Preventing snow entry into Make-up air unit
Preventing snow entry into Make-up air unit
(OP)
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.
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RE: Preventing snow entry into Make-up air unit
Lots of info and things like this
https://www.munters.com/en/munters/products/mist-e...
Most will need power I think but they are some guides and lots of options.
https://www.google.com/url?sa=t&source=web&...
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RE: Preventing snow entry into Make-up air unit
RE: Preventing snow entry into Make-up air unit
I think I need something that separates the snow from air and then only melts the snow. the MUA is direct-gas fired. It would be extremely expensive to heat that amount of air with electric resistance heat.
Thanks for linking the document. Some of their proposals will be hard to do architecturally. We have to go through an architectural review process for changes visible on the outside. and some ideas (having pre-filters that need to be changed when full_ don't really solve the problem since that kind of does what we do now, except with additional filters. filters are expensive and labor-intensive. I think their idea of the large snow entrance attached to the wall is the best, but also the worst looking.
A preheater would have to be listed to get wet and work without filter. But if I have a pre-heater that heats air to above 32°F, I could use that to heat to heat to 50°F and not use an MUA.
google gave me other ideas (like snow stopper screens etc.). but the system needs to be self-cleaning or the snow accumulated will reduce airflow. The louver (Greenheck ESD 635) is extremely good against water penetration. but snow is different. It will have an exterior bird-screen.
Some results show a heat-traced screen. Problem with any exterior melting solutions is that the water will drip down and freeze on the building or the ground. Depending on amount of snow, that can create an ice heap.
ASHRAE seems to recommend under 500 fpm of free air velocity. The problem is, that is just not always enough.
RE: Preventing snow entry into Make-up air unit
You might not need as mush, but that's a lot of air...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Preventing snow entry into Make-up air unit
There also should be some control that recognizes if there is snow and not just heat the air to above 32° all the time. Snow that goes into the MUA only happens so many hours a year and if that tis heated with electricity, don't want to do that all winter. If that theater is inside, I still need a drain pan and route the water somewhere.