Tell us a bit more about your application, the type of duct system you are using, etc. There are advantages to both but it is hard to make a recommendation without more information.
Don't use blow through if you need to have final filters at the discharge of the fan and also provide humidification. When you humidify the filters will get wet.
In general if you have requirement for high airchanges per hour and the sensible and latent loads are not that high, blow thru will require less coling tons because the fan heat will be on the entering side of the coling coil, not on the leaving side. But be warned per above.
Steamguy2: actualy i am not looking for a specific application, but I want to know the general guides for selection AHU, based on comparing the advantage & disadvantage of each type.
lilliput1: thanks, your reply is very useful to me
Placement of the humidifiers downstream of the final filters is preferred for the reasons specified above. Allowing 15' or any other reasonable amount of distance between the humidifier will help if it is placed in front of final filters, but it doesn't keep the damage to the filters from happening if the humidifier or its control should malfunction.
As to blow through versus draw through, it (blow through design) is helpful in allowing placement of the heating coils (especially when hot water coils with their higher pressure drop)in the reheat position. Most fans are better utilized with higher pressure drops placed downstream of the fan.
The placement of the fan motor (unless in an unconditioned space not a part of the conditioned load, such as an external mount in a fully ducted system)does not affect the amount of heat imparted to the system, only whether it is absorbed by the coil before or after passing through the conditioned space. Sometimes, such as in dehumidification systems, placement of the fan motor within the airstream after the cooling coil is an advantage.
Personally, I will never again use a blow-thru coil upstream of final filters if the coil leaving air condition is close to saturation. While I can't state the cause with certainty, seeing green growth on those filters for the first (and last) time was enough for me. Kudos to the description of motor location in the previous post. I "fell" for that low discharge temperature argument mentioned even earlier, but will not make that mistake again. Same goes for locating any humidifier upstream of final filters, in fact this is prohibited in some jurisdictions - for good reason. I also would not use a blow-through coil if it would be possible to otherwise use a draw-through and then convert fan discharge velocity pressure to static pressure with a well-designed duct transition. Blow-through coils also suffer from moderate to severe variations in face velocity unless a pressure equalizing plate is located between the fan and the coil. Finally, I am not aware of any reliable discussion regarding placement of larger pressure drops upstream or downstream of a fan. To my knowledge, the fan doesn't know and doesn't care. Just my two cents after many years.
well, I've got 2 units recently running in a hospital ..... a third unit shortly and all are dual duct, dual fan with humidifiers and final filters ....... what knd of fans do you use (DWDI?) ..... i use plenum type
I would typically use a plenum type fan as well for any air handling unit that included a discharge sound attenuator, final filter bank, humidifier or anything else that prohibited a direct duct connection to fan discharge.
In the State of Washington, hospital regulations require that steam injection type humidifiers be located a minimum of 15' downstream of the final filter bank. I agree.
In this region as well, I can't recall a new dual-duct system for at least 10 years, probably longer. The last one I rememeber was a VAV changover type that used both ducts for cooling in the summer. My boss at the time did that, and he received ample recognition. Although they go overboard in some areas, energy codes around here make us count BTU's quite carefully. We use single-duct terminal reheat with demand-based LAT and HWS temperature reset, CV in areas with pressure relationships, and VAV using FTU's in other locations. I too use twin supply fans, however, they are typically located in completely independent side-by-side custom AHU's equipped with isolation and cross-over dampers such that either side can feed both duct systems at about 75% of design air flow when the adjacent unit is out of service for any reason.
Regarding the "green" final filters downstream of a blow-thru DX coil, I recall discussions with the filter manufacturer about the influence of filter air pressure drop upon vapor pressure - no kidding. This was not news to them, but we never received a definitive answer and I was not in a position to conduct a research project.
For this same client, I later employed a dual temperature CWS sytem and 2 chilled water coils in series for the surgery AHU - again, a side-by-side arrangement. Base load chillers fed first coils and remainder of campus at normal CWS temperatures. Part load chiller fed second coils at depressed CWS temperatures during the summer, and the entire campus during the winter. Great way to go if geography permits. The improved ability to clean two shallow coils is another bonus.
