above the louver and the transition moves up. This hopefully would make it harder for snow to travel up. but there is no room for much elevation difference. the solution should be reliable, robust and not require much maintenance (like manually removing snow every once a while)
What other measures could I take to collect snow before entering MUA and getting rid of it? No matter how low of a velocity the air will be, some snow is so light, it will get in. Any snow collecting in there, would have to melt and flow out. but if it is cold outside, that would freeze up the louver. Some ideas (and problems:
- build some snow trap in the duct to MUA (like a Tee where air goes straight, and snow falls down into the branch) and have a stainless steel drain pan at the bottom of the Tee-branch). This pan could be heated (electric), or just not insulated and heated by the room. I would have to drain the condensate somewhere and there is no drain nearby. Just running the condensate outside also wouldn't work due to freezing
I'm not sure many designer actually are aware of the issues. It is a real issue operators experience with a variety of designs. Admittedly, some of the older designs have too small (IMHO) louvers. But even if a louver is large enough, some snow still will get in.
I'm designing an MUA replacement and so far my idea is to use an as large as possible rain-tight louver. Free air velocity is a bit under 400 fpm (face velocity under 300 fpm). There is a physical limit how large intake -louver I can install. In addition the MUA will be a bit above the louver and the transition moves up. This hopefully would make it harder for snow to travel up. but there is no room for much elevation difference. the solution should be reliable, robust and not require much maintenance (like manually removing snow every once a while)
What other measures could I take to collect snow before entering MUA and getting rid of it? No matter how low of a velocity the air will be, some snow is so light, it will get in. Any snow collecting in there, would have to melt and flow out. but if it is cold outside, that would freeze up the louver. Some ideas (and problems:
- build some snow trap in the duct to MUA (like a Tee where air goes straight, and snow falls down into the branch) and have a stainless steel drain pan at the bottom of the Tee-branch). This pan could be heated (electric), or just not insulated and heated by the room. I would have to drain the condensate somewhere and there is no drain nearby. Just running the condensate outside also wouldn't work due to freezing
I'm not sure many designer actually are aware of the issues. It is a real issue operators experience with a variety of designs. Admittedly, some of the older designs have too small (IMHO) louvers. But even if a louver is large enough, some snow still will get in.
