Moisture on Final Filters in Blow-through AHU's 1

[LeaderP]

I've been involved in trouble shooting/critiquing the design of the AHU's at a large new hospital in the midwest where the engineer used a blow-through fan with the chilled water cooling coils and final HEPA filters downstream of the fan. The units were field erected and poorly constructed so there's a lot of bypass of the warm moist mixed air around the coils and through the gaps between the vertically stacked coils. I did not observe moisture carry-over off the fins but the final HEPA filters are loaded with water to the point they had to remove several filters to get some airflow. I suspect the moisture in the by-pass air has condensed on the cold filters. Most of it appears to have condensed on the underside of the cold intermediate drain pans. I talked to some friends at HVAC equipment manufacturers and they dug up an old internal memo where one of their engineers hypothesized that the pressure drop in the final filters caused cooling of the nearly saturated discharge air coming off the coils(50.0F DB/49.9F WB)and is the cause of the water on the filters. He cited Boyles Law(pV=MRT)and if P2 is lower than P1, it stands that T2 is lower than T1. I've never heard of this being a problem but also rarely use blow-through. With draw-through, the fan heat ends up increasing the spread between leaving dry bulb and wet bulb. The equipment mfgr. engineer said you could expect a 0.5F drop for each 1 in.w.c. drop. Anyone ever experienced this problem?

 

[JEFF1]

The first explanation (bypass air condensing on cold filter media) sounds like a more probable explanation to me, especially if the construction is poor and there is a lot of bypass air. Condensate could even be forming in the cold air downstream of the coil before it reaches the filters. I've heard of (but not witnesses) situations where someone tries to blow outside air into a conditioned space, resulting in the formation of fog in the conditioned space.<br>
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The other possibility is complicated. I once tried to determine if this was the cause of water that was being found downstream of the fan (relatively high pressure, maybe 4" w.g.). In this case, the theory was that the increase (not decrease) in pressure was causing condensate to form. The result turned out to be unrelated to this - it was some form of leak, I think, but don't remember the details. But the thermodynamics is more complicated than Boyle's law. The psychrometric chart varies with air pressure. I don't remember for sure, but I think that when air pressure rises, the air can hold less moisture, and when it falls, it can hold more moisture. This sounds right - Le Chatelier's principle says that when you create a "stress", physical processes will tend to relieve that stress. So when you "squeeze" the air, it can't hold as much moisture. I can't say for sure this is right since I don't have psychrometric charts handy, but a look at the psychrometric charts at different pressures will answer the question. (The pv=RT aspect is also at work, of course. But does the temperature also change a very slight amount because of the air friction?)

 

[Tom584]

LeaderP,<br>
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The air leaving your cooling coils is at 100% relative humidity (saturated) if there is any condensation occuring in the cooling coil. If the air handler cooling coil valve is modulating, the filters will be colder than coil discharge air when the cooling coil valve closes. Condensation could then easily occur on the filters. When the cooling coil valve opens again, the saturated air discharge will do little to dry them out. The process would seem to continue until air flow is reduce due to water in the filters and maintenance is called.<br>
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I must live under a rock, but I have never heard of the filters being on the discharge side of the cooling coil. It is pretty difficult for the filters to keep your coil clean when configured as you described. Who made your air handlers and what model are they? <p>Tom Worthington<br><a href=mailtoworthi@astro.as.utexas.edu>pworthi@astro.as.utexas.edu</a><br><a href= > </a><br>

 

[LeaderP]

Found the answer to my own question on a CD I recently received from FARR Filter Co.

Unfortunately I can't paste it here but if you contact you local FARR sales representative, they can get you the CD.

The article is titled &quot;Wet Filters in HVAC Systems&quot;



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